Pilot is lit but the heater won't fire up

I had this ame thing happen and it was easy to fix.. I backwashed the filter.. I was told if the filter or pump is plugged the heater wont fire up . Its a protective device to keep the heater from burning up with no water running thru it.. I see your post was back in Sept . maybe you dont need this answer anymore..

A few things dictate when the main burner turns on. One is a thermocouple not getting hot enough, the other is a bad thermostat ot thermostat wire, and another is the gas valve. First look at the gas valve located on the heater and make sure it is in the On position and not in the Pilot position. Remove your thermostat and jump the two wires together. The heater should light. If not, go back to the gas valve and find the wires up to the thermostat, the should be attached to terminals marked TH and MV or something similar. Jump the terminals. The heater should start. If not, you may need to replace the thermocouple.