Condensation problem in AC ducts in winter

jb96December 30, 2010

I have a dedicated AC system with the air handler in unconditioned attic (heat is provided by hydronic baseboard). When outside temps drop down below about 40 (pretty much Nov thru March) condensation starts forming in the ducts and drips from some of the ceiling registers. First winter after moving into this home I just tried closing the dampers on the registers, but that had limited if any improvement since air still passes thru the vents. As a temporary fix each year since then (4 years now) I tape plastic over all the registers each winter. This solves the problem, but looks horrible. I have worked with 2 HVAC contractors but neither of them have been able to completely resolve the issue yet. The ducts are all now insulated to code (replaced some to bring up to code) and the metal registers in attic are all low-e wrapped and spray foamed.

This is not just a little condensation on the vents, the ducts will actually fill with gallons of water. The warmer I set the heat the worse the condensation. My theory is that there is a slight and slow circulation occurring between different floors in the home to balance out minor temp/pressure differences. The slow movement of air gives it time to condense in the ducts, regardless of how well insulated they are. RH is about 40% and we keep the thermostat set at about 68 all winter, so there are no environmental extremes. This is a 60's era split level home located in mid-Atlantic region.

Since sealing the vents works, I'm looking for a more attractive solution. Does anyone know if someone makes (reasonably priced) ceiling registers that will completely seal tight when the damper is closed?

Or any other expert advice?

Another odd thing is that there are many other homes in our neighborhood with same floor plan and similar AC systems installed about the same time, yet our home seems to be the only one with this problem.

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would you be able to add a damper to the main branch duct to seal off the "circulation" of air instead? This way you are closing the system in 1 place, and only have 1 thing to open/close every season. I would ask your dealer about something like that and see what they think.

Also, I would recommend double checking your humidity level. Are you windows fogging up too? I find it surprising that you have such a condensation problem ONLY on the duct system.

I think that if you humidity is OK, then there might be something else going on that has not been thought of yet. Where is all this water coming from is my question?

    Bookmark   December 31, 2010 at 8:49AM
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Damper sounds like a good solution, if sealing off a main branch will stop the circulation. I think it may since there is a single large feed that supplies the 1 1/2 level of the house, which is where the most condensation occurs. I have been doing some experiments of uncovering certain vents the past few days and seems that the return from the 1st floor may be the main source of the uplifting air current. So maybe a damper on the return would fix. This would be a pretty significant (costly) install so I will want to be certain before having the work done.

Humidity is a pretty constant 40% thru the winter. Some minor condensation on a few windows on the coldest days but nothing uncommon.

    Bookmark   December 31, 2010 at 11:01PM
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I have had a similar issue and finally fixed it. I have a 3 year old home that is well insulated and tightly sealed. It is a 2 story home with a basement with geothermal heat/cooling. The second story of the home contains only guest bedrooms that are rarely used.

The second floor has its own dedicated hvac and I set the stats to cover extreme temps (set temp = 60 in winter, 82 in summer). These settings cover the rare extreme temperature cases but also enure that the hvac unit doesn't normally need to run to save on heating/cooling costs. It turns out that in the winter the warm air rising from the first floor actually keeps the rooms on the second floor warm enough that I don't normally need to run the hvac unit upstairs. However, not running the hvac upstairs is what caused the condensation problem like you have.

The key to the issue is that the insulted ducts for the second floor rooms run throughout the attic. Even though the ducts are insulated and my attic is over insulated, the air and surfaces inside the ducts will get very cold if the hvac is left off. When the warm air rising from the first floor gets into the duct inlets a lot of condensation forms. But I notcied that the condensation only formed when the outside temps fell below about 20 degrees.

The first time this happened, I soaked up the water on the carpet and then turned on the heat and everything dried up. I also noticed that as long as I ran the heat up there I did not get condensation even when the temps outside dropped to zero. I figured out that as long as I kept warm air moving through the ducts that the condensation would not form. But I did not want to run the hvac up there all winter because it is a waste of money.

I had an idea that whenever the temps dropped below 25, I went upstairs and turned on the hvac fan but left the heat off. The idea was that continuously moving the warm air from the second floor rooms (which naturally get heat from the first floor) through the attic ducts would be enough to keep the inside of those ducts warm enough to not create the condition for condensation to form. This process proved to work for me but was a bit of a pain. I had to always remember to go up and turn the fan on at night and then shut the fan off in the morning. But it worked.

AFter one season on manually turning the hvac fan upstairs on and off I decided to automate the system. Since I have a zoning up there, I checked the manual for my zone contoller (EWC Controls Ultra Zone 4) and found out that is has a dry contact input for a carbon monoxcide sensor. This feature allows the automatic detection of CO (with a seperate CO sensor) and allows the hvac fan and a freash air inlet to be automatically turned on to circulate fresh air through the system. I used that CO dry contact input to hook up a battery operated progammable timer with a dry contact closure so that every night at 6pm the hvac fan turns on, and every morning at 6am the fan turns off. I only turn this timer on during the months when nightime temps drop below 25 (generally Jan & Feb). This fixed my problem and saved me the hastle of doing it manually.

When this problem first surfaced I thought about sealing up the registers upstairs but decided against that approach becuase there are times when I do run the hvac (when we have guests or when it is really cold outside) and so I would have to run up there and take the seals off. There are quite a few regsiters and it just wasn't a practical solution even though it probably would have worked. The challenge is making sure that the seals are air tight.

    Bookmark   January 3, 2011 at 8:18AM
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I second sniffdog's opinion..Air circulation will help alleviate the problem. Keep in mind though that if the ductork is not properly insulated, the air coming OUT of the registers will be colder that the air going IN as the attic cools the duct. In some cases there may be enough humidity in the house air to cause it to condensate as it cools in the duct though that would be an extreme case. You might want to consider an in-line booster fan as opposed to running the HVAC fan-just a thought.

    Bookmark   January 4, 2011 at 10:58AM
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"Air circulation will help alleviate the problem."

No, it will only increase the amount of condensation.

The warm moist air from inside the house is not going to warm the ducts enough especially if they are metal (higher mass) once they get cold.

More condensation will form, and Mr. Newton's gravity tells you were it will go.

You should make sure the ducts in the unheated area are well insulated with a solid vapor barrier.

You can construct insulated walls in the attic to create a semi-conditioned space around the HVAC equipment (remove any insulation for the ceiling below the enclosed area to help leak heat into the area above) or close it off completely.

Close all the supply vents and block the return vent.

If you still see any condensation you may need to seal all the vents and return better.

The best thing I have located is the flexible magnetic sheeting used to make signs to stick on cars.
You cut it to the outside dimensions of a return or vent and stick it over the metal.

Unico systems come with plastic plugs to close off the system from the inside of the house when in heating season.

Letting warm moist air into a duct board system (better for unconditioned space since there is less thermal mass) and allowing water to condense out is NOT a good idea.

    Bookmark   January 4, 2011 at 11:24AM
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My ducts in the attic are insulated flex ducts and have a vapor barrier around them. A consistent relatively warm air flow (room temps are 63-65 deg F without heat running) through the ducts keeps them warm enough so that condenstaion does not form.

    Bookmark   January 4, 2011 at 2:20PM
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