1. ## Heater Math Needed!

We just opened our brand new 20x40 inground vinyl pool. Pool builder tells me that the pool is 26,000 gallons. We have the Pentair 125,000 Heat/Cool pump. Current water temperature is 66 degrees and we want to get it to 80 by Saturday. The daytime temperature is forecasted to be 75 degrees for Thursday/Friday. Morning/overnight temperatures are around 45 degrees. We also have the automatic cover so hopefully that will keep most heat in. Based on lower temperatures at night and morning, I was thinking of just running heat pump from about 9am until evening. With this being my first experience with a pool and heat pump, I have no idea what to expect. Is it even possible for the water temperature to reach 80 degrees over the next 48 hours with those ambient air temperatures? I wasn't sure if a heat pump has the capability to get the pool water hotter than the ambient air temperature is outside. I realize there is a probably a lot of math to this question but would appreciate any insight on how to calculate this.

2. ## Re: Heater Math Needed!

Congrats on the new pool! There is only a little math needed. Yes, a heat pump can heat water above ambient temp.

Here is the easy math formula
26,000x8.3 = 215,800 lbs of water
215,800÷100,000btu = 2.16 hours per degree
2.16x14degrees = 30.24 hours

100,000 btu assumes your heat pump operating at 80% efficiency

I think I would run it the whole time to be safe. This does not take into consideration heat loss to colder overnight temps, wind, evaporation, rain, the heat pump being less efficient at cooler overnight temps, etc

3. ## Re: Heater Math Needed!

I hesitated to answer this as I was not sure on the efficiency of heat pumps. But, I thought the amount of heat they can add is greatly reduced as the air temperature drops? With air in the 40s or even 50s, it may not be able to add any meaningful heat to the water.

4. ## Re: Heater Math Needed!

I am with Jason on this one... Heatpumps are not great for doing what you want. They are better for maintaining temps. The best way to find out how long it will take, is to run it and record temps each hour for a while and see what you are getting. Heat pump efficiency is not rated the same as a gas heater would be. In other words the efficiency is not constant. With colder air temps the amount of heat available to pull out of the air diminishes to a point that they become very inefficient. If I am not mistaken, some will automatically shut down when the ambient temp gets below a certain point.

5. ## Re: Heater Math Needed!

Thanks for the replies and my plan is to leave it on all day as the temps should be in the mid 50s at 9am up to 75 later today. I'm hoping to gain about 1 degree per two hours but that may be wishful thinking. This is the heat/cool pump so it will automatically reverse the cycle if it gets too cold. I did notice the cover held heat pretty good overnight. I went to bed and the temp was 66 and woke up and it was 64.

6. ## Re: Heater Math Needed!

Modern pool heat pumps are much more efficient at 50 degrees than they used to be. Some even spec down to 45 degrees. Home heat pumps are able to heat now with ambient temp below freezing and low temp heat pumps can heat with temps below 0f.

There is definitely an efficiency curve that drops as the outside temperature drops. I agree that the best way to determine heat pump performance in a specific application is to heat the pool and note the hourly water temp rise along with the ambient temperature. Humidity is also a factor in heat pump efficiency.

I've been chatting with another TFPer about heat pumps. They were recently working on heating their pool for an event and maintained 72° water temp overnight with no cover and a 47 low temp. So, running it overnight will at least maintain temp, if not add some with a cover in the 45-50 ambient temp range.

7. ## Re: Heater Math Needed!

Originally Posted by dkite22
Thanks for the replies and my plan is to leave it on all day as the temps should be in the mid 50s at 9am up to 75 later today. I'm hoping to gain about 1 degree per two hours but that may be wishful thinking. This is the heat/cool pump so it will automatically reverse the cycle if it gets too cold. I did notice the cover held heat pretty good overnight. I went to bed and the temp was 66 and woke up and it was 64.
Your heat pump should be able to be set to heat, cool or auto. And your auto setting should have a 5ish degree or more temperature differential from cooling to heating. i.e., heat set point 84, cool set point 90 so the heat pump isn't fighting itself on temps.

8. ## Re: Heater Math Needed!

Thanks again for the feedback as it is much appreciated. I'm away from home all day so I'm not able to monitor the temperature. However, I will be home all weekend and will be able to do that to get a better understanding of what to expect. I guess I can leave it on at night and hope that it does something at 45-50 degree ambient air!

9. ## Re: Heater Math Needed!

Congrats on the new pool. We just had ours completed a few months ago. Our pool builder told us that the first time we heat our pool it will be the last. He said the price to heat it will not be cheap. My kids are already using our pool here in SoCal with out the pool heated. Let us know how it turns out.

10. ## Re: Heater Math Needed!

Eostrike...I don't think it will be that bad...Last year, it cost me, on average,\$0.20/hr to run my heater. Not sure what gas prices are in your area. Look them up and see what they are per therm and in your case multiply by 4 for an hourly cost

11. ## Re: Heater Math Needed!

You may be right, I can only go based on what my pool builder said. He said the size of my pool would take two to three days to heat. Heck it takes 45 minutes to heat my spa and this is with a 400k btu furnace.

Lucky it it gets warm here in SoCal, my guests can go in a slightly cold pool as I still need to pay for the complete backyard so they can come over and enjoy .

Originally Posted by danpik
Eostrike...I don't think it will be that bad...Last year, it cost me, on average,\$0.20/hr to run my heater. Not sure what gas prices are in your area. Look them up and see what they are per therm and in your case multiply by 4 for an hourly cost

12. ## Re: Heater Math Needed!

Your pool holds roughly 250,000 lbs of water. at 80% efficiency your heater returns about 320,000 btu's per hour. Based on that math, you should see roughly a 1.0 -1.5 deg/hr temp rise in the pool so over a 10 hour period you will see roughly a 10 - 15 deg temp rise. Assuming a gas consumption of \$1.50/hr for that time frame it would cost you about \$15.00-\$20.00

13. ## Re: Heater Math Needed!

I wanted to give an update to the group. When I turned the heat pump on Thursday around 9am, the water was 66 degrees. I have kept the heat pump on the whole time, including the 45 degree outside air during Thursday night. As of right now at 5pm on Friday, it is at 79 degrees. I am able to check my electrical consumption online and it breaks it out by the quarter hour. My heat pump is costing me exactly \$1.00 every hour as it is about \$.025 every quarter hour. That is way more than I would have expected. I believe heat pumps have to work harder when the ambient air is cooler so I'm sure this will diminish as outside air gets to 85-90 degrees. However, in April, Kansas temperatures average around 70 during day and 45 at night. I'm hoping that automatic cover will keep heat in pool and the pump only has to run a few hours a day.

14. ## Re: Heater Math Needed!

That is a nice temp rise! What is your rate per kWh? We pay 10.25c/kWh which would be a draw between 9 and 10 kW. That seems too high and unlikely. The heat pump doesn't work harder at lower temps you just get less heat output for the same energy usage.

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