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F/A furnace main burners quitting mid-cycle

miltimjmiltimj Member Posts: 7
Greetings everyone - this is my first post on this forum..  I have what seems to be a very specific symptom: (edit: see my follow-up several posts down for an updated symptom.. I don't think this original post is accurate, or else they're both symptoms)

Sometimes during a call for heat, while the main burners are firing and everything is going fine, the main burners quit and the blower stops.  The T-stat still calls for heat and the exhaust fan keeps running (forever), but it never comes back on until I cycle the t-stat (call for heat) off and on again.  (What also works is cutting power to the furnace and turning back on, but that obviously doesn't narrow it down as much.)

Furnace:  Heil NTC6100KFA1 (forced air)

Gas valve:  Honeywell SV9502H2704

Any ideas what this could be and/or what to troubleshoot?


  • miltimjmiltimj Member Posts: 7
    edited November 2010
    New filter

    When I first experienced the problem, the air filters were indeed very dirty (forgot about them!)... but have since replaced them, so those must not be it.  Is there an easy way to test electrically which limit is open?  I'm guessing a continuity tester from the circuit board?

    Thanks for the suggestion.
  • IronmanIronman Member Posts: 5,560
    Fault Code?

    Does the board flash a fault code?
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • Tim McElwainTim McElwain Member Posts: 4,357
    If the limits were open

    the fan would be running. It sounds like a problem with the pressure switch not switching over due perhaps a restriction in the tubing or moisture. Take a measurement of the suction pressure across the pressure switch/
  • SlimpickinsSlimpickins Member Posts: 322
    The dreaded "smart valve"

    You didn't include the serial number which would tell what year furnace you have. If it's a 1994/95 the problem is more than likely the Smart Valve. Most of these have already failed. Even the newer ones seem to have a high rate of failure. That's where I'd start. I have a Smart Valve test kit which is a harness with a molex connector that plugs inline with the existing connector. The wires have an exposed little section of bare wire where you can check your voltages. Good luck.
  • miltimjmiltimj Member Posts: 7
    New information

    I need to add to the symptoms.. the main burners were on, functioning normally for at least 20 minutes (catching up on the cold house temp), when I decided to stop the call for heat (shut off at t-stat), waited about 5 minutes, then turned back on at t-stat.  The exhaust fan turned on immediately, the pilot came on about 30 seconds later, but then no main burners. If I remember correctly, there's supposed to be a click from the gas

    valve just before the main burners come on, and I didn't hear that this

    time. After about 90 seconds of the pilot on (but no mains), the pilot goes out.  Five minutes later, the cycle repeats.

    I think this has been happening occasionally as well - I just didn't put my finger on it. The above cycle is consistent with the gas valve instructions, pg 12, here:

    So according to the flowchart, it seems that the problem is the  "pilot flame is not sensed during trial for ignition".

    Now for the crazy part - I've done this before, but didn't know if it was coincidence. Well, on the fourth cycle through the above (pilot flame not sensed) after 75 seconds waiting for it to be sensed, for some reason, I instinctively start to blow (with my mouth) across the pilot and main burners. The gas valve clicked and the main burners fired on!  I don't think that's coincidence.  I'm going to try to reproduce again.

    Anyway, any idea how that type of thing can be fixed?  I think SLO-115's suggestion of cleaning the flame rod with sand cloth sounds about right, but with my inexperience of how that works, I'm not sure if that's true, or even how to do that properly.  I guess I'll research that specific method/task.

    I appreciate the help!
  • miltimjmiltimj Member Posts: 7
    edited November 2010
    serial numbers

    S/N of the furnace is L0036 18150, and there's a hand written/painted "2-4-02" above the specs listing.

    Not sure where the S/N of the SmartValve is, but below the model number (and a UPC code), it says:  0736D4C11V  20086695

    The smart valve actually has already been replaced once (Nov 2007) - that's why I suspected that first, but it didn't make sense that it quit in the middle of a call for heat.  According to the sequence of operation flow chart, once it's burning, it should keep going until the call for heat ends.  Unless it thinks the flame signal is lost?
  • miltimjmiltimj Member Posts: 7
    No LED

    I don't see any LED or LCD on the board that would display or flash any kind of fault code.
  • miltimjmiltimj Member Posts: 7
    Reproduced again - pilot flame not sensed

    Okay, I reproduced it again - from everything off, I called for heat. Pilot came on, but no main burners (no click from gas valve).  I let it go 90 seconds, it shut off, then waited the 5 minute recycle time.  Pilot came back on.. I waited 60 seconds (to be sure it wasn't going to work on its own), then blew across the pilot and main burners to get the pilot flame to bounce around a bit...  the gas valve clicked, I stopped blowing, and 2-3 seconds later the burners ignited.

    Any ideas how to fix this?  I wouldn't think it's the SmartValve's "fault" since it works when it's actually sensed.  It must be whatever is sensing the flame is not close enough to it (I'm just not sure what it is that senses... the flame rod?)  Will research more on this later today when I have time, unless one of you knows off the top of your head.

    Thanks again!
  • IronmanIronman Member Posts: 5,560
    edited November 2010
    Stupid smart Valve

    It's been a few years since I had to deal with one of these, but I believe that the ignitor electrode doubles as the flame sensing rod. The fact that the valve senses flame when you blow across the pilot indicates that the problem's in the pilot, not the valve. The pilot's flame is not sufficiently impinging on the rod and the hood to form a current path from the rod, through the flame, to ground.

    Several things to check:

    1. Make sure there is sufficient flame. Pilot orifice may be restricted or head is dirty.

    2. All connections are tight. Electrode wire and terminals, ground wires, etc.`

    3. Rod is clean and free of rust.``

    4. Pilot head (hood) is not rusted.

    5. The porcelain in the rod is not cracked.

    6. No gas leaking from pilot tube connection.

    7. Too much gap between rod and hood.

    8. If in doubt, replace pilot head assembly.

    Just checked Honeywell's manual and the pilot has an adjustment - Page 4.

    Also, there is a fame rod and it shows the proper gap - page 8.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • Tim McElwainTim McElwain Member Posts: 4,357
    Try replacing the

    Smart Valve Sensor/Rod assembly, it sounds like you are having a flame failure after it lights. When this occurs has the igniter gone out or is it still lit, that is the small igniter that lights the pilot?

    You may need to have someone come with a microamp test harness to see what you microamp signal is doing. I would also hook up a combustion analyzer to see what is happening with the burner flame. There could possibly be a cracked heat exchanger.
  • Tim McElwainTim McElwain Member Posts: 4,357
    The Smart Valve has been replaced

    as it is a SV9502 which is a Generation II Smart Valve.

    There are checks that can be done even without the harness. What is the voltage coming out of the pin connector to the Smart Valve on the EFT output on the harness?
  • Tim McElwainTim McElwain Member Posts: 4,357
    The part number

    for the Smart Valve Igniter Assembly is Q3400A-1008.
  • miltimjmiltimj Member Posts: 7

    A few updates to this:

    The pilot flame is of decent size (at least an inch tall and inch wide) and lasts the full 90 seconds until the valve shuts it off from not sensing it

    I thought that maybe the flame rod wasn't sensing it due to some corrosion on it, so took it out and cleaned it with steel wool.  I'm used to using steel wool for copper plumbing, so was probably not gentle enough with it... bottom line, I put it back in and now the pilot doesn't even come on.

    So it looks like I'll have to replace the flame/ignitor assembly anyway.  Even bigger problem - I need to leave for work Friday-Sunday, and since the pilot is not working at all now, had to end up calling a service tech to repair.

    I truly appreciate the suggestions, info, and insight you all provided! I'll post back with the end result to close the loop for future reference in case anyone else has the same problem.
  • Tim McElwainTim McElwain Member Posts: 4,357
    If you are going to service

    Smart Valve a couple of things to have that will make troubleshooting easier are:

    1. A spare Smart Valve Igniter Assembly Q3400A-1008

    2. Smart Valve Flame meauring Kit Part # 395466

    3. Smart Valve Input Voltage Test Harness Part # 396085

    I also sell a complete guide to servcing and troubleshooting Smart Valve. Contact me at [email protected]
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