Lennox forced air furnace

Here is a little story and a tip.

This morning when I went to turn up my thermostat I heard the induced draft fan on my Lennox gas furnace start and then I went to check my E-mail and such. After about an hour I was feeling rather chilly so I went and looked at the thermostat.

Well, it was showing second-stage heat and a temperature of about 63 but the furnace blower was not running. I went out in the garage and sure enough the ID fan was running but there was no flame. I cycled the power and still no flame. I removed the blower compartment door and while holding in the safety switch I again cycled the main power and waited for the furnace to light.

The tattletale LEDs were showing normal operation yet the burners would not light. I was not getting any glow from the igniter either. I went in the house and dug out the installation and operation manual to try and troubleshoot. Finally I went back to the furnace and looked for a loose or cracked hose on the draft pressure switch.

Understand that this furnace is only about 1-1/2 years old and had been "serviced" by the installing dealer in May of this year and "everything was fine" according to the serviceman.

Anyway, the hose to the draft switch was okay so I did the ohmmeter check of the igniter and that also checked okay. I then connected a manometer to the draft hose from the ID fan and got no reading.

AHA! The problem must be with the draft fan or the connection to the draft proving switch. I removed the draft fan and a sheet metal screw fell out. Hmmm, must have fallen down the stack outlet during manufacture or else during the installation. I then checked the hose nipple for the draft switch and FOUND IT PLUGGED!

Now the Lennox instruction manual states that annual service includes removing the draft fan and also cleaning the heat exchanger, neither of these things were done by the man that "serviced" my furnace.

I cleared the plugged nipple and re-installed the fan and draft switch. I replaced the blower compartment door and turned on the power. The furnace went through its regular start-up cycle and everything was back to normal.

Note that this problem DID NOT SHOW with the diagnostic LEDs.

 

On the LED chart, does it say anything on like failed switch?

I know Trane has a chat show a fault code for pressure switch.

 

Yes, Jay, it does have codes for both the high and low pressure switches. The code I was getting was the normal operation with a heating demand initiated at thermostat.

That's what initially threw me. Once I determined the igniter resistance was within tolerance I knew it had to be a draft problem or other limit switch.