Ok have an inspection that is coming back to haunt me. I inspected a system on a home and it looked fine. Good splits. It was spring time here in Florida so the outside temps were in the 80s. The air handler is located in the garage hanging from the ceiling. The owners have just recently occupied the home and in using the system found the air handler is sweating on the exterior. Didnt happen when I inspected however its only on for an hour or so. Have gotten a couple calls from them concerning this. Explained my position, limits of the inspection and I think all is well. My question is what causes air handlers to sweat and is it something I should be looking for? the “thing” not the condensation.
I don’t know about “sweating”, but if the unit is low on freon, the coils will freeze and ice-up. Then melt and drip. But you would typically need to run it for more than an hour. That’s been my experience.
I’ve seen that several times, I’ve also sen where it was so severe they built a tray under the unit to catch the condensate and drain it outside.
Every now and again I see the exterior of the air handler unit covered in condensation. Being that its hot here in Florida, Its cold inside the air handler and it seems logical it would happen I have kind of brushed it off. But as I think about it more I realize the majority of units dont seems to sweat and now Im wondering if its a lack of insulation, balance, freon, air flow or what. Additionally, is it something I need to be picking up on. Just trying to pick a few brains here.
Causes and Remedies
Improper or No insulation of Ducts: The simple physics behind sweating is: “Whenever there is a contact between the hot humid air and cold metal surface then there will be sweating or condensation on the metal surface.” So, it is very important to have a proper insulation for the air handler ducts in case the portion of the duct is coming in contact with the unconditioned air outside.
Sealing the Crawl Space Properly: If you have the air handler unit mounted at the crawl space of your house, then sealing the crawl space properly in order to prevent the humid outside air and/or ground moisture coming inside will definitely help you reduce/eliminate the sweating problem.
Trying a Dehumidifier: If the outside relative humidity is too high in your area, then probably you will have to install a dehumidifier at your crawl space to get rid of the problem.
Air Flow: Due to the chocked filters, or blocked duct, or maybe for whatever reason, if the air flow is restricted, then there will be chance of sweating. As the air flow is reduced, the temperature of the air goes below the dew point and causes sweating. So, you may think of changing the filters or cleaning the air handler ducts to solve the problem.
Icing of Evaporator Coils: The ice deposition over the evaporator will cause the temperature to decrease locally at air handler unit and may cause the sweating.
Leakage of Ducts: The leakage of the cold air from the duct will increase the chance of sweating. You should be able to detect a visible leak by placing your hand there, or else you can get help from a set of pressure gauges.
Blockage of the Air handler Drain Pipe: If the air handler drain pipe get chocked and/or if the inclination of the unit is not right then the chilled water inside will be staying inside for longer and will cause the outside air to sweat over the surface of the air handler unit.
Temperature Setting: For each relative humidity (RH) value there is a temperature value below which the condensation will start. If you set the air conditioning temperature higher, then the temperature of the return air will also be higher and there will be less chance of sweating at the AHU.
Hope this helps
Condensate tray under air handler is more for in case the condensate line clogs and backs up the water can be collected before entering the home and either sent out the home via another line and/or the unit switched off via the overflow switch. However, it does catch the occasional sweating unit.
I had an air handler case sweating badly in a hot attic last year. A/C tech wrapped it with a duct insulation (don’t know if that is the answer). I recommended better top mount spot ventilation…hip roof. Didn’t hear back on it.
Yea, I explained the unit might need more insulation but it was only an educated guess and to get a HVAC tech to confirm and or recommend the fix.
I can not see from here but if you think about what you are asking (what causes condensation) that is your answer the air temp is lowered to dew point and if it is making contact with the outer surface of the AHU it will condense out. You did not say if the unit was a down flow, a upflow or horizontal flow just that it was hanging from the ceiling. It would ber most helpful to know what area of the furnace has the condensation the coil box, I can not image the return air section having any condensation the temp is to high
Warm and cold air differential. Simple physics.
that is true.However if they restricted the air over the coils as in blocking of some of the registers and or bad or clogged filters that will freeze it up as well.
Are most air handlers in Florida installed in the Garage? If so…why would it not sweat? You’ve got that hot, humid air down there around a cold metal box with the coil. Every time I go to Florida I sweat…maybe cause of my cold heart.
Justin, that just makes too much sense.
and your post did not include the phase of the moon! 
Were your clients with you during their inspection?
I just find it odd people, knowing how long you examined systems, the conditions of the weather outdoors, and everything else short term - would call you back, wanting what exactly? You’re not their land lord, for sure.
Is this a common occurrence with Home Inspectors to receive a call back?
Personally, it would never haunt me. I would politely tell them they aren’t my tenant and there are most likely Professional HVAC businesses that can handle their concerns.
Todd - a couple of things to note; 1) with heat pump air handlers - the cases are insulated throughout / however, air handlers of a furnace / coupled with split AC installation; the case of the AH - furnace does not have an insulated case portion. therefore making the exterior of the case cold. and 2) as one person said above, condensation / dewpoint is the issue. Much like a cold drink sweats on the outside, so does / will the uninsulated case. But, it is all based on differential temperatures and the humidity of the air on the warm side. Here is a dewpoint calculator that should help to clarify when water / condensation occurs - http://www.dpcalc.org/
I have a lot of experience with air handlers sweating. The thing is if the air handler is sweating and it is not caused by the drain line stopped up. It is a environmental problem. ie. The temperature and the humidity in the area are very close to one another. Causing the Dewpoint to rise. A air handler with 410a refrigerant makes 48-52 degree air inside the cabinet off the air handler. With very minimal insulation in the cabinet. The cabinet of the air handler is now below the dew point temperature. This is what causes the air handler to sweat. The solution for this is very simple. Reduce the moisture in the air around the air handler. A good dehumidifier piped to the outside dose a very good job doing this. I had a air handler in a attic in a new constitution home on the beach. The air handler was sweating very bad. I was working with a very knowledgable builder. I proposed a dehumidifier. And the builder agreed to let me put one in on a trial basis. We ended up leaving it there because the air handler and everything else in the attic was bone dry. That is the proper way to prevent the air handler from sweating. Also if there is water on the outside of the air handler you can bet the farm there is water inside the air handler. This must be fixed. Or you will have mold and mildew problems. This post is from a professional Hvac business owner. Who has extensive knowledge in the business with 25 years of experience. Please use this knowledge to help your customers or friends correct this ongoing problem. Insulating a air handler is a band-Aid that will come back, and when it dose it will be a moldy mess.