Surprised how much heat my pool drops overnight this cold snap

Started by btu_frank · · 2 replies
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#1
Had a few nights down in the 40s here and figured with the pool covered I'd only lose a couple degrees by morning. Lost almost 6 one night. Did the math after, 12k gallons times 8.34 times 6 degree drop is right around 600k btu walking out into the night air, cover and all. Makes you appreciate why I run the gas heater to recover fast in the morning before my swim and let the heat pump hold steady the rest of the day, splitting the load like that keeps the gas bill sane. Didn't expect the surface loss to be that big a chunk even with a cover doing its job. FN

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#2
no heater here so cant speak to the gas/heat pump split but that math seems off to me, 600k btu walking out through a cover seems like a lot of that is evaporation not just straight surface radiation, cover slows evap a ton but doesnt stop it especially if theres any wind gap around the edges. i dont have a cover at all and figured thats just why im losing heat every night, but if a covered pool is still dropping 6 degrees id bet the cover fit is doing less than you think. how tight is it sealing around the edges, any gaps where it meets the deck
#3
Had almost this exact conversation with myself two winters ago when a cold front came through Marietta and I watched my pool drop 5 degrees overnight with a solar cover on, and I went down the same rabbit hole of blaming radiation until I actually thought about what a cover does and doesn't stop. Tyler's right that evaporation is the real thief here, not straight conduction to the night air. Water loses roughly 1000 BTU per pound just to evaporate, that's the latent heat of vaporization, and any exposed surface or gap is going to bleed heat that way way faster than plain surface cooling would account for. A bubble cover cuts evaporation a lot when it's flat on the water with good contact, but if yours is a floating solid cover or has any lift from wind, or gaps at the steps and skimmer, you're still getting a meaningful vapor exchange happening under or around the edges.

Frank, worth checking your cover for exactly what Tyler's asking, gaps at the deck line, and also whether it's actually laying flat on the water or bridging with air pockets underneath, because trapped air under a cover behaves differently than a cover in direct contact with the surface. My old solar cover had started curling at one corner from sun exposure and I didn't clock it as a problem until I did basically your same math on a cold night and got a number that didn't add up for radiation alone.