A/C makes air feel cold and damp

[jcoronat]

Hello, everybody.

Does anyone know what's happening in the following situation?

My house air conditioner (a 21,500 Btu Amana thru-wall unit) is off. The house temp is mid-to-high 70s. Relative humidity inside the house may be 60 percent or more. So I turn on the air conditioner and after a short time, the air in the house becomes cool—even cold—but feels damp, and sometimes actually wet. Feels and smells wet; and the relative humidity (measured on a hygrometer) might even have gone up to 70 percent or more. This of course makes it even more uncomfortable, and I have to turn on a stand-alone dehumidifier in order to bring the humidity down to a reasonable level.

Now, I understand (roughly) how relative humidity measures moisture content relative to a theoretical maximum for a particular air sample: lower the temperature while holding the moisture fixed, and relative humidity has to go up. But why doesn't the air conditioner (which is pumping out cold air) remove enough moisture from the air while it's cooling it to keep the relative humidity at *least* at its previous level?

Before I install central HVAC in my house (I've already gotten estimates from contractors) I want to ensure that the new system takes care of this problem. I notice the same cool, damp feel in the air in our public library downtown, and think that their system is somehow not set up right.

Any ideas?

 

[coldmaster]

Cold and damp is classic oversized. Load is less than 2 ton in the space. Take a good look at the load calculation before you throw down for new central system.

 

[jcoronat]

AbbyNormal, I like that philosophy of sealing and insulating the attic envelope. I'm looking into that now.

Coldmaster, this is interesting. Posts above, esp. by AbbyNormal, suggest that lower fan speeds increase the amount of removed moisture on each pass through the evaporator coils, lowering relative humidity. Indeed, doing that did improve the quality of life, esp. after I also cleaned the evap coils.

If, in addition, one dials back the thermostat so that the compressor runs less frequently, would the overall result of those two adjustments be equivalent to having a smaller-capacity unit?

Somehow, I suspect that it's more complicated than that, since when I did turn down the thermostat (when it was still warm up here in Minnesota!), the unit seemed overall to become less effective, resulting in a warmer and more humid space. Is that indicative of anything?

 

[coldmaster]

You want that compressor to run more, not less. the longer the compressor runs, the longer the cooling coil is cold, the more moisture is pulls out of the air...satisfy the space too quickly and the compressor shuts off and stops "making water."

The unit should cool down the room in hours, not minutes. 20,000 BTU is a big sucker...how big is that room?

 

[jcoronat]

The main living area is about 33 x 22, but during the hot periods, it really has to cool the entire 1300-sq.ft house (one floor). And maybe this is the problem; if I circulated the air throughout the entire house all the time the A/C was running, rather than just during especially hot weather, the temperature in the main area wouldn't drop so fast, and the humidity would gradually lower. What do you think about that?

 

[AbbyNormal]

If you can control how outside air moves through a strucutre you will have no problems with humidity.

There is a lot of truth to 'oversized air conditioning drives up the relative humidity'; however my experience is more like "oversized air conditioning cannot handle excessive infiltration of humid air". So a tight structure and you can have a very dry space in the humid tropics.

I used to live in NW Ontario and I did a lot of geothermal back in the day. These were sized for heating and were grossly oversized for air conditioning. There were no problems with humidity control because the homes were tight, mainly 'R2000' homes and the ambinet dewpoint was not all that challenging.

One job was a geo retrofitted to a home built in the 70s and it was not air tight. There was a humidity complaint but it was caused by a "Humidex" which is a packaged exhaust fan meant to ventialte with dry air in the winter to keep windows clear of condensation. The owner had the "Humidiex" set for 50% RH in the summer and it just ran steady, causing a high amount of infiltration. As soon as he turned it off, no more humidity problems in the summer with an air conditioning system over sized by 100%



Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[coldmaster]

That shouldn't be too much air for that room.

 

[kdxbob]

Why not just run the dehumidifier when the outside temps are low, ie, close to the temp you want the inside. Yeah, it uses electricity and adds heat to the room, but it would control the humidity. By the way, big systems frequently have re-heat coils that kinda do the same thing.

I'd also be tempted to open the vent on the AC unit so you would reduce the moisture in the incoming air and pressurize the room reducing the infiltration of hot moist air.

 

[jcoronat]

AbbyNormal, the more you talk about humid air infiltration, the more I think that it's at the root of the problems in my house. It doesn't hurt that I've long been attracted to the idea of completely sealing the shell on this house and ventilating only by design.

My plan, as it's evolved, is hold off on central HVAC for now, and instead to replace the windows, strip and discard the existing siding on the outside of the house, apply rigid foam board to the outside of the studs from the footing to the roof line (covering over the old vents completely), tape the seams, and install new siding. The roof deck can then be insulated either with rigid foam board over the existing roof topped by a second roofing material; or, what's more interesting, a sprayed urethane foam on the existing roof and a seamless elastomeric membrane applied over the top of that.

I'm thinking that once I get a thermal envelope enclosing the shell, the leaks in the interior surfaces won't be so critical, the temperatures in the attic space will be manageable, and I can work on stopping moisture from the crawl space.

 

[jcoronat]

Actually, kdxbob, I often do run the dehumidifier even when it's cool outside. When I want the heat, I'll run it alone; and if it's too much for the outside temps, I'll run the A/C along with it to keep the house from heating up too much.

So it's interesting what you say about the reheat coils. I've often wondered whether big systems did something like that in order to give you dry air, but not overly cold, air. Thanks for the input.