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A/C makes air feel cold and damp

A/C makes air feel cold and damp

A/C makes air feel cold and damp

(OP)
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?

RE: A/C makes air feel cold and damp

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

RE: A/C makes air feel cold and damp

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.

RE: A/C makes air feel cold and damp

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.

RE: A/C makes air feel cold and damp

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

RE: A/C makes air feel cold and damp

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

RE: A/C makes air feel cold and damp

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.

RE: A/C makes air feel cold and damp

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.

RE: A/C makes air feel cold and damp

(OP)
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.

 

RE: A/C makes air feel cold and damp

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

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RE: A/C makes air feel cold and damp

(OP)
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.

RE: A/C makes air feel cold and damp

turn off the attic fan

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

RE: A/C makes air feel cold and damp

(OP)
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)?

RE: A/C makes air feel cold and damp

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?

RE: A/C makes air feel cold and damp

(OP)
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.

RE: A/C makes air feel cold and damp

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.

RE: A/C makes air feel cold and damp

(OP)
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.  

RE: A/C makes air feel cold and damp

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.

RE: A/C makes air feel cold and damp

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.

RE: A/C makes air feel cold and damp

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.

RE: A/C makes air feel cold and damp

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.   

RE: A/C makes air feel cold and damp

(OP)
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?
 

RE: A/C makes air feel cold and damp

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?

 

RE: A/C makes air feel cold and damp

(OP)
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?

RE: A/C makes air feel cold and damp

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.

RE: A/C makes air feel cold and damp

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

RE: A/C makes air feel cold and damp

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.

 

RE: A/C makes air feel cold and damp

(OP)
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.   

RE: A/C makes air feel cold and damp

(OP)
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.  

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