# Thread: Is my heater/hr cost math correct?

1. ## Is my heater/hr cost math correct?

My last NM Gas bill last month was for \$158.42 and I used 220 therms.
If my math is correct, that brings my cost per therm to \$0.72.

Today, the gas company switched the meter and I ran my 400K Pentair heater for exactly an hour with nothing else in the house.
I see that I used about 3200 Cubic feet, which translates to 32 therms, according to this site: http://www.kylesconverter.com/energy...of-natural-gas

32 therms for one hour at cost of \$0.72 per therm, equals roughly \$23.

The pool heater ran for 5 hours to bring the brand new pool's water from 69° up to 80°, which is roughly 2° per hour.

Is my math correct that this 5 hour run will cost me roughly \$100 ?!?

2. ## Re: Is my heater/hr cost math correct?

A 400k BTU heater uses about 4 therms per hour. Maybe you are not reading the meter correctly.

3. ## Re: Is my heater/hr cost math correct?

Originally Posted by mas985
A 400k BTU heater uses about 4 therms per hour. Maybe you are not reading the meter correctly.
I hope you're right, but take a look at my starting point reading:

and one at my 1 hour mark:

I just looked again, end of the day after heater ran 4-5 hours, and I'm at ~10,700 cubic feet, if I'm looking at this correctly.

4. ## Re: Is my heater/hr cost math correct?

Each dial is per revolution. The last dial on the right is 1000 cu-ft per revolution or 100 cu-ft per number increment. So that is about 320 cu-ft which is 3.2 therms. But again, the heater should be using about 4 therms but it is possible the gas line is not big enough which can limit the burner rate.

5. ## Re: Is my heater/hr cost math correct?

Gotcha; thanks.
I thought it was like a tach in the vehicle, where the dial is a single digit indicator and the multiplier is displayed above/below.

So today's run was not 100 bucks -- it was only \$10. That's great news and a reason to run the heater to get the pool up to temp.

Speaking of which, what's a good temperature to keep the pool at for a bunch of kids playing around and good 'ol me swimming laps.
I must say that I'm usually on the cold side and am easily spotted in the crowd of t shirt wearing people as the one in a sweater.

6. ## Re: Is my heater/hr cost math correct?

There are no absolute guidelines for water temperature, as it involves a lot of personal preference. Still something in the 78 degree range is often good for people who are being very active. The less active you are, the warmer you will want the water. Air temperature also enters into it, as getting into a cool pool from cool air can be difficult for many people.

7. ## Re: Is my heater/hr cost math correct?

Kids will swim in anything no matter what the temp is of the water. Unless there is ice on it.

8. ## Re: Is my heater/hr cost math correct?

Originally Posted by ps0303
Kids will swim in anything no matter what the temp is of the water. Unless there is ice on it.
Paul, you've got that right.

My pool was set on Feb. 9th. The temperature during the day was in the 40s.

My daugher didn't get home until after 6PM. I'm guessing air temperature was in the 30s, with water (coming from 683' deep well) in the 50s.
You should have seen her mom trying to talk my 9 year old out of going in

9. ## Re: Is my heater/hr cost math correct?

Yes Mario, there is 100,000BTU in every therm. Your hearted is a 400K unit. (400,000 BTU). The max the unit will run thru it is 4 therms an hour.

10. ## Re: Is my heater/hr cost math correct?

Hi Mario.
If your heater is like mine, it's been adjusted for the higher elevation. From my Raypak 409 manual:
Reduce input 4% for each 1000 feet above sea level when installed above 2000 feet elevation
400K BTU * 0.88 = 352K BTU input

I think the heater efficiency was around 80-82%.

I get around a 1°F rise every 2 mins for my hot tub. I used to think that was crappy but after doing the math tonight - it's pretty close.

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