A few more details would help. Model # ? is it an insert or direct vent. ? Thermostat or switch ? Main valve is dropping out and pilot remains lit than I would lean towars wiring or gas valve. How old is the unit and when was it last serviced by a pro. ??
:gah:

 

I have seen this before, I had to replace the thermopile again!

 

check all wire connections, reconnect all spade connectors, if you have a "light switch" on the wall for turning the unit on and off. you may wanna replace that as well

 

it may have a O2 sensor cutting it off. check to make sure it is getting enough air for combustion

 

Deon, O2 sensors are "oxygen depletion sensors" which are special safety pilots used on ventfree gas appliances. The only ventfree appliances Heat&Glo makes are electric.

 

Have you checked for a vent safety switch?

 

Gas Pressure:

Have you checked the gas line pressure to the gas valve? If it started after the gas meter was turned off, it could be a bad gas regulator in the gas meter.

It could ne any of the things mentioned but you should make sure that you have adequate gas pressure. Is it Nat. Gas or LP?

Its unusual that the burner stops but the pilot stays lit. If the thermocouple senses no flame, it shuts down everything. Its not supposed to leave itself burning.