furnace-no ignition

[pitteach]

I have a Lennox G26Q3-75-1 that will not ignite. With a call for heat, the combustion blower runs contiuously and that's it. The igniter does not come on and I suspect that the gas valve is not opening. The blower motor operates on the fan only mode from the thermostat. The honeywell ignition system update has been installed already. Any ideas?

Thanks

[Ian Gills]

The transformer may have gone or a fuse on the board might have blown.

Make sure you disconnect the power before you go in there.

[jadnashua]

Closed combustion burners have a pressure switch that ensures the air supply and vent are working properly. There's usually a vacuum hose going to the switch. If it has cracked or come loose, the switch won't open, and the burner won't fire. There are other reasons why it might not work - blocked vents or flue, or cracks. The diagram on the device will often have a schematic of the safety interlocks. You can usually figure out where the interlock is open, and isolate it to the appropriate control. If it actually tries to fire and doesn't detect flame, the flame sensor may be dirty or shot. Sometimes, there's a flashing light on the control board that tells you something if you have the manual.

[pitteach]

It was the pressure switch. All is well. Thanks for the responses.