On our central heat unit the pilot is lit, but when we turn the thermostat up the unit will not come on. The fan blows when its at the on position. Please help. It's cold

If the gas doesn't come on at all, either the gas valve isn't getting the signal from the thermostat, or it is bad. Check for 24 volts AC across the terminals of the gas valve. If you have power, give it a good thwack, and see if it come on. Even if it does, you still should plan on replacing it soon.

If you don't have power, try jumpering the R and W terminal where the wires from the thermostat connect. Also, make sure there is a wire running from the other terminal on the gas valve to the transformer.

We have had are central heat off for months now its time to turn back on and three of the two pilots are working is that bad