Spot heating with gas heater vs constant temp with heat pump

[Poolnewb105]

I was wondering if you guys thought it would be cheaper to spot heat a pool using natural gas (200k btu heater at $1/therm) for the season or to maintain a steady temperature using a heat pump (140k btu heater with COP of 5.5 at $.21/kw). We like it at 88 degrees when we go in and usually use the pool 2-3 times/wk from memorial day to labor day.

The pool is 20k gallons ans has a solar cover. It gets about 8 hours of direct sun a day.

Thanks!

 

[JoyfulNoise]

Typically speaking, maintaining a warm body of water (thermostatting) is cheaper than spot heating but the analysis really requires a good idea of usage patterns and environmental conditions. The COP of the heat pump is going to vary quite a bit with relative humidity and air temperature and so you likely won’t get the 140kBTU/hr you expect out of it.

In all cases, you are going to lose the most heat from your pool during the overnight hours as temperature inversion causes evaporative heat loss. The latent heat energy of vaporization for water is quite large and so even modest temperature inversions can cause significant cooling. Therefore, you really need to cover the pool overnight and when not in use to retain as much heat as possible. Without a pool cover, you’re going to waste a lot of heat.

 

[ajw22]

The answer depends on complex variables that are outside of your control. Some years you may have a warm spring, summer, and fall and other years it can be cool. Performance of a HP will vary with ambient temperature and shut down when temp gets below 50. See chat below How often do night time temps get below 50 during the pool season by you?

A NG heater gives you guaranteed heat regardless of ambient temperature. A HP gives variable heat output and if it can't maintain the temp you want you are stuck. It will not be a question of cost when the HP is maxed out.

1 BTU raises 1 lb or water 1 degree. Your 20K pool has 166K lbs of water. A 200K heater will raise your water around 1 degree an hour. A HP at max output of 140K BTUS will raise your temp about .84 degrees an hour and decline to around .5 degree an hour as the temperture drops towards 50F.

Running your HP will take 7.2kw at .21/kwh = $1.50 to raise your water .5 to .8 a degree.

200K BTUs is .2 therm. At $1/therm that is .20 to raise your water 1 degree.

Not sure I have the power needed for the HP correct but that is the type of analysis you need to do. Going to think about this more.

Heat Pump Performance

• ajw22
• Feb 23, 2019

Pentair Ultratemp Heat Pump BTU Output by OAT

 

[Poolnewb105]

The answer depends on complex variables that are outside of your control. Some years you may have a warm spring, summer, and fall and other years it can be cool. Performance of a HP will vary with ambient temperature and shut down when temp gets below 50. See chat below How often do night time temps get below 50 during the pool season by you?

A NG heater gives you guaranteed heat regardless of ambient temperature. A HP gives variable heat output and if it can't maintain the temp you want you are stuck. It will not be a question of cost when the HP is maxed out.

1 BTU raises 1 lb or water 1 degree. Your 20K pool has 166K lbs of water. A 400K heater will raise your water around 2 degrees an hour. A HP at max output of 140K BTUS will raise your temp about .84 degrees an hour and decline to around .5 degree an hour as the temperture drops towards 50F.

Heat Pump Performance

• ajw22
• Feb 23, 2019

Pentair Ultratemp Heat Pump BTU Output by OAT

Thank you for this analysis. The model I was looking at is the low ambient version which goes down to 40 degrees but like you said, it gets a whole lot less effective at those Temps. They aren't too common though during our pool season.

 

[ajw22]

Thank you for this analysis. The model I was looking at is the low ambient version which goes down to 40 degrees but like you said, it gets a whole lot less effective at those Temps. They aren't too common though during our pool season.

Heat Pumps can not handle a low ambient condition. Pentair HPs shut down at 50 degrees. Some others may run at lower temps.