11:05PM | 12/06/03
Member Since: 12/06/03
1 lifetime posts
My furnace won't fire up. I changed the thermostat with no results. When I shut the gas valve off, then reset it, it does light the pilot automatically but not turn the burners on. What am I overlooking? Could it be the thermo coupling?

plumber Tom

05:56AM | 12/08/03
Member Since: 05/10/03
801 lifetime posts
Check something simple 1st. A single pole switch is usually installed somewhere near the heater, but that's not always the case. Sometimes the switch can be located at the top of the basement stairs (emergency switch) Check the breaker or fuse. Alot of older single pole switches will not be labeled "on" and "off"

R Man

12:56PM | 12/10/03
Member Since: 12/09/03
7 lifetime posts
Just had a similar problem with my Lennox. It was the air sensor. There is a small hose from the blower to the round disk-shaped sensor. Cut power to the unit and pull off the hose at the blower. There is a nipple or stem protruding from the blower. Take a thin coat hanger and fashion it into a poker. Use it to push the clog out of the nipple.

If the nipple isn't clogged, pull off the hose completely and check it for obstructions. Also poke the nipple at the sensor. If that doesn't work, pull the combustion air duct off the top of the unit and make sure it's drafting. You should feel air turbulance at the duct opening. If it's not drafting, you probably have a bird's nest or leaves stuck in there.

If it's not the air sensor, then it may be something serious like the control module. Run the furnace and let it shut down of it's own accord, open up the combustion chamber on the bottom and check the LED code. It will probably be blinking, or perhaps have an error code. Check this against the troubleshooting guide in you owner's manual.

The unit will not run with the combustion chamber open, so you will have to put the access panel back in place to see if its working.

plumber Tom

04:26PM | 12/10/03
Member Since: 05/10/03
801 lifetime posts
Post back with the make and model. R Man the part you are referring to is called a pressure switch. A coat hanger should never be used to clean the hoses on this switch. If A DIY'r cannot and SHOULDNOT attempt to troubleshoot any gas appliance without proper training and experience. The user manual that was supplied with your unit will tell you the sequence (order) of operation. Yes if the pressure switch detects a blockage or a negative draft condition it will prevent the gas valve from opening. If you have a newer higher AFUE furnace check and clean the flame sensor. If you have no idea what these parts are call in a trained technician.


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