I have a Lennox G40DF-24A-045-07 furnace that blows, but does not heat. During operation it starts up and when it receives a signal to heat (as indicated by the LED codes) the draft inducer motor turns on (I can see it turning), but the igniter never comes on. After a couple minutes it stops and the LED codes indicate a pressure switch problem.

I have removed the pressure switch and checked. When a small amount of suction is applied I can hear the switch, and I also verified this by using a meter to do a resistance measurement. The resistance across the terminals is normally infinite, but when a small amount of suction is applied the resistance drops to 0 ohms.

I reinstalled the pressure switch and jumpered the contacts during the call for heat cycle (only momentarily for troubleshooting). With the jumper applied the igniter came on and the furnace fired up and began blowing hot air.

Based on this I would assume that the pressure switch is good, and that the draft inducer motor is good since it is turning on. Is there still a possibility that either one could be bad, and if so, is there anything else that can be done to check? I wanted to ask before I started taking things apart to look for blockages.

Thanks in advance for your help.

 

Try taking the tube of the inducer motor housing and make sure that the fitting is clear.

 

I have checked the tubing and made sure that's clear. I also checked the port on the inducer with a paper clip. It seemed to be clear since the paper clip went in a decent amount before hitting something hard (felt like metal). I didn't want to push to hard though and risk damaging something.

 

Welcome to the forums.

Make double sure the nipple on the inducer is clear. There is very little vacuum there and every bit needs to be supplied to the pressure switch. The pressure switches are highly reliable and are very rarely the cause of the problem.



Your manual.... http://www.hvacpartsshop.com/manuals/G40DF_Series.pdf

It's pretty hard to guess if the inducer is producing enough negative pressure. We use a manometer to actually check the level.

 

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The fact that you struck something suggests that your paper clip was too large. A small drill bit is often better for checking to BE SURE that the inducer port is clear, or to clean it.

The drill bit should typically not bottom out, but should go far enough in so that you ARE SURE that the port is clear. The only thing that might obstruct such a test is usually the inducer motor fan itself.

 

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A bad pressure switch is quite rare. So a pressure switch that fails to close almost always indicates a problem with the furnace ---usually a defect in the flow of combustion air into, through and out of the furnace. A wide variety of issues can cause such a problem.

You can pull the vent pipe off the furnace to see if that allows the pressure switch to close. A plugged or otherwise defective chimney can cause this problem.

Check to be sure that the hose to the pressure switch is in good condition and not plugged with water or debris.

If those easy checks don;t identify the problem, pull out the inducer motor and fan, which can usually be done pretty easily by removing some bolts which hold the inducer motor to the furnace. Check to see if the inducer motor fan is in good condition, and that the fan turns freely and comes up to speed when the motor turns on.

 

Thank you to everyone for their advice. I checked the port on the inducer with a drill bit and gave it a little extra push when it hit something and that cleared it out. The furnace works like a charm now. Guess I was just being to gentle.

 

Add cleaning this port to your list of things to do to prepare for winter. If you clean it out once/year, it's unlikely that this problem will prevent the furnace for operating reliably.

The way to make that easy to do is to find a nice spot inside the furnace compartment to leave the drill bit so that you can easily clean the port in the future.