Pool Heating Math

Hello,

<Deep Breath>...

Thanks in part to this cooler Summer I have finally convinced my Wife we need a heater. I've read, "There is no such thing as too big of a heater." I think this might not be true. Someone please correct my newbie math:

Questions, first, explicit details follow:

1. Is my math even close to correct?
2. If we turn the heater on when the intake water is 82 degrees will the water returning to the pool be 98 and could that burn someone standing near a return?
3. Last question is about efficiency. I believe 1 gallon of propane is about 100,000 BTUs. Does this mean the heater will use 4 gallons per hour of operation?

Pool information:

Viking Gulf Shores model fiberglass outdoor IG pool built so that long sides are approximately North and South. House is less than 10 feet to East and timberline is up slight hill no more than 50 feet to the East. Climate is St. Louis, Missouri USA.

Officially the Gulf Shores is a 19,000 gallon pool, but we often fill it 1-2 inches above the ideal depth for skimmer effectiveness. Estimate 20,000 gallons.

For thermal efficiency guesses: Proposed heater is 400,000 BTU propane installed outside patio fence approximately 50 pipe feet from where water will enter the pool up to 4 underwater returns. All pipes will be underground between heater to pool.

Pump is Hayward 1.5 HP IG Superpump with 1.5 inch plumbing. Filter is 72 ft^2 Hayward DE. Pressure is usually 11-20 PSI depending on return and intake routing, if pressure gauge is accurate. I do not have a flow meter installed currently, but expect to have one added as part of this upgrade. The pump is rated at up to 70 GPM, but I expect I am circulating about 50 GPM.

The pool will be covered with a 12 mil solar "blue top/black bottom" cover except when in-use.

Now for the math and assumptions:

1. A heater advertised as 400,000 BTU introduces at most 400,000 BTUs of heat per hour not including thermal losses.
2. If 50 gallons flows through the heat exchanger per minute that is 3,000 gallons per hour.
3. 400,000 BTUs per hour into 3,000 gallons (25,000 pounds) of water is about a 16 degree rise at the returns which will mix with the other 17,000 gallons of water.

Hope I give you enough information to answer my questions.

Thank you in advance!

-Bill

Obvious problems:
1) You're not just heating the water, you're heating the walls and the earth around the walls. I don't know how far the heat will penetrate, but 6" to a foot is reasonable. So how many cubic feet of dirt and fiberglass do you have to heat?
2) The BTU content of propane is based on a 100% conversion efficiency. Never gonna get that. I don't know what the maximum efficiency is, but it will be lower. You'll use a lot more propane than you figure.
3) Even if insulated, 50' from the heater to the pool will lose a fair number of BTUS. Gotta figure that, too.
4) Since 3000 gallons is about 15% of 20,000 gallons, your ESTIMATED 16 degrees for the 3000 gallons comes down to about a 2.5 degree rise/hour for the entire 20,000 gallons--my guess.

I don't guarantee my numbers by any means, but.....they are worth thinking about.

Carl,

Thank you for the reply!

If I do not use propane then I have to use a heat pump, but the largest heat pump I can find is less than 200,000 BTUs at top efficiency and conditions.

My decision is to spend 2X as much for the heater, have a lot more parts to break, but hopefully make back savings on energy with a heat pump - if electric rates do not rise over the life of the heater. I used to own a diesel car thinking that form of energy would be cheaper and paying more for the diesel engine option was a good idea. Ha... wrong!

The pool installer claims he has plenty of unhappy customers with heat pumps because in St. Louis, MO USA there are many days when environmental conditions are not right for a heat pump to work well. I think it has to be above 50 degree or maybe even warmer.

All comments appreciated. Thank you!

You last point is key and requires an answer to the question: How cold outside are you looking to overcome and actually swim in? For me, in NJ, there's NO WAY I want to go swimming in my pool when it's below 75, much less when it's below 50. For MY needs (and mine alone), I would never use a heat pump to heat my pool when it was in the 40's (that's what below 50 implies). I don't know where the "sweet spot" for heat pumps is, but I don't think it's there.

If, OTOH, you are looking to keep your pool evenly warm and maybe extend your swim season at both the front and back end, that's a different decision tree. I use solar and it allows us to swim, in NJ, usually from early May till late September, rather than than traditional Memorial Day to Labor Day season. For that, a heat pump should EASILY cost far less to run than a gas heater. But if you want to heat and swim in your pool in the 40's and below, then a natural gas heater is probably the cheapest to run, followed by a propane heater.