Category: Tech Tips

Tech Tips

Why Measuring Static First Isn’t Always Productive

May 25, 2017 By No Comments


There is a big move in residential and light commercial HVAC toward measuring static pressure regularly during commissioning, service and maintenance. And don’t get me wrong

Measuring static pressure is VERY important 

The challenge comes in when techs begin taking measurements without understanding where to take them, what they mean, or worse… they use measurements as an excuse not to do a proper visual inspection.

So before we go on, let’s cut to the chase. You need to visually inspect blower wheels, blower taps and settings, blower direction, belts, pulleys, evaporator coils, filters and condenser coils as well as look for any other abnormal return or condenser restrictions. 

Do this BEFORE you take detailed measurements and you will save yourself a lot of time and heartache. 

So what is “static pressure” anyway? 

Think of the air side of the system like a balloon. Static Pressure is the inflating (positive) or deflating (negative) pressure against the walls of the ducts / fan coil / furnace in relationship to another point which is usually atmospheric pressure 14.7 PSIA (at sea level) or 0 PSIA. 

When you blow up a balloon there is a positive pressure against all sides inside the balloon in relationship to the atmospheric pressure around the balloon. 

Static Pressure in residential and light commercial HVAC is generally measured in Inches of water column. We often measure it with an accurate digital manometer zeroed out to atmospheric pressure before use.

Static Pressure is not air flow. You could have static Pressure and have no airflow whatsoever. If you think of it in electrical terms, you can read voltage (potential) between two points and have no actual movement of electrons. It is a measure of difference in energy states between two points not a measure of quantity.

If you took a blower, attached a duct to it and blocked the end of the duct with a cap and turned the blower on you would have 0 CFM of airflow in the duct and very high static pressure. The exact amount of static pressure would be based on the ability of that particular blower motor and wheel to build up pressure. 

So when we are measuring static we are measuring pressure in the duct system not flow.

The more powerful the motor, the more pressure it can create and the more pressure / resistance it can overcome.

Think of a blower motor like a compressor, when it is off the pressure on both the inlet and outlet are the same. In the case of a compressor the pressure when off will be the static pressure of the refrigerant (let’s say 132 PSIG static pressure for R22 at 75° ambient) in the case of a blower it will be atmospheric pressure.

When the compressor turns on the suction pressure drops below 132 PSIG and the head pressure rises above 132 PSIG. The compressor creates this difference in pressure both above and below the static, saturated refrigerant pressure.

When a blower turns on it also drops the pressure of the return side below atmospheric pressure (14.7 PSIA) and it increases the supply side pressure above atmospheric pressure. 

We measure this static pressure at various points to find out how much resistance to airflow there is at various points in the system. 

For example we may measure the pressure drop across the evaporator or the filter or a particular run of duct or across a fire damper (to see if it’s slammed shut)


We also measure at the top and bottom of the appliance (furnace or fan coil) to find the Total External Static Pressure (TESP) which helps us calculate airflow when we compare to fam tables as well as helps us understand if we duct or system issues.

On a brand new, perfectly functioning system this works great.

But on an older system with a dirty blower wheel or a fan coil with a dirty coil this no longer serves its original purpose. 

If the blower wheel is dirty, the blower loses its ability to move air effectively, and therefore also loses its ability to create the pressure differential between the return and supply. 

We are trained to think that low static equals good and high static equals bad 

In the case of a dirty blower wheel or a clogged evap on a fan coil the TESP will be LOW and there will still be low air flow (low CFM).

As far as the refrigerant circuit and capacity is concerned the static pressure is meaningless, it is all a matter of how many CFM of air are traveling over the coil surface area. We use static Pressure as a diagnostic and benchmarking tool when taken together with an understanding of the system, blower specs and settings and duct design. Static pressure by itself means very little in the same way that measuring voltage or head pressure by themselves mean very little.

The point of this article is not to downplay the importance of static pressure or to explain how to measure it. The point is to remind you of two important facts.

  1. Check your blower wheel, blower direction, coils, filters, blower settings and other obvious airflow restrictions and issues first. 
  2. Before measuring your static think carefully about where you are placing your probes and what you expect to see / what you diagnosing with the measurement.

— Bryan

For a detailed explanation of static pressure you can go HERE

Or see a great video by Jim Bergmann using a Testo manometer HERE or Corbett Lundsford on static HERE

Tech Tips

The “5 Pillars” of Residential A/C Refrigerant Circuit Diagnosis

May 23, 2017 By No Comments

Taking all five of these calculations into account on every service call is critical. Even if further diagnostic tests must be done to pinpoint the problem, these five factors are the groundwork before more effective diagnosis can be done. I would also add static pressure as an important reading that should be checked regularly (Keep TESP between .3″wc and .7″ wc on most systems) but I would still place it slightly below these five.

Some of these are “rules of thumb” and obviously are for reference only. Refer to manufacturer recommendations when setting a charge.

Suction Pressure / Low Side
Suction pressure tells us several things. The first thing it tells us is what the boiling temperature of the refrigerant in the evaporator is. If the suction pressure is below 32° saturation temperature, the evaporator coil will eventually freeze.

As a general rule, the higher the temperature of the air passing over the evaporator, the higher your suction pressure will be. A good rule of thumb for suction pressure is 35°  saturation below indoor ambient +/- 5° (Return temperature measured at the evaporator coil). This temperature differential is often called an evaporator split or design temperature difference (DTD),

This means that when the temperature of the air passing over the evaporator is 80°, the low side saturation temperature should be 45° when the system is set for 400 CFM per ton output. Remember the temperature scale next to the pressure scale on the gauge represents saturation or if you don’t have the correct sale on (or in your gauge if you have a Digital manifold) you would need to use a PT chart.

This 35° rule only works at 400 CFM per ton, when a system is designed for 350 CFM per ton the DTD will be closer to 40° +/- 5° 

Make sure you know the actual CFM output of the system before you calculate DTD. It can vary significantly based on the setup of the particular blower.

Head Pressure / High Side
When used in conjunction with liquid line temperature, we can know what state the refrigerant in the liquid line and that the compressor is pumping / creating the required compression ratio. We can also know something about the state of the metering device as to whether or not refrigerant is “backing up” against the metering device. A good rule of thumb for head pressure is a 15° – 20° saturation above outdoor ambient +/- 3° for most modern systems. These saturation / ambient calculations are only indicators; they are not set in stone. Keep in mind, when I say ambient; I am talking about the air entering the evaporator for suction pressure and the condenser for head pressure.

Jim Bergmann brought to my attention that different equipment efficiencies will have different target Condensing Temperature Over Ambient (COTA) readings.

6 – 8 SEER Equpiment (Very Old) = 30° COTA

10 SEER Equipment = 25° COTA

13 / 14 SEER Equipment = 20° COTA

15 SEER+ Equpiment = 15° COTA

Superheat
Superheat is important for two reasons. It tells us whether or not we could be damaging the compressor and whether we are fully feeding the evaporator with boiling, flashing refrigerant. If the system has a 0° superheat, a mixture of liquid and vapor is entering the compressor. This is called liquid slugging and it can damage a compressor. A superheat that is higher than the manufacturer’s specification can both starve the evaporator, causing capacity loss, as well as cause the compressor to overheat. So how do we know what superheat we should have? First, we must find out what type of metering device the system is using. If it is using a piston or other fixed metering device, you must refer to the manufacturers superheat requirements or a superheat chart like the one below.

If it is a TXV type metering device, the TXV will generally try to maintain between an 7 – 17 degree superheat on the suction line exiting the evaporator coil (12° +/- 5°) 

TXV target superheat setting may vary slightly based on equipment type.

Subcooling
Subcool tells us whether or not the liquid line is full of liquid or not. A 0 degree subcool reading tells us that the refrigerant in the liquid line is part liquid and part vapor. An abnormally high subcool reading tells us that the refrigerant is moving through the condenser too slowly, causing it to give up a large amount of sensible heat past saturation temperature. A high subcool is often accompanied by high head pressure and, conversely, a low subcool by low head pressure. Subcool is always a very important calculation to take because it lets you know whether or not the metering device is receiving a full line of liquid. Typical ranges for subcooling are between 5 and 14 degrees, but always check the manufacturer’s information to confirm.

Evaporator Air Temperature Split (Delta T)
The evaporator air temperature split (Delta T) is a nice calculation because it gives you a good look at system performance and airflow. A typical air temperature split will be between 16 and 22 degrees difference from return to supply. Keep in mind, when you are doing a new system start up, high humidity will cause your air temperature split to be on the low side. Refer to the air temperature split and comfort considerations sheets for further information.

For systems that are set to 400 CFM per ton, you can use a target Delta T sheet like the one shown below

If the leaving temperature/delta T split is high it is an indication of low airflow. If it is low it is an indication of poor system performance / capacity.

Again, this only applies to 400 CFM ton. 350 CFM per ton or less are more common today than ever and in those cases the above chart won’t apply.

Diagnosing With The Five Pillars
The way this list must be utilized is by taking all five calculations and matching up the potential problems until you find the most likely ones. A very critical thing to remember is that a TXV system will maintain a constant superheat, and a fairly constant suction pressure. The exceptions to this rule are when the TXV fails, is not receiving a full line of liquid or does not have the required liquid pressure/pressure drop to operate. This situation would show 0° subcooling and in this case, will no longer be able to maintain the correct superheat. Before using this list, you must also know what type of metering device is being utilized, then adjust thinking accordingly. Also remember, in heat mode, the condenser is inside and the evaporator is outside.

Low Suction Pressure
• Low on charge
• Low air flow /load – dirty filter, dirty evaporator, kinked return, return too small, not enough supply ducts, blower wheel dirty, blower not running correct speed, insulation pulling up against the blower, etc.
• Metering device restricting flow too much – piston too small, piston or TXV restricted, TXV failing closed
• Liquid line restriction – clogged filter/drier, clogged screen, kinked copper
• Low ambient (Low evaporator load)
• Kinked suction line (after the kink)
• Internal evaporator restriction
High Suction Pressure
• Overcharge
• High return temperature (Evaporator Load)
• Metering device allowing too much refrigerant flow – piston too large, TXV failing open, piston seating improperly
• Too much airflow over the evaporator (Blower tapped or set too high)
• Compressor not pumping properly – bad suction valve, bad discharge valve, bad or broken crank
• Reversing valve bypassing
• Discharge line restriction
Low Head Pressure
• Low on charge
• Low ambient temperature / low load
• Metering device allowing too much refrigerant flow – piston too large, TXV failing open, piston seating improperly
• Wet condenser coil
• Compressor not pumping properly – bad suction valve, bad discharge valve, bad or broken crank
• Reversing valve bypassing (heat pump units)
• Kinked suction line
High Head Pressure
• Overcharge
• Low condenser airflow – condensing fan not operating, dirty condenser, fins bent on condenser, bushes too close to condenser
• Metering device restricting too much flow – piston too small, piston or TXV restricted, TXV failing closed
• High ambient temperature
• Liquid line restriction – clogged filter/drier, clogged screen, kinked copper
• Non-condensables in the system
Low Superheat
• Overcharge
• Low air flow / load – dirty filter, dirty evaporator, kinked return, return too small, not enough supply ducts, blower wheel dirty, blower not running correct speed, insulation pulling up against the blower etc.
• Metering device allowing too much refrigerant flow – piston too large, TXV failing open, piston seating improperly
• Low return air temperature
• Abnormally low humidity
• Internal evaporator restriction
High Superheat
• Low on charge
• Metering device restricting too much flow – piston too small, piston or TXV restricted, TXV failing closed
• High return air temperature
• Liquid line restriction – clogged filter/drier, clogged screen, kinked copper
• Compressor not pumping properly – bad suction valve, bad discharge valve, bad or broken crank
• Reversing valve bypassing
Low Subcool
• Low on charge
• Metering device allowing too much refrigerant flow – piston too large, TXV failing open, piston seating improperly
• Dirty condenser coil – not a very low subcool, but low considering how high the head pressure will be
• High ambient surrounding the condenser
• Compressor not pumping properly – leaking suction valve, leaking discharge valve, bad or broken crank
• Reversing valve bypassing
High Subcooling
• Overcharge
• Metering device restricting too much flow – piston too small, piston or TXV restricted, TXV failing closed
• Low ambient surrounding the condenser
• Liquid line restriction – clogged filter/drier, clogged screen, kinked copper
• Wet condenser coil
• Dirty Condenser Coil on New High Efficiency Condensers (Increased Condensing Temp Can Actually Result in Higher Subcooling)
• Having an H.R.U. in the discharge line (old school I know)
• Internal evaporator restriction
• Kinked suction line
High Evaporator Air Temperature Split
• Low air flow – dirty filter, dirty evaporator, kinked return, return too small, not enough supply ducts, blower wheel dirty, blower not running correct speed, insulation pulling up against the blower etc.
• Abnormally low humidity (WB Temp)
• Blower not running the correct speed or running backward
Low Evaporator Air Temperature Split
• Undercharge
• Severe Overcharge – because saturation temperature is increased with overcharge
• Metering device not functioning properly – restricting too much flow or allowing too much flow
• Too much airflow through the evaporator – blower not running correct speed
• Heat strips running with air
• Abnormally high humidity
• Liquid line restriction
• Kinked suction line
• Compressor not pumping properly – bad suction valve, bad discharge valve, bad or broken crank
• Reversing valve bypassing

 

This is an incomplete list designed to help you. Always keep your eyes and ears open for other possibilities. Diagnosis is an art as well as a science.

— Bryan

Tech Tips

The Difference Between Variable Speed and X13 Motors

May 22, 2017 By No Comments

First, let’s cover the basics. X13 is a brand name for the Regal Beloit / Genteq brand of constant torque motors, there are other manufacturers who make them but the term “X13” has become pretty much synonymous for the fractional horsepower HVAC constant torque motor.

Also, this article is specifically discussing the common residential/light commercial motors. There are other types of variable and constant torque motors and equipment not being addressed here.

Both variable speed and X13 motors are ECM or “Electronically Commutated Motors”, This just means the DC power that drives them is electronically switched from positive to negative. Both are more efficient than the typical PSC motor with ECM motors commonly being about 80% efficient and PSC being about 60%.

Both X13 and variable speed motors are DC, 3 phase, permanent magnet rotor motors that use back EMF to determine motor torque and adjust to load conditions.

The primary difference is the type of inputs to the motor control. A variable speed motor is programmed for a specific piece of equipment to produce a set amount of airflow based on the particular static pressure profile of that system as well as based on the inputs from the air handler circuit board or system controller. In other words, a variable speed motor can ramp up down based on the static pressure as well as the staging of the equipment, pin/dip switch or controller settings for desired airflow output and “comfort profiles” that can be setup to allow the blower to ramp up or down for enhanced dehumidification and comfort.

An X13 motor is programmed to produce a set motor torque based on which input it is receiving 24v. This means that while an X13 motor is more efficient than a PSC motor and does a better job of ramping up to overcome static pressure increase it does not have the level of control that a variable speed has and it also does not produce an exact airflow output across the full range of static pressure.

This is why when you check the blower charts on a unit with a variable speed motor the CFM will remain the same over a wide range of static points but when you look at an X13 system the CFM will drop as the static pressure increases.

— Bryan

Tech Tips

This is Not a Soft Start

May 21, 2017 By No Comments


When I started in the trade I would see these devices (above) in the field and I would hear guys call them a PTC or a “Soft Start” and I just accepted it and moved on. In fact, I’ve been calling PTCR (Postive Temperature Coefficient Resistors) a “soft start” for most of my career.

Turns out I was wrong 

A PTCR is just a resistor (thermistor) that increases in resistance as it heats up. It connects from run to start in parallel with the run capacitor and allows a surge of current the start winding of a motor when it starts. When the PTCR heats up, the resistance inside increases and it essentially takes itself out of the circuit. 


A PTCR by itself does allow a spike of current to the start winding but it does not create a phase shift. This is why some devices add in a start capacitor and just use the PTCR as the “relay” to take it out of the circuit like the products shown above. It is a start device… but there is nothing “soft” about it.


Then there are the more traditional hard start kits that use the tried and true start capacitor and potential relay instead of a PTCR. This serves the same basic purpose, increase current and phase shift to the start winding for a fraction of a second and then remove it from the circuit. It just does the “removal” part of the equation in a more precise manner.

All of these technologies serve to increase current to the start winding quickly then drop out

The idea is to get the motor to 75% to 100% of running speed as QUICKLY as possible by increasing start winding current and phase shift.

There is good reason for this. When a motor is stationary or running at low speed, it’s windings act as low resistance resistive loads, essentially really high amperage heaters. The longer the motor spends trying to start at full voltage the higher the current it will draw and the hotter the windings get.

Hard start devices can do nothing to actually reduce the current the motor draws when it is at locked rotor (stalled), the hard start device simply gets it out of that stalled / low RPM as quickly as possible. 

Many of you may note that when you measure inrush current with a hard start in place that it will show lower than when it is not in place. This is simply because a hard start shortens the time the motor remains in locked rotor, not because it actually reduces the starting amps. 

There are also some concerns about the added torque that a hard start provides over such a short time period and the side effects of that “torque shock” to the internal components of the compressor as well as the connecting copper lines.


Soft Start

Different methods of “soft starting” has been in use in large 3 phase motors for a long time. The purpose is the start a motor more slowly and therefore reducing the current inrush.

The devices shown above are single phase soft start devices. They reduce the voltage during starting to reduce the current associated with start. These devices carefully control the voltage and therefore the current applied to the start and run windings to provide a lower initial current during start and slowly increase up toward full speed. These devices require advanced algorithms to do this which makes them significantly more expensive than the traditional hard start technologies and they are not capable of producing a large shifted / current  boost to a start winding like a hard start. 

This means that while a soft start is a great device to help reduce light flicker, decrease start amps and increase compressor life, it is unlikely that it will start that old, stuck compressor.

Soft starts and hard starts both serve a purpose but what they do and how they do it couldn’t be more different. 

Have an old, locked compressor? Hard start is likely the best bet. If you have a complaint that lights are dimming on compressor start? A soft start will give a better result.

— Bryan

Here is a pretty dramatic demonstration of hard start and soft start HERE

And a great application guide on soft start HERE

Tech Tips

How to Determine the Friction Rate for Residential Duct Design

May 17, 2017 By 2 Comments

This article is written by one of the smartest guys I know online, Neil Comparetto. Neil is a little nervous about writing a tech tip so make sure to give him lots of positive affirmation on this one. Thanks Neil!

Recently I posted a question in the HVAC School Group on Facebook, “when designing a residential duct system what friction rate do you use?”. As of writing this, only one answer was correct according to ACCA’s Manual D.


I feel there is some confusion on what friction rate is and what friction rate to use with a duct calculator. Hopefully, after reading this tech tip you will have a better understanding.

So, what is friction rate?

Friction rate (FR) is the pressure drop between two points in a duct system that are separated by a specific distance. Duct calculators use 100′ as a reference distance. So, if you were to set the friction rate at .1″ on your duct calculator for a specific CFM the duct calculator will give you choices on what size of duct to use. Expect a pressure drop of .1″ w.c. over 100′ of straight duct at that CFM and duct size / type.

Determining the Friction Rate

First, you need to know what the external static pressure (ESP) rating for the selected air handling equipment is. ( external static pressure means external to that piece of equipment. For an air handler, everything that came in the box is accounted for, including the coil and typically the throwaway filter. For a furnace the indoor coil is external and counts against the available static pressure)

Next you have to subtract the pressure losses (CPL) of the air-side components (coil, filter, supply and return registers/grilles, balancing dampers, etc.). Now you will have the remaining available static pressure (ASP). ASP = (ESP – CPL)

Now it’s time to calculate the total effective length (TEL) of the duct system. In the Manual D each type of duct fitting has been assigned an equivalent length value in feet. This is done with an equation converting pressure drop across the fitting to length in feet (there is a reference velocity and a reference friction rate in the equation). Add up both the supply and return duct system in feet. It is important to note that this is not a sum of the whole distribution system. The most restrictive run, from the air handling apparatus to the boot is used. Supply TEL + Return TEL = TEL

The formula for calculating the friction rate is FR= (ASP x 100) / TEL
This formula will give you the friction rate to size the ducts for this specific duct system. If you test static pressure undersized duct systems are very common, almost expected. This is because a “rule of thumb” was used when designing the ducts.

This is just an introduction to the duct design process. I encourage you to familiarize yourself with ACCA’s Manual D and go build a great system!

— Neil Comparetto

Tech Tips

ECM, VFD and Inverter – What’s the Difference?

May 15, 2017 By 1 Comment

Ever since Nikola Tesla invented the modern induction motor we have been struggling with varying the speed of motors in an efficient and reliable way. The trouble in the HVAC industry is that there are several different types of technologies in play and they can easily get confused.

ECM (electronically commutated motor)

In residential and light commercial HVAC we have seen ECM (Variable Speed / X13) motors for years, primarily in blower motors but sometimes even in condenser fan motors. The first thing to know is that an ECM motor is “Brushless” DC motor. Most traditional DC motors require brushes to provide power to the motor rotor (spinning part). Brushes are notorious for wearing out over time making DC motors unreliable in constant duty applications. An ECM motor uses a permanent magnet rotor which eliminates the need for power to be fed to the rotor through brushes.

An ECM motor is a DC 3 phase motor with a permanent magnet rotor where the cycle rate is controlled by the motor module. Here is a great video on how they work.

 

VFD (Variable Frequency Drive) 

For existing A.C. (Alternating Current)  3 phase motors the only way to change the speed reliably and efficiently is the alter the “frequency” of the power applied to the motor to something other than 60 hz (60 cycles per second). A VFD intercepts the power applied to a motor, changes it to DC power with a bank of diodes (rectifier) also called a CONVERTER. It then smooths out the power using capacitors before feeding that power to a bank of transistors called an INVERTER which is constantly switching the power from DC back to a form of power called PWM (Pulse Width Modulation) which replicates frequency change to the motor. The drive needs to be able to provide this PWM power at the correct voltage and current in order to control a 3 phase motor properly.

Inverter / Inverter Drive

Many A/C systems are coming with Converters, Capacitor Smoothing (Intermediate Circuit) and then the Inverter all built in to the equipment itself to drive a compressor or compressors. This inverter technology is essentially an intelligent and specifically designed VFD built into the equipment itself. The Carrier Infinity system is one of many systems that utilize inverters.

These technologies are constantly evolving and changing and while they may be similar, the different names describe different types and applications of technology all designed with e end goal of making motors go more than one speed with the best efficiency and reliability.

— Bryan

Tech Tips

Does Refrigerant Get Old or Wear Out?

May 12, 2017 By 2 Comments


Over the years I have heard technicians say that refrigerant can wear out or “lose it’s blend” by sitting in a tank.

This does not happen… at least not like that

What can and does happen is called “Fractionation”. Refrigerant blends that are composed of a mix of refrigerants with different vapor and liquid PT characteristics known as Non-azeotropic, Zeotropic or in some cases near-azeotropic. All fancy words to mean that these refrigerant blends must be added or removed completely or in the liquid state to prevent more / less of one refrigerants in the mix to be added or removed than the other.  If the refrigerant is allowed to fractionate and some of it is added in the vapor only state both the refrigerant left in the tank, and the refrigerant added to the system will no longer have the designed properties of the listed refrigerant.

If one of the refrigerants in the blend leaks out faster, what you have left isn’t the same refrigerant

While all blends should all be charged in the liquid state, some refrigerants are more likely to be impacted by fractionation than others.

For example, R-410a  (50% R-32 & 50% R-125) has very little “glide” between liquid and vapor and so while it is a blend, it is less likely to fractionate severely when charged in the vapor phase (which you still shouldn’t do). 

A refrigerant like R-407c ( a mixture of R32/125/134a) will fractionate much more easily resulting in far greater pressure / temperature swings and poor performance when it occurs.
Fractionation will often happen for three reasons

  1. A technician charged the system in vapor phase (tank upright) instead of in liquid phase (upside down)
  2. The tank had a small leak while stored upright
  3. The system has a significant leak.

The particular case of fractionation being caused by a system leak depends on many factors including what part of the system the leak occurs, the physical location of the leak and how much refrigerant leaked out. There was a study done at Purdue that shows that fractionation after leakage can be a factor in high glide systems like R407c.

The ramifications of this depend on the specific situation, but in some cases, the only viable option will be to completely recover and recharge with a virgin charge. This is not because refrigerant has “lost its mix” from sitting, but rather because some of the”mix” has left the tank or system at a different rate, leaving an improper mix behind.

— Bryan

 

Tech Tips

Testing, Recovery and Charging a Sealed System

May 10, 2017 By No Comments

There are many appliances with compression refrigeration circuits that do not have ports installed for testing, recovery, charging and evacuation.  These can include window units, PTACs, fountains, refrigerators and much more.

This presents a challenge any time you suspect or know there is a refrigerant circuit issue. How can you diagnose and make a repair when you can’t connect?

First, when an appliance is a sealed circuit DONT OPEN IT UNLESS YOU HAVE A GOOD REASON TO DO SO.

If you do need to open the circuit, first look for factory “process tubes” often on the compressor or on the lines near the compressor.

You can then either add a piercing valve to the tube or you can often pinch off the tube with a pinching tool, Leave the tool in place, cut the process tube / stub near the end and solder / braze in a schrader to the end.

Common pinch off tools


 Once the new port is added you simply remove the pinching tool and form it slightly back into shape. It does not need to be completely rerounded because the size of these types of systems will generally be small so recovery speed will not be a huge factor.

You can then recover the the charge and once the charge is fully recovered you can add in additional soldered / brazed in ports.

Another method is to use a piercing line tap valve to access the system like the ones shown below.


These should also only be left on as a temporary measure to get the refrigerant recovered. You then must repair the “pierced” hole, usually by putting a tee schrader in the same spot where the piercing was.


Once you have good, solid access points in place (don’t forget to flow nitrogen), now you can proceed to evacuation, weighing in a factory charge and then performing further diagnosis.


If done correctly the appliance will have no leaks at the ports and will be much easier to service next time.

Keep in mind when working on systems with very small charges that hoses can hold a lot of refrigerant. You may consider using Smart Probes with tees and only one short 1/4″ hose from the charging tank to the suction probe to reduce losses.

Also make sure the hose is well purged before starting the charging process. Keep the air and moisture out.

— Bryan

Tech Tips

Motor Service Factor

May 9, 2017 By No Comments

Service factor is an interesting motor rating that you will see on many motor data tags. It simply means how much additional “work” a motor can do or “load” it may be placed under for short periods of time without failure or overload.

For example. The FLA or Full Load Amps of the motor above is 10.8 amps at 115 volts

The Service Factor or S.F. is 1.5, which makes the Service Factor Amps 16.2 (rounded down to 16 on the motor tag) because 10.8 x 1.5 = 16.2

Don’t confuse SFA with LRA (Locked Rotor Amps). LRA is the current the motor will draw when the rotor is stationary, such as during startup. Service Factor is simply a short term “fudge factor” that the motor has for short periods of higher than normal load.

When a motor is running above its Full Load Amps and in the Service Factor range it may function but its operational life will be shorter and it will generally run at lower efficiency and power factor.

— Bryan

Tech Tips

Air Velocity Quick Tips

May 8, 2017 By No Comments

As a service technician, we are often expected to understand a bit about design to fully diagnose a problem. Duct velocity has many ramifications in a system including

  • High air velocity at supply registers and return grilles resulting in air noise
  • Low velocity in certain ducts resulting in unnecessary gains and losses
  • Low velocity at supply registers resulting in poor “throw” and therefore room temperature control
  • High air velocity inside fan coils and over cased coils resulting in higher bypass factor and lower latent heat removal
  • High TESP (Total External Static Pressure) due to high duct velocity

Duct FPM can be measured using a pitot tube and a sensitive manometer, in duct vane anemometers like the Testo 416  or a hot wire anemometer like the Testo 425. Measuring grille/register face velocity is much easier and can be done with any quality vane anemometer, with my favorite being the Testo 417 large vane anemometer

First, you must realize that residential, commercial and industrial spaces tend to run very different design duct velocities. If you have ever sat in a theater, mall or auditorium and been hit in the face with an airstream from a vent 20 feet away you have experienced HIGH designed velocity. When spaces are large, high face velocities are required to throw across greater distances and circulate the air properly.

In residential applications, you will want to see 700 to 900 FPM velocity in duct trunks and 600 to 700 FPM in branch ducts to maintain a good balance of low static pressure and good flow, preventing unneeded duct gains and losses.

Return grilles themselves should be sized as large as possible to reduce face velocity to 500 FPM or lower. This helps greatly reduce total system static pressure as well as return grille noise.

Supply grilles and diffusers should be sized for the appropriate CFM and throw based on the manufacturer’s grille specs like the ones from Hart & Cooley shown above. Keep in mind that the higher the FPM the further the air will throw but also the noisier the grille will be.

— Bryan

 

Scroll to top

Daily Tech Tip

Subscribe to our daily tech tip to receive daily tips and advice!
Email address
Name