I have a day and night heater not sure how old, the recent problem is that the heater will work fine for a while then it will turn on and no heat. The thermostat must be working because it turns the heater on,just no producing the heat. I seem to think it might be the gas valve is not opening letting the gas go to the burners. The pilot light stays on though and the fan blows when the thermostat turns on the heater.

I have an Amana rooftop HVAC made in 1990. The ignitor lites but the heater
Will not fire.I am not getting a reading at the 24vac to the gas valve. There
Has to be a 24vac transformer in the unit somewhere, but I cannot find it.