I have a 27' above ground pool that is about 17,000 gallons. The wife found a used Hayward H100ID on Craigslist for $250 and it seems like a good price but it seems small for my pool. I live in Northwest IN about 30 minutes south of Chicago and we use a solar blanket and right now the temp of the water in the morning is in the low 70s and she would like to be in the mid to high 80s. I would like to do solar but I don't have a good spot except for the roof which is two stories up and I don't think the HOA would approve. What is the minim size heater that would be good for my size pool?