One of our techs called me the other day and gave me a story of woe.

He had been working on a system and he had the following readings

  • Low superheat
  • Low suction pressure
  • Low head pressure

He reassured me that the system airflow was correct and wondered what could have been wrong.

I asked him how he could be sure his airflow was correct and he told me that he had “checked everything”. By that he meant he has looked at the coil, blower wheel, filter and inspected the ducts, NOT that he had measured the airflow.

This isn’t a tip on how to measure airflow but there are many ways it can be done with varying levels of accuracy in the field. From a hot wire anemometer in duct to an air flow hood measuring airflow can be done and is certainly better than just guessing, especially when you get stuck on a diagnosis. My favorite way to measure airflow is to use factory fan tables and static pressure but that method just doesn’t work when anything in the system has been altered from factory test conditions (dirty blower wheel, wheel or motor replaced etc…)

While there is validity to visual inspection and to airflow measurement there are some issues that can be tough to notice that can lead to the symptoms the tech was observing.

Low Load

While we often think of the combo of low suction, superheat and head pressure as being caused by low airflow it actually falls under a larger heading of low evaporator load. This simply means that the quantity of heat being picked up in the evaporator is lower than the refrigerant mass flow rate requires for desired operation.

This can be caused by low air temperature passing over the coil, low air flow, or an undersized coil.

Here are some things to look out for that can cause these symptoms that are more uncommon.

Missing Blower Cutoff Plate

The blower housing cutoff plate helps to direct the airflow from the wheel out of the housing. It’s there so the blower wheel can be removed but if it’s missing it can greatly reduce airflow.

Incorrect Blower Wheel

We’ve seen several occasions where a homeowner or handyman has replaced a blower wheel with a wheel off of another system where it is too small. This will generally be visually obvious but is certainly worth looking out for.

Incorrect Evaporator Coil

We had one instance where we were consistently seeing symptoms of low load and later found that someone had put in an Evaporator coil that was a smaller tonnage than the original.

Oversized Compressor

Sometimes a compressor will be replaced with a compressor a size or two larger than the original. This will show low suction and superheat but will show higher than usual head pressure rather than lower like a typical low load evaporator condition.

Incorrect Blower Motor

In the old days you would simply match HP, RPM and Voltage on a Motor and you would get a fairly consistent result. There are now off the shelf ECM/X13 Motor replacement kits that can produce very different results from the original factory motors depending on how they are programmed.

Concealed Duct Issues

Issues like a collapsed inner duct liner or an old filter pulled deep into a return can be tough to find visually. I will generally use a combination of measuring total system airflow and measuring static pressure at various points in the duct system to help find these concealed issues.

Air Bypassing or Recirculating

Open bypass dampers are a common source of issues but there can also be cases where there are gaps around the coil where air can pull around the coil without adding heat to the coil like usual.

Blower Spinning Backwards

This is an extreme case but I’ve had techs chasing their tails on many occasions just to find out the blower was running backwards. Some older ECM motors would fail and run backwards though I haven’t seen that issue occur recently.

Oil logged evaporator

Over time an Evaporator can become logged with oil that can impede the transfer of heat through the tubing walls. This can look like a low load condition and often accompanies low refrigerant velocity CAUSED by low load over time. This was more common in older mineral oil systems especially when the system has had a compressor changed or oil added over time. The only way to fix it is to flush the coil internally or use an additive designed to help with oil return.

The way to find these more uncommon causes is to

  • Measure total system airflow against design
  • Use static pressure to help isolate issues
  • Look for signs of past repairs or newer parts and confirm the replacements are correct and setup properly

— Bryan

This article is written by one of the smartest guys I know online, Neil Comparetto. 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

Recommended Duct Velocities (FPM)

Duct TypeResidentialCommercial / InstitutionalIndustrial
Main Ducts700 – 9001000 – 13001200 – 1800
Branch Ducts600 – 700600 – 900800 – 1000

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, induct 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

Measuring airflow is easy… measuring airflow accurately is quite a bit more difficult. In many cases when we as technicians measure airflow we are trying to get to the almighty CFM (Cubic Feet per Minute) volume measurement. You can take CFM readings fairly easily with a hood like the Testo 420 shown above, but even a hood has some limitations when the goal is to measure total system CFM vs. register / grille CFM.

In this series of videos Bill Spohn from Trutech tools demonstrates all of the tools you can use to measure airflow from hot wire and rotating vane anemometers, to flow hoods, to smart grids and pitot tubes, all the way down to using a GARBAGE BAG.

I had the privilege of seeing this presentation in person (I am the one behind the camera) and I wanted to share it with you. It is well worth your time.

— Bryan


When tightening down a blower wheel or a fan blade on a motor shaft ONLY tighten it on the flat of the shaft.

If you have more than one screw, but only one flat surface on the shaft then only tighten the one set screw.

Also…

Refrain from overtightening set screws, they need to bite into the shaft but you don’t need to mangle the poor thing. Both setting on the curve and overtightening of these conditions can make it hard or impossible to remove the blade or wheel later. Simple, but important.

— Bryan

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