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When servicing an air conditioner or heat pump, we often notice a low airflow problem which needs to be fixed. If there is no indoor air filter or there is air skirting around the air filter because its not snugly fit, the dust will collect on the side of the indoor evaporator coil. Eventually, enough dust collects onto the coil to significantly reduce the amount of airflow crossing through the coil fins. This reduces energy transfer, increasing electricity bills and reducing the air conditioners effectiveness. You will often see low superheat and a frozen vapor refrigerant line and coil. Remember, a normal to low superheat and a low vapor line saturated temperature indicated low airflow but hopefully techs are measuring airflow before measuring refrigerant pressure!
During air conditioning mode the evaporator coil is wet, creating an environment where dust just sticks right onto the wet coil until it becomes a mat covering the coil. On most air handlers (also known as fan coils), it is easy to visually inspect the bottom side of the coil for blockages. On a furnace and AC system, it is significantly different. Below are 5 methods to determine if the evaporator coil is clogged with dust when you can't visually see it!
Remember, we need roughly 350-450 CFM per 12,000 BTU/HR of system capacity. Also, every 12,000 BTU is equivalent to 1 ton of cooling capacity. Because of this, a 3 ton system would need around 1200 CFM if we were figuring 400 CFM per ton. Make sure to read our "Refrigerant Charging and Service Procedures for Air Conditioning" book to learn more about airflow measurement methods and measuring the refrigerant charge.
Method 1: Use a Video Scope
In reference to the video scope, you get what you pay for. You want one that has a light at the head and it is best if the head is moveable (articulating) to look around after the borescope is inserted into the system. Some video scopes also record the video display so you can share it with your customer.
The photo below shows us using a Teslong video scope.
One method to insert the borescope into the upstream or underside of the coil is to remove the furnace limit switch while the power is off. This provides a pathway into the furnace heat exchanger area. After inserting the borescope in an upwards manner, the display shows the underside of the coil. Use the articulating dial to move the camera position.
Here we can see that the back half of the coil is definitely clogged and will need to be cleaned.
Another method to gain entry to the under side of the coil is to drill a hole into the side of the furnace cabinet. Just be sure that the drill bit does not penetrate too deeply and accidentally hit the heat exchanger or the refrigerant coil.
Make sure to put a plug in any holes in the furnace or reinstall the thermal limit switch once you’re finished. Make sure to turn the power back on and test the unit too.
Method 2: Remove the Front Cover Plate to Visually Inspect the Coil
For this method, you can visually view the underside of the evaporator coil without using any special tools other than a stubby 1/4" or 5/16" ratchet/nut driver and a flashlight.
To start, turn the power off to the system and remove the coil box cover. Next, on an A-coil, you will see a tin cover plate screwed to the coil segments. After removing the screws, either remove or pull back the cover plate to look inside. You can use a basic flashlight or a mirror along with a flashlight to visually inspect the coil. You can also use a phone with a light to take video or photos, but do not accidentally drop the phone inside!! Sometimes you cannot fully remove this cover plate because of the refrigerant tubing so a phone can be used to see the hard angles with its camera. Often, you can just insert it only partially so there is less chance of dropping it!
One nice thing about this method is that if you are able to completely remove the cover plate, you will be able to clean out the coil that way. Just make sure that you reinstall the plate after you’re finished, or else the airflow will go through the coil instead of around it!
Method 3: Removing the Electrical Compartment and the Blower Motor
This next method can be used on 80% efficient furnaces because they only have one single heat exchanger. We remove the electrical compartment and the blower motor, allowing us to look in and up past the heat exchanger at the evaporator coil. This method is helpful because it doesn’t require any special tools or equipment, however it does involve more work and time then some of the other methods.
Before starting the process of removing anything from the unit, make sure all power in the system is turned off. Then, begin removing the electrical panel and blower motor. Make sure to take a picture of how all of the wiring is set up before removing anything. This will allow you to easily return things to how they were beforehand!
Unscrew the screws that hold the blower motor in place and pull it completely out of the unit. This will leave enough room to get inside the unit and look up at the evaporator coil. Set the blower motor aside with the electrical panel.
Crawl into the now opened section and look up and past the heat exchanger coils at the bottom side of the evaporator coil to see if it’s clogged. You can use your phone to take a photo or video to get a better look. Be sure to use proper PPE such as eye protection before looking up inside the unit and a mask to not breathe in the dust.
When you're done, be sure to put back the blower motor and electrical panel the same way that it was, using the picture you took beforehand as a reference if necessary. Test the unit after turning the power back on to make sure the unit works in the same fashion as it did before hand.
Method 4: Shift the Coil Back or the Furnace Forward to Expose the Coil
This next method will work if the unit you’re trying to check has either flex duct and/or an extra line set length. In this case, it may be possible to carefully shift the coil back to expose the underside of the evaporator coil. Make sure not to crack any copper braze joints during this process. Only perform this method if there is extra tubing length and the coil moves very easily. This is to avoid causing any refrigerant leaks due to a cracked joint. The drain line may need to be temporarily moved or cut as well. Make sure to wear proper PPE for safety.
In a similar manner, if the furnace has a gas flex line or a union on the gas line, and a metal exhaust, you may be able to slide the whole furnace forward, exposing the underside of the evaporator coil. Be sure to have the gas and power turned off when using this method.
An advantage to using this method is that if you are able to get enough of the evaporator coil exposed, you could potentially clean the coil while keeping the majority of the system in place.
Once you’re done, return everything into its place and seal it all back up. Be sure to check that your gas line and exhaust are all secure before you’re complete. Leak check the gas line with non corrosive bubble leak detector. If the condensate line was cut, make sure to glue it back together with a coupling.
Method 5: Measure the Static Pressure Drop Across the Coil
For this method, a single port or dual port manometer and static pressure tips are used 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 and inserting the static pressure tip into the air stream at this location. Turn the power off to the system and place the switch and wires away from any metal where it may short out, when the power is turned back on during testing. The outdoor disconnect can be turned off during this test so the outdoor unit does not turn on. Turn power back on to the furnace and set the thermostat to air conditioning mode for the highest fan speed. The picture below shows a measurement of 0.85” WC.
The second measurement is downstream of the coil and in the supply plenum. To make this hole, use a step bit and drill a 3/8" diameter hole in the sheet metal. In this scenario, the measurement is 0.18” WC while the fan is running.
Subtract the second measurement from the first to determine the static pressure drop. 0.85” WC - 0.18” WC = 0.67” WC.
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 blockage must be removed 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 while the outdoor unit is not being powered. Because refrigerant is not cycling through the coil, humidity in the air will not 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.
If you're interested in seeing a more detailed step-by-step process for Method 5, check out this article: https://www.acservicetech.com/post/measuring-the-static-pressure-drop-across-an-evaporator-coil-to-see-if-it-is-clogged
If you want to learn more about these 5 methods, watch this video!
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If you want to learn about refrigerants and how they work in a system, check out our “Refrigerant Charging and Service Procedures for Air Conditioning” book . Test your knowledge with our 1,000 question workbook along with the answer key! We also have quick reference cards for use out in the field! Bundle Packs are a great way to save and get faster shipping! Check out www.acservicetech.com/store
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Published: 7/24/24 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
If you're concerned about your indoor air conditioning coil being clogged with dust, there are a few key signs to look out for. First, reduced airflow is a common indicator. If your smart AC isn’t cooling as efficiently, it might be due to a clogged coil. Another method is checking for unusual noises; a dusty coil can cause the system to work harder, leading to strange sounds. You can also inspect the coil visually if accessible, but remember to turn off the power first. Regular maintenance, like using a smart AC system that monitors performance, can help prevent these issues before they escalate.
Regular maintenance is crucial to ensure your air conditioning system runs efficiently. A clogged coil can significantly reduce cooling performance and increase energy consumption. Always employ HVAC Contractor Riviera Beach experts for more info regarding HVAC systems.