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?

 

[quark]

There may be moisture carry over due to a clogged filter. Also check the drain pan and drain line for clogging.

 

[Compositepro]

An air conditioner is a heat pump. If all it did was remove heat from your room the relative humidity would approach 100%. If the evaporator coil that cools the air is much cooler than the air then water will condense on it and will be removed by a drain. When subcooled air at 100% RH mixes with warm room air the relative humidity of the mixture goes down.

To make a window air conditioner do more dehumidification turn down the fan speed. The air going through it will drop to a lower temperature so more water will be removed. The thermostat will still control the room temperature to the same set-point but the humidity will be lower.

Make sure the unit is clean. Dust/algae build-up on the evaporator fins acts like wet towel that will not allow the condensed water to drain-off. I have found that window units need to be pressure washed at least once a year. Most air conditioners are thrown away because they need cleaning and its easier to buy a new one.

 

[trashcanman]

Another cause could be that the unit is oversized. If so, it quickly cools the dry bulb temp down and shuts off the compressor. Thus, it does not run very long and little moisture is removed.

 

[DonPhillips]

And I suspect when the unit runs, it is drawing some outside air, bringing in some of the moisture from the outside.

Don Phillips

http://worthingtonengineering.com

 

[rmw]

Great answers. Also, if you run the fan all the time the moisture that is left in the evaporator coil (exacerbated by the presence of dirt and/or alge that hold the moisture on the coils as stated earlier) is returned to the room when the compressor is off.

rmw

 

[AbbyNormal]

I have been doing quite a study down here on humidity control in tight structures.

I am now of the opinion that rather than saying 'an oversized AC drives up RH" we should think of it as "an over sized AC cannot handle excessive infiltration of humid air"

First steps to get RH down. Set the thermostat fan switch to Auto, let the air handler run only when the compressor run, constant fan drives up RH.

Have a technician check your fan speed, it can be set lower and this increases dehumidification when it runs

Leaking ducts, especially supply ducts in unconditioned spaces like a vented attic or a crawlspace cause a lot of infiltration.

Make sure you are not running exhaust fans for extended periods of time. Shuit them off when you are finished in the bathroom

Fans that pwer vent attics are a problem, they can depressurize your home and cause a lot of infiltration.

Central return air systems relying on undercut doors cause infiltration as well, rooms get pressurized and force air out through the building envelope, you get a lot of infiltration in in the vicinity of the central return.

If you have attic duct work, make sure the ceiling penetrations around the diffuser boots are well sealed.

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

 

[AbbyNormal]

duh, just read it was a through the wall PTAC, close the vent on it

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

 

[jcoronat]

Great information, all of you.

This unit has an outer cabinet or sleeve that installs in a wall or window, and an inner assembly of all the motors and coils mounted on a heavy pan, which slides in and out of the sleeve for servicing.

I took the mechanical portion outside in the beginning of the season and cleaned the condenser coils, and hosed out the pan. I couldn't at the time see any particular drainage construct -- maybe some flutes molded into the pan? I think I forgot to notice. From what I could see, the condensate falls to the pan from the evaporator coils and runs toward the back (outside) of the unit where, if there's enough of it, a fan ring flips it up onto the condenser coils.

The unit cooled better after the condenser-coil cleaning, but a persistant mold smell and the damp-air effect were getting worse; so, more recently, I cleaned the evaporator coils with a can of foaming evaporator-coil cleaner while the unit was still in the wall. Both the smell and the damp-air syndrome are better, but not completely gone yet. May need another can.

The comments on air infiltration were perceptive -- precient, even. This house is very leaky, and so I rarely run the a/c with the vent open -- why bother? Plus, I have attic and crawl-space issues (including an attic power-vent) that have to be dealt with, especially before I install central air.

I found the info on the relationship between the capacity and air speed through the unit to be enlightening. It explains a lot. Except, unfortunately, why, for nine hundred bucks (five years ago) the Amana people couldn't put more thoughtful controls on a unit this size. As it is, they don't allow intermittent fan operation, but I have been running it on low fan speed (continuous whenever the unit is on) these days, and that actually does seems to help. Nice.

So I appreciate the education; it's made a difference in how I approach this thing. Thanks again.

 

[IRstuff]

One thing that happens with car A/C is a moldy/dank smell, usually caused by condensation and baterial growth further downstream. Spraying Lysol into the intake and exhaust pots can sometimes help.

TTFN

FAQ731-376

 

[jcoronat]

Funny you should mention that. I actually tried Lysol with the room air conditioner, but the flavor I'd chosen (Fresh Mountain Breeze, or something) when concentrated and mixed with the mold smell stunk and I had to go to the professional cleaner. However, at the same time, I also bought a can of A/C cleaner for the car because my wife was becoming increasingly sensitve to that environment, too. Haven't used it yet. We'll see how it goes. Thanks for the tip.

 

[AbbyNormal]

turn off the attic fan

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

 

[jcoronat]

Got some issues there, but I can work on that. Looking ahead, though, to the central air installation, and supposing that I still may have higher-than-normal moisture infiltration that I cannot yet do anything about: how do I determine the moisture load in this house so as to size the central A/C properly?

In other words, is there some chart, say, that relates the average (un-conditioned) RH in a space to some kind of modification factor that one then applies to a preliminary design selection for an air-conditioning unit (to up- or down-size it)?

 

[hynhvac]

ive never heard of such a chart (then again, im still young) but if you do have moisture problems due to a leaky house, i would recommend that you first fix the infiltration problems where feasible. the moisture problem could soon become a mold problem which in turn could become a health problem. i recently started on a project that had moisture/mold problems that now have the health inspectors involved.

that said, one question...

by central air to u mean ducted or ductless split?

 

[jcoronat]

I'm thinking ducted, because for one thing, the unit will use two or three heating fuel sources (heat pump, electric resistance element, LP gas), and it seems wise to just include the heat, the A/C, and the air filtration into a single unit.

 

[Compositepro]

Attic ventillation is very important for keeping the attic cooler in summer and reducing A/C requirement. If it is creating a suction in the attic that is drawing air from the house the problem is inadequate attic vents to allow air to come in freely.

 

[jcoronat]

So what I'm hearing here is that infiltration of moist air may be the main culprit in the prevailing high inside humidity with which the A/C seems unable to cope. Lower fan speeds and colder settings on the A/C may help, but stopping infiltration and providing adequate attic intakes to feed the power vent may ultimately solve the problem.

 

[AbbyNormal]

composite

Attic ventilation is important to avoid icycles

What ever pressure differential causes air to pass into an inlet is also applied to recessed light fixtures and a myriad of other ceiling penetrations.

In humid climates a power vented attic can be a disaster.

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

 

[Compositepro]

Abby, I understand your valid concerns. Antic ventilation can lead to some problems but in general it solves more problems than it creates. Also, in general, the solution to condensed moisture caused by humidity is ventilation. The cause of the problem is cooling a room to below ambient outside dewpoint. Ideally the cold space should be kept slightly positive in pressure so that humid air is not drawn into contact with cold surfaces.

I'm just suggesting that a better approach than not ventilating the attic is to properly ventillate it. With no ventillaion you can still have high humidity in the attic which will condense on light fixtures.

 

[AbbyNormal]

When you seal the attic and insulate the roof deck and gable walls, you keep the heat and the humidity out in the first place

You are correct that it is desirable to be under positive pressure when cooling. The power vented attic often has the effect of making the space below negative. It is the path of least resistance and if the attic fan pulls hard enough to draw air in through vents in the soffits or gables, it pulls hard enough to draw air from the space below as well.

You vent attics to stop ice dams and icycles. In a poorly insulated home, it has the effect of 'flushing the heat out', in a moderately insulated home, the cooling savings from flushing the heat out often equla the power to run the fan.

An easy test in the summer is to shut the fan off and see if humidity drops off

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