Arkansas HVACR NewsMagazine May 2025 Issue

HVACR NewsMagazine May 2025

Tech News

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.

Measuring the Static Pressure Drop Across an Evaporator Coil to See if it is Clogged!

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