Choosing correct heater size

[Mario P]

How do you determine the proper size heater for swimming pool?
I was suppose to get the 350K BTU heater that's no longer available.
Pentair now offers 300K or 400K units.
The cost is only ~$65, but PB already delivered 300K unit.
I want to get some data on proper sizing of my 16'x40' FB pool, with all plumbing running with 2" PVC.
I have a hard time figuring out the proper size.
I know that any heater will eventually get my pool to temp, but what's the most economical?
Is the right direction to go with 300K unit and run the heater/pump longer, or do I blast the 400K unit and run it that much shorter?
Running gas is not a determining factor as we already have to redo the gas pressure to deliver either 300K or 400K to the pool.

 

[Richard320]

Heaters are BTU per hour. The size of the plumbing has little to do with it. (gas line excepted) A 400K BTU heater will raise the pool temperature 33% faster than a 300K BTU in a perfect world. In reality, it's probably faster than that. You lose a certain amount of heat to the air and the surrounding soil, and that's a function of time. Wait long enough, and the pool will cool down to the ambient temperature. So a pool that takes 8 hours to warm up will lose a lot more heat to the surroundings than a pool that takes 6 hours, everything else being the same. So it would likely be more like 9 hours versus 6 hours.

All that is my way of saying, I'd go bigger. My pool is considerably smaller than yours, and I have a 400K heater. It might be warmer where you live, and a cover will help a lot, but faster is still better. If you're hosting a pool party, you don't want to be waiting around for the thing to heat. The cost to heat will be roughly the same - a BTU is a BTU, whether it's 100 for an hour or 10 for 10 hours.

Estimating your pool at about 24000 gallons, that's roughly 192000 lbs of water. 192K/300K = 38 minutes per each degree rise. 400K lowers that to about 29 minutes. Say the water is a chilly 69 and you want it to a comfy 83. 9 hours running full tilt versus 6:45, excluding losses.

 

[Mario P]

Thank you Richard.
That's a lot of help.

I emailed my PB and he responded that he'll swap the 300K unit that was delivered for the 400K.

I really like my PB.