I have a 1990 model Coleman/Evcon 3500a811 furnace. It won't always come on when triggered by thermostat. We have changed thermostat and replaced disconnect. Need to know what else could be causing problem. Please help.

In units that old, it triggers the main gas valve. The valve opens, ignites from the pilot and then the fan relay comes on at a set temperature. Do you have a pilot light lit?

In units that old, it triggers the main gas valve. The valve opens, ignites from the pilot and then the fan relay comes on at a set temperature. Do you have a pilot light lit?

I'm sorry I forgot to specify that my furnace is electric. But thanks for your rapid response. Can you help?

In an electric furnace there is a series of relays. 24volts from the transformer, through the thermostat, & to the coil of one or more relay(s). When the relay closes power is applied to the heating element(s) & fan. Without a wiring diagram or being able to actually see the furnace controls, I can't begin to guess the configuration of your particular furnace.

Grady is spot on. I will just add that many times the relays on electric units will be called sequencers on the wiring diagram. Odds are you have a bad sequencer/s.