Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.

If you've found help here, check back in to let us know how everything worked out.
It's a great way to thank those who helped you.
Need to contact us? Visit

Weil Mclain boiler (maybe solved?)(yup, solved)

weil_failweil_fail Member Posts: 69
edited December 2017 in Gas Heating

I posted before about my boiler issues (short cycles, failed ignitions). the failed ignitions only happen when the system has run recently. if I leave it off for a while, it starts ok. I began just replacing components that were old, like thermostat/wire, rollout fuse was cracked, relay was very hot. however, after replacing the pilot module, which was actually kind of expensive ($40), I decided I should use my meter and rudimentary HVAC skills to actually find the problem of my intermittent boiler issues. Thanks to the very helpful people here, I think I've been able to narrow it down. if you guys could be so kind as to help me with a sanity check of my conclusions, that would be great.

what I tested:
since the system would try to re-ignite after the short cycle, that ruled out any of the interlocks like water temp, damper, rollout, etc. I also tested these individually by jumpering them out one at a time, just to be sure. since I replaced the relay and pilot module, I can rule out the flame rod, piezo igniter, and pilot tube as well as the relay. I can feel the circulator pump running, so I don't think that's the issue (though, I'm not sure how to do a more conclusive test). so, my primary suspicions were the gas valve and the control board. so, I tested the sequence on a good startup by placing my meter between PV and MV. this allowed me to see 0v before startup, then 25V when power was applied to PV, then back to 0v when power was connected to MV, at which time the burner came on. I also test each with respect to the PV/MV "return" just to be sure, but having only one multimeter it was hard to test timing when measuring with respect to PV/MV return. it seemed like whenever the valve had 24V, that it supplied gas just fine. but at the end of a short cycle, both PV and MV would be 0V while the system tried to reignite (clicking piezeo spark is all that came on). this seems to point me to the control module, and not the gas valve.

are there any other tests I should do before ordering a new control board? I was thinking about directly connecting (for 1 second) 24V directly to the gas valve to see if it would turn on, while holding a grill lighter to the pilot/burner area.

FYI, I have some electrical debug skills, and have installed and repaired a couple of heat-pump systems. though, this is my first gas boiler repair.

also, I believe this is the circuit diagram for my Weil-Mclain CG boiler:

edit: solved: control module


  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,079
    If it is sparking and no power to pv I would suspect the control board. Be real careful when jumping out anything and bypassing safety's. We don't want it to go boom.

    An amprobe clamped around the circulator pump wire would prove weather it is running or not
  • weil_failweil_fail Member Posts: 69
    yeah, definitely careful with bypassing. would never leave anything bypassed longer than is necessary to debug, and I did not bypass the water temp, but I checked it by waiting for the end of a short-cycle and testing it for short/open. it was still a short with the terminals off, so it had not tripped. (open= trip, short=normal). I also made sure the damper was open when bypassing it, and had my CO detector nearby.

    the circulator is definitely running, but I don't have a good way to assess health of the circulator.
  • HVACNUTHVACNUT Member Posts: 3,410
    You shouldn't be checking for 24v between PV and MV. PV/MV is not a return, it's the common side of the control circuit.
    On the Ignition Control Module, test incoming 24v between 24v and C, or ground after flame loss and recycle. If you dont lose 24v there, then incoming power is not interrupted, so you can cross off anything before that.
    If that checks ok, watch for 24v between MV and PV/MV (COMMON) on the ICM. If 24v drops out, then immediately tries to recycle, it's probably an internal fault in the ICM. But ONLY since you stated you checked the other safeties in series before the gas valve.
  • weil_failweil_fail Member Posts: 69
    hey HVACNUT, return is just another word for common where I come from. yeah, I wouldn't rely on checking between PV and MV, since having one of them open could give a AC voltage measurement with respect to a floating connection. however, in this instance, it was a valid measurement. like I said, I also checked them with respect to "common", but having only one meter made the measurement of timing difficult without checking between PV and MV. if you ever need timing, and only have one meter, give it a try ;)

    thanks, I'll check the board's incoming 24V.

    I appreciate your help!

  • weil_failweil_fail Member Posts: 69
    I tested and the control board (ICM) had 24V during the failed ignition and remained at 24V after (was actually about 26V).

    should I go ahead and order the control module?
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,079
    You have to decide if the module is or is not powering the gas valve. If it powers the valve and ignition is sparking then I would think the valve is faulty.

    If it sparks and does not power the valve then I would think module.

    Make sure your 24 volt connections are good. If you drop below 21-22 volts your transformer could be overloaded or weak. Also check the line side of your transformer (120) volt If the line is low the low voltage and flame signal will be low as well
  • weil_failweil_fail Member Posts: 69
    if anything the 24V connection is high. seems to be around 26V
  • Jamie HallJamie Hall Member Posts: 12,944
    weil_fail said:

    if anything the 24V connection is high. seems to be around 26V

    Is that under load? Or no load? You need to check it under load.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • weil_failweil_fail Member Posts: 69
    it never dipped below 24 while under load. ranged from about 24 to 26
  • Tim McElwainTim McElwain Member Posts: 4,369
    Why did you move from your original posting on this problem? That only causes confusion here on the Wall. I gave you a step by step check on your problem did you follow it?
  • HVACNUTHVACNUT Member Posts: 3,410
    edited November 2017
    Sorry @weil_fail, in my 30+ years, I've never heard the common called a return.
    And what does checking across 2 seperate 24v load terminals accomplish, without the common?
  • weil_failweil_fail Member Posts: 69
    edited November 2017
    Tim, sorry for causing confusion. I felt like starting a new one would be smoother, since I had eliminated so many things, and I accidentally posted the wrong control module diagram in the beginning of the last one, and there was a lot of responses for clarification. I just wanted to make a fresh start. yes, I checked the whole startup sequence and everything looked good when it ran.

    when it failed, I got everything but the voltage to MV or PV, those were at 0 during fail.

    HVACNUT, no worries, common is the better term to use. return is not commonly used, lol

    MV and PV are both 24V, but not at the same time. so, before call for heat, you measure 0V. when the main burner is not turned on yet, and the pilot is igniting, it will read 24V. when the main burner is being activated, it will go back to 0V since both MV and PV are the same voltage with respect to common.

    or to put it another way (with respect to Common):
    T0: MV= 0, PV=0
    T1: MV= 0, PV=24
    T2: MV=24, PV=24

    MV and PV with respect to each other:
    T0: 0
    T1: 24
    T2: 0
    T2+, move one probe to Common to verify 24V (this was obvious since the the burner was on)

    this confirms the startup sequence is correct, using only one volt meter. maybe there is a better way, but that's what I could think of, and it worked. be aware that this technique could give a false negative on the voltage since you may be reading 24V with respect to open air on the relay, since you don't have common. so, if you either get 0V the whole time, or some weird floating value, then you have to try something else because you can't trust the measurement. but if you get 0, 24, 0, you can trust the measurement. when checking a bad startup, for example, I did not rely on this method measured each, which would have made timing measurements hard, but neither MV nor PV EVER got off 0V during a failed start, so timing didn't really matter.

    in short, it either tells you the timing is good, or the test is inconclusive.
  • HVACNUTHVACNUT Member Posts: 3,410
    > @weil_fail said:
    > when it failed, I got everything but the voltage to MV or PV, those were at 0 during fail.
    Were you measuring at the ICM or the gas valve when it read 0?
    The spill switch is manual reset correct?
    Again, if it immediately tries to recycle, I'd think its a faulty ICM.
  • weil_failweil_fail Member Posts: 69
    measured at the ICM.

    I think the spill switch is manual reset. it has a red button on it. I don't think that is the culprit, though. I've jumpered it out and had no change. I also don't think it would try to reignite if a safety tripped.

    yeah, I ordered a new ICM. $100, so lets hope that solves it.
  • weil_failweil_fail Member Posts: 69
    [solved] it was indeed the control board.
Sign In or Register to comment.


It looks like you're new here. If you want to get involved, click one of these buttons!