Author: Bryan Orr


This is a basic overview of the refrigeration circuit and how it works. It isn’t a COMPLETE description by any means, but it is designed to assist a new technician or HVAC/R apprentice in understanding the fundamentals.

First, let’s address some areas of possible confusion 

  1. The Word “Condenser” Can Mean two Different Things Many in the industry will refer to the outside unit on a split air conditioner, heat pump or refrigeration unit as a “condenser” even though it will often contain the condenser, compressor, and other parts. It’s better to call the outside component the “condensing unit” or simply the “outside unit” to reduce confusion.
  2. Cold and Hot are Relative terms Cold and Hot are both an experience, a description, a comparison or an emotion. Cold is a way to describe the absence of heat in the same way that dark describes the absence of light. We will often use the words cold and hot to compare two things “Today is colder than yesterday” or to communicate comfort “It feels hot in here”. These are useful communication tools, but they are comparisons not measurements.
  3. Heat and Absolute Zero Can be Measured We can measure heat in BTUs and light in lumens, we cannot measure cold or dark. Absolute cold is the absence of all heat.  -460°F(-273.3°C) (cold) is known as absolute zero, -460°F(-273.3°C) is the temperature at which all molecular movement stops. Any temperature above that has a measurable level of heat. While this is a known point at which all molecular movement stops, it has not (and likely cannot) be achieved.
  4. Boiling Isn’t Always Hot When we say it’s “boiling outside” we mean it’s hot outside. This is because when we think of boiling we immediately think of water boiling in a pot at 212°F (100°C) at atmospheric pressure, which is 14.7 PSI (Pounds Per Square Inch)(1.01 bar) at sea level. Boiling is actually just a change of state from liquid to vapor, and the temperature that occurs varies greatly based on the substance being boiled and the pressure around the substance. In an air conditioner or a refrigeration system, refrigerant is designed to boil at a low temperature that corresponds to the design of the system. On an average air conditioning system running under normal conditions with a 75°(23.88°C) indoor temperature, the evaporator coil will contain refrigerant boiling at around 40°F(4.44°C). In air conditioning and refrigeration when we refer to “boiling”, “flashing” or “evaporating of refrigerant” we are talking about the process of absorbing heat, otherwise known as cooling.
  5. Cooling and Heating Cannot be “Created” We are not in the business of making heat or creating cool; it cannot be done. We simply move heat from one place to another or change it from one form to another. When we “cool” a room with an air conditioner, we are simply absorbing heat from the air into an evaporator and then moving that heat outside to the condenser where it is “rejected” or moved to the outdoors.
  6. Heat and Temperature Aren’t the Same  Imagine a shot glass of water boiling away at 212°F(100°C). Now imagine an entire lake sitting at 50°F(10°C). Which has a higher (hotter) temperature? That answer is obvious-I just told you the shot glass had 212°F(100°C water in it so it is CLEARLY hotter. But, which contains more heat?  The answer is the lake. You see, heat is simply energy and energy at its basic form is movement. When we measure heat we are measuring molecular movement; the movement of molecules–atoms stuck together to make water or oxygen or nitrogen. When molecules move FASTER they have a HIGHER temperature and when they move SLOWER they have a LOWER temperature. Temperature is the average speed (velocity) of molecules in a substance, while heat is the total amount of molecular movement in a substance. The lake has more heat because the lake has more water (molecules).
  7. Compressing Something Makes it Get Hotter (Rise in Temperature) When you take something and put pressure on it, it will begin to get hotter. As you pack those molecules that make up whatever you are compressing, they get closer together and they start moving faster. If you drop the pressure the molecules will have more space and will move slower causing the temperature to go down.
  8. Changing the State of Matter Moves Heat Without Changing Temperature  When you boil pure water at atmospheric pressure it will always boil at  212°F(100°C). You can add more heat by turning up the burner, but as long as it is changing state (boiling), it will stay at 212°F(100°C). The energy is changing the water from liquid (water) to vapor (steam) and the temperature remains the same. This pressure and temperature at which a substance changes state instead of changing temperature is called its “boiling point”, “condensing temperature” or more generally “saturation” point.
  9. Superheat, Subcool, Boiling, and Saturation Aren’t Complicated  If water is boiling at sea level it will be 212°F(100°C). If water is 211°F(99.44°C) at sea level we know it is fully liquid and it is 1°F(-17.22°C) subcooled. If water is 213°F(100.55°C) at sea level we know it is vapor and superheated. If something is fully liquid it will be subcooled, if it is fully vapor it will superheated, and if it is in the process of change (boiling or condensing) it is at saturation.

Where to start 

Take a look at the diagram at the top of this piece and start at the bottom left. Are you looking at the part at the bottom left? OK, now read this next line OUT LOUD:

Compressor > Discharge line > Condenser > Liquid Line > Metering Device > Expansion Line > Evaporator > Suction line and then back to the Compressor

When I first started in HVAC/R trade school this was the first thing my instructor forced me to LITERALLY memorize forward and backwards before he would allow me to proceed.

While I am not always a huge fan of rote memorization as a learning technique, in this case, I agree with committing this to memory in the proper order.

These four refrigerant components and four lines listed above make up the basic circuit that every compression refrigeration system follows. Many more parts and controls may be added, but these basics are the cornerstone on which everything else you will learn is based. Once you have these memorized we can move on to describing each.

Compressor

The compressor is the heart of the refrigerant circuit. It is the only mechanical component in a basic refrigeration system. The compressor is like the heart that pumps the blood in the body or like the sun that provides the earth its energy. Without the compressor to move the refrigerant through compression, no work would be done and no heat would be moved.

The compressor creates a pressure differential, resulting in high pressure on the high side (discharge line, condenser & liquid line) and low pressure on the low side (suction line, evaporator and expansion line).

There are many different types of compressors, but you will most likely see Scroll and Reciprocating type compressors most often. A reciprocating type compressor uses pistons, valves, and a crankshaft. Reciprocating compressors operate much like car engines; pulling in suction vapor on the down-stroke and compressing that vapor on the up-stroke. A scroll compressor does not have any up-down motion like a reciprocating compressor. A scroll compressor uses an oscillating motion to compress the low-pressure vapor into high-pressure vapor.

The compressor pressurizes low-pressure vapor into high-pressure vapor, but it also causes the temperature of the gas to increase. As stated in the gas laws, an increase in pressure causes an increase in temperature and a decrease in volume. In the case of refrigerant cooled compressors, heat is also added to the refrigerant off of the kinetic (bearings, valves, pistons) and electrical (motor windings) mechanisms of the compressor. Compressors require lubrication; this is accomplished through oil that is in the compressor crankcase, as well as oil that is carried with the refrigerant. Liquid entering the compressor through the suction line is a very serious problem. It can cause liquid slugging, which is liquid refrigerant entering the compression portion of the compressor. Liquid slugging will most likely cause damage to the compressor instantly. Another problem is bearing washout or “flooding”. This occurs when liquid refrigerant dilutes the oil in the compressor crankcase and creates foaming, and it will greatly reduce the life of the compressor because it will not receive proper lubrication and too much oil will be carried out of the compressor and into other parts of the system. The compressor also (generally) relies on the cool suction gas from the evaporator to cool the compressor properly, so it’s a delicate balance to keep a compressor from being flooded and also keep it cool.

Condenser

Condensers come in all different types, shapes, and sizes. Regardless, they all perform the same function: rejecting heat from the refrigerant. The refrigerant entering the condenser was just compressed by the compressor, and this process increased the temperature by packing the molecules together which added heat to the vapor refrigerant due to the motor and mechanical workings of the compressor. This process in the compressor also greatly increased the pressure from a low-pressure in the suction line entering the compressor, to a high-pressure vapor leaving the compressor.

The condenser has three jobs:

  1. Desuperheat the refrigerant (Drop the temperature down to the condensing temperature)
  2. Condense (saturate) the refrigerant (Reject heat until all the refrigerant turns to liquid)
  3. Subcool the refrigerant (Drop the temperature of the refrigerant below the condensing / saturation temperature)

The condenser’s job is to reject heat (drop the temperature) of the refrigerant to its condensing (saturation) temperature, then to further reject heat until the refrigerant fully turns to liquid. The reason it must fully turn to liquid is that, in order for the refrigerant to boil in the evaporator, it must first have liquid to boil.

The way in which the condenser removes the heat from the refrigerant varies. Most modern condensers flow air over the tubing where the refrigerant is flowing. The heat transfers out of the refrigerant and into the air. The cooling medium can also be water. In the case of a water source system, water is circulated across the refrigerant in a heat exchanger.

In either case, the condenser relies on the removal of heat to another substance (air, water, glycol etc..). For instance, if you turned off the condenser fan so that no air was flowing over the condenser coil, the condenser would get hotter and hotter. This would cause the pressures to get higher and higher. If it kept going that way it would trip the internal overload on the compressor or cause other damage.

The hot vapor from the compressor enters the condenser and the superheat  (temperature above condensing temperature) is then removed. The refrigerant then begins to change state from vapor to liquid (Condense). The refrigerant maintains a constant temperature until every molecule of vapor is condensed. The temperature of the liquid again starts to fall. This is known as subcooling. When we measure subcooling we are measuring degrees of temperature rejected once the refrigerant has turned completely to liquid.

Temperature above the saturation temperature is called superheat. Temperature below the saturation temperature is called subcool or subcooling. So when something is fully vapor (like the air around us) it will be superheated, and when it is fully liquid (like the water in a lake) it is subcooled. 

Metering Device

The metering device is a pressure differential device that creates a pressure drop to facilitate refrigerant boiling in the evaporator coil.

The metering device is located between the liquid line and the evaporator. The liquid line is full of high-pressure liquid refrigerant. When the high-pressure liquid hits the small restrictor in the metering device, the pressure is immediately reduced. This drops the pressure of the refrigerant to such a degree, that the saturation temperature is lower than the temperature of the air surrounding the tubing that the refrigerant is in. This causes the refrigerant to start changing from liquid to vapor. This is called “boiling” or “flashing”. This “flashing” brings the refrigerant down from the liquid line temperature to the boiling (saturation) temperature in the evaporator, and in this process a percentage of the refrigerant is immediately changed from liquid to vapor. The percentage of the refrigerant that changes during flashing depends on how great the difference is. A larger difference between the liquid line temperature and the evaporator boiling temperature results in more liquid lost to flashing and reduces the efficiency of operation.

There are a few different types of metering devices. The most common ones being the Thermostatic Expansion Valve (TXV/ TEV) and the Fixed Orifice (often called a piston)– as well as electronic expansion valves, capillary tubes, and others.

Evaporator

The evaporator is also known as the cooling coil, because the purpose of the evaporator is to absorb heat. It accomplishes this through the refrigerant changing from liquid to vapor (boiling). This boiling process begins as soon as the refrigerant leaves the metering device, and it continues until the refrigerant has absorbed enough heat to completely finish the change from liquid to vapor. As long as the refrigerant is boiling it will remain at a constant temperature; this temperature is referred to as saturation temperature or evaporator temperature. As soon as the refrigerant is done boiling, the temperature starts to rise. This temperature increase is known as superheat.

When the indoor air temperature or the air flow going over the coil is higher, the evaporator pressure and temperature will also be higher because more heat is being absorbed into the coil. When the air temperature or airflow over the coil is lower, it will have lower pressure and temperature in the coil due to less heat being absorbed in the coil.

The refrigerant leaves the evaporator, travels down the suction line and heads back to the compressor where the cycle starts all over again.

Refrigerant Lines 

Suction Line = Line Between the Evaporator and the Compressor

The suction line should contain low-pressure superheated suction vapor. Cool to the touch on an air conditioning system, and cold to the touch in refrigeration.

Discharge Line = Line Between the Compressor and the Condenser 

The discharge line should contain high temperature, high pressure superheated vapor

Liquid Line = Line between the Condenser and the Metering Device

The liquid line should be high pressure, slightly above outdoor temperature subcooled liquid

Expansion Line (When applicable) = Line Between the Metering Device and the Evaporator

On most systems, the metering device will be mounted directly to the evaporator making the expansion line a non-factor. Some ductless mini-split units will mount the metering device in the outside unit making the second, smaller line and expansion line. The expansion line is full of mixed vapor/liquid flash gas.

Yes, this was long, but more than anything else just keep repeating over and over: compressor>discharge line>condenser>liquid line>metering device>expansion line> evaporator>suction line and on and on and on…

Jim Bergmann did a great whiteboard video on the MeasureQuick YouTube page that explains the basic refrigerant circuit

-Bryan

I need to warn you…

This is the actual process we use at the company I own for our typical “silver” residential maintenance. I’m sure you will find some things you do differently. Take it for what it is and I’m happy to get any feedback you may have.

  • Read the call notes, property notes and customer notes. Check the last service call and last notes and readings so you are aware of the service history.
  • Check the filter size and ensure you have the PROPER filter if possible
  • Wear shoe covers in the home, ask the customer if they have noticed anything unusual with the system.
  • Inspect and set the set the thermostat to run, shine a light in the return to check for filters, blockages, debris or damage.
  • Visually inspect the system operation to make sure all of the components appear operational before beginning the maintenance. Note anything out of ordinary you observe to the owner and address / diagnose before proceeding.
  • Remove disconnects / shut off breakers. Check for proper breaker sizes and inspect disconnect wires, lugs and pulls.  
  • Remove the condenser top and panels and place them carefully in the grass away from damage.
  • Remove any debris from the bottom of the unit. Inspect wires and compressor terminals while doing this. Use a vacuum to remove dirt and leaves as required. If grading is poor, use a shovel to scrape dirt/leaves away from the base of the unit.
  • Wash the coil well, starting from inside out and from top to bottom. Only use coil cleaner according to the labeled dilution and only use it when the coil requires it.
  • Check the inside of the condenser one final time for any potential wiring or copper rubouts and repair / isolate as required. Note any rust on the compressor, roto locks or accumulator.
  • Inspect the crankcase heater if the system has one. Confirm operation by amperage or ohming out.
  • Look for any signs of refrigerant oil (potential leak points)  
  • Replace the top carefully, ensuring that you don’t pinch any wires. Rewire the fan.
  • Inspect all wiring connection in the condenser control box for tightness and damage. Check contactor points and note condition.
  • Move inside, if the air handler has a finished floor around it, lay down a drop cloth. Always keep the work area inside clean during and after work.
  • Check the evaporator coil condition and cleanliness top and bottom. If the coil is a slant coil orientation and has any dirt on it clean the surface with evaporator coil cleaner, pump sprayer, rag and a soft brush.  If the coil is dirty and an A or another coil type that cannot be easily cleaned in place you may quote a removal and clean if the coil has no known leaks and is less than 7 years old.
  • Connect a wet vac to the drain outside
  • Remove the panels from the air handler and begin pouring water into the drain pan all the while helping any sludge in the pan get removed by moving it toward the pan outlet. Use duct tension straps or zip tie ends / small bottle brushes to help pull sludge out from the sides of the pan under the coil and clean all channels.
  • Run a minimum of 2 gallons of water through the pan and then empty the vacuum. Run another gallon through and repeat the process until the pan is visibly clean and the water in the vacuum is clean.
  • Run 1 more gallon through the drain one done and ensure it runs out.
  • Spray and wipe down the inside and outside of the air handler, including wires with a safe anti-microbial solution.
  • Remove any dust from the blower motor body and end bell with a vacuum, rag or soft brush, being careful not to force dust further into the motor. When the dust buildup is severe, use compressed air or nitrogen and a vacuum to remove it.  
  • Test the blower capacitor by removing the leads, testing with a capacitor tester and reconnecting the leads.
  • Inspect the blower wheel for cleanliness. If it is dirty check the particular maintenance type to see if removing and cleaning the blower is extra or included.
  • Check the blower motor bearings for play.
  • Inspect all wires for rub out inside and outside the air handler. Inspect and disconnects and check any lugs for tightness.
  • Check low voltage wiring and dip switch/pin settings. If the system has an advanced interface you will need to check for proper settings at the controller.
  • Check coil feeder tube location and condition. If tubes begin to rub out, isolate them and strap them together using foam tape and zip ties.
  • Inspect the float switch for proper installation and wiring. Test the float switch to ensure it breaks the circuit when the float rises. If there is no float switch, quote to install one at the end of service. 
  • Check air handler panel insulation, glue or retape as required.
  • Spray down the coil surface lightly with antimicrobial and add 3 pan tabs to the front of the drain pan away from the outlet.
  • Install a new air filter with date and your name.
  • Replace the bottom air handler panels and turn the air handler breaker on.
  • Run the heat and test the heat strips on and off using an amp meter. Note heater and blower amps.
  • Shut off the air handler, replace all panels and double check the drain cleanout cap is in place and the float switch is in the correct position.    
  • Put the system into cool mode and turn the air handler back on (condenser disconnect still off / out)
  • If the system has been checked during previous calls and has no history of leaks then use the historic data to perform a Non-invasive refrigerant test. If this is the first maintenance or if the system has a history of issues then connect gauges.
  • Connect an amp clamp to compressor common and observe as you turn on the condenser breaker/disconnect.
  • Test the L1 & L2 voltage and ensure it is in the acceptable range
  • Allow the system to run for at least 10 minutes. During this time you can Begin Cleaning up
    1. Start filling out the service call/collecting model and serial numbers if required
    2. Inspect suction insulation and thermostat wire outside for damage or poor splices
    3. Perform the “under load” capacitor test
    4. Read across the contactor points to see if there is any voltage drop in the contact points
  • Once the system has run for 10 – 15 minutes perform either the non-invasive test protocol or check and note suction, head, superheat, subcool and delta T according to the “5 pillars” tests and any manufacturer guidelines.
  • If the system is a heat pump, test the opposite mode (usually heat) to ensure the reversing valve shifts properly and the system runs.
  • Note and suggested repairs or improvements to the customer and get their response.
  • Clean up fully and double check drains refrigerant caps and disconnects. Call in standby.
  • Wrap up paperwork with the customer.
  • Ask the customer if there is anything else you can do to make their experience better and if they can think of any way you or the company could improve.
  • Complete all call notes, finish timesheet entry, neaten van in preparation for the next call.

   — Bryan

P.S. – If you have a good maintenance procedure for gas, oil or other types of systems would you be willing to share it? you can email it to me at hvacrschool.com / I can give you credit or make it anonymous, whichever you prefer.

P.S.S – I just put out my first attempt at a tool list. It is sortable by sector and my thoughts on which tools are most critical but I would like your feedback HERE IT IS

  

We had a really great conversation on the HVAC School Facebook Group about some belt tension best practices and it turns out that even a lot  of really smart and experienced techs are not aware of all the factors related to belt tensioning.

Myth #1 is that amperage is used to set belt tension. Now don’t get me wrong, checking amperage before and after changing belt tension is an excellent practice to ensure you are not binding the bearings from over tension, it does not tell you whether or not the belt is at optimum tension.

I think Browning summarizes it best in this statement from their Browning tool box technician app

Ideal tension is the lowest tension at which the belt will not slip under peak load conditions

Getting a belt too tight shortens the life of the belt and bearings and can cause high amperage. Leaving a belt too loose will shorten the belt life and result in loss of airflow and noise.

Many techs confuse the sheave adjustment, designed to alter the pulley ratio and the airflow with the belt tension adjustment. These are not the same thing and serve separate purposes.

The adjustable sheave allows the pulley faces to adjust closer or further from one another, resulting in a belt that rides closer to the hub when looser (halves further apart) or closer to the edge when tighter  (halves further separated) THIS ADJUSTMENT IS FOR FAN SPEED ONLY NOT TENSIONING

With a properly tensioned belt the belt should not slip significantly when starting, it should not be noisy and it should not bounce around. If you tighten the belt check the amps before and after and the motor should not overamp.

The correct tension method is to get the belt close to the correct tension by feel with a deflection of 1/64 of an inch for every 1″ of distance between the two pulley centers. You can then use an app or a chart like THIS ONE to find the proper force to generate this deflection.

You would then use a belt deflection tool like the one shown above to test the deflection force required and adjust accordingly. The video below demonstrates this.

I like what Jeremy smSmithtated in the group “Belt tension has less impact on motor amperage than pitch diameter of the sheave and how that affects total airflow.Use the Emerson tool and the app (or paper chart if you’re all stone age) Record tension and other data (sheave diameter, center to center length, rpm and proper tension) on blower housing.”

Check those belts.

— Bryan

Download the podcast Directly HERE

As always if you have an iPhone subscribe HERE and if you have an Android phone subscribe HERE

Condenser Flooding / Motormaster Podcast Companion

This article and podcast is courtesy of Jeremy Smith, one of the most knowledgeable and helpful refrigeration techs I know.

It’s my feeling that, no matter how well explained, this topic really requires a treatment that is more in depth and one that can be absorbed slowly with the ability to continually return and re-read certain sections to allow for best understanding of the subject matter.

As discussed in the podcast, as the outdoor temperature drops, the capacity of the condenser increases dramatically causing it to be, essentially, oversized for normal operation.   To counteract that, we use a valve (headmaster) or valves (ORI/ORD) to fill the condenser with liquid to effectively reduce the amount of coil that is actively rejecting heat and condensing refrigerant.   This also maintains a high enough liquid pressure feeding our TEV.   This prevents wild swings in TEV control because it is a pressure operated mechanical device.

First things first, let’s open up Sporlan’s 90-30-1 … seriously go ahead and click it , it will open in another tab so you can go back and forth.

This is a document I reference all the time when dealing with condenser flooding problems.  If you’re tech savvy, save it on your mobile device.  If you’re more of a low-tech guy, listening to a podcast and reading an internet publication on your flip phone or whatever, go ahead and print this out, laminate it and keep it in your clipboard.   Heck, even if you are a high tech guy, sometimes nothing beats a hard copy of this the first few times you work through it.

If ,after the podcast, you haven’t read through this to familiarize yourself with it, take the time to do so.   It seems like a really complicated procedure to work through, and the first few times that you do it on your own, it can be.  With practice, however, you’ll get used to it.

We’ll work through a condenser flooding calculation here in slow time, outlining all the different calculations taken into account.

First lets lay out the basic info we need.  The measurements and counts will vary, of course, depending on the equipment that you have.

If we have an R22 unit, 44 condenser passes ⅜” in diameter each are 38 ¾” long with 42 return bends.   Our evaporator temperature is 20°F, current temp is 35°F and the lowest expected ambient is -20°F.

Now, that seems like a lot of information, but we’ll break it all down.

First, we need to figure the total length of the condenser tubing in feet.   So, we take 44 x 38 ¾ and get 1705” of tubing.   1705 ÷ 12” per foot gives us 142.083 feet of tubing.   Now, that’s just the straight tubing.   We’ve got return bends to account for.

Refer to our Sporlan document.   In TABLE 1, you’ll find an equivalent foot length per return bend.   In the case of a ⅜” return bend, it’s. 2 feet per bend, so 42 x .2 gives us 8.4 feet more.

Add those together for total length of 150.483.  Back to TABLE 1 look in the R22 section under ⅜” tubing and follow the line for -20°F across.   You’ll find a density factor of 0.055.   This number is how many pounds of liquid refrigerant is needed to fill one foot of tubing at that temperature.   So, 150.483 x 0.055.  This gives us 8.28 pounds.  This is the amount required to fill the entire condenser with liquid, but we don’t really need to fill the WHOLE coil….

Back to the document..TABLE 2 this time.

Across the top, find 20° evaporating temp, now follow that down to the -20°F row.   This gives us a percentage.   82%  so, this unit at -20% will have 82% of its condenser filled with liquid.   So let’s take 8.28 x 0.82 to get our flooding charge.

6.78 pounds.

Now, what does this number really mean.   This is the amount of refrigerant we need to add to a system that we’ve JUST cleared the sightglass on when the ambient temperature is 70°F or higher.  If our ambient temperature were 70 degrees or warmer, we could add just that amount past a clear sight glass and walk away, satisfied in knowing that the unit will run properly no matter what the weather throws at it.

Remember, though, that our current ambient is 35°F.   So, now what?

Time to stop.  Get your Sharpie out and WRITE THIS NUMBER DOWN!   Record it on the unit somewhere.  Somewhere easy to see but somewhere that the sun doesn’t degrade the ink over time.   That way, you only have to go through this one time.  If you’re doing a new installation and startup, do the next guy a favor and write both this AND the total system charge down somewhere so that I don’t have to guesstimate the charge when it all leaks out.

Now, let’s go back to TABLE 2 and look at the 35°F row.   We find that at 35°, we need to have 63% flooded.   Well, we’ve got a clear sight glass and it’s 35° ambient so, we’re already 63% flooded.

Since the most we need is 82% flooded, 82%-63% gives 19% so, we take our total, 8.28 x 0.19 to get 1.57 pounds.  At our current conditions, that’s all the flooding charge that we need to add because we’ve already got some flooding going on to have a clear sightglass because we’re under the 70 degree mark and the low ambient controls are in play and doing their job.

Some techs claim that just spraying water on the coil will flood the condenser enough to allow the use of that as a charging technique.    Let’s think about it for a minute.   What variables come into play with a method like that?  Variables that we can’t control…  for starters, what is the wet bulb temperature of the air entering the condenser?  How well is the condenser wetted? With the stakes being what they are, I’m not excited about the prospect of using this because I’m probably going to be the guy who winds up on the roof when it’s -20 and the wind is howling and this unit is low on gas because someone tried to use this method to figure a flooding charge, didn’t get enough gas in the unit and now it’s short.    I’ve still got to my due diligence as a service tech, do a full leak check, not find anything, and walk away wondering if I missed a leak somewhere all because someone else didn’t take a couple minutes to do a little work to do the job properly.  This is a totally preventable service call.

What about TABLE 3, you ask?  Very astute and that tells me that you’re reading ahead. Excellent.  I have never had to use it.

It gives a different flooding percentage for units with an unloader and low ambient controls where they’ll be running in low ambient conditions.  With the unloader, remember that we’re really moving less heat, changing the condenser dynamic and making it even MORE oversized than it would be if there weren’t an unloader, so more refrigerant needs to be added to properly flood the condenser.

— Jeremy Smith

P.S. – You can checkout the Testo 770-3 multimeter we mentioned in the middle by going here

An important rating on motors is the AMBIENT  temperature rating that the motor can operate at.  This rating means the temperature of the air around the motor, not the temperature of the motor itself or even the temperature of the outdoor air since the motor is often in a condenser air stream that is higher temp than outdoors. In HVAC/R we will commonly see condensing fan motors at 60°c (140°f), 70°c (158°f) and 80°c (176°f) and blower motors will often be rated at40°c (104°).

In residential and light commercial HVAC it is fairly common for condensing fan motors that are experiencing issues to overheat and go out on internal thermal overload during the heat of the day which then drives up the head pressure until the compressor goes off on thermal or on a high-pressure fault. In some cases, the system will cool off overnight and run again once the tech arrives, or if the customer shuts it off and it cools off the issue may not show up again right away causing a nuisance intermittent callback.

The temperature of the motor shell itself will vary motor to motor but commonly will be 30° – 60° warmer than the outdoor temperature during normal operation depending on factors such as if the sun is shining on the top, the efficiency of the motor and how long they have been running.

In many cases, you may be able to compare a motor you suspect to be overheating against other units nearby with the same motor operating in nearly the same conditions. Look at the photo below compared to the one above. Both of these are similar motor taken a few minutes apart but you can see that one motor is running quite a bit warmer than the other. Sure enough, the hotter motor is noisier and has more side to side play in the bearings.

If you have reason to believe a motor is running hotter than it should there are a few things that can cause the issue to watch out for.

  • High condensing temperature – If the air around the motor is hotter the motor will also be hotter. Watch for dirty condensers and overcharge.
  • Direct Sun – Pretty obvious but if the radiant heat from the sun is right on the motor it will run hotter.
  • Voltage – Check and make sure the motor voltage is in the proper range while it is running (under load).
  • Capacitor – Make sure the capacitor is the correct size for the motor, both weak and oversize capacitors can cause overheating.
  • Bearing Issues – When bearings start to fail there may be increased noise or side to side shaft play (but not always)

A thermal camera can produce a great look at the temperature of the motor but keep in mind that depending on the motor surface there will be some inaccuracy of the temperature reading due to varying emissivity so it’s best to use it to compare motors rather than trusting a single reading as a pass / fail test.

— Bryan

P.S. – These images were taken with a FLIR One Pro that I got at a great price at Trutech tools using the offer code getschooled

 

This article is written by regular contributor, experienced rack refrigeration tech and RSES CM Jeremy Smith. Thanks Jeremy. Also… There is a new podcast out about what murders compressors HERE


A technique that you can use to diagnose compressor problems and to help differentiate them from other possible issues is the use of compressor performance analysis.

Manufacturers do extensive testing of their compressors before they sell them, and a part of that testing is available to you as a troubleshooting tool. The compressor performance chart. I’ll primarily refer to Copeland compressors as they are what I service most, but I’ve been able to find charts and data from other manufacturers through their websites and tech support lines.

Let’s look at a real-world example. I went to do a follow-up check after a major leak and recharge on a set of freezers. On arrival, the cases, which had been running now for 14 or 15 hours since having been repaired, weren’t as cold as expected. Checking the unit, here is what I found:

Copeland compressor
2DA3-060L-TFC
R404A
27# suction
185# discharge
209v (average of all 3 legs)
13.9A draw.
Unit at 18-20°F

The suction line was cool to the touch and the sight glass had a thin ‘river’ of refrigerant in it. The high suction pressure really jumped out at me here as worthy of more consideration.

Now, a high suction pressure in this instance can be caused by high load (note the high unit temperature) or it can be caused by a compressor problem. Looking over the data here, I was concerned about the health of the compressor and its ability to pump properly. I did a quick “pump down test” and found it inconclusive. The compressor pulled to 24” Hg easily and held there. Still, I wasn’t happy with this, so I pulled out my smartphone and opened the Copeland Mobile app.

A quick note on pump down ‘tests’. They really aren’t effective on most modern compressors. I performed the test and included the results here to illustrate exactly that fact.

Entering the model of the compressor leads you to select the application (R502 low temp which is closest to R404a low temp). Selecting the “Diagnostics” tab brings you to a screen where you and input pertinent data and the app then outputs both the expected amperage at your conditions and the percent deviation
from the norm.

In this case, my expected amperage was significantly higher than my observed amperage, so the high suction was definitely caused by a compressor problem.
I recovered the refrigerant from the machine and removed the compressor head and valve plate for internal evaluation.

Finding a single broken suction reed, the rest of the internals were intact making this a good candidate for a new valve plate. I Installed new valve plates, evacuated and restarted the machine and re-evaluated operation.

Had this been a hermetically sealed compressor, I would have had no choice but to condemn and replace the compressor. This time, the amperage was within 5% of specifications (sorry, didn’t get a screenshot) and I continued to monitor unit operation until equipment reached 0° F, verified and completed proper
charging of the unit and called it a day.

Why not use RLA (Rated Load Amps) (? Or use LRA÷6 (Or is it 8?) to diagnose?

The simple answer is that they just aren’t sufficiently accurate enough for me dealing with high stakes, high dollar equipment and they shouldn’t be accurate enough for you, either.

Let’s return to my real-world example…

The compressor has a listed RLA of 25.8 and a LRA (Locked Rotor Amps) of 161.0. Now look back at the original screenshot of the app. It calls for an amp draw of 17.09A at that set of conditions. If we compared that to the RLA, even the correct amperage looks low. If we use common LRA divisors 161 ÷ 6 gives us 26.83A and 161 ÷ 8 gives us 20.125A. Maybe a little better than the RLA method but still off by a significant amount. Enough to cause concern and possibly lead to an incorrect diagnosis.

Not a one of these methods gives us an accurate expected amperage for this machine. That inaccuracy can lead us to draw a bad conclusion and potentially wasting time and money pursuing a “bad” compressor that is in fact, working exactly as it should.

Like most things in HVAC/R using a fixed operational target without considering the specific conditions can lead to misdiagnosis and a lot of wasted time. You would be surprised what is available within manufacturer specs if you take the time look.

— Jeremy Smith, CM

When I first started in the trade we used to run into shielded control wires on the Carrier Comfort Zone 1 zoning systems and also on a Carrier VVT system I used to maintain at a bank. I knew it has something to do with electrical “noise” and that communicating systems often called for it but I never looked any further into it.

Over the last decade there has been a lot of different residential communicating systems that have come out. Some require shielded cable, some recommend it and others don’t mention it all.

The fact is that whenever controls work on a low voltage “signal” rather than a simple “on/off” control they are more susceptible to induced charges from other nearby conductors, electronics and even transients from electrical storms.

A shielded cable has a  metallic jacket that surrounds the individual conductors and routes the induced charges to ground, keeping it away from the conductors inside.

As an example of this, I installed a Carrier Infinity system at my own house WITHOUT using shielded cable and almost every time there are lightning strikes nearby the unit will throw a communications fault, since I’m in florida that happens quite often.

If you do have the wisdom to run shielded cable you need to remember to bond (ground) one side of the shield securely to a good equipment ground on one end and ONE END ONLY. If you ground both ends you risk the sheild becoming a path in the case of a ground fault which could cause some bigger issues. If you ground both ends you can also create a “ground loop” that can cause the very noise you set out to eliminate.

In some cases, you can perform a similar function by grounding leftover/unused conductors on one end if you failed to run a shielded cable. There is no guarantee it will solve the issue depending on the severity because the other conductors don’t fully surround the conductors being utilized.

The lesson being, when working with communicating “signal” controls run shielded cable whenever possible. I was looking around and found this spec sheet from Southwire on their shielded 8 wire.

— Bryan

Breaking the Sound Barrier

So what do you think of when you hear an “ideal gas”? R22, R12 maybe… Natural? Take a look at the F-18 above… It is breaking the sound barrier and that cloud is a shockwave… This has nothing to do with this article but I think it’s pretty darn cool!

An ideal gas is a gas that obeys the ideal gas law, it’s ideal because it’s good at following rules. These ideal gasses walk in a straight line, they don’t run on the playground and they never fish without a proper permit. More like an ideal gas behaves in a predictable way with changes in volume, pressure, temperature and mass.

The problem is, a truly “ideal” gas really doesn’t exist.

While many gasses behave close to ideal at normal temperatures there is no gas that obeys the ideal gas laws in all conditions.

The ideal gas law is –

P=  Absolute Pressure (gauge pressure + atmospheric pressure)
V = Volume (How much space the gas occupies)
n = Mass measured in “moles” (the number of molecules)
R = The universal gas constant (varies depending on the units of measure being used Example: [lbf ft/(lb mol oR)]= 8.3145 )
T = Absolute Temperature (temperature in a scale that starts at absolute zero like Kelvin or Rankine)

The ideal gas law is really a combination of several different laws into one.

The result is that many gasses that we work with behave in about the same way with changes in mass, volume, temperature & pressure. This is the case because the primary force at play in a nearly ideal gas like nitrogen or CO2 is simply the velocity of the molecules bouncing around in the container and against one another like tiny little ping pong balls.

If the molecules react, or interact with one another through attraction or repulsion due to their intermolecular forces then they can cease to behave as an ideal gas. A perfect example is when a gas is in contact with its liquid form (saturation) it no longer obeys the gas laws. This is why most gasses behave more and more like an ideal gas the hotter they get (within a range) because the hotter they are the greater the force of molecular velocity (temperature) will be relative to the intermolecular interaction of the molecules.

Once the gas gets to the “supercritical” state all bets are off once again. So like most good kids, even the most ideal gasses have their limits where if pushed they become little molecular rebels.

— Bryan

 

Keep in mind when reading ANY article about electrical theory or application is that it only scratches the surface of the topic. You can dedicate years of your life to understanding electrical theory and design the way many engineers do and still know just enough to be dangerous.  In HVAC we rarely need to have a DEEP understanding of electrical design but there are a few cases where a little understanding can go a long way to identifying issues before they cause trouble and that is the intent of this short article.


What is Three Phase Power?

Power is generated at the utility in three phases that are 120 degrees out of phase with one another at 60hz (Hertz). This simply means that 60 times per second each individual leg of power makes one peak and valley (a full circle), and all three of the phases together split the cycle into thirds (trisect).

This video is the best visual demonstration I have seen of three-phase power and how it works.

What Does an HVAC Tech Need to Know About Three Phase Power?

Three-phase motors don’t’ require a run capacitor because the 120-degree phase difference is ideal for efficiently spinning a motor so a “start” winding and phase shifting capacitor isn’t needed.

The biggest concern for the techs and installers with three phase is getting the phasing correct so that motors run in the correct direction. While this doesn’t matter for reciprocating compressors it is important for condenser fans and blowers and it is absolutely CRITICAL for scroll and screw compressors. Changing the direction of rotation is just as simple as swapping any two phases.

Keep in mind when installing replacement parts and equipment that if you keep the phases connected in the same way you will generally be in good shape. It is still a good practice to use a phase rotation indicator like the one above to confirm proper rotation. In most cases, clockwise phase rotation is what you are looking for but I’m sure there are exceptions to that. Alternatively, you can disconnect the compressor that could be damaged by improper phasing and start up the blower to see if it runs the correct direction before bringing the compressor(s) online. One caveat is that when motors use a VFD (variable frequency drive) the phase rotation will automatically correct making them an unreliable test in those cases.

Balancing Phases

Electricians are responsible for balancing the amperage of single-phase loads (both 120v single leg and 208v two leg loads typical on a wye three-phase system) both so the neutral doesn’t carry high amperage on the 120v loads and so the one leg of power doesn’t carry significantly more or less load than the other two. As the amperage load on a particular phase goes up, there is more opportunity for voltage drop depending on the size of the load, size of the transformer and service feeding the space and well as wire size and connection quality. This can become a challenge when there is a mix of single phase outlets, 208v appliances, and three-phase equipment.

Let’s say someone connects a bunch of space heaters on phase A, as well as a few smaller HVAC systems between phases A and B and almost nothing on phase C. If you have a large RTU that uses three-phases phase C will tend to have less load and therefore higher voltage while the load on phases A and B will fluctuate based on when the smaller systems and space heaters go on and off.

This can cause overheating of conductors and damage but it can also cause voltage imbalance which is a real cause for concern for an HVAC technician.

3 Phase Voltage Imbalance

Voltage imbalance is a motor killer. It causes poor motor performance and increased winding heat which leads to premature failure. In the case of HVAC blowers and compressors, this additional heat ends up in either the refrigerant or the air which must then be removed, further decreasing efficiency.

To test for three-phase imbalance always check from phase to phase not from phase to ground. You simply check the voltage from each of the three phases to one another and find the average (add all three and divide by three). Then compare the reading that furthest from the average and find the % of deviation. For most of you I know that sounds like a giant pain so we made this easy calculator for you.

The US Department of Energy recommends that the voltage imbalance be no more than 1% while other industry sources say up to 4% is acceptable. In general, you will want to make SURE the imbalance is below 4% and work to rectify anything over 1%.

What Can I Do About It?

You want to first look for the obvious. Melted wires, loose terminals and lugs, undersized wire, pitted contacts, poor disconnect fuse contact etc… Obviously, if you aren’t licensed or allowed to open a panel you won’t always be able to fully rectify the issue yourself but you can go a long way towards the diagnosis.

When checking voltage it is generally best to do it with the system running as close to the motor you are checking as possible. This is the actual voltage the motor is “seeing” and is what matters to the operation of the motor. You can then test back towards the distribution point, if you see a big increase in voltage as you test back towards the source you know you found a voltage drop and a cause or contributor to the issue.

From there the issues of amperage load imbalance in the panel, service size and utility issues must be considered once all the basics are covered. Most of all, if the imbalance is severe (over 4%) you don’t want to leave your motors running or you risk damage and expensive repairs.

— Bryan

In the HVAC and building performance industry, you will hear the terms CFM (Cubic Feet per Minute), ACH (Air Changes per Hour) and ACH50 (Air Changes per Hour at 50 Pascals) thrown around a lot and it’s important that you understand the differences.

CFM

CFM is a measurement of volume (not mass) flow rate, the cubic feet of air moving per minute. To convert CFM to cubic feet per hour you simply multiply by 60 and to convert cubic feet per hour to CFM you divide by 60. When measuring air flow we generally convert air velocity (speed) to CFM by simply measuring the velocity  (FPM) and multiplying that by the opening we are measuring.

For example, if we are measuring an air velocity in a duct of 700 FPM (feet per minute) and the duct is 24″ x 24″ (2’x 2′) that would be a square footage of 4 x 700 = 2,800 CFM (Cubic Feet Per minute). If we needed to calculate cubic feet per hour we would multiply that by 60 (minutes in an hour) and the result would be 2,800 CFM x 60 = 168,000 CFH (Cubic Feet per Hour)

ACH

Like we talked about in a recent article air changes per hour is often used to calculate ventilation requirements for a room or structure.  Let’s imagine there was a dining room that required 5 air changes per hour (CFH) for proper ventilation. If that dining room was 10′ x 10′ x 10′ that would equal 1,000 cubic feet of internal volume in the space. This means that we would need to provide that room with 5,000 cubic feet of “new” air every hour to hit that number. 5,000 CFH ÷ 60 minutes per hour = 83.33 CFM of airflow to hit that target.

When discussing ACH within a space for ventilation it can often get confused with discussing ACH with outdoor ventilation air for healthy air dilution via mechanical ventilation or with undesigned infiltration through a loose envelope shell.

ACH for ventilation of specific rooms within a structure, ACH for designed outdoor air ventilation and ACH50 for envelope infiltration testing should not be mixed up or you will find yourself being very confused   

All of these ACH measurements are simply a calculation of the cubic volume of a space dived by air volume moving in and out of a space in an hour. The question is, is the air from inside the structure or outside of the structure and when/how is it being brought in.

ACH50

A blower door is used primarily to measure infiltration of outdoor air into a structure. In order to compare the “tightness” one structure to another. The common standard for measurement is 50 pascals of negative pressure in the structure in reference to the outside.

In other words, you use a big fan placed in an exterior door with a fine-tuned manometer and you run the fan at a rate that gets the building down to 50 pascals of negative pressure.  You then calculate how much air is moving through the blower (in CFM) and convert CFM to CFH by multiplying by 60. You calculate the internal cubic feet of the structure and divide that number into the calculated CFH to come up with the ACH50 number. This is simply the air changes per hour of the structure at 50 pascals of negative pressure. A higher number means more infiltration (loose)  and a lower number means less infiltration (tight).

This ACH50 number ONLY applies when the house is under a pretty strong negative pressure, it doesn’t actually tell you how much air is moving in and out of the space under normal operating conditions.

— Bryan

 

 

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