Calculator for Comparing Time & Cost of Heater vs. Heat Pump?

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wgillette

wgillette

Silver Supporter
Oct 23, 2021
83
Raleigh, NC
Pool Size
26000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Is anyone aware of a calculator or Excel file that provides a more specific/detailed comparison of heat pump vs heater for a specific scenario? It's my first spring with this pool and with sunny days and spring break next week, I'm looking to heat the pool to a more tolerable temperature. I'd like to be able to compare which is more cost effective - the heat pump or heater? I want to think the heat pump is, but it depends on air temperature, so it might be more cost effective to use the heater to bring the pool to temp and then use the heat pump to maintain.
 
wgillette said:
Is anyone aware of a calculator or Excel file that provides a more specific/detailed comparison of heat pump vs heater for a specific scenario? It's my first spring with this pool and with sunny days and spring break next week, I'm looking to heat the pool to a more tolerable temperature. I'd like to be able to compare which is more cost effective - the heat pump or heater? I want to think the heat pump is, but it depends on air temperature, so it might be more cost effective to use the heater to bring the pool to temp and then use the heat pump to maintain.
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There are a few out there:

 
Those are not scenario specific. They are either sizing calculators (how big of a heat pump) or seasonal comparison calculators (e.g. here's how much a heat pump can save you over a season). I have not found one with enough inputs/controls. e.g. current air temp, current water temp, desired water temp, size of heat pump (BTUs), size of heater (BTUs), cost/kWh, cost/therm. The outputs would be hours of heating (heat pump), cost of heating (heat pump), hours of heating (heater), cost of heating (heater).
 
wgillette said:
Those are not scenario specific. They are either sizing calculators (how big of a heat pump) or seasonal comparison calculators (e.g. here's how much a heat pump can save you over a season). I have not found one with enough inputs/controls. e.g. current air temp, current water temp, desired water temp, size of heat pump (BTUs), size of heater (BTUs), cost/kWh, cost/therm. The outputs would be hours of heating (heat pump), cost of heating (heat pump), hours of heating (heater), cost of heating (heater).
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You may need to visit a few. You might get gas from one and pump from another. You may have to take some with a grain of salt, and it might be valuable to compare results from one site to the other. In some combination, you can come to a reasonable approximation.

The pentair calculator (2nd on the list in the search) does a reasonable job and allows you to input MOST of your needs (maybe not current air temp, but their average usage for the season for sure). I think it does a good job, and makes a good assumption about the efficiency of gas, average temperatures and allow you to adjust cost/kWh and cost/therm. Heat Pump Calculator

From the 6th one on the search I shared: (for my region)...(Calculator | THERMEAU®) While this is high across the board for my region, when I did comparison is relatively accurate to my analysis I did myself vs. actual (for my region/rates) (ignoring thermeau results).

1680317546197.png

It will take a little bit of time to triangulate, but it is possible. It is highly dependent on utility rates and location.
 
wgillette said:
Is anyone aware of a calculator or Excel file that provides a more specific/detailed comparison of heat pump vs heater for a specific scenario? It's my first spring with this pool and with sunny days and spring break next week, I'm looking to heat the pool to a more tolerable temperature. I'd like to be able to compare which is more cost effective - the heat pump or heater? I want to think the heat pump is, but it depends on air temperature, so it might be more cost effective to use the heater to bring the pool to temp and then use the heat pump to maintain.
Click to expand...
So, maybe I missed something in the original question. Are you saying that you have both a heat pump and gas heater connected to your pool? If so, that is a really extreme edge case. There may be some scenario where air temp, pool temp and utility rates might give one or the other an edge, but that is not the way calculator builders think. Most calculators think about annual usage vs. therm/elec cost vs. target pool temp. Nor is it the way I think. I think annual operating costs, not cost to operate this month. Sorry for my error in thinking and assuming. 😊 Generally, I worry about the macro (annual cost) and don't suffer the micro (this week heating the pool). In the end, location and rates will win, in either case. Hope that makes sense.

Maybe this will help? (see section 1.4)

Calculators - Further Reading

www.troublefreepool.com www.troublefreepool.com
 
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The calculations in question are really not that complicated and in the end just reduce to understanding what the units mean.

Gas heaters are rated by the number of BTU/hr of natural gas (or propane fuel) they consume. So the "gotcha" there is you need to multiply that rating by the efficiency of the heater (typically somewhere in the 80-90% range) to get the heat that will actually end up in the pool water. A BTU is defined as the amount of heat required to raise the temperature of one pound of water one degree F. So divide the number of pounds of water in the pool by the BTU/hr of the heater (after the efficiency correction) and you will get the number of degrees/hour you can expect the water temperature to rise by.

BTUs being an inconveniently small unit to actually bill you for, the gas company charges you by the "therm" Which is define as 100,000 BTUs. So your typical 400k BTU pool heater burns 4 therms/hour.

Heat Pumps (at least in the US) are also rated in terms of BTU/hr, but in their case the BTU/hr is the heat delivered to the water, so no further correction for that needed for figuring out how quickly they can heat your pool. But the amount of heat they can deliver is very dependent on the difference in temperature between the water and the air so that does need to be accounted for.

Heat Pumps also have an efficiency rating called COP. This is the ratio of the electrical energy put in to the unit to the amount of heat that is delivered to the water. The wonderful thing about heatpumps is that this number is greater than 1, typically about 3, meaning that they deliver 3 times as much heat to the water as the electrical energy consumed (the extra heat being extracted from the air). Since electrical power is usually measured in kWh I'll leave it as an excise the reader to find the conversion from kWh to BTU. When looking at the heat-pump number it is also important to look at the spec sheets closely as the COP (just like the heating capacity) is strongly dependent on the temperature difference between the water and the air that you are trying to pull the heat from. (Think of a heat pump as pushing the heat uphill from a cold place to a warmer one, the higher the hill the more push needed.)

Finally when trying to figure out the question "How long will it take to warm up my pool" you have to consider the amount of heat being lost to the environment if the pool is warmer than the surrounding air and earth. This is where the higher available capacity of the gas heaters can win as higher capacity is better able to outrun the losses.
 
PoolStored said:
Are you saying that you have both a heat pump and gas heater connected to your pool?
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Yes. I have a heat pump and gas heater, and I have a pool and spa. Most of the time the gas heater is spa only, but when it's Christmas and the extended family is around, it's fun to swim sometimes. ;). However, I primarily have the heat pump for extending the usage of the pool. Now is a good example, on days when it's sunny and 70's it would be fun to swim; except the water temp is still in the 60s. So the overanalytical side of me wanted to know, when heating a pool from 64 to 84 with average air temperature of 60 degrees (highs in 70s, lows in 50s), at which point does the cost of the heat pump intersect with the cost of the gas heater. e.g. is 12 hrs of the heater the same as 12 days of the heat pump? At which point I've "lost" 12 days of swim time.

I know that maintaining the temperature is most cost effective with the heat pump - so I'm not worried or questioning that. I know the gas heater will heat the water quicker. I'm trying to determine when the cost/times curves converge.
flynwill said:
it is also important to look at the spec sheets closely as the COP (just like the heating capacity) is strongly dependent on the temperature difference between the water and the air that you are trying to pull the heat from.
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Yes, and this is a little maddening as the primarily advertised COPs seem to be based on 80/80/80 (air, water, humidity). For my own heat pump (UltraTemp 140) the ratings are quite different. On this site, they show at 80/80/80 it's 140,000/5.6 but at 50/63/80 it's 85,000/4.0, so of course that will impact the calculations.

flynwill said:
"How long will it take to warm up my pool" you have to consider the amount of heat being lost to the environment if the pool is warmer than the surrounding air and earth.
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Yes. I see a lot of heat pump calculators have an option for "pool cover", which I have. It does make a difference, which is good. I'm less concerned with exactly how long it will take vs. being able to model the overall time and cost to heat to a given temp.
 
@wgillette Just went through my pool files and I did the same search last spring. Here is the calculator I have. Might help. (file in post at this link).

Natural Gas Heating Time & Cost Calculator Spreadsheet

There have been several recent posts inquiring how long it would take or how much it would cost to heat a pool / spa using a natural gas heater. In response, I have taken on the challenge of putting together this spreadsheet to help answer those questions. It’s now ready to be test driven...
www.troublefreepool.com www.troublefreepool.com
 
You can probably get a pretty accurate model of the heat loss by heating the pool and then observing the 24h drop in temperature. Convert the loss to BTUs and then divide by the difference between the average pool temperature and the average air temperature in the period. You have a cover so that number should be pretty consistent over a range of temperature differences. (If you didn't have a cover then the evaporation cooling would vary wildly depending on wind and relative humidity).

With that you can then model the cost of the Heat Pump and the Gas Heater at a number of different cases of difference between water and air temp.

You might even find that there are cases where it makes sense to run both together (if that is an option in your plumbing setup).
 
PoolStored said:
Just went through my pool files and I did the same search last spring. Here is the calculator I have. Might help.
Click to expand...
Thanks for sharing! I was actually starting to build my own so it will be great to have something to compare to (I want to learn the math behind all of this) and possibly extend this to include heat pumps. I'm thinking it would be possible to build a calculator for a gas heater, a decent calculator for a heat pump and then layer the comparison on top of them.
flynwill said:
You might even find that there are cases where it makes sense to run both together (if that is an option in your plumbing setup).
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Oh, I wish that were possible. I thought it was, the Pentair Intellicenter Manual mentions hybrid mode, however that is only for a hybrid/combo gas/heat pump all-in-one they have. It seems like there is an ability for the Intellicenter to switch between the heat pump and gas heater, but there is not a lot of documentation on how it does this. It looks to only be one way - the heat pump will hand off to the gas heater if the air temperature gets too cold, but it doesn't switch back.
 

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PoolStored said:
The pentair calculator (2nd on the list in the search) does a reasonable job and allows you to input MOST of your needs (maybe not current air temp, but their average usage for the season for sure). I think it does a good job, and makes a good assumption about the efficiency of gas, average temperatures and allow you to adjust cost/kWh and cost/therm. Heat Pump Calculator
Click to expand...
Is this calculator working for you? For me it gets stuck here:
1680366842192.png
I've used it before and it's worked. I've tried a couple of browsers but can't seem to get it to work on any of them.
 
wgillette said:
Thanks for sharing!
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No worries. I tried to do the same thing when I opened last year. That is how I found the spreadsheet last year. How much was it going to cost me to run the heater. It was fun, but the conclusion I came to was that it didn't matter. I wanted to swim with the grandkids. It cost a total of $1300 to heat the pool, May 15 to Oct 29. I just turn it on.

Yes, the pentair calc works for me. It is a nice one and you can adjust both your electric and gas costs and efficiency assumptions on the last page. Try clearing your browser cache.
 
If you go after the heat pump side, note that your efficiency will vary with air temp and RH.


1680362617638-png.479761
 
I forgot that @mas985 has a number of spreadsheets including this one, which does the comparison you might like...

docs.google.com

Pool Heat Transfer Tools v024.xlsx

docs.google.com docs.google.com

Here is the thread with all his spreadsheets:

Hydraulics 101 - Have you lost your head?

Edit - Before reading this article, you may want to first familiarize yourself with this Pool School article on Pump Basics Hydraulics 101 - Pump Head Curves, Plumbing Head Curves and Operating Points Hydraulic Head In fluid dynamics, head is a concept that relates the energy in an...
www.troublefreepool.com www.troublefreepool.com
 
PoolStored said:
It was fun, but the conclusion I came to was that it didn't matter.
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I get that, I'm of a similar mindset - it's why I built the pool with both heaters. I wanted to be able to heat it and use it and not really worry about it. I think it's because it's my first spring with the pool that I'm the most interested in it, and I'd like to avoid some crazy surprise bill at the end of a month since it can all be calculated.
PoolStored said:
Try clearing your browser cache.
Click to expand...
I tried that and even tried an InPrivate browser thinking it was a browser extension blocking it. I think it's a privacy setting somewhere that's interfering with it. Annoying as it's worked before. Oh well, I'll figure it out.
PoolStored said:
If you go after the heat pump side, note that your efficiency will vary with air temp and RH.
Click to expand...
Yep - where did you find that table? I was looking for that for my Pentair equipement. I followed the link you posted to the brochure but didn't see that table.
PoolStored said:
I forgot that @mas985 has a number of spreadsheets including this one, which does the comparison you might like...
Click to expand...
Oh, this is another good set of spreadsheets. The heater one looks pretty sophisticated.
 
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@wgillette There is another thread going on right now that you might find interesting...

Heat Pump vs. Natural gas heater Cost Efficiency

I have a Pentair Ultratemp 140 Heat pump (140,000 BTU, 230v @ 50amps) and a Pentair 400,000 BTU natural gas hearer (4 Therms per hour). My current cost for electricity (just electricity-no other fees) is $ 0.145640 per kWh. My cost for natural gas is $1.296980 (just gas - no other fees) per...
www.troublefreepool.com www.troublefreepool.com
 
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