Tag: blower

Fan laws and fan curves are a deep subject with a lot of nuance and variation. Just to get our heads around the subject let’s focus on two different types of fans that we see all the time in HVAC, the prop/axial type and the radial/centrifugal type. Centrifugal / Blower With a typical PSC blower motor (non-variable/ECM) as we increase the pressure differential across it due to any variety of factors (small ducts, dirty filter, dirty coils) the blower moves less air and it uses less power to do it. The easiest way to test this is to measure the amperage of a PSC blower with a blower door off and then measure again with it on. The current will be higher with the panel off because the static pressure is LOWER and the blower is moving more air. Take a look at this chart which shows the huge impact static pressure (and input voltage) have on airflow If you do the same test (door on then off) on an ECM constant torque or constant airflow motor the amperage will go DOWN with the door off but this is because of the motor characteristics ramping the RPM down not because of the blower wheel properties. Take a look at this chart for an air handler that uses an ECM motor. The lines of airflow to static pressure are pretty constant until the static gets above the 0.5″wc If you were to check amperage on an ECM blower you will notice it draws higher amperage the higher the static pressure across it gets due to the motor ramping up to maintain the designed flow or torque. The PSC motor is the opposite, if we increase static the airflow, amperage and wattage all drop due to the characteristics of the centrifugal blower. To Summarize –

A blower wheel decreases in power used as static pressure increases UNLESS there is a ECM motor changing the RPM to compensate

Axial / Prop A prop fan performs in an opposite way in relation to pressure. As pressure differential across it increases the power used INCREASES even as the airflow it produces decreases. This means that if you block a condenser coil the fan will move less air and draw higher amps… illustrating again why keeping condenser coils clean has a big impact on performance. To Summarize –

A prop fan “loads” more based on pressure while a centrifugal fan “loads” based on mass flow

Again… this is a simplification but for a technician understanding these relationships can help you diagnose and understand system issues. Read specific system fan charts and curves for a better look at how a particular fan performs. — Bryan

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, common sense 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 airside 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 airflow. 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 the 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 fan 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 airflow (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

In residential air handler/fan coils it is common to use a high voltage interlock between the blower and the electric heat strips to ensure that the blower comes on whenever the heat is on.

The problem is that it CANNOT work the other way around where the heat comes on with the blower.

Heat strips are generally going to draw 20+ amps depending on the voltage and KW rating which means you CANNOT power them through a typical relay like a blower relay or board which are generally rated for 15 amps or less.

The way the interlock is wired is really quite obvious but is easy to forget because it’s the reverse of what we are used to with a relay.

In short, we connect the blower to the “common” terminal on the relay, L1 power to the normally open (n.o.) terminal and the load (out) side of the heat strip contactor /relay/sequencer to the normally closed (n.c.) terminal.

This diagram from Carrier shows the blower connected on the common terminal and constant power coming in on black to the normally open terminal from the right side of the transformer primary.

Using this 90-340 relay as an example, the blower would connect to 1, power to 3 and the heat strips to 2.

I made a video on it as well if you need it. The result is that the blower runs with the heat but the heat doesn’t run with the blower.

— Bryan

For those of you who use the MeasureQuick app for system diagnosis and performance testing, you may have noticed the “fan efficacy” results and wondered what it is.

It is simply the CFM output of the system divided by the wattage used by the blower. It is only for the blower motor and has nothing to do with the other components when done properly.

Fan (blower) efficacy is called out in various codes and standards such as California Energy Commissions requirement that all blowers perform at or below a 0.58 fan efficacy. This means a blower that is moving 1000 CFM cannot use more than 580 Watts of power to do so.


The tricky part is measuring fan efficacy is getting accurate measurements of system CFM and blower amperage. Equipment manufacturer fan charts can be used along with an accurate TESP (total external static pressure) measurement to figure out the CFM when the system is new and clean. When using these charts it’s important that the system is setup and run according to what is shown on the chart, one wrong pin setting or input can lead to vastly different airflow than the chart shows resulting in a fan efficacy that is way off.

Other options like measuring airflow at the return with a hood, anemometer duct traverse or the Trueflow from TEC can be used for measuring system CFM, but all have their own challenges.

Blower Wattage

When measuring blower amperage the panels must be in place which can be difficult to accomplish on some system types making a wireless connected ammeter very handy where the meter can be put in place and the panels put back on for testing.

Traditionally techs calculate wattage by measuring voltage and amperage and multiplying them together. This is actually VA not Wattage becasue it does not account for power factor. The only way to accurately measure wattage is to use a watt or power quality meter like the Redfish IDVM550 which calculates wattage by multiplying the VA by the power factor for the final wattage.

ECM Motors

ECM (electronically commutated motor) motors are more efficient than traditional PSC motors but their efficacy will generally vary based on the static pressure they are subjected to. Becasue most ECM motors are either constant airflow or constant torque rather than constant speed they will increase in wattage as the static pressure increases. This means that the fan efficacy will decrease on these motors as filters and coils become dirtier.

— Bryan



Just so you don’t get bored and quit reading let’s go straight to the point.

When the blower runs for more than a few minutes after the system has cycled off in cool mode the air may continue to be “cooler” (lower sensible temperature) coming out of the supply but the heat content of the air will remain unchanged. 

The only reason I say “may” be cooler instead of “will” be cooler is that we are assuming there is moisture on the coil and/or in the pan and the indoor RH is less than 100%.

Translation: When you run the blower once the system has gone off in cool mode you will continue to cool for a while, but that extended cooling comes from the evaporation of water out off of the coil and out of the pan. This results in sensible cooling and greater sensible efficiency but also increased indoor humidity.

Translation of the translation: It may feel cooler but there ain’t any less heat in the air by the time you figure for humidity.

Translation of the translation translation: If you live in a humid place run shorter off-cycle run times and think twice before running the fan in the “on” position. If you are in a dry place then let it blow until your heart is content.

Whenever cooling occurs by direct evaporation of a substance into an airstream (think a swamp cooler) it occurs at no net decrease to the heat content in the air. The heat is just going from sensible (what you can measure with a thermometer) to latent resulting in higher relative humidity air.

If you go below this line it is going to get nerdy… BEWARE

Now let’s talk about why, but first some terms.

Heat = Molecular energy or total molecular movement within a substance
Temperature = Molecular velocity, the speed that the molecules are moving
Adiabatic Process = A change in temperature without a change in heat content

Think of adiabatic process like this – You have a whole room full of ping pong balls bouncing around in a zero-gravity room. The balls are molecules, their total motion is the amount of heat and the speed they move is temperature. If you were to change the size of the room by bringing in one of the walls the balls the balls would bounce faster because the available space was decreased so the “temperature” would increase but the number of balls and the total motion would remain constant (this is what happens to refrigerant in a compressor by the way). If you were to move a wall outward and increase the size of the room the speed of the of the molecules would decrease, resulting in less speed and lower “temperature”. All the while the number of balls and the total motion remained constant (which is what occurs at the outlet of the metering device). In both of these examples temperature (Sensible heat) changes but the total heat content does not change, these are both examples of an adiabatic process due to compression and expansion of contained molecules.

An adiabatic process can also occur in uncontained systems like open airstreams, and evaporation of water is one such example.

Evaporation of water is a process where heat is absorbed into water molecules as they evaporate from liquid water and become entrained in the air as a vapor displacing some of the nitrogen and oxygen in the air. When that heat is absorbed from the air into the water it results in lower sensible temperature, but the water is still CONTAINED IN THE AIR. This means that while the air may be cooler it still has all the heat contained in it in the form of water vapor.

Now for the real shock..

Water vapor is NOT more dense than dry air at the same temperature it is actually less dense / lighter than dry air, however, is does contain more heat (enthalpy for you nerds like me). This means that when you run the blower after a cooling cycle the moisture on the coil and in the pan are evaporated back into the space and depending on the RH of the air it will lead to sensible cooling but latent gains. This means cooler but higher RH and this is due to the higher heat content of higher RH air at the same temperature.

Once again, depending on where you live this may be positive or negative.

In Arizona or Colorado? Run that blower after the cooling cycle.

Florida? May wanna shut it off right after the cycle or maybe 90 seconds at most and leaving the fan in the “on” position will likely result in a small increase of indoor RH.

— Bryan



P.S. – I also did a Facebook Live Video about it today

also… Here are some great videos on the subject by Jim Bergmann

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

When I started in the trade in 1999 there were still a lot of oilable blower motors in service. As part of the maintenance, we would remove the housing, oil the motor plus vacuum / wipe it down.

As oilable motors have become extinct I see fewer and fewer techs pulling the blower housing. Here are some reasons you may want to consider doing it more often.

  • Cleaning the motor itself can help it run cooler and last longer. A hot motor not only is more susceptible to winding breakdown but also to bearing/lubricant failure. Grab a vacuum, soft bristle brush, and a rag and get the dust buildup off the motor. If you have any dust that gets stuck inside, use some low-pressure nitrogen or compressed air to get it clean.
  • Get in there and look carefully at the wheel. A wheel that is even slightly dirty can have a significant effect on air output. If it’s dirty,  recommend cleaning.
  • Check the blower bearings, it’s easier to do when it’s out
  • On high-efficiency furnaces pulling the blower is a good way to check the secondary heat exchanger. On 80% furnaces, you can check parts of the primary exchanger and even the evaporator coil with a mirror or inspection scope.
  • Pulling the blower gives you the ability to wipe down the inside of the furnace or Fan coil.
  • You can check blower mounting bolts and set screws as well as blower alignment and balance more easily.

Obviously, when and why you pull the housing will vary from contractor to contractor but I advocate it being done more often than it is now.

What say you?

— Bryan

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