pool heater ?

hello, my teledyne-lars 250,000 btu pool heater is acting up i think. when i first got this heater it seemed like if i turned it to full hot ,it would almost put the water in the 90's, not where i wanted it,but i then knew where to set switch . now when i turn it full blast ,it cycles on and off through the night and after running for 12 hours last night it went from 70 to 76 degrees. i think it should have gotten warmer than this . do you agree? could i have a bad limit switch or mayb the rehostat swithch is bad? i have a good troubleshooting chart from the manufacturer and am an electrician so i don't think testing things will be too hard. i was just wondering if anyone experienced this problem? thanks for any info Randy

How big is your pool? A 250K BTU/Hr heater (assuming no heat loss to the air) should be able to raise about 60,000 gallons by 6 degrees in 12 hours, but if the air is cold and there is no cover, it could be a lot less than that.