Lennox G20 Furnace will not ignite and has no blower and 2A fuse open


  #1  
Old 10-15-12, 08:43 AM
L
Member
Thread Starter
Join Date: Oct 2012
Location: United States
Posts: 3
Upvotes: 0
Received 0 Upvotes on 0 Posts
Lennox G20 Furnace will not ignite and has no blower and 2A fuse open

Hi, I have a Lennox Whisperheat G200E3-75-4 Unit at my elderly mothers house that I just thought I'd test last weekend: it failed, as you might expect due to my posting here. It would initially start to auto-ignite, then the burner would immediately shut off, which seems fairly common from posts here. I cycled power and the internal gas valve switch several times per the ignition sequence instructions on the front of the unit, with the same results each time, but it now appears that the unit does not attempt to auto-ignite and the 2A fast-blow fuse on the control board is open and replacement fuses blow upon power application. There also does not appear to be any 24V at the thermostat, though initially (when the unit was trying but failing to auto-ignite, the fan blower could be set to on via the thermostat; now it is unresponsive. Any help would be very much appreciated.
 
  #2  
Old 10-15-12, 09:09 AM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 4,469
Upvotes: 0
Received 1 Upvote on 1 Post
The most common reason for fuses to blow is a short circuit to ground in the thermostat wire. It's not unusual for wire to be cut by sheet metal which also will ground the wire and cause a short.

If you have a multimeter, check the thermostat wire for a short to ground (the C terminal on the circuit board).

If you don't have a multimeter disconnect the thermostat wires, connect a new fuse and connect the R and W terminals with a wire to see if the furnace starts normally.
 
  #3  
Old 10-15-12, 09:20 AM
L
Member
Thread Starter
Join Date: Oct 2012
Location: United States
Posts: 3
Upvotes: 0
Received 0 Upvotes on 0 Posts
OK, thanks, I do have DMM and will check that this weekend. We had a service guy doing some other work there that day and he did pull the cover off the thermostat to try to help figure out the problem - there is a chance that a thermostat wire got shorted in the process, especially given the before/after difference in behavior that I had noted. Thanks again for your very fast reply. I will follow up with what I find out.
 
  #4  
Old 10-15-12, 06:00 PM
hvactechfw's Avatar
Member
Join Date: Jan 2011
Posts: 5,491
Upvotes: 0
Received 4 Upvotes on 4 Posts
make sure that furnace heat exchanger is checked by a professional (known for cracks).
 
  #5  
Old 11-12-12, 07:10 AM
L
Member
Thread Starter
Join Date: Oct 2012
Location: United States
Posts: 3
Upvotes: 0
Received 0 Upvotes on 0 Posts
Update - Fuse Problem Fixed; Original Problem Remains

After spending a fair amount of quality time with my DMM, and after studying the schematic and furnace manual, I found the fuse issue. I had already ruled out the thermostat (removed the old mercury-containing one, verified the short to ground on the load-side fuse terminal was still present with it removed (including disconnnecting the connection wires to it at the furnace circuit board), and replaced the thermostat with the similar-appearance "new" (but simple internally) round Honeywell unit). In looking at the unit schematic, the first connection made on the fuse load side is to 'limit control" switch S10, which from the G20 manual, is located on the front face of the heat exchanger unit. It turns out that a plumber who had attempted to help me figure things out after we had shut off and then returned the gas service, had apparently removed and reinstalled this switch (which is mounted on a small fiberglass panel and secured by 2 screws). The problem was that it was reinstalled upside-down and there is a small vertical asymmetry in the board mounting screw locations - this caused the board to be slightly lower than its intended position and allowed one side of the attached switch element to rest against the sheet metal cutout edge. Inverting this board, reinstalling a 2A fast-blow fuse, and repowering got me back to square one, with the unit attempting to light, and then immediately ceasing operation (blower, thermostat operation, etc. is restored). From other posts here, I suspect this is possibly the flame presence sensor, though I gently cleaned it with 0000 steel wool to no avail. I greatly appreciate all the help already provided here on this forum and would also appreciate any additional thoughts on the fail-to-light problem (I do have a replacement 30W33 ignition module kit, which includes a flame sensor, but deferred installing it to minimize changes until got the fuse issue diagnosed) . I will also get the heat exchanger checked for cracks. Thanks again for the responses.
 
  #6  
Old 11-12-12, 08:19 AM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 4,469
Upvotes: 0
Received 1 Upvote on 1 Post
Excellent analysis of the problem! But please don't revive an old thread. It's confusing.


Repost on a new thread, please.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: