(206) 949-5683, Top Rated Plumber, Seattle
Results 1 to 7 of 7

Thread: Furnace will not light.

  1. #1
    DIY Junior Member pitteach's Avatar
    Join Date
    Jan 2008
    Location
    Hamburg, NY
    Posts
    27

    Default Furnace will not light.

    I have a Lennox 75k Btu 90+ furnace that will not light. (Approx 15 yrs old)
    This is the first attempt of the season.

    Here's what I've got:
    Turn on tstat and call for heat.
    Exhaust blower kicks on.
    After a few seconds, the intermittent ignitor starts firing.
    It keeps firing for almost a minute and while it does, the main blower kicks on but it does not light.
    The main blower stays on for about a minute, shuts down and the ignitor stops.
    The exhaust blower stays running continuously.
    It seems like there is no gas coming to the manifolds.
    The gas valve is in the on position.

    Any ideas? I am surprised that the main blower would come on without any heat. Could this be a clue? The unit ran fine when shut down last winter and I have just replaced the tstat before this happened.

    Any ideas would be greatly appreciated.

  2. #2
    Homeowner Thatguy's Avatar
    Join Date
    Aug 2008
    Location
    MD
    Posts
    1,460

    Default

    If there's voltage across your gas valve and current through it while your ignitor is sparking, it probably means the valve is stuck closed. This could cost you a half a kilobuck.

  3. #3
    DIY Junior Member pitteach's Avatar
    Join Date
    Jan 2008
    Location
    Hamburg, NY
    Posts
    27

    Default

    OK, where can I check voltage at the gas valve? There are 3 leads to the valve (mv, mv/pv, and pv).
    Am I looking to get 24v across all of these leads?
    Can I assume that the pressure switch is ok since the ignitor is coming on?

  4. #4
    Homeowner Thatguy's Avatar
    Join Date
    Aug 2008
    Location
    MD
    Posts
    1,460

    Default

    Quote Originally Posted by pitteach View Post
    OK, where can I check voltage at the gas valve? There are 3 leads to the valve (mv, mv/pv, and pv).
    Am I looking to get 24v across all of these leads?
    Can I assume that the pressure switch is ok since the ignitor is coming on?
    After disconnecting the gas valve [some multimeters put out enough current on the ohms ranges that they will damage electronics] check voltage on and continuity between all the valve terminals, 3 measurements for each. Also check for continuity between the 3 terminals and the valve housing.

    I'd say 24v +/- a few volts is OK on the valve. The pull-in/drop-out coil specs for gas valves is very hard to get. Some work on millivolts.

    I don't know the logic diagram for your furnace but it sounds like the only thing necessary after the sparking is for the gas to flow.
    If you post a schematic, which may be found on the inside of a panel, some possibilities can be eliminated.

    For my '82 Bryant the manufacturer sent me a several page factory manual for free, including Schematics and Theory of Operation. That's how I diagnosed my gas valve problem.

    http://en.wikipedia.org/wiki/Furnace
    Last edited by Thatguy; 10-04-2010 at 03:26 PM.

  5. #5
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Location
    Cave Creek, Arizona
    Posts
    25,615

    Default

    The blower will come on in anticipation of the burner igniting, and when it does not it shuts down. There are MANY possible reasons for the valve not to open, and each has its own diagnostic procedure.

  6. #6
    DIY Junior Member pitteach's Avatar
    Join Date
    Jan 2008
    Location
    Hamburg, NY
    Posts
    27

    Default Success

    After multiple tries, the furnace is on.
    I'm still not sure what happened but after a bit of fidgeting, it fired up.
    The gas to the house had been shut off at the meter for a couple months, so maybe I had some air in the line.
    I disconnected the pressure switch hose at the blower and reinstalled it and after a few misfires, it lit and is working fine.
    Thanks for the help.

  7. #7
    Homeowner Thatguy's Avatar
    Join Date
    Aug 2008
    Location
    MD
    Posts
    1,460

    Default

    Quote Originally Posted by pitteach View Post
    After multiple tries, the furnace is on.
    I'm still not sure what happened but after a bit of fidgeting, it fired up.
    The gas to the house had been shut off at the meter for a couple months, so maybe I had some air in the line.
    I disconnected the pressure switch hose at the blower and reinstalled it and after a few misfires, it lit and is working fine.
    Thanks for the help.
    You should take this opportunity to measure every voltage, current and resistance in your furnace while you still can. These values are extremely hard to come by for DIYers.

Similar Threads

  1. Ducane furnace will not light then shuts down
    By Dean M in forum HVAC Heating & Cooling
    Replies: 2
    Last Post: 09-28-2010, 02:52 PM
  2. DIY - Split receptacle to light switch and light fixture with 14/2?
    By AJM in forum Electrical Forum discussion & Blog
    Replies: 17
    Last Post: 11-24-2009, 04:36 PM
  3. Outside Light
    By white tiger in forum Electrical Forum discussion & Blog
    Replies: 3
    Last Post: 11-14-2008, 10:45 AM
  4. pilot light wont stay light on furnace
    By possum in forum HVAC Heating & Cooling
    Replies: 2
    Last Post: 10-07-2008, 12:48 PM
  5. PEX and UV Light
    By sbd001 in forum Plumbing Forum, Professional & DIY Advice, Tips & Tricks
    Replies: 1
    Last Post: 11-27-2005, 08:05 AM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •