Is this ridiculous?
We live in a 1600 sqft 2 level end unit rowhouse with structural historic masonry walls from the 1890s.
Housing is really expensive where we live, figure between $400-500 per square foot, so we’re trying to balance efficiency & comfort with sacrifices of this valuable square footage. We have a crawl space and attic, and intend to condition both so we can run ductwork without replacing the joists on the second floor.
Looking at the numbers, avoiding 2 floors of chases nets us a couple thousand dollars in useable square feet. Minisplits are one solution, but we’re committed to the hidden mechanicals so we’re debating a heat pump system with 2 air handlers, 1 in attic 1 in crawlspace.
Q1) Is this nuts?
Q2) Can we find equipment small enough, to be correctly sized for our house–keep in mind, no wall insulation, All attic insulation is polyiso on roof and air sealing on sides of attic–or is everything going to be too large?
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One question: what is your heating system now?
Sealed combustion gas furnace+forced air. We'd replace the old AC unit with 2 heat pumps in this scenario, but probably keep the furnace in the crawl space for very cold stretches.
Is there something wrong with the furnace that's prompting you to replace it? Unless your gas rates are very high or your electric rates are very low, it's unlikely that converting to conventional heat pumps or ducted mini-splits will save you any money, and unless the existing system is grossly oversized, you probably won't gain much if any comfort either--central forced air is central forced air, regardless of whether it's heated with gas or electricity. While converting your crawlspace and attic to conditioned space will help with equipment and ducts located in those spaces, you also have to keep in mind that you're increasing the volume of space that the equipment has to work to condition.
As to your questions, 1) It only seems nuts if you're replacing perfectly good equipment, and 2) Without you letting us know your Manual J calculated heat load, there's no way to say, but an uninsulated structural masonry house is going to have a higher heat load than even a crappy code-min 1975 ranch tract house, so I'm going to go out on a limb and say that you will probably have no trouble finding equipment that matches your house's heat loss.
First things first. Start with a heat load calculation for each of the conditioned spaces separately, using a realistic indoor temp (do you REALLY need it to be 73F at dawn on the coldest day of the year?) and the 99th percentile bin for the outdoor design temp for you location:
http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Outdoor_Design_Conditions_508.pdf
(If you are outside of North America, with a location we could estimate the 99% design temp using other weather data sources.)
To estimate the U-factors for the exterior walls we would need to know the particulars about the construction, such as the number of wythes of brick, whether it's a cavity wall or solid brick, and whether there is lath under the interior finish plaster, etc.
If unknown, the window U-factors can be estimated by how many panes, sash framing material, and any low-E coatings, etc.
Essential to figuring the U-factor for the roof we need to know thick is the roof polyiso, and is it on the underside of the rafters, or it above the roof deck?
The it's
U-factor x square feet x (inside design temp - outside design temp) = BTU/hour.
Do this on a room-by-room basis, using the U-factors & areas of each type of surface (window, wall, roof, door, etc.).
Only exterior wall /roof surfaces count, and you should count the crawlspace exterior wall area down to a foot below grade as a fudge-factor for ground losses.
A 2-zone multi-split with pair of mini-duct cassettes and one compressor unit might get you there.
Alternatively a pair of single-zone mini-duct mini-split units may work.
But making that call depends on the room-by-room load numbers, which will be quite different in a place like Thunder Bay Ontario than it would be in say Birmingham England, Perth Australia, or Washington D.C..
It all starts with the load calculations. Short of a full-on Manual-J, a crude I=B=R type spreadsheet (as outlined above) isn't too bad for sizing the equipment.