How do you determine the correct gas pool heater
BTU's for above ground pools?
The goal is to have a heater that has enough capacity to heat the
pool to your desired level in a reasonable amount of time.
Maintaining that temperature is a lot easier once the pool is up to
temp.
First, the capacity of most pool heaters are rated in BTU's or
British Thermal Units. One BTU is the amount of heat required to
raise the temperature of one pound of water, one degree F. And since
there are 8.33 gallons of water per pound, it takes 8.33 BTU's to
raise one gallon of water, one degree F.
Now let's calculate:
1. Determine the number of gallons in your pool (G).
2. Determine the amount in temperature that you want to raise the
pool temp (the easiest way to figure this is to use the air temp as
the minimum and the desired pool temp as the maximum) (Delta T).
3. Calculate the number of BTU's needed (BTU) per gallon of water
by multiplying step 2 x 8.33. Multiply this number times the gallons
of water in your pool (G). This is the number of BTU's to raise your
pool temp from the minimum to desired temp.
4. Determine how FAST you want to be able to go from the minimum
temp to the desired temp (in hours) (H).
5. Divide the total number of BTU's from step 3 by the hours in
step 4 to yield the BTU'S per hour that your heater will need to
deliver.
6. Multiply Step 5 by a error factor of 20% and add to step 5.
This is approximately the size of heater that you will need.
Example:
1. Assume pool volume, G = 10,000 gallons 2. Assume 80F desired
pool temp and 60F air temp. Delta T = 80  60 = 20F 3. BTU's per
gallon x Delta T: 8.33 x 20 = 166.6 BTU's per gallon. BTU/Gal x
Gallons (G): 166.6 x 10,000 gal = 1,666,000 Total BTU's. (Wow, seems
like a lot!) 4. Assume I want to be able to warm the pool in two
days of continuous operation = 48 hours. 5. Total BTUs / Hours:
1,666,000 / 48 = 34,708 BTU's per hour. 6. (BTU/Hr x 20%) + BTU/Hr:
34,708 x 0.20 = 6,941 BTU's 6,941 + 34,708 = 41,649 BTU's per hour.
Now, most pool heaters come in a round number of BTU's like
50,000, 100,000, 150,000, etc. You should select the closest size to
your needs (usually on the higher side is best).
Hence, for this example, a 50,000 BTU heater would be more than
adequate.
Obviously, you could also work these calculations backward to
determine what Delta T you could handle given a certain size heater
and pool size.
