Hello,
I’m going to start a house this spring that will be a large two-story that calls for having two furnaces and AC units. One will be in the basement and the other in the attic.
Here in the Midwest, basement units are the norm and I don’t regularly see attic units. I seem to be getting conflicting answers on several questions that I have, so I was hoping to get some additional input.
1. Should a condensing furnace be put in the attic? If so, should the equipment be insulated or should it be located in an insulated room?
2. Even if a condensing furnace is not a good idea, should the 80% furnace be insulated or should it be located in an insulated room?
3. Is insulated flex ducting or uninsulated sheet metal ductwork better? Either will end up under the blown cellulose for the attic.
Thanks in advance,
Erk
Replies
I can't answer your questions other than to say you might consider insulating your rafters instead of the attic floor, that way your ductwork will be in conditioned space which is a good idea.
Who will be doing your HVAC work? They should have an engineer or at least somebody experienced enough to help you plan this.
If put in an unheated space, a condensing furnace must have heater cables added to the condensate trap. Our new condensing furnace came with specific instructions for this. (Of course, if power fails in winter you may end up with the trouble/expense of replacing a cracked trap assembly.)
The big problem with having a furnace in unheated space is that ductwork is rarely that tightly sealed, and you end up losing a lot of heated air to the attic space and vice-versa. However, if ductwork is VERY CAREFULLY sealed then this shouldn't be a problem.
Of course, the ductwork also needs to be insulated, both to avoid losing heat in winter and to prevent condensation in the summer, when the AC is running.
A common solution is to have an inch of foam sprayed to the underside of the rafters/sheathing to make the attic space semi conditioned. This also eliminates the need for attic venting. We did that all the time in a cold spot in Wyoming.
Flex duct is so quick to run and less prone to create noise that it's what our HVAC guys usually go with unless we're maximizing a small space.
Good heating
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