[alanmav]

My gas heater fires up the burners ok but the water still comes out the same temp as it went in. Is there some kind of thermostat, like a car has, that is closed? How about a bypass valve?

 

[SKIP4661]

You may want to have it checked by a pro. The burners may not be firing at full capacity.

 

[sninny]

there may only be a 2 or 3 degree temp rise between in and out. Pool heaters have internal bypasses built in to keep the temp rise down to prevent condinsation from developing. Then of course you may have a real problem such as calcium build up it the tubes of the heat exchanger.