Running the heat pump low and steady vs blasting it right before soak time

Started by spool_derek · · 3 replies
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#1
Been going back and forth on this all summer. Right now I let the AquaCal just idle around 88 all week so it's ready whenever, versus the other option of leaving it off and cranking it to 102 a few hours before we actually want hot tub mode in the evening.

On paper the steady low setting should be easier on the compressor and probably better COP since it's not fighting a big delta, but it's also running way more total hours. The blast approach is fewer hours but working harder each time. Solar cover's on the whole time either way so that's not the variable.

Honestly haven't tracked kWh close enough between the two to have a real answer, just going off gut feel. Anybody actually run numbers on steady state versus spot heating for something spool sized, curious if it's even worth agonizing over at 15k gallons.

3 Replies

#2
No heat pump here so I can't give you real numbers, but whoever said the steady state wins on heat loss through the water/air interface not just the compressor cycling is right, and I think that gets left out a lot in these threads. Bigger delta between water and air temp means faster heat loss overnight even with the solar cover on, so cranking it from 88 to 102 in a few hours you're not just working the compressor harder, you're also losing more btus to ambient the second it gets above your baseline. For what it's worth I'd bet the steady low setting wins most nights unless you're only using the spa maybe once a week, at which point the hours saved probably make up for the inefficiency. Your mileage may vary depending on your air temps at night in your area though.
#3
Jason's right about the delta losing more btus, but at 15k gallons and once or twice a week use the math still favors spot heating on total kWh, just do it. Steady state only wins if you're actually using the spa close to daily, otherwise you're heating water nobody's sitting in for hours at a time. Mine only gets cranked when we know we're using it that night.
#4
no heat pump on the swamp so this is over my head numbers wise, but the delta thing is funny to me cause even just running my pump longer on the hottest days here i can feel the water temp swing way more than i'd expect for how insulated i thought a pool would be. phoenix air temp doesnt even drop that much at night in summer so idk if that changes your math any, always wondered if that makes steady state even more worth it out here vs somewhere with bigger night swings.