Low Airflow – Beyond the Obvious

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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

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