We had our pool installed about 6 years ago. We decided at the time to install solar panels which have worked awesome. We figure they have definitely paid for themselves and then some. At the time we built our pool we had oil heating for our home. We have since converted to NG. With that being said, we are considering adding a natural gas heater to our pool. The *only* reason being is that towards the middle part of August, the sun is fairly low on the horizon, even though our backyard gets full sun from 9-sunset, the sun is just not as strong. We usually end up losing about 5 degrees overnight and regain only about 3 degrees. There was one August it rained a cold rain for one week straight and we never regained a pool temp above 78-80. Our entire August had been lost. Yes, 80 is too cold for us...spoiled, I know.
So my question is, how big of a heater/# of btu's do we need? We are only looking to supplement the solar so at the most we would need to raise our pool by 15 degrees in September. People have told us 400k/btu's, but we are trying to figure out if that would be if it was our only source of pool heat and is overkill for this situation.
If I calculate the surface area of our pool, which is a grecian-lazy-l, it comes to 680sf ((18+24)*.45) * 36. The depth averages 5.5 feet.
The one other problem is the piping on the equipment pad will need to be reconfigured and of course most pool companies won't do the reconfiguration unless we buy the heater through them, which is an extra $1k over an internet supplier. Will need to try to find someone to do it on the side.
One last question which I know will vary is brand. My sister in SanDiego has a Pentair MinMax, my sister in Arkansas has a Jandy(don't know model) and we were looking @ a Hayward H-400, but the reviews on Amazon for Hayward say to stay away.
I can do further research on the brand, but my main question is how big of a heater do we actually need to suit our purpose.
Thanks for your time.