Lennox Gas Furnace Problem

Reply

  #1  
Old 12-23-03, 01:00 PM
Member
Thread Starter
Join Date: May 2003
Posts: 14
Lennox Gas Furnace Problem

I've got an older Lennox furnace, model G12Q4E-110C-6 (serial number 5880B 09405). It is an electronic start - no pilot lite. For the most part, there's no problem, but it seems that if it hasn't cycled on and off for 8 to 10 hours, it won't come on.

I've got a programmable thermostat that's set to 67 deg. for most of the night, then it bumps up to 72 at 5:00 a.m. so everything's toasty when we wake up. About 80% of the time it doesn't come on, so I have to go to the thermostat and manually move the target temperature below the ambient temperature, then back up to 72. Sometimes it comes on, sometimes I've got to do it again. It never takes more than twice. The starter spark runs for 5-10 seconds, which seems like a long time to me.

I've been told this may be a thermocouple problem, which I could easily replace because it appears easy to get to (assuming, of course, that what I'm seeing is actually the thermocouple). Any theories?

Thanks
 
Sponsored Links
  #2  
Old 12-23-03, 03:21 PM
hvac4u's Avatar
Member
Join Date: Aug 2001
Location: NW atlanta
Posts: 3,147
no thermocouple on that unit. what you are looking at is either the spark electrode or flame sensor. you may be able to pull the spark wire off the board and cut an inch off, that cures the problem on many lennox models, not sure of yours.
 
  #3  
Old 12-24-03, 02:03 AM
rav12's Avatar
Member
Join Date: Oct 2003
Location: California
Posts: 668
Could be a faulty thermostat. You can try troubleshooting by using a multimeter to see if a call for heat is ocurring by measuring the voltage on the heat wire (assuming it is not an advanced thermostat that uses serial data comms to communicate to the furnace. If this is the case you will have to check on the control board). If the call to heat is not ocurring when it should the thermostat may be faulty.

If it is, the fault is most likely in the furnace and more troubleshooting will be required.
 
  #4  
Old 12-24-03, 11:31 AM
Member
Thread Starter
Join Date: May 2003
Posts: 14
Thanks for the replies...

Is it possible the flame sensor is going bad? The first time it tries to light, I hear the gas valve click, the burner lights immediately, then the valve clicks and the burner goes off. At that point the lighter spark runs for about 15 seconds (clearly audible), then gives up. It sounds like a small flame is burning while the spark is running.

Once it runs once, it seems to cycle fine all day. It's just the early morning wakeup that it seems to goof up on.
 
  #5  
Old 12-24-03, 11:46 AM
hvac4u's Avatar
Member
Join Date: Aug 2001
Location: NW atlanta
Posts: 3,147
clean the sensor lightly with steel wool, see what that does. it could be dirty, check all grounds to be sure they are tight. flame sense works through the ground.
 
  #6  
Old 01-04-04, 04:11 PM
Member
Thread Starter
Join Date: May 2003
Posts: 14
I finally got around to pulling the flame sensor out and cleaning it (it's been getting up to 60 or 70 around here since Christmas).

It was very easy to get to and remove, and there was a film of stuff on it that came off pretty easily. The heater hasn't been on for quite a while, which normally means it won't start on the first try. I did start the first time after I cleaned it, so hopefully the problem's been fixed. We'll know for sure tomorrow morning because it'll be below freezing outside.

Thanks all.
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes