Badly executed heat, what to do now? (To mini or not to mini)
I’m performing some repairs on my house and discovered that most of the central heating duct work was poorly executed and needs rework Or does it?
I have a fundamental question to decide that I think the GBA community could help me with:
– Should I repair the central heating system
– Should I largely abandon it for mini splits?
– 1976 split level (~6 elevations)
– Climate 5b
– Low slope black epdm roof (relevant to cooling)
– Furnace: 95% 2 stage ECM variable. 75k/115k btu. Fan rated 1100 to 2000 cfm.
– Two older a/c mini splits are around but don’t have to be part of the plan. SEER 10 and 15k btus and need servicing. Both are located in spaces without central returns.
– 3200 sf interior sf + 400 sf unventilated indirectly conditioned crawl space
– Above average tightness: an older blower result of ~0.3 nACH.
– Wall: generally R19+5
– Roof: Mostly R-30, 25% R22, 15% R46
– Windows are mixed 70s double, 80s double, and recent low-e argon double.
– Cost of installation is a primary consideration at this point. Mini splits require paid labor, fixing ducts may not.
– House currently is not well cooled. WIth a flat black roof and a zero overhang roof line, cooling comfort is a problem. I’d love to say new windows and more insulation will address that but I don’t think it is true. We prefer lower design temps (heat and cool).
– Daylight basement is usable but too cold in the winter (no slab insulation), two upper level additions made in the late 80s are much too hot in the summer to use without cooling.
The summary of the ducting defects is none of the returns are right/work/exist and all of the ducts are unsealed, unscrewed, falling off, and a few that don’t exist/work are needed. They are 95% inside the envelope (there is one 4″ round that goes through the garage ceiling).
Fixing the supplies is doable, though it likely means some drywall removal. Fixing the returns is going to be a challenge due to furnace location and truss directions.
Manuals J and D:
– I paid to have a J and D done. The D is still being tweaked but I have the J results.
– I also spent an hour doing a more back of the napkin J using load-calc.net. The results are similar.
Heating: both say 57k
Cooling: pro 41k , load-calc 44k
This isn’t really a question about whether my manual J is accurate (more about what to do) but if the numbers seem crazy, please speak up. I’m going to ask another question about a few specific manual J things.
Heating: I am not surprised that the J says the furnace is oversized. It brings the house to temp quickly and doesn’t struggle during design weather.
Cooling: Existing cooling is inadequate. We do know that we like minis so more/better is an option. I think 43k could be low with the black roof.
Why not repair the ducts? Valid choice, although it leaves me needing to figure out the cooling, possibly central or possible minis later. And maybe the right answer is do both ducts and separate cooling. So the question might be more subtly stated as exactly how much to fix the ducting.
Why not improve the envelope? I will likely be able to reduce both loads over time with DER-style work but some areas are tough. The daylight basement and crawl space have no slab insulation and would be very expensive to retrofit or be unacceptable height wise (slab to truss is 91″ in basement). All of the spaces added by a previous owner in the late 80s are substandard for insulation and glazing coatings but to DER these spaces might be a lot of $. Anything north of 30k and I’d rather re-roof and add PV.
Opinions? Love some opinions based in experience. While I love the knowledge fountain that GBA is, it isn’t a replacement for experience. (just as experience isn’t a replacement for knowledge)
Extra credit: Phased approach allowing a slow injection of cash rather than $20k+ right now.
Sorry this is so long.
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