Heat Loss from Ducts in Unconditioned Crawlspace
We bought a beautiful home in sunny California (Zone 3C) that has long runs of R-8 ducts in an unconditioned crawlspace, with a very recent 24,000 btu heat pump that can’t quite keep up.
If I’m calculating things right, the heat loss from just the ducts is roughly this:
(10″ diameter flex duct * 1 ft / 12 in) * pi * (100 ft duct length for supply and return) * (0.125 U-value for R-8) * (28 degrees temperature difference at the design condition of 40 degrees outside and 68 degrees inside)
= 920 btu
According to CoolCalc, the whole area served by this heat pump only needs 18,000 btu.
Assuming this is accurate, our 24,000 btu heat pump should be more than adequate.
It gets a little worse. It’s likely that the ducts are losing some additional heat from any crimps from the supports hanging them from the floor joists, and there might be a little air leakage anywhere the joins aren’t perfect. Even if it’s double the loss, that’s still under 2,000 btu. That’s not enough to explain what’s going on.
The house must be leaking more than 4,000 btu more than what CoolCalc says it should be. Where is the heat going?
1. ($) Insulate the ducts better. Unfortunately, I don’t think there’s much we can do. We have a tiny crawlspace, only about 20″ high, and 10″ R8 duct takes up most of that. Next idea:
2. ($$) Encapsulate the crawlspace. If we could reduce the temperature differential between the crawlspace and the outdoors, that could help. There’s just not enough heat loss to explain the problem though. Even if duct loss was zero, that’s small compared to the performance gap we’re seeing.
3. ($$$) Replace the very new new 24,000 btu air handler with a new 36,000 btu unit. This would work, and it’s what any HVAC contractor might recommend. But it’s not addressing the root of the problem, which is air infiltration or loss.
4. (-) Wear extra layers of clothes on cold days. Get a space heater. This would work, but again, it doesn’t address what’s wrong.
5. ($$) Air seal the attic and crawl space. Maybe we have a chimney effect that’s causing much greater loss than what the Manual J / CoolCalc predicts.
Anyone know if poor air sealing can cause more than 4,000 btu of heat loss? With what we’re seeing, it would have to be 6k or 8k btu.
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