July, 2017 | HVAC School

Open and Closed Case

July 31, 2017 By Bryan Orr 7 Comments

I know I’m gonna get some eye rolling here but it needs to be said.

When we teach electricity to new techs we use a lot of “water” metaphors. We talk about volts like PSI, amps like flow and Watts like GPM. Even the word “flow” gives us a vision of water moving.

Then we talk switches and circuits and we say “open” to mean no path / no flow and “closed” to mean a path or flow.
That’s the opposite of water…

With water, we “open” the tap when we want flow and close it to stop the flow.
With a switch, we “close” when we make a circuit and we “open” when we break a circuit.

Someone pointed out to me that describing an “open” switch or circuit like a drawbridge may be better. Cars (electrons) can move when the bridge is closed and cannot move when the bridge is open.

It struck me that this water metaphor may be one reason newbies struggle to grasp relays.
Or maybe I’m just overthinking it.

— Bryan

Tech Tips

Air Filter Static Pressure Drop

July 30, 2017 By Bryan Orr No Comments

I’m a big dummy when it comes to my own air conditioning maintenance. I talk about the importance of changing air filters to customers and techs but I never stay up on replacing my own.

Yesterday I walked into my mechanical room and my 2-ton air handler sounded like a vacuum cleaner about to implode.

My filter was nasty… nasty to the point that I wasn’t willing to leave the filter in. So I pulled it out and think to myself “I’ll just grab a filter from the office tomorrow”. well… I forgot and I live 35 minutes from my office.

So today I grab a filter from my nearby hardware store, a common brand and pull it out of the plastic wrap to install it. Sure it was a MERV 11, but that was the only option other than the cheap, spun fiberglass “bug catcher”.

I know what you’re thinking, I should have known better

I’ve got to give it to this filter manufacturer for actually printing the static pressure drop on the filter (shown above).

My system is setup for 350 CFM(594.65 m3/h) per ton so it’s required running at right around 700 CFM(1189.3 m3/h) which means on my system this filter is going to add 0.26″wc(64.69 pa) of extra static to the return side of the blower.

With most systems being rated at 0.5″wc(124.4 pa) TESP (total external static Pressure) this makes up more than half of that, before any ductwork, grilles, registers, balancing dampers or coils in the case of furnace systems.

On a PSC blower motor this extra static from this filter would result in lower airflow, poor system performance and poor air distribution.

With an ECM motor this extra static can result in higher blower motor power consumption and condensate drainage issues / difficulty maintaining trap.

While some systems may be able to deal with the extra static, many will have issues ESPECIALLY on older systems that have PSC motors and furnaces with coils.

This is why larger filter cabinets with lower pressure drop filters often make sense or oversized filter back return grilles.

When choosing a filter remember that airflow (Pressure Drop) is just as important to consider as filtration (MERV rating)

— Bryan

Tech Tips

Why Compression Ratio Matters

July 28, 2017 By Bryan Orr 1 Comment

In HVAC/R we are in the business of moving BTUs of heat and we move these BTUs on the back of pounds of refrigerant. The more pounds we move the more BTUs we move.

In a single stage HVAC/R compressor, the compression chamber maintains the same volume no matter the compression ratio. What changes is the # of pounds of refrigerant being moved with every stroke(reciprocating), oscillation (scroll), or rotation (screw, rotary) of the compressor. If the compressor is functioning properly the higher the compression ratio the fewer pounds of refrigerant is being moved and the lower the compression ratio the more pounds are moved.

In A/C and refrigeration the compression ratio is simply the absolute discharge pressure leaving the compressor divided by the absolute suction pressure entering the compressor.

Absolute pressure is just gauge pressure + atmospheric pressure. In general, we would just add the atmospheric pressure at sea level (14.7 psi)(1.01 bar) to both the suction and discharge pressure and then divide the discharge pressure by the suction. For example, a common compression ratio on an R22 system might look like-

240 PSIG Discharge + 14.7 PSIA = 254.7
75 PSIG Suction + 14.7 = 89.7 PSIA
254.7 PSIA Discharge ÷ 89.7 PSIA Suction = 2.84:1 Compression Ratio

The compression ratio will change as the evaporator load and the condensing temperature change but in general, under near design conditions, you will see the following compression ratios on properly functioning equipment depending on the efficiency and conditions of the exact system.

In air conditioning applications compression ratios of 2.3:1 to 3.5:1 are common with ratios below 3:1 and above 2:1 as the standard for modern high-efficiency Air conditioning equipment.

In a 404a medium temp refrigeration (cooler) 3.0:1 – 5.5:1 is a common ratio range

In a typical 404a 0°F to -10°F(0°K to -5.5°K) freezer application 6.0:1 – 13.0:1 is a common ratio range

As equipment gets more and more efficient, manufacturers are designing systems to have lower and lower compression ratios by using larger coils and smaller compressors.

Why does the compression ratio number matter?

When the compressor itself is functioning properly the lower the compression ratio the more efficient and cool the compressor will operate, so the goal of the manufacturer’s engineer, system designer, service technician and installer should be to maintain the lowest possible compression ratio while still moving the necessary pounds of refrigerant to accomplish the delivered BTU capacity required.

The compression ratio can also be used as a diagnostic tool to analyze whether or not the compressor is providing the proper compression. Very low compression ratios coupled with low amperage and low capacity are often an indication of mechanical compressor issues.

Compression ratio higher than designed = Compressor overheating, oil breakdown, high power consumption, low capacity

Compression ratio lower than designed = Can be an indication of mechanical failure and poor compression

Understanding compression is critical to understanding the refrigeration process. Don’t be tempted to skip past this because it is a really important concept.

Look at the pressure enthalpy diagram above. Top to bottom (vertical) is the refrigerant pressure scale, high pressure is higher on the chart. Horizontal (left to right) is the heat content scale, the further right the more heat contained in the refrigerant (heat, not necessarily temperature).

Start at point #2 on the chart at the bottom right. This is where the suction gas enters the compressor. As it is compressed it goes to point #3 which is up because it is being compressed (increased in pressure) and toward the right because of the heat of compression (heat energy added in the compression process itself) as well as the heat added when the refrigerant cooled the compressor motor windings.

Once the refrigerant enters the discharge line at point #3 it travels into the condenser and is desuperheated (sensible heat removed). This discharge superheat is equal to the suction superheat + the heat of compression + the heat removed from the motor windings. Once all of the discharge superheat (sensible heat) is removed in the first part of the condenser coil it hits point #4 and begins to condense.

Point #4 is a critical part of the compression ratio equation because the compressor is forced to produce a pressure high enough that the condensing temperature will be above the temperature of the air the condenser is rejecting its heat to. In other words, in a typical straight cool, air cooled air conditioning system the condensing temperature must be higher than the outdoor temperature for the heat to move out of the refrigerant and into the air going over the condenser.

If the outdoor air temperature is high or if the condenser coils are dirty, blades are improperly set or the condenser coils are undersized point #2 (condensing temperature) will be higher on the chart and therefore will put more heat strain on the compressor and will result in lower compressor efficiency and capacity.

As the refrigerant is changed from a liquid vapor mix to fully liquid in the condenser it travels from right back left between points #4 and #5 as heat is removed from the refrigerant into the outside air (on an air cooled system). Once it gets to #5 is is fully liquid and at point #6 it is subcooled below saturation but ABOVE outdoor ambient air temperature. The metering device then creates a pressure drop that is displayed between points #6 and #7. The further the drop, the colder the evaporator coil will be. The design coil temperature is dictated by the requirements of the space being cooled as well as the load on the coil but the LOWER the pressure and temperature of the evaporator the less dense the vapor will be at point #2 when it re-enters the compressor and the higher the compression ratio will need to be to pump it back up to point #3 and #4,

This shows us that the greater the vertical distance between points #2 and #4 the higher the compression ratio, which means that both low suction pressure and/or high head pressure result in higher compression ratios, poor compressor cooling, lower efficiency and lower capacity.

In some cases, there isn’t much that can be done about high compression ratios. When a customer sets their A/C down to 69°F(20.55°C) on a 100°(37.77°C) day they will simply have high compression ratios. When a low temp freezer is functioning on on a very hot day it will run high compression ratios.

But in many cases, you can reduce compression ratios by –

Keeping set temperatures at or above design temperatures for the equipment. Don’t be tempted to set that -10°F freezer to -20°F(-5.5°K to -11°K)or use that cooler as a freezer
Keep condenser coils clean and unrestricted
Maintain proper evaporator airflow
Install condensers in shaded and well-ventilated areas

Keep an eye on your compression ratios and you may be able to save a compressor from an untimely death.

— Bryan

Tech Tips

Pull The Top

July 27, 2017 By Bryan Orr 5 Comments

The photo above is from a video one of my techs took of proper condenser cleaning. I must say, he did a GREAT job of cleaning the coil and he was very careful with the top. However I STILL would have liked to see the top get completely removed during a full maintenance. Pulling the top usually just requires disconnecting the fan wires, cutting a few wire ties, taking out some screws and then removing the fan grille or the entire top and laying it top down in the grass.

This is ACTUALLY how I performed a maintenance, even before I started my own business.

Here is why –

If you wash from the outside – in you are not doing the best possible cleaning. Everyone knows that washing from the inside out is a superior method of cleaning.
If you lay the fan on top of the unit (like shown above) you risk twisting / damaging the wires, scratching the paint and bending the fan bade.
When you pull the top entirely you can more easily clean the dirt and leaves from the inside of the condenser, this should also be part of a proper maintenance because that dirt can reduce coil capacity as well as hold moisture against the base, compressor and accumulator resulting in corrosion.
With the top off you can get a better view of any wire rubouts or potential wire rub outs and address them before they cause a problem.
You can also visually inspect the compressor terminals for signs of heat and corrosion, potentially preventing a major issue such a terminal failure / “blowing a terminal”.

Obviously it will take about 5 mins longer and you will need to rewire it properly with the terminals snugly installed.

So what do you think?

— Bryan

P.S. – Here is the video in case you want to see what I mean and yes… he knows that cleaner isn’t always required when washing a coil but he used it for demonstration purposes

Tech Tips

Don’t Forget the Suction Filter / Drier

July 26, 2017 By Bryan Orr 1 Comment

Courtesy of Emerson

It is important to have refrigerant that is free from debris and contaminants and we control these issues on many different fronts.

Proper tubing handling preventing copper shavings, dirt and water from entering while installing
Flowing nitrogen while brazing to prevent carbon build up
Deep vacuum of 500 microns or less to remove air, nitrogen and moisture
Installation of a liquid line filter drier to keep contaminants from hitting the metering device

But in all of this we can forget the role that suction driers can play in protecting the compressor and the compressor oil.

In air conditioning, we rarely install suction driers unless there is known acid contamination such as in the case of a compressor burnout. Interestingly Copeland actually recommends suction driers in ALL applications in bulletin AE24-1105 R5 . While I certainly don’t think that we need to change our practices and begin installing suction line filter driers on every single installation, it does get you thinking about the role a suction drier can play in protecting a compressor.

In a typical burnout application where acid is present, it is a good practice to –

Remove / Flush as much contaminated oil from the system as possible considering the application including any oil traps, separators or accumulators
Install a high capacity acid removal suction and liquid drier or removable core(s)
Some contractors will add acid neutralizers such as Acid Away in certain applications
Return after running the system for a while and test for acid and replace high capacity filter/driers with new ones if required
Once acid is no longer present, return and remove the suction filter/drier and install a standard liquid line drier or core

These practices above are good, general practices to follow, but you may consider replacing the suction drier with a standard, high capacity, low-pressure drop suction drier with two pressure ports instead of just straight piping it. This will provide you an extra layer of protection for the compressor should any acid or contaminants from the burnout make their way to the compressor.

Table Courtesy of Emerson

If you do choose to LEAVE a suction drier in a system there are a few things to consider.

Just like with a liquid line filter drier, make sure to install a suction filter/drier that is large enough for the system capacity. Read the info on the drier or the manufacturer’s data to make sure it is large enough so you don’t start off with a restriction
Make sure you don’t burn the paint on the drier when installing. Because suction driers on air conditioning will often be exposed to the elements you want to make sure the paint is intact so they don’t rust.
Use a suction filter/drier with ports on both sides and measure the pressure drop whenever you service the unit. make sure the pressure drop does not exceed the levels shown in the chart above.

All in all, having a suction drier in the system is a good thing, so long as it isn’t contaminated, rusty or restricted.

— Bryan

P.S. – Sporlan has a great catalog of filter/driers HERE

Podcast Episodes

Measuring Air flow Podcasts w/ Jim Bergmann

July 25, 2017 By Bryan Orr No Comments

In this two part podcast series, Jim Bergmann covers measuring air flow for HVAC in including

Static Pressure
ECM & PSC motors
Air Mass vs. Volume
Measuring air velocity vs. volume
Air flow hoods and vane anemometers
Hot Wire Anemometers
Air Turbulence and much more…

Tech Tips

Static Pressure – Why Measure It?

July 23, 2017 By Bryan Orr 4 Comments

This article is written by Neil Comparetto. Neil is one of the smartest and most thoughtful techs I know online. Thanks Neil.

Why measure static pressure? Because it’s fun

I enjoy drilling holes in things. I rarely leave a house without drilling a hole in something. I also believe it’s an essential step to commissioning and diagnosing a forced air piece of equipment. Let me explain why.

Commissioning

I think we all can agree that proper airflow is necessary across the indoor coil. You should set the airflow before adjusting the refrigerant charge, right? Yes. Well, how do you know what fan speed to set the blower at?

Whether it’s a PSC, X-13, or an ECM motor you have fan speed options. The easiest way to set the blower speed is to measure TESP (total external static pressure), cross reference the TESP to the manufacturers blower chart in the installation manual, and adjust the blower speed. Sure, there are other ways of estimating or measuring airflow, but for commissioning a system in cooling, static pressure and a blower chart is easy and accurate enough.

350-400 CFM(594.65 m3/h – 679.6 m3/h) per ton works in my neck of the woods. If you are in a very dry climate, or at high altitude the CFM per ton requirements may be higher, often 450 – 500 CFM( 764.55 m3/h – 849.51 m3/h) per ton.

Even when commissioning a furnace in heating I like set up my airflow first, or at least know how many CFM the blower is moving. Typically it’s between 130 and 150 CFM(220.87 m3/h – 254.85 m3/h) per 10,000 input BTUH.

How many times have you serviced a system installed by others, there is no evidence of airflow being measured, and the blower speed is set too high? My guess, everyday.

There’s a better than good chance the airflow is wrong, and has been since day one.

Benchmarking

The airflow is set, now what? Take it to the next level, benchmark your pressure drops.

The pressure drops across the return duct, air filter, indoor coil, and supply duct can be valuable pieces of information when servicing the equipment in the future.

Imagine knowing exactly what the pressure drop across the coil was when commissioned 7 years ago. In a typical arrangement of the evaporator coil on top of a furnace a visual inspection of the coil for cleanliness can be difficult.

Knowing the original pressure drop can save time diagnosing, and justify taking further action. TESP by itself will not give you this information, only that there is an issue somewhere in the supply air side of the system. (It is recommended to record the dry and wet pressure drops across the coil, they will be different).

Same applies for the air filter. I typically install 5” media filters on our installations. On a furnace I aim for a .10” pressure drop across the filter when new.

Air handlers can typically handle a higher pressure drop across the filter (because the coil is included in the TESP rating), but at the cost of filter efficiency. Generally these filters are good for 6-12 months. Knowing the before and after pressure drop of the filter in this system will help you determine how frequently it needs to be changed.

Knowing the supply and return duct pressure drop can be useful as well. It’s not unheard of for vents to be closed, return grilles blocked, internal liners to collapse, flex duct to get smashed, or even disconnected. A static pressure reading of the ducts referenced to the pressure drops when commissioned can quickly tell you if there are any discrepancies, and better yet what actions to take.

Does benchmarking lengthen the time it takes to commission the system? Yes. Does it give you the information necessary to quickly and accurately diagnose airside issues during future servicing? Yes it does. In reality, once you do it a few times and develop a system it doesn’t take much longer at all.

If you have not listened to Bryan Orr and Jim Bergmann’s podcast on checking the refrigerant charge without gauges please do. They make a case for an even more comprehensive benchmarking procedure. Listening to it was one of those ah-ha moments for me.

Servicing

Take TESP to verify airflow against benchmarks and / or blower charts. In my experience most of the time airflow is incorrect. If this system is new to you and will be part of a service agreement I recommend that you check all four pressure drops (return, filter, coil, supply) for reasons mentioned earlier.

I find a lot of air filters that are too restrictive (small) on service calls. Air filters can be low hanging fruit if the equipment is not getting to proper CFM. It’s not uncommon to get .30” pressure drops on new filters.

If you find issues with the existing duct system, and it’s exposed, static pressure readings can help pinpoint where the restriction is.

Many times the restriction is obvious. A nasty reverse elbow then it turns twice, transitioning from 20” to 10” into some kind of cap-and-tap contraption. Sometimes the restriction is internal, or not obvious. Collapsed duct liner or a closed damper can be found with static pressure strategically measured across portions of the duct system.

If there are issues with the duct system let the homeowner know. This conversation might expose some comfort problems that they are experiencing. At the least it will make them (and you) aware that there are issues that may need to be addressed when it’s time to replace the system.

I’m not advocating to check static pressure every time you run a service call. I know that’s not always practical. I am advocating for installing the pressure ports, and benchmarking on commissioning as well as measuring airflow against the benchmarks during service when the call type calls for it. Future service techs will thank you and you will come to a faster and more accurate diagnosis.

— Neil

Tech Tips

Don’t use Tank Caps on Systems

July 21, 2017 By Bryan Orr 1 Comment

This is a video tech tip from our friend Brad at HVAC in SC.

In this video Brad demonstrates that using a cap off of a refrigerant tank on a system can depress the Schrader core on the system in some cases. Make sure to use proper caps that have seals or brass flare cones.

Transcript

If you’re ever on a service call and you need to use a replacement valve cap, do not use the ones that are on the refrigerant tanks. Here’s a perfect example the one on the right is just a standard plastic cap you can see the circle o-ring in there the one on the left you can sort of see the dimple in the center it’s just got a flat piece of rubber across the bottom of the valve cap. What actually happens is where you see that depression is what’s pushing against the valve core. As you can see this is all oil and what’s happening is when they screw this on it depresses the valve cap and slowly leaks as you can probably hear. So that’s how this system lost its refrigerant charge, just make sure you’re using the proper caps make sure they have rings inside of them.

— Brad Hicks

Tech Tips

Drip Loops and Straws

July 20, 2017 By Bryan Orr 1 Comment

Many of us are aware that X13 and Fully variable motor failure has peaked over the last few years and there are several reasons for that. One of the reasons is fairly simple and can be traced back to two simple installation and service practices that can be easily implemented.

Seal all air handler / Furnace / Coil penetrations
Use drip loops on wires entering the motor

Eliminating “Straws”

Straws are openings in the cabinet that are unsealed that “suck” moisture into the system and can cause condensation on the interior surfaces. These can be copper penetrations, drain port openings and electrical penetrations.

This is a bigger factor on fan coil systems and package units than it is on furnace/coil systems because in a fan coil or package unit warm/moist air can more easily be drawn in after the coil and before the blower.

When unconditioned air enters into the system due to these penetrations it can cause mold, short circuits, and corrosion. This moisture can also gather on wires and drip into electrical connections causing issues with motors and control panels.

Make sure to seal any penetrations into the conditioned compartments of equipment with proper rubber grommets or in some cases silicone or thumb gum can be used.

Wire Drip Loops

Anytime a wire enters a plug, board or motor it is best to either locate the connection facing down to prevent water from entering or make a drip loop before the connection point. This allows moisture to drip off of the wire before entering the connector or device.

These issues have been identified as causes of X13 and ECM motor failure and checking these two areas can be very helpful in preventing future failure.

— Bryan