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If you suspect that an evaporator coil is clogged with dust but cannot visually see it, such as when you can't directly inspect on a furnace and evaporator coil, static pressure measurements can be used to determine if the pressure drop is too high. This is a quick way to determine how blocked off the under side of the coil is.
The higher the pressure drop, the more restricted the coil. The more restricted the coil, the lower the CFM of airflow crossing the coil. The lower the CFM crossing the coil, the lower the amount of heat removal from the building. If the coil is very clogged with dust, it can also cause the coil to freeze up. This is due to a lack of heat from the lack of airflow crossing the coil. If there is lack of heat, the refrigerant won't boil into a vapor and superheat (increase in temperature). If there is no superheat, the vapor compressor could have liquid refrigerant entering it and damage it!
This method involves using a digital water column manometer and a static pressure tip to measure the static pressure at two locations. If you have a dual port manometer and two static pressure tips, even better!
In the example below, the unit we’re testing is an upright gas furnace with an evaporator coil that is resting on top of the furnace. Because of this, the blower motor is below the evaporator coil pushing the air upwards. Since that blower motor has to exert a certain amount of pressure in order to move the air across the furnace heat exchanger and AC coil, static pressure measurements can be read. We can measure the static pressure before and after the coil to see if the pressure drop across the coil is too high. The pressure before the coil will be the higher measurement while the one above and after the coil will be a lower measurement. To get the pressure drop across the coil, we subtract the lesser measurement from the higher measurement. If the static pressure drop across the coil is too high, this means there is something restricting the airflow and causing resistance. This restriction is usually dust, hair, mold and/or loose insulation that has gathered on the underside of the coil.
Below are steps in order to measure the static pressure drop across the evaporator coil
Step 1: Turn the power off to the furnace and remove the front furnace cabinet cover. Next, locate the thermal limit switch. Here it’s easily visible, but it could also be mounted back behind the gas valve. Unscrew and remove the thermal limit switch from the hole. Place the switch and wire assembly in a location in a safe spot where it will be protected when the power is turned back on. When it is turned back on, the electrical connection will not short to ground. Temporarily place tin tape over the hole.
Step 2: Connect the static pressure tip to the positive port of the digital water column manometer. Then insert the static pressure tip into the hole, through the tin tape, while keeping the point facing downward toward the blower motor.
Step 3: Zero out the manometer and make sure the units on the manometer are set to inches of water column ("WC). Check that the removed thermal limit switch isn’t going to be shorted out on any metal, since the power will need to be turned back.
Step 4: Temporarily pull/turn off the outdoor electrical disconnect because the outdoor condenser does not necessarily need to be turned on during this test. Then turn the power on to the furnace. Set the thermostat to air conditioning mode and turn the temperature down. After the blower motor is up to speed and the static pressure is constant, record the measurement. In this example, the static pressure before the coil is 0.857” WC, (inches of water column) This is a very high static pressure for the supply side of the system.
Step 5: In order to take the second measurement, drill a small 3/8" diameter hole, using a step bit, into the supply plenum above the evaporator coil. If using a dual port manometer, add the second static pressure tip to the negative side of the digital water column manometer. Insert the static pressure tip into the hole with the tip facing downwards to measure the static pressure of the supply plenum downstream of the coil.
Step 6: The image below shows a pressure drop of 0.67” WC across the coil because the dual port digital manometer is automatically subtracting the downstream measurement of 0.18" WC from the initial measurement of 0.85" WC to equal 0.67"WC.
If you don’t have a dual port manometer, you can use one with a single port to take the readings. Take the first measurement from the point above the blower motor but before the coil. In our case, we are using the thermal limit switch hole. The picture below shows a measurement of 0.85” WC.
The second measurement is downstream of the coil and in the supply plenum. This measurement is 0.18” WC.
Subtract the second measurement from the first to determine the static pressure drop. 0.85” WC - 0.18” WC = 0.67” WC. This is the same calculation we got from using our dual port manometer.
Step 7, Conclusion:
0.67” WC is an extremely high pressure drop to have on a system which tells us that there is some sort of blockage on the underside of the coil. This must be cleaned in order for the system to operate correctly with proper airflow. Normally, the highest pressure drop that a manufacturer may specify are 0.4 or 0.5" WC. However, a typical static pressure drop will usually be between roughly 0.1-0.35” WC. This is a dry pressure drop reading because the outdoor unit is not powered so no refrigerant is cycling through the coil causing humidity to condense onto the coil. A wet coil pressure drop may be roughly .05" WC higher than a dry measurement.
Evaporator coil manufacturers list both a dry and wet coil pressure drop in "WC, based on the airflow CFM crossing the coil. Compare the actual "WC pressure drop to the manufacturer's data pressure drop to see if the unit is in spec. If the actual "WC measurement is higher or significantly higher, the underside of the coil needs to be cleaned by moving it to get access or recovering the refrigerant in order to remove and clean it. Sometimes, there is a front panel on the coil which can be temporarily removed for cleaning.
If the static pressure measurements check out and are within spec, it may be possible that another airflow problem exists such as in the return ducting. Make sure to turn the power off and remount the safety switch and turn back on the outdoor disconnect. Then make sure to turn the furnace back on and test both heat and ac before leaving the jobsite.
If you want to learn more about measuring static pressure and pressure drop, watch this video:
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Published: 7/3/24 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 17 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
