Electric Heater without cover

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J

Jack_King

Member
Oct 10, 2024
9
NC
Hello,

Warm temps are here in NC and my wife/kids have been itching to use our new (to us) pool in the house we moved into last year. We figure that a heater could get us ~1 month extra on both ends of the swim season.

Our pool has a electric heat pump that is still in place but it is broken and non-repairable and we are debating about whether it is worthwhile to replace it or not. Gas is available at the house but a new line ~150ft long would have to be run underground to the equipment pad and I don't think this is in our budget.
The one thing holding me back from purchasing a new heat pump is that we do not have a pool cover of any kind. The pool is a very unusual free-form shape with a water feature on one side and would be extremely difficult to cover. The prior owners never covered it.

I know this question has been asked before and a cover would be better, but wanted to know if there is anyone out there with experience using an electric heater with and without a cover. Will I just lose a few degrees at night that I can easily get back during the day? Or is it a completely stupid idea to run the heat pump without a cover at night?

Thanks!
 

Dual Heat Pumps on LI?

I've been looking into installing a pool heater after using our pool for the first time last year. It's ~45-50K gallons and gets nearly full sun during the day on Long Island, NY. The pool is comfortable during the summer on sunny days (~78-80) but gets chilly after the sun sets. The curved L...
www.troublefreepool.com www.troublefreepool.com
 
Jack_King said:
Hello,

Warm temps are here in NC and my wife/kids have been itching to use our new (to us) pool in the house we moved into last year. We figure that a heater could get us ~1 month extra on both ends of the swim season.

Our pool has a electric heat pump that is still in place but it is broken and non-repairable and we are debating about whether it is worthwhile to replace it or not. Gas is available at the house but a new line ~150ft long would have to be run underground to the equipment pad and I don't think this is in our budget.
The one thing holding me back from purchasing a new heat pump is that we do not have a pool cover of any kind. The pool is a very unusual free-form shape with a water feature on one side and would be extremely difficult to cover. The prior owners never covered it.

I know this question has been asked before and a cover would be better, but wanted to know if there is anyone out there with experience using an electric heater with and without a cover. Will I just lose a few degrees at night that I can easily get back during the day? Or is it a completely stupid idea to run the heat pump without a cover at night?

Thanks!
Click to expand...
You can get a bubble wrap cover for any shape pool for not much money. (At least not much money compared to electric heater bill)
 
Bperry said:
You can get a bubble wrap cover for any shape pool for not much money. (At least not much money compared to electric heater bill)
Click to expand...
Yes, this is true. But the cover/reel is also an enormous hassle as well.
We have family in south FL with a tiny pool that they heat and use in the winter, and dragging the cover on/off the pool is a reel (pun intended) PITA that kinda kills the fun of spontaneous swims.
If you have an auto-cover, it's a no brainer.
 
JamesW said:

Dual Heat Pumps on LI?

I've been looking into installing a pool heater after using our pool for the first time last year. It's ~45-50K gallons and gets nearly full sun during the day on Long Island, NY. The pool is comfortable during the summer on sunny days (~78-80) but gets chilly after the sun sets. The curved L...
www.troublefreepool.com www.troublefreepool.com
Click to expand...
I read the whole thread, very interesting. That's an enormous residential pool he's got!

Obviously there is a massive operating cost to leave it uncovered, but having dealt with a bubble cover and reel before on a tiny pool; I'd rather have to pay the premium or delay swimming in general than deal with the cover.


I'm trying to interpolate and come up with some rule of thumb from the examples you provided. Would you say that an electric heat pump heating w/o a cover is feasible/reasonable so long as overnight temps are in the mid to high 60s?
 
Jack_King said:
I know this question has been asked before and a cover would be better, but wanted to know if there is anyone out there with experience using an electric heater with and without a cover. Will I just lose a few degrees at night that I can easily get back during the day? Or is it a completely stupid idea to run the heat pump without a cover at night?
Click to expand...

The answer is the same if you have a Heat Pump or a gas heater. It is all about BTU's from the heater versus lost to the environment.

At best a 140K BTU heater will heat your pool at about 0.9 degrees an hour. Probably more like 0.7 degrees an hour.

My 400K gas heater heats my pool at about 1 degree an hour.

My pool loses about 4 degrees on a cool night. So it takes me about 4 hours to recover the heat lost. I never swim before the air warms up around 1PM. I turn my heater off when I am done with the pool for the day and it turns on at 9AM. That gets the pool back up to temperature for the afternoon.

If I am not going to use the pool for a few days or if a cold spell hits and the weather is not good for swimming, I leave the heater off. No reason to waste money heating the pool. With the heater off for a few days the water can lose 8-10 degrees. BTW, my target swimming temperature is 86F.

You will learn how much heat your pool loses over a typical night and what temperature your water stabilizes at with the heat off after a few days. In my pool it is the mid-70's.

I pay $1/therm, and running my heater costs $4/hour. You can calculate what it will cost to run your heat pump and determine if you are better off heating all the time or heating on demand. With a Heat Pump, it can take 5 to 6 hours to recover four degrees of night loss in your pool.
 
Jack_King said:
I'm trying to interpolate and come up with some rule of thumb from the examples you provided. Would you say that an electric heat pump heating w/o a cover is feasible/reasonable so long as overnight temps are in the mid to high 60s?
Click to expand...
Minimum air temperature?

Pool surface area?

Water temp to be maintained?

Wind speed.

Humidity?
 
JamesW said:
Minimum air temperature?

Pool surface area?

Water temp to be maintained?

Wind speed.

Humidity?
Click to expand...
Thank you. I would say:
  • 350ft2 is pool surface area
  • 85F is desired water temp
  • Avg. Wind is ~6mph
  • Avg Humidity is 70%.


  • Min air temp: thinking like 60-65F, but this is also what I'm trying to solve for. At some air temp, I'm going to be losing as much to environment as heater can add. From the avg wind spd and graphs above looks like this point is ~50F air temp for a 125-140k BTU heater.

    So at 70F air temp maybe I'm increasing it ~0.5F per hour with a 140k BTU heater. :(
 
At 60 air and 85 water, you will lose about 415 btu/hr per sqft or about 145,250 btu/hr for 350 sqft at 6.81 mph wind velocity.

At 60 air and 80 water, you will lose about 315 btu/hr per sqft or about 110,250 btu/hr for 350 sqft at 6.81 mph wind velocity.

A 140,000 btu/hr heat pump will produce about 105,000 btu/hr at 60 degrees.

Air.....Water.....Btu/Hr gain.
50........80.............90,000
60........80...........105,000
65........80...........115,000
70........80...........130,000
80.......80............140,000

1741971511615.png
 
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Air.....Water.....Btu/Hr loss.
65........80................92,750
65........81.25.......100,188
65........82.5..........107,625
65........83.75.......115,063
65.......85.............122,500

For 80 water and 65 degree air at 6 mph, you will lose about 265 btu/hr per sqft, which is 92,750 btu/hr.

For 85 water and 65 degree air at 6 mph, you will lose about 350 btu/hr per sqft, which is 122,500 btu/hr.

The heat pump can produce about 115,000 btu/hr at 65 degrees air temp.

So, you will probably top out at about 84 degrees.

Air.....Water.....Btu/Hr gain.
50........80.............90,000
60........80...........105,000
65........80...........115,000
70........80...........130,000
80.......80............140,000

1741975202781.png
 
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You have two separate considerations.

1) Heating and increasing temperature.

2) Maintaining a specific temperature.

19,000 gallons is 158,460 lbs of water.

At a heating rate of 115,000 btu/hr, you will gain about 0.72 degrees per hour.

However, as you begin to heat, the pool also begins to lose heat at a rate proportional to the temperature of the water above its natural equilibrium temperature.

For example, at a water temp of 75 degrees, you might be losing half of the heat added, so your net gain rate is now 0.36 degrees per hour.

At 80 degrees, the loss might be 75% of the heat added and now the net gain rate is 0.18 degrees per hour.

At about 83 to 85 degrees, the loss rate equals the gain rate and you are now in maintenance mode.
 
This gives an idea of the water temperature vs. time based on the fact that as the water temperature goes up, the net heating rate decreases.

Net Heating Rate = Heat Gain - Heat Loss.

Assumes heat loss at 85 degrees is equal to the heating gain, which is a net heating rate of zero.

Water temp.....Heating rate per hour.

65..............................0.72

70..............................0.54

75..............................0.36

80..............................0.18

81..............................0.144

82..............................0.108

83..............................0.072

84..............................0.036

85..............................0.00
1741989354509.png
1741989229913.png
 
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The curve is Asymptotic to 85 degrees, which means that it approaches 85 degrees but never actually reaches that level because the temp rise approaches zero as the curve gets closer to 85 degrees.

65 to 70 = 8 hours = 8 hours total.

70 to 75 = 11 hours = 19 hours total.

75 to 80 = 19 hours = 38 hours total.

80 to 81 = 6 hours = 44 hours total.

81 to 82 = 8 hours = 52 hours total.

82 to 83 = 11 hours = 63 hours total.

83 to 84 = 19 hours = 82 hours total.

84 to 85 = 55 hours = 137 hours total.
 
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Temperature = 84 + (65-84)e^(-0.0441Time).

X = Time in Hours.
Y = Temperature.

Y = 84 + (65-84)e^(-0.0441X).

X = -22.6757 ln((Y - 84)/-19)

You can use this formula to calculate the time to reach a temperature or the temperature at a specific time.

For example, to get to 82 degrees will take about 51 hours.

To get to 83.5224 degrees, it will take 83.5224 hours.

To get to 83.9885 degrees will take 1 week (168 hours).
1742056594985.png

1742056283462.png



1742056577958.png


Y = 84 + (65-84)e^(-0.0441x), x from 0 to 80 - Wolfram|Alpha

Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.
www.wolframalpha.com www.wolframalpha.com
 
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If you had no heat loss, it would look like this.

For the first 5 degrees, the heating is pretty close and then the lines begin to diverge.

Both methods give about 6.9 hours to go from 65 F to 70 F. (6.8899 Hours vs. 6.92475 Hours).


1742086247497.png
1742088329591.png

70 = 65 + 0.7257x - Wolfram|Alpha

Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.
www.wolframalpha.com www.wolframalpha.com

70 = 84 + (65-84)e^(-0.0441x) - Wolfram|Alpha

Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.
www.wolframalpha.com www.wolframalpha.com
 
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