wanting some opinions


New member
Apr 25, 2017
lynchburg, va
I have a pool, roughly 35000 gallons, and I was thinking about putting a spa heater on it. I am not trying to heat it to 92 degrees, rather than just grab a 5 degree increase to get it over the 79 to 80 degree threshold that takes forever due to not a lot of hours in direct sunlight daily. and I am not planning on heating around the clock. just when the pool will be in use. other words I am not wanting to sink a lot of money in a heater.


TFP Expert
Apr 29, 2016
Quaker Hill, CT
By spa heater do you mean an electric coil heater in the 5 or 11kw power range? While it might not cost a lot to buy it will cost a ton to run and won't make a dent in the temp of your pool.

With a pool your size you need a proper pool heater like a 400kBTU gas heater or 110-140kBTU heat pump or large solar array.


Mod Squad
TFP Expert
Platinum Supporter
LifeTime Supporter
Aug 10, 2012
Welcome to TFP!

I have a small heat pump, 50k btu, and my pool is 20k gallons. It runs on a 20a, 240v breaker. It adds 1 degree every 3-1/2 hours or so. I bought it for the same reason you want one. I found myself needing 3, 5, 7 more degrees to be able to swim comfortably. I installed it last September and it allowed us to swim 3 out of 4 weekends in October and start swimming March 13th this year. We normally can't swim until mid April at the earliest. At the same time I got an 8mil, clear solar cover, since I'm paying for heat I didn't want to lose it all every night. Our pool was 90 degrees this afternoon.

More discussion about solar, heat pump and heating my pool in my pool thread, link in sig.


TFP Expert
LifeTime Supporter
Apr 27, 2012
Grand Rapids, MI
Heres some info onnNat Gas Heating for comparison:
To give you an idea of the energy volume it takes to quickly heat 35,000 gallons, which is no small feat:

1. Multiply the total gallons of water in the pool by 8.3 (pounds per gallon)
This will give you the total weight of the water
(35,000 gal x 8.3 = 290,500 lb)

2. So an 80% efficient 400k BTU heater would get you 320,000 btu of energy an hour (divided by your volume), a.k.a. 1.1 degree increase per hour - in ideal conditions with reasonably warm air temps...otherwise there are additional calculations

3. If your natural gas rate per CCF (close to a therm) in VA is like mine in Michigan, your hourly cost of NG for heat up on a 400 btu gas heater that's 80% efficient would be about $2.46/hr (if your rate is .77 per therm).

So, IMHO, even if you DO get the right size heater (eg a 400k btu ng heater) you are probably better running it up once then retaining/preserving heat, eg just Thermostatting to desired temp and keeping a good cover on at night, in day if you want cover off, using an evaporation barrier like Cover Free to reduce evaporation (which is 70% of heatloss) OR keeping a solar cover on during day also to get sun effect heating, etc. eg. Heat, then preserve heat at all costs ;)

Sometimes its cheaper to maintain heat 24/7 - especially in summer - than to constantly just "run up" when you want to use it.

For example, our gas costs on a 24,000 gallon pool were pretty much the same once we started Thermostatting at 88 degrees instead of running it up just before use...and the bonus was the pool was always the temp we wanted. Until this year we never covered it at all, but did use an evaporation barrier (Cover Free) that made a big difference.

On my 266 btu heater with 24k gallons, if I wanted a 5 degree increase daily, my daily cost would be about $7.50 or $225/ mo in ideal summer conditions. Thermostatting to constant 88 cost me under that in July and August, and about $100 more in June and September (and about $200 more - eg $420 in cold April/early May and October with lots of nights freezing.) With that said, Thermostatting doesn't always make sense....eg I now operate throughout winter in a done, using a cover, on 3 scheduled daily heat runs preserving temp at 95 degrees for physiotherapy exercises every morning.

Which is how I've come across the calculations and observations on energy use ;)