Dear Mr. Allen:
Thank you for writing to the Forum. You are right that the heater is not as efficient at night. If the heater has 135,000 BTU's when it is 80 degrees outside then it might have close to 100,000 BTU's if it was 50 degrees outside. It is still 3 to 5 times more efficient than gas at the low temperatures though. Many pool guys in New York still tell people to run their water pumps 24 hours a day. That costs another $50.00 a month in electricity. Since the season without a heater is so short many people didn't care. But now that you have a heat pump you might get 5 months out of the pool and so I don't agree with 24 hours and think 8 to 12 hours is more reasonable. (If the water is really cold there might be some times that you need to run the heat pump 24 hours) What we are trying to do is circulate all the water once a day. So if you pool has 20,000 gallons and your water pump puts out 35 gallons of water a minute that is 2,100 gallons an hour and it would take the water pump about 10 hours to do circulate all the water in the pool. Most pumps do 50 to 100 GPM though. Then you get a timer to run the water pump maybe 9 am to 9 pm and then the heat pump will only run during the warmest part of the day. The heat pump also has the time clock over ride but that could cause the heater to run at 2 am if you wire it. If you still want to run the water pump 24 hours then you can get a simple 24 hour water heater timer at Home Depot for $60.00 or so and then have an electrician run a control wire from the heat pump through it and then you can set the heat pump to run anyhour that you want and still run your water pump 24 hours a day. You can also buy a refrigeration thermostat and set it to shut open the circuit when the outside temperature gets below 50 or 60 degrees if you want too. I can send you one for about $150.00. Things are starting to get complicated with that though and I do like to keep things simple.
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