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If a furnace or air conditioning system develops an airflow problem, there are several items to visually inspect. If visual inspection does not locate the issue, proper testing tools are required to determine where the problem is and how severe it is.
First visually inspect the air filter to see if it is dirty or clogged. Replace if it is too restrictive or clogged. The back of the evaporator coil could become clogged if there is no air filter or the filter is dirty. The evaporator is usually hard to visually inspect because it is positioned tight to the furnace and/or other system components. There could also be a collapse in part of the ductwork that could be the problem. The ductwork may also have been designed too small to begin with.
The right tools (a basic digital water column manometer and a static pressure tip) will help to determine whether the airflow problem is in the ductwork, at the air filter, or at the evaporator coil. Below is the step-by-step procedure to use when searching for an airflow problem. I will also show how to calculate the delivered CFM of the system in our example.
Definitions/Terms:
Static pressure is the amount of pressure a fan has to apply to move air through a duct system. Static pressure is exerted equally on all sides of a duct system. When measuring static pressure, the unit of measurement used is inches of water column (in WC).
The total external static pressure is the measurement taken on both sides of the blower motor by using a basic digital water column manometer and a static pressure tip. (See photo below.)
Steps to Taking a Measurement on the Return Side
Use a unibit to drill a 3/8” hole in the return duct, close to where the air enters the return for the furnace. (See photo below.) This must also be downstream of the air filter and upstream of the blower motor. Install a duct plug in this hole after testing.
The unit being measured in our example is a vertically installed indoor gas furnace with an evaporator coil above the furnace. The filter is installed in the FRAG (Filter Return Air Grille) located on the interior wall inside the building.
Insert the static pressure tip into the hole. Point the arrow on this tool AWAY from the furnace. (See photo below.) The water column manometer will measure the static pressure in the return after the furnace is turned on. (NOTE: The static pressure tip does not have a hole on the front. The holes are on the side. The static pressure is read as the air moves past the holes.) This reading is the static pressure (water column measurement) for the return including the filter, which is upstream from this location. In this case during air conditioning mode, the reading is 0.21" WC in the return. (See photo below.)
Steps to Taking a Measurement NEAR the Heat Exchanger
To get the total external static pressure, a static pressure reading must be taken on both the return side and the supply side of the blower. The return side measurement was already taken at .21" WC.
Next, the supply side measurement must be taken. On a furnace, do this near the heat exchanger area where the high temperature limit switch is. This location is used because there is already a hole in the furnace here.
With the power to the furnace OFF, remove the screws securing the high temp limit switch from the unit. Remove the high temp limit switch. Set it aside in a location where it will not short out during testing, when the unit is turned back on.
Cover the hole where the switch was located with tin tape. Make a hole in the tin tape to allow the static pressure tip to be inserted. (See photo below.)
Insert the pressure tip into the hole with the arrow pointing downward toward the blower motor.
Turn ON the power to the furnace and turn the blower motor on in air conditioning mode. Any mode can be used for these tests as long as the return and supply are both being measured. These modes include fan, heat, or air conditioning speed. Fan mode is easiest to check. In this case, the outdoor unit disconnect is off and the indoor unit is running in air conditioning mode.
In this example, the water column measurement on the supply side of the furnace reads about +0.85, which is much higher than on the return side. When the water column measurement on the supply side is higher than the measurement on the return side, this indicates that the airflow problem is on the supply side of the system.
Remove the tin tape before replacing the heat exchanger area high temp limit switch.
Steps to Taking a Measurement at an Alternate Location
If the high temp limit switch location is not suitable for taking a static pressure reading, use a unibit to drill a hole on the side of the furnace toward the top, but below the evaporator coil . The short unibit will not penetrate in far enough to damage the heat exchanger. However, care must be taken not to use a longer bit which could potentially reach the heat exchanger. Also, avoid the coil and pan within the evaporator coil box.
Insert the static pressure tip into the hole with the arrow and tip pointing downward to take the measurement. (See photo below.)
In this example, the measurement is about +0.82, which is lower than the reading taken at the high temp limit switch. This change is because this location is further away from the blower motor. Once again, if the water column measurement on the supply side is greater than the measurement on the return side, this indicates that the airflow problem is on the supply side of the furnace.
To get the TESP (Total External Static Pressure), combine the static pressure measurements from both sides of the system, closest to the blower motor. The readings taken closest to the blower motor were 0.21 on the return side and 0.85 on the supply side. Therefore, the total external static pressure is .85 + .21 = 1.06" WC
Compare this measured TESP to the max TESP displayed on the rating plate. The furnace's blower motor is usually rated to operate and obtain the proper air flow with no more than 0.5 inch TESP. Compare the actual TESP measured to the max TESP on the rating plate. In this case, the TESP is higher than the max TESP indicating a major airflow restriction. (Remember, we already know that an airflow problem exists on the supply side.)
Now measure the static pressure on the other side of the evaporator coil to determine the pressure drop across the coil.
Drill a hole in the supply plenum duct above the evaporator coil to take a static pressure reading. In this example, the measurement in the supply plenum is 0.20" WC. This measurement reflects everything downstream from this location. Compare the reading before the coil (0.85" WC) with the reading after the coil (0.20"WC).
0.89 - 0.20 = .65" WC pressure drop A significant difference between the two measurements indicates that the coil is clogged with dust. This coil must be cleaned to remove the clog. (NOTE: Most evaporator coils range from approximately 0.1" WC to 0.35" WC. The range depends on the air flow speed, the surface area of the coil, and how dense the coil fins are.) The picture below shows how clogged the under side of this coil is.
Determine the Rated Static Pressure Drop of the Evaporator
Use the model number of the coil to look up the coil's performance data sheet online. This will show what the static pressure drop should be for the coil.
The performance data chart below has a left hand column for the model number (unit size) (See image immediately below.) and a row with the CFM at the top of the chart. (See second image below.)
As you cross the chart for each model, the information provided helps determine what the dry and the wet coil static pressure should be. (See image below.) (NOTE: Dry means the outdoor air conditioning unit is not running and the coil is completely dry. Wet means there is moisture on the coil, because the outdoor unit is running, so the static pressure will be higher.)
How to Determine the Airflow Speed of the HVAC UniT
Take the total external static pressure (TESP) and input it into the performance data of the furnace. This data can be found in the installation literature or in the service manual of the furnace itself.
Most furnaces are rated for a 0.5 inch water column as the highest measurement (the highest total external static pressure TESP). (See image below.)
The system we measured had a TESP of 1.06" WC. On the chart below, this amount gets input into the TAP column. Unfortunately, the chart only goes to .70, but 1.06 has been inserted at the bottom of this column. This unit was tested in air conditioning mode at the highest airflow speed. The unit was running in air conditioning mode with a dry coil and the outdoor unit off while the tests were being performed. When looking at the picture below, the estimated CFM is roughly 800.
The example in the chart below shows that if a system with .50" external state pressure is running on the highest fan motor speed, it has a delivered CFM of 1371.
Once the air flow speed is known, take the performance data for the evaporator coil and input the actual CFM into a Performance Data chart. Line up the model type (horizontal) with the CFM (vertical ) to determine what the static pressure should be for the pressure drop across the coil. (See image below.)
The reality is that we only had 800 CFM and not 1371 CFM, so the static pressure drop across the evaporator coil should have been even lower at .128"WC. Make sure to clean the coil and then remeasure the static pressure drop across the coil. Also, determine the new airflow CFM.
using a video scope to see an Evaporator coil clog
A video scope allows you to see the extent of a clog on an evaporator coil without having to remove the coil from the system. With the power to the furnace OFF, remove the screws securing the heat exchanger temperature limit switch from the unit. Remove the heat exchanger temperature limit. Insert the tube of the video scope into the hole where the heat exchanger temperature limit was. The camera is on the end of this tube.
The image below is of a clean evaporator coil which is not clogged with dust.
The image below is of an evaporator coil which is clogged with dust. This image shows the coil is covered with dust. It is difficult to see the coil because of the dust buildup.
The photo below shows an evaporator coil that has been removed from the system. It is covered with dust and mold on both sides of the coil.
Looking for a video on Finding the Airflow Problem on a furnace or air conditioner? Check out our "AC FREEZING, FURNACE OVERHEATING? Quickly Find the Airflow Problem on a Furnace or Air Conditioner" video below!
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Published: 06/21/2022 Author: Craig Migliaccio
About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 15 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel
