Closing attic based AC vents/ducts during winter
I’d really like some of the expert science based advice as I haven’t found a clear answer to this question. We have natural gas forced hydronic as our primary heating source in a 2 story colonial in southern New England. Last year we had a ducted heat pump installed with R-8 insulated ducts in the attic and the basement. It’s significantly cheaper to use the boiler in the colder months so I plan on switching over and turning the heat pump off. I understand how much more efficient units inside the building envelope would have been and it was not an option for us.
I’m not worried about the additional heat load/cost of keeping the ducts open but rather a mold situation (sweating on the inside) developing inside the ducts. I’ve given this a good amount of thought and come up with two possible avenues:
1) Close the ceiling registers and insulate them as best as possible (insulated covers). Air trapped in the ducts would become somewhat outside the building envelope aside form the inevitable leakage. However, any hot air that leaks in would be much more likely to condense as the temperature inside the duct would be much closer to the outside temp. [attic temperature sensor for december attached]
2) Leave the vents open. The hottest air would migrate into the attic and the ducts would stay part of the building envelope. Hopefully this would keep the temperature inside the duct high enough that condensation is unlikely.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I don't think you'll have any problem either way, but you do have a simple option to "solve" this issue regardless: run the heat pump's fan to circulate air in the ducts periodically. That should be all you need to do to be absolutely sure you don't have a problem.
BTW, "hydronic" is for circulating hot water boiler-type heating systems, "hydroponic" is for growing plants with water only and no soil.
Bill
Thanks for the response Bill, I edited my error with the hydronic vs hydroponic. Do you mind educating me on why I shouldn't worry about this? Just trying to learn.
The internals of the duct are technically inside the building envelope, since they are sealed and insulated from the outside air. What you have is a case of slow, or non moving air inside those ducts, which is where there is a possibility for condensation to occur. You should have minimal, if any, stack effect taking "new" air into the ducts, and minimal convective flow on a mostly horizontal duct run (making an assumption here). That means the air in the duct is stagnant, so the only moisture that can condense out is what is already inside the duct -- for the most part. That "most part" is because you will have some new moisture introduced to the stagnant air in the duct by way of diffusion, but it should be minimal IF the ducts themselves are well air sealed.
That means you shouldn't have a huge problem if the ducts are left connected but without air movement. Moving air through them periodically will warm them up a bit, and take out (by evaporation) any condensation that may have formed. That's why some periodic air movement through the ducts using the blower should prevent any issues, but why you have minimal risk if you just leave everything as is too.
Bill
Thanks again BIll. You're correct in that most if not all of the duct runs are essentially horizontal. I assume what you said still applies even though the attic may have R30 and the R8 duct runs are on top of the fiberglass batts not below?
I think to be safe I'll set up the fan on that air handler to run every now and then on low fan speed. Any suggestion on how often? I was thinking every night for 15 minutes or so.
Less R value means it will get colder. The basic physics is the same though.
You could try a nightly run. You won't need to run for a long period of time, you'd be more likely to need to run more frequently. It's common to run for 5-10 minutes every hour. Experiement a bit, see what seems to work. The ideal cycle from an energy consumption standpoint is as infrequent as possible, and a short run. I'd try to make sure a cycle is never less than 5 minutes though, 10-15 is probably pretty good.
Note that you may find you don't really need to run any air through those ducts at all.
Bill