Make sure you have a nice blue flame in burner the more yellow that it is the less heat it put out. To adjust the flame there are air control slides on the burners there the flame comes out. Open or close them to get the proper setting. You will see the flame go yellow and blue, the bluer the better. The gas pressure could be off which you have to check with a manometer 3 lbs per water column on natural gas and 11 lbs. on l.p. gas. Check the gas valve inlet screen for dirt. If the burner system is operating properly then you have a bad or dirty t-stat
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Make sure you have a nice blue flame in burner the more yellow that it is the less heat it put out. To adjust the flame there are air control slides on the burners there the flame comes out. Open or close them to get the proper setting. You will see the flame go yellow and blue, the bluer the better. The gas pressure could be off which you have to check with a manometer 3 lbs per water column on natural gas and 11 lbs. on l.p. gas. Check the gas valve inlet screen for dirt. If the burner system is operating properly then you have a bad or dirty t-stat