I ran into the same situation--no heat on Christmas eve when I first turned the thing on. You must have the electrical ignition. Make sure that your gas valve is turned on--also make sure that the side panel of the heater is on tightly--there is a small metal button that is depressed by this--it needs to be depressed in order for the thermostat to work. Also, make sure any fuses are working (you can buy new ones for $1.5 at a hardware store). Good luck. I ended up going through all of this, and a PG&E guy told me I'd need to find a Lennox heater specialist.