Will blocking off the air conditioner returns and registers in the winter make a difference?
I have a 1992 4,500-square-foot Colonial in NH: It has central air with the handler in the unconditioned attic. Will blocking off the returns and registers in the winter make a difference?
We usually use about 1200-1500 gal of oil which I don’t think is too bad. The upstairs bedrooms however seem cool and drafty.
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There are several issues here.
First of all, how is your house heated? I'm going to guess that your house has a boiler and hydronic heat distribution, and that the forced-air ductwork is used for cooling only, not heating.
If that is the case, feel free to seal the registers on your ceiling. If you do that, you are likely to save energy.
Your house probably has a few problems. First of all, it's usually a bad idea to install ductwork or an air handler in an unconditioned attic.
Second, with a little bit of weatherization work, I imagine that you would be able to reduce your winter heating bills. It's painful to buy 1,200 gallons of oil every year.
Sealing the ducts' register-boots to the gypsum and sealing all of the duct seams/joints as well as taping all the seams on the air handler with FSK tape will make a difference. If the ducts are R6 or less putting shrink-wrap air-barriers on the register grilles will stop the parasitic convection. A blower door test would zoom in pretty quickly on where the biggest leaks are, but start by fixing all of the obvious stuff first. Unsealed connections to the gypsum around duct-boot edges can easily add up a couple square inches of leak into the attic insulation per register, and may even cause localized wetness & mold issues if the register grille itself isn't too tight to the gypsum.
Assuming you are also heating hot water with an indirect, 1200-1500 gallons/year implies of over 65,000BT/hr (at a presumed outside design top of -5F), or about 12-15 BTU/foot of conditioned space whidh is middle of the road for 1990s 2x6 construction. If you're losing a lot of the top of the house to convection through uninsulated leak ducts it'll probably be measurable in the oil bill. (If you have a zip code and a K-factor from mid and late-winter oil fill ups we could sanity check the heat load.)
Most 1990s homes have no foundation insulation and leak air at the foundation sill & band joists too, despite the presence of foamy sill gaskets. I usually adds up to a double-digit percentage of the oil bill even if you don't actively heat the basement. (More than 20% in some cases.)
In any on-grid house in New England heating with oil it's cost effective to heat one or more large zones with ductless mini-split heat pumps. At the 5-year average NH pricing for electricity and oil a mini-split would cost about half what it costs to heat with oil in an 85% efficiency burner.