We inherited a Hayward H250 natural gas heater with our pool and this season is the first time we're attempting to use it (first season with our pool as well). We've got a vinyl in-ground, about 18500 gallons. No solar cover. We ran the pump 24/7 while heating.
We live in Toronto, Ontario and temperature ranged from 60 degrees at night to 77 during the day (ignoring humidity effects) the past couple days.
I started heating the pool 8pm Friday and the water measured about 71 degrees using a floating digital thermometer that measures down about 4 inches. Fast forward 36 hours to Sunday 8am and the pool water now measures 82 degrees.
That's 11 degrees in 36 hours. Seems way too low from what I've been reading.
The heater does give some trouble starting. It's not the push button type, it fires electronically but it takes a few attempts to get going. Once it's going however it stays running. I don't think there are any adjustments I can make on the heater to vary it's output - it's either on or off if I understand it correctly.
There is a bypass on the heater which is how I run it when not heating. When the water flows through the heater there is only a 1 PSI increase in the pressure so it doesn't look like the heat exchanger is clogged or anything.