I have a partially finished basement that is connected into my house's 2-ton A/C system. The humidity in the space is consistently high during the summer (60%rh and up), and I believe that this cool, moist air being drawn into my HVAC system is making it work harder and inhibiting it's ability to cool/dehumidify the living areas (which are uncomfortable on hot days). I don't really care if the basement is warm, though I do wish to control the humidity so nothing gets ruined and to keep the cave cricket population down.

What I've tried:

  1. My first inclination was to cover the basement return vent, but the return duct is huge (16inch diameter) and this made me worry that I would drastically change the pressure dynamics of the system and possibly damage the blower motor if I disconnected it. Of the 3 return ducts in the system, it is by far the largest - the others are each only 6 inches (this weirdness confuses me).

  2. I have sealed around the foundation/rim joists with spray foam to prevent outside warm/moist air intrusion, this reduced the humidity somewhat.

  3. I have added two 50-pint dehumidifiers to the basement, one running continuously and another set to go off at high RH. This seems to work to reduce the RH in the basement, but I don't know if this is helping me cool the house, as it is producing hot air, all day long, that is feeding back into the HVAC return.

  4. Covered the supply vent. I'm not sure if this does anything, the return will just suck cool floor air down the stairs from the 1st floor. It makes me feel better about wasted cold air, so there's that.

I have hired 3 different HVAC professionals to help me with this, and the end result is I now know 3 different ways to rip everything out and start from scratch ;) No hard feelings, I think it's funny.

Any ideas? Could I safely disconnect the supply and return?

EDIT: A little more info about the duct layout:

  • the A/C system was added on to an existing forced-air heating system where each (1st floor) room has a single supply vent at floor level, and there is a single return for the whole floor, at floor level in a central hallway. Supply ducts are 4inches in diameter. The single 1st floor return is a box with a large grill, and there are two 6 inch ducts coming from that box and entering into the air handler in the basement below.

  • the basement return is also a box with a large grill, connected to a 16 inch duct that connects directly into the air handler. It is on a wall at the bottom of the basement stairs.

  • 3
    Figure out where the moisture is coming from. It sounds like a general "wet basement" problem due to an exterior issue. As a first step, check rain gutters and downspouts, make sure they discharge far from the house, and check the grading around the house. If the basement walls are getting wet behind the finishing, you could end up with a mold problem.
    – fixer1234
    May 18, 2017 at 20:52
  • Good comment. I observe a fair bit of efflorescence near the floor in the unfinished part, no clue what's behind the finished walls, and I guess this means there is moisture at least outside the foundation. No water anywhere though, walls or floor. Any tips on finding moisture source in a basement? I just picked up a psychrometer to check RH in different areas of the room, not sure if that is helpful for walls and floors though.
    – mikewaters
    May 18, 2017 at 21:20
  • 2
    If you're seeing efflorescence, that's from water. It doesn't get better on its own, it only leads to bigger problems. Look for obvious exterior sources and try the easy exterior fixes. If that doesn't fix it, I would try a cheap interior fix before tackling the foundation on the outside. One approach that is often adequate, or at least helpful, is to put a sump pump in the worst area. You cut a hole in the floor, embed a sump bucket in a deep gravel pit, and pump the water out well away from the house. The water collecting against the foundation will have a place to migrate to.
    – fixer1234
    May 18, 2017 at 21:48
  • I'm not sure how old the efflorescence is - I just bought this place 3 years ago, and there had been a recent (within 5 years) gutter modification and a few feet of red rocks added to most of the house perimeter. The efflorescence appears to be behind some sort of wall treatment, like a foundation seal coat, which is now flaking off in places. I'll take your advice though and carefully check where the water is going during the next heavy rain. My 10 year plan includes removing all of the efflorescence and re-sealing, huge project though.
    – mikewaters
    May 19, 2017 at 1:39
  • 1
    There's a great thermodynamics question. If your A/C condenses a pound of water out of the air, did it need to expend 970 BTU to do it? That is, does removing humidity from air tax the system's overall capacity? May 19, 2017 at 2:17

2 Answers 2


I have read and re-read all the questions and answers every one put forth and here is another try at helping you resolve your A/C problem. You said that the temperature drop is about 10 degrees. You need to buy a pen style thermometer similar to a road pro RPCO 841 (amazon $11.00), and check the temperature drop at the furnace, close to the coil. If the temperature drop is actually 10-12 degrees, that is too low. A low temp. drop could be a worn out A/C unit, a dirty outside coil, low refrigerant charge, too much air flow at the furnace, a mismatched restrictor or thermostatic valve at the coil inlet, etc. I would check out as many as you can yourself, since I rarely trust any service tech. (I was one for 40 years). For the outdoor coil, turn off the A/C unit, look at the outdoor coil closely for blockage of dirt, hair (dog hair), etc. If it is dirty it will need to be cleaned, or just have someone clean the coil. You need someone that knows what they are doing so the do not ruin the coil or get shocked by the electricity. Do you know how old the unit is? To answer Harpers question Of May 19 -- yes high humidity and it's removal does greatly reduce the capacity of your A/C unit. The single large return in the basement is the "new normal" way to run returns in a house. I hate that installation because it is the CHEAP way to do a return. Returns for the top floor should be located at the ceiling and in every room, although this can't always be done. Another question I have is "how much insulation is in the walls if any and how much is in the ceiling". What type construction is your house. Measure the outside dimensions of the top floor, add a best-guess for the conditioned basement space. Trying to figure out what is wrong is hard for anyone on this site since they are not there. Lastly, use the psychrometer to check the basement humidity. I should use paragraphs in my writing but I was a service tech and not that good at writing. You may have gotten good advice from the A/C companies you have contacted or you may need to may still need to find that good one. Hope this helps

  • Wow, great answer. I have two questions: 1. I have bought a UEI DTH35 psychrometer, should I use this to check the TD at the coil? 2. Should I allow unrestricted airflow to the basement return from the main floor? There is a basement stairs door that has a very tight fit; if it's open when the A/C kicks on, the door slams shut from the suction. You have been extremely helpful, more than any of the techs I've hired.
    – mikewaters
    May 28, 2017 at 13:00
  • I never used a digital psychrometer like the one you have, so I can not give a definitive answer. I have been retired for 10 years and am from the "old school" where we did things much differently than the way people do things today. My choice was a wet bulb/ dry bulb psychrometer. I looked yours up on google and guess you can use it to measure the temp drop. As far as closing the basement door "NO", since most of your return air is from the basement. The 2-6" pipes are not enough return from the main floor.. I hate the way "BUTCHERS", install equipment today, just get it in and get paid.
    – d.george
    May 28, 2017 at 17:37
  • I once had a next door neighbor that had dogs. The hair from the dogs plugged the coil enough that the unit would not cool the house. He had numerous A/C companies tell him he needed a new unit. We cleaned the coil and the unit again worked great. Around Pittsburgh, Pa. older houses require about 1 ton of A/C for 600 sq. ft. living space, and newer homes about 800-900 sq. ft. per ton of A/C.The basement adds a small amount to the unit's load since cold air is heavy and sinks to the lowest level. You may also want to run the fan all the time. Mine has run for 19 years.
    – d.george
    May 28, 2017 at 17:49
  • I was referring to the condensing coil (outside unit)
    – d.george
    May 30, 2017 at 11:36
  • I didnt even realize the condenser had fins that could get dirty, I looked at the tubes inside and they looked fine. I'll check the fins asap. I paid somebody to give the system a checkup in March (he tried to sell me Mitsubushi mini split), I would hope that he checked this,
    – mikewaters
    May 31, 2017 at 16:07

I need a better explanation of the duct system in your home. How many supply and return registers are in the house and where are they located (what room and where in the room, high or low on the wall). You mentioned a 16" diameter return duct. Is this duct for the basement only, and is it attached to a large return grill? Is this the only return duct for the house? Are there any other return ducts, If so how many, where are they located and how big are they? Now for other problems. What is the square footage of your house, upstairs and conditioned area down stairs? When you say the living area is warm when the A/C is running, (how warm)? What is the outside temperature and what is the room temperature. Where in this country do you live? If you can measure the temperature drop across the cooling coil, (supply air vs. return air. It should be at least 15 degrees F. or more. Lastly I and many others that have given advice think that you have a moisture problem in the basement (water leaking thru the walls etc), that will need to be corrected. If you can answer the questions I asked above a better a better solution can be offered.

  • Thanks D, I've added some notes about the ducts. I am unsure of my sq footage, but the house is a small post-WW2 ranch in Long Island, NY. On a hot summer day we struggle to stay below 75 degrees inside, but it's an uncomfortable/humid 75. For the amount of electricity I burn keeping this running in August, I should be able to wear a sweater if I want ;)
    – mikewaters
    May 19, 2017 at 10:42
  • Have you checked the temperature drop across the cooling coil.(the supply and return ducts next to the furnace). The temperature drop should be about 15-20 degrees across the coil. More or less than that could indicate a system or air delivery problem.
    – d.george
    May 25, 2017 at 11:36
  • On a recent hot day, I did; it was around 10 degrees, more or less depending on which vent I chose. I have two vents with wider diameter ducts, those two had a higher TD, maybe 12 or 13 (I don't know if the vent size is related). Also, those vents are about 12 feet from the single first-floor return, and it's a straight shot, as they are all on floor-level. Maybe this is relevant.
    – mikewaters
    May 26, 2017 at 13:25

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