Heat Pump BTUs - 8,500gallon pool

[marsui]

I have 3- 2x20 solar panels and a solar cover, but august on Long Island has been breezy with low humidity, my pool makes it to 84 by the end of the day, but by then its just not warm enough to offset the cooler low humidity air we have been having, so I want to add a heatpump so I can maintain 87-88 degrees to counteract.

I'm considering a ComforTemp 53,000 BTU Heat Pump which heats up to 10,000 Gallon Pools, however I'm wondering if I should oversize more. I know once September hits the solar panels usually maintain a 78-80 pool temp, which is far too cold to enjoy with the cooler air - rainy stretches drop me down to 76 usually and also my pool will be mostly shaded by mid sept due to trees 40 feet away from the south. I'm hoping just to expand my swimming season from June -aug, to like May to Sept.

I've already read through many posts on here and I know I should install the heatpump after my solar panels and that I will probably have to raise the temperature on the heatpump higher so it doesnt think the pool is warm because of the water coming off the solar panels.

Just wondering for those of you who bought a heat pump did you oversize or wish you oversized. I was thinking maybe step up to a 65K BTU. I know heat pumps work kinda slow and I'll have to run it often to maintain the temperature, I'm going to hire an electrician to run a new line anyway so I want to do it right the first time, also planning on running a second 220V line to prepare for a possible variable speed pump upgrade in the future (right now I have 1.5hp Hayward single speed motor).

 

[DrewLG]

To keep your pool at a set temperature, the heat pump must produce enough heat to offset your pool's heat loss. So to size the heat pump appropriately, you need to know how quickly your pool loses heat.

A 53000 BTU/hour heat pump can raise the temperature of your pool by 1 degree F every 1.3 hours. So if your pool loses heat more slowly than that, 53000 BTUs is sufficent.

However... Heat pump output (and efficiency) decreases as the ambient temperature decreases. How cold will your air temperature get? That heat pump puts out 53000 BTUs/hour when the air is at 80F, but if the air temperature drops to 50F it'll only produce 29000 BTUs/hour. At 29000 BTU/hr, it will need 2.5 hours to raise your pool temperature by one degree. So if the pool loses heat faster than that, the heat pump won't keep up in cooler weather.

 

[marsui]

To keep your pool at a set temperature, the heat pump must produce enough heat to offset your pool's heat loss. So to size the heat pump appropriately, you need to know how quickly your pool loses heat.

A 53000 BTU/hour heat pump can raise the temperature of your pool by 1 degree F every 1.3 hours. So if your pool loses heat more slowly than that, 53000 BTUs is sufficent.

However... Heat pump output (and efficiency) decreases as the ambient temperature decreases. How cold will your air temperature get? That heat pump puts out 53000 BTUs/hour when the air is at 80F, but if the air temperature drops to 50F it'll only produce 29000 BTUs/hour. At 29000 BTU/hr, it will need 2.5 hours to raise your pool temperature by one degree. So if the pool loses heat faster than that, the heat pump won't keep up in cooler weather.

So In may temps range between like mid 60s to low 80s. In September its more like upper 60s to 80s, I'd say the days in both months in the 60s are minimal. Maybe ill do the 65K BTU just to have it heat a little faster, I'd like to avoid spending too much more than that though.

Thank you for your reply, I'll do some research on BTUs.

 

[marsui]

To keep your pool at a set temperature, the heat pump must produce enough heat to offset your pool's heat loss. So to size the heat pump appropriately, you need to know how quickly your pool loses heat.

A 53000 BTU/hour heat pump can raise the temperature of your pool by 1 degree F every 1.3 hours. So if your pool loses heat more slowly than that, 53000 BTUs is sufficent.

However... Heat pump output (and efficiency) decreases as the ambient temperature decreases. How cold will your air temperature get? That heat pump puts out 53000 BTUs/hour when the air is at 80F, but if the air temperature drops to 50F it'll only produce 29000 BTUs/hour. At 29000 BTU/hr, it will need 2.5 hours to raise your pool temperature by one degree. So if the pool loses heat faster than that, the heat pump won't keep up in cooler weather.

Can you share the formula you used to determine 1 degree every 1.3 hours?

 

[DrewLG]

Can you share the formula you used to determine 1 degree every 1.3 hours?

Sure.

One BTU is the amount of heat that will raise the temperature of one pound of water by one degree Fahrenheit. The heat pump in question has an output of 53000 BTU/hour, so it can raise the temperature of 53000 pounds of water by one degree F per hour.

Your signature says your pool holds 8500 gallons. One gallon of water weighs 8.34 pounds, so your pool holds 70890 pounds of water.

70890 pounds of water / 53000 pounds of water heated one degree F per hour = 1.3 hours per degree F.

 

[marsui]

So Island Recreational said it wasn't worth it to oversize the heat pump too much, their recommendations were:

32,000 BTU - 5,000 gallons
53,000 BTU - 13,000 gallons - 25amp
65,000 BTU - 15,000 gallons - 32amp
80,000 BTU - 20,000 gallons - 40amp

I have 8,500 gallons, so I went with the 65,000 BTU, I was on the fence about going with an 80,000 BTU, since the electrical was the same (40 amp circuit).

I bought the 65,000 BTU but they said I could change it before delivery based on the site survey.

I also have solar water panels (3 -2x20') panels but I will most likely have to upgrade my motor from 1.5HP to a 2 HP or more, I only have 10gpm with the solar, 30gpm without the solar (I have a bypass valve).

I also have solar electricity, but I use more electricity than I produce, my roof is maxed out.

The guy was saying it literally costs 1$ per day to run the thing and going up would only decrease my initial heating time by 4 hours. I would like to swim mid-may through end of september, and the pool doesn't drop below 76 in september with my solar water heaters. So I would need an additional 10 degrees in september.



"70890 pounds of water / 53000 pounds of water heated one degree F per hour = 1.3 hours per degree F."

my calculations

70890 / 65000 = 1.09 hours per degree of water (1 hour 5 minutes)
70890 / 80000= .886 hours per degree of water (52 minutes)


Should I stick with the 65,000? This is already costing me an arm and a leg so if I can save extra 500$ by staying with 65,000 Id feel better lol

 

[JamesW]

Note that the heat pump rating is with 80/80/80 (80˚ F Air, 80% RH, 80˚ F Water).

If the outside air temperature is about 65 degrees, you only get about 80% of the rated output.

For example a heat pump sold as a 130,000 btu/hr heater can only do 82,000 btu/hr at 50˚F Air, 63% RH, 80˚F Water.

At 130,000 Btu per hour at 80˚ F Air, 80% RH, 80˚ F Water.

At 124,000 Btu per hour at 80˚ F Air, 63% RH, 80˚ F Water.

At 82,000 Btu per hour at 50˚F Air, 63% RH, 80˚F Water

Using an air temperature of about 65 degrees, I would estimate that the water should gain about 103,000 Btus per hour,

 

[JamesW]

The guy was saying it literally costs 1$ per day to run the thing and going up would only decrease my initial heating time by 4 hours.

The 65,000 btu/hr heat pump will take about 3.25 kw.

At $0.20 per kwh, that is $0.65 per hour.

 

[JamesW]

You also have to account for the heat loss.

If the air is 65 degrees, the actual gain rate will probably be about half of what you expect.

 

[JamesW]

To get 1 degree per hour, you would need a heat pump in the 90,000 btu/hr range if the pool is covered or about 140,000 btu/hr if the pool is uncovered and the air temperature is 65 degrees.

 

[marsui]

To get 1 degree per hour, you would need a heat pump in the 90,000 btu/hr range if the pool is covered or about 140,000 btu/hr if the pool is uncovered and the air temperature is 65 degrees.

Alright good points, you convinced me, I do keep the pool covered, 90K btu only requires a 50amp which isn't that much more so I'm going to go that route. Id rather oversize and not be mad at myself later, once the line is run I have to live with it. I hate it over the summer having 2-3 rainy days before a party killing my pool's heat, water solar panels can't recover in one day.