How much propane does a Pentair Mastertemp 400 heater consume while Running?

[Steve-D]

We had 2 120 gallon propane tanks installed for our (new this year) pool and a backup generator. How fast does the heater (Pentair Mastertemp 400) burn through that? We didn't use the heater all that much this past season and when we tried to run it for one last swim the heater would turn on (blower pushing cold air from the exhaust vent), you could hear it ignite and feel hot exhaust, but then it would start blowing cooler until the exhaust was cool air with the faintest hint of the smell of propane.

Are my tanks empty or too low to provide enough pressure to keep the heater running? When we had the tanks installed and filled I didn't think to ask how to read the gauges. This is what I see:

 

[mknauss]

One gallon of propane has 91,502 BTU. So in round numbers, your heater will consume just over 4 gallons of propane per hour of operation.

I am not sure on the gauges.

 

[DrewLG]

Are my tanks empty or too low to provide enough pressure to keep the heater running? When we had the tanks installed and filled I didn't think to ask how to read the gauges.

The gauges show volume of propane as a percentage of a full tank. There's a pointer that I can't see in your photos -- maybe because it's pointing to 0 and is hidden beneath the black remote-sensor case? Here's what it looks like when the pointer's visible (this one is pointing to 64%):



The gauge maximum is 80% because tanks are never filled above 80% -- the 20% empty space is to allow for expansion on hot days. So when your tanks are "full" they contain around 96 gallons each. If the tanks ever get completely empty (which may have happened in your case), safety codes require that the whole system must be inspected before they can be filled again, so you should try not to let them get below 10% full.

When your tanks are 10% full, they contain 12 gallons each. So you can use 96 - 12 = 84 gallons from each tank before you have to refill them. That's 84 x 2 = 168 gallons total from both tanks.

If the information in your signature is correct, your heater needs 2.2 gallons of propane to raise the temperature of your pool by one degree Fahrenheit. So 168 gallons is enough to raise the temperature of your pool 76 degrees. Which means you can raise the temperature 7 times from 70 to 80 degrees, or 4 or 5 times from 70 to 85, or 2 or 3 times from 60 to 90. After that, you'll need to refill your tanks.

 

[JamesW]

the 20% empty space is to allow for expansion on hot days.

It's not really for expansion, it's because you need vapor/gas that can be drawn off.

The lowest should be about 20% which is necessary because you need a certain amount of wetted wall surface area to boil the propane.

As the gas is drawn off, the liquid boils and produces more gas.

 

[DrewLG]

It's not really for expansion, it's because you need vapor/gas that can be drawn off.

Oh, right. Of course.

The "expansion" explanation never really made sense to me, but I admit I just accepted it. Thanks for the correction.