How much will it cost to heat my pool for this weekend?

[wayner]

I live in Toronto. I turned my heater off last weekend. The water temperature is now around 68F. This coming weekend is a long weekend as it is Canadian Thanksgiving and the weather looks good at 20C and higher. I would like to be able to go swimming this weekend. How much will it cost me to heat my pool?

My heater is a Raypak natural gas (I think around 260,000 BTU or about 260,000 kJ) and I pay $0.12/m^3 of gas (including taxes). How would I calculate, theoretically, how much gas (in m^3) I will burn to raise my pool temperature to 84F? My pool holds approximately 100,000L (or 100m^3) of water. So how much will it cost to increase 10m^2 of water by 16F or 9C?

Can I suggest that a calculator like this be added to PoolMath.

 

[JoyfulNoise]

I’ll assume your 100,000 pool volume is in liters which is roughly 26500 gallons.

You need about 3.5 million BTUs to raise your pool water 16F. 1 cubic foot of natural gas has approximately 1,000 BTUs of energy. If your heater is 80% efficient (that’s an assumption), then you’ll need about 4,000 cubic feet of natural gas. Your heater (and pool pump) will need to run 16 hours to produce that much heat energy. Unfortunately you will lose A LOT of heat if the air temps are very low at night. So you’ll really need to use a pool cover to keep the evaporative heat losses to a minimum.

I’ll let you do the conversion to metric units

 

[phonedave]

One cubic meter of natural gas is about 35,000 BTU give or take

So, with a 100% efficient heater and no losses to the air, you would need 3,500,000 / 3,5000 = 1,000 cubic meters. That is $120. Now take into account all of the losses to the air and the efficiency of your heater and you are looking at $250 or so.

As a point of reference, my friend used to have a in-ground vinyl pool almost the same size as yours. His was in the shade and he had a high ground water level (which helps to pull heat out of the pool). He said, toward the end of the season, if he wanted to get the pool up to temp for a party, it would cost him about $200 in propane.

 

[wayner]

(This is in reply to JoyfulNoise's response)Thanks. 4000 ft^3=113m^3. That will cost me about $13.50, without accounting for additional heat losses. I do not have a solar cover, or other cover other than the winter cover.

But that is easily worth it for me. If it cost around $100 then it would not have been worth it. Especially when you amortize the original capex of the pool and surrounding area, which was well into six figures, over the amount of swimming days that you will get over 20-25 year lifespan of the pool

 

[JoyfulNoise]

(This is in reply to JoyfulNoise's response)Thanks. 4000 ft^3=113m^3. That will cost me about $13.50, without accounting for additional heat losses. I do not have a solar cover, or other cover other than the winter cover.

But that is easily worth it for me. If it cost around $100 then it would not have been worth it. Especially when you amortize the original capex of the pool and surrounding area, which was well into six figures, over the amount of swimming days that you will get over 20-25 year lifespan of the pool

Without a cover during the night, your pool will likely lose all the heat added during the day. So it’s going to cost way more than $13.50. That was partially my point, you have to cover the pool.

A pool can easily lose 6-8°F overnight from evaporative cooling.

 

[JoyfulNoise]

One cubic meter of natural gas is about 35,000 BTU give or take

So, with a 100% efficient heater and no losses to the air, you would need 3,500,000 / 3,5000 = 1,000 cubic meters. That is $120. Now take into account all of the losses to the air and the efficiency of your heater and you are looking at $250 or so.

As a point of reference, my friend used to have a in-ground vinyl pool almost the same size as yours. His was in the shade and he had a high ground water level (which helps to pull heat out of the pool). He said, toward the end of the season, if he wanted to get the pool up to temp for a party, it would cost him about $200 in propane.

Your division is off by a factor of 10 … 3,500,000/35,000 is 100.

 

[Halzg]

Another way to do it is note your gas meter reading and pool water temp, then turn on the heater. After one hour, check the meter and temp again. That will give you a m^3 per hour burn rate and temp increase. From there you can calculate how many hours you need to run the heater to get the temp you want. Hours * m^3 per hour gives you total gas volume, * your rate =total cost. However that still doesn’t account for evaporative and conductive losses.

 

[wayner]

I had a solar cover on my parents' pool growing up about 40 years ago. But I didn't like two things about the cover (1) it took up space on the deck and was unsightly, and (2) it was difficult to remove unless you had two people so the cover acted as a deterrent to using the pool. When I had my pool built 15 years ago I did not get a cover. The irregular shape would make it difficult to have a cover.

If I were to do it over again I might get a rectangular shaped pool and put in a cover with an automated roller system. How much do those things cost? I imagine it is a tradeoff between the capex for that system and the opex of burning more natural gas.

This is what my pool looks like.

 

[wireform]

Auto covers are in the area of $18k the last I saw.

 

[wayner]

$18,000 buys a ton of natural gas.

Auto covers are in the area of $18k the last I saw.

Then they don't make economic sense for me. My gas bill for June-Sep seems to average around $1000 total for the four month period when I am heating my pool. If I assume 80% of that is for the pool (I have gas water heaters, cooktop and BBQs that would be used during the summer) then I am spending about $800/yr to heat the pool.

 

[wireform]

It also keeps the pool much cleaner by not letting all the tree foliage from spring through the fall, out of the pool. You're never losing chems from rain pump off. It's definitely a luxury but keeps pool maintenance to the bare minimum.

 

[wayner]

I am luck that I don't have that many trees nearby that shed debris. I can generally empty my skimmer once per week and I am good. I guess I would be able to turn down my SWCG as I would lose much less chlorine. But I don't typically have to add any chemicals other than that - maybe acid twice per year.

 

[Deb04]

I’ve never done the energy calc, but I have heated my pool from 68 to 84 late in season for a nice weekend when daughter and family were coming over. Took a whole tank, which costs about $250. Totally worth it to hear my granddaughtEr laughing and playing.

 

[derekm]

In PEI. Same size pool and Raypak heater (but propane, so fuel costs are approximately double). I’ve been using my pool daily, but colder weather here this weekend and frost warning tonight may end it. I’ve been losing 5 degrees F each night WITH a solar cover (rectangular pool, manual roller). I would not attempt it without. I heat to 79 or 80 F. 84 would be a lot more $$$. If I were doing it I would pick ONE weekend day and heat for it. Overnight losses may preclude a second day being viable.

 

[Pool_Medic]

For extended use a thermal heat blanket is the route to go, several clients have them and it’s crazy to roll them off and see the steam rising.

 

[wayner]

In PEI. Same size pool and Raypak heater (but propane, so fuel costs are approximately double). I’ve been using my pool daily, but colder weather here this weekend and frost warning tonight may end it. I’ve been losing 5 degrees F each night WITH a solar cover (rectangular pool, manual roller). I would not attempt it without. I heat to 79 or 80 F. 84 would be a lot more $$$. If I were doing it I would pick ONE weekend day and heat for it. Overnight losses may preclude a second day being viable.

But heating for a second day will be much cheaper as you just have to maintain losses. I will heat up for Sunday and Monday as temps will be 20 and 22.

 

[phonedave]

Your division is off by a factor of 10 … 3,500,000/35,000 is 100.

Ah, I put the comma in the wrong place when I typed it and that threw me.

$12 seems awfully low though.

 

[JoyfulNoise]

Ah, I put the comma in the wrong place when I typed it and that threw me.

$12 seems awfully low though.

It’s going to be way more than $12 if/when thermal losses are accounted for.

 

[derekm]

But heating for a second day will be much cheaper as you just have to maintain losses. I will heat up for Sunday and Monday as temps will be 20 and 22.

Depends on your overnite loss of course. Mine would be MUCH lower with solar cover.

 

[Halzg]

Calculating the losses is a pretty complex thermodynamic problem. I try to simplify my heater usage by determining that it costs about $4 an hour to run my heater, and if the air temp is in the 70s F, I will gain about 2^F per hour. There will come a point, where the air temp is cool enough, without a cover you won’t be able to raise the water temp. You will simply be “boiling” the water into the atmosphere.