Given:
How much propane does it take to increase the spa’s temperature by 1 degree? Or to increase the temperature of the spa from 60 to 100 degrees?
Extra credit:
On my current contract, propane is $2.04/therm. How much does it cost to heat the spa from 60 degrees to 100 degrees, and maintain that for one hour ?
I guess nobody is interested in this question, but I’ll carry on with some guesses:
A gallon of propane is about 1 therm or 100,000 BTUs. My Jandy manual says it heats 250,000 BTU/hr.
Does this mean it takes 2.5 gallons of propane to run the heater for 1 hour? Or does this ignore inefficiencies that need to be taken into account?
600 gallons of water weighs 5,007 lbs. It takes 1 BTU to raise the temperature of 1 lb of water 1 degree. So it takes 5,007 BTU’s to raise the temperature 1 degree. To raise the temperature 40 degree (60 to 100) is 200,280 BTU’s.
Your heater is rated at 250,000 BTU/hr. It is not clear if this is input or output. If it’s output, it will heat the water in about 48 minutes. If it is input and assuming 80% efficiency, it will take an hour. This is neglecting heat losses.
Propane has a heat content of 91,547 Btu/gallon (liquid). So it will take about 2.75 gallons to heat the spa from 60 to 100 degrees, assuming an 80% efficient heater.
How much it takes to maintain it at that temperature depends on how much heat is lost to the surroundings. I’m not able to answer that.
Thank you! I didn’t realize a BTU was tied to water weight, that’s much clearer.
My Jandy manual lists the BTU’s as the “firing rate” so I assume that’s output, not input.
I think firing rate is input. You might want to check the nameplate on the heater, it should list the input BTUH. In any event, 2.75 gallons should be the worst case. If the heater is more efficient, then you would use less fuel.
I’m curious: is 80% a reasonable assumption for heater efficiency? I would have thought that they’d be nearly 100% efficient. Efficiency losses from most machines are in the form of waste heat, but there’s no waste heat for a heater. Where does the waste go? Incomplete combustion?
There is waste heat. It goes up the flue. Your standard gravity vent gas appliance depends on the heat of the flue gas to exhaust the products of combustion.
Higher efficiency can be realized by designing a heat exchanger/fan that will extract more heat from the flue gas. Of course, the flue gas will be cooler and won’t vent properly. In order to get rid of the flue gas, mechanical means are used (fans) on the combustion side. In addition, if the flue gas cools below a certain point water will condense and run down the flue pipe back to the heater. This water is usually a bit on the corrosive side, so the heater must be designed to handle this, in addition to requiring a condensate pan, drain, and perhaps a pump. High efficiency heaters (93-95%, I think) use PVC pipe as a flue, because the gas is cool enough and probably for corrosion resistance.
Electrical heaters, on the other hand, are 100% efficient. (I’m pretty sure about that, but I could be wrong).