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Weil McLain GV90 no flame because of clogged condenser drain?

lchh1975 Member Posts: 3
I have a Weil McLain GV90+4 LP conversion that stopped delivering heat, of course it had to happen just in time for Thanksgiving.

Symptoms were blower and circulators run, sounds of water gurgling but the flame won't come on. After a cold start it would go through the startup sequence as expected, but when it's time to ignite there would be a sound of gas valve opening and immediately closing, three times is a row, then "FLAME" led would start flashing, but the POWER led would remain steady. Multiple restarts didn't help, although couple times it started up went to about 80 deg and then shut down. A few times PRESSURE led would flash or turn off but I presumed it's side effects of flame not coming on.

So I started with the new igniter, the old one measured fine too. I checked other points and sensors per troubleshooting charts in the manual, everything seemed fine, ground present, no shorts, 120VAC and 24 VAC where expected. At this point I thought it must be gas valve or control board (replaced recently btw). So it's Thanksgiving and while I'm trying to think where do I go to buy a bunch of space heaters today because the temps here are in the teens and in the meantime I figured I'll clean what I can while I'm at it.

Inlet and exhaust were clean. The clamp on the inlet hose was loose but ok, and then I suddenly noticed the condenser drain pipe was completely clogged. I removed it, cleaned it, put it back together and... the boiler starts up and been working fine for almost 15 hours now. The drain was clogged with brownish powder (rust?). When I removed it it drained maybe quart of water from the condenser.

So my first question is could that clogged condensate drain cause the flame fail? Or is it a red herring and something else is/was going on.

Another question, how seriously should I be concerned about that rust? Where is it coming from? Should I clean it ASAP or wait until the end of season?

And finally, the manual calls for filling the condensate trap with water and makes it sound like it's a big deal (Page 61) but my setup doesn't seem to have condensate trap at all. The drain nipple is right at the recuperator. Is this normal?


  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 5,594
    edited November 2021
    That is the cause. There are sensors that measure air pressure thru the combustion chamber. The air pressure caused by the combustion fan. If the water can't get out then the water will build up enough to clog the vent pipe.. Clogged vent pipe means no exhaust. No exhaust means no flame. So you fixed it

    Read the maintenance portion of the manual. there are some other thing you should do every year before you start up for the season.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
    EBEBRATT-Ed Member Posts: 13,422

    I don't know what the age of the boiler is but condensing boilers especially are complicated and should be serviced at some time period.

    If you have no service history with this boiler certainly get it serviced in the spring. Then have it checked the following spring. You will probably find it only needs to be done every 3 years or so.

    Based on what we are seeing it looks like the max life for condensing boiler is 15-20 years and that may be a stretch. And that is if they are serviced. It also seems that the MFGs are making parts unavailable after 15 years or so
  • Tom_133
    Tom_133 Member Posts: 847
    So the other thing that can happen is the sensor in the pic with the two yellow wires can be the culprit. In fact yours is the older version. That one can be tough to diagnose due to it holding out the fan from even coming on. You may wanna get the new revised version and replace it. https://www.supplyhouse.com/Weil-Mclain-511-624-664-Flue-Gas-Thermal-Fuse
    Montpelier Vt
    HVACNUT Member Posts: 5,023
    You say you have no trap but in your pic you labeled it a "clogged". Where I'm from it's called a trap.

    The trap clogged and the condensate from the flue gasses backed up into the recouperator and tripped the pressure switch. The "pressure switch" flashing LED was an excellent clue BTW. 

    Of course get it serviced. If a qualified tech had already performed an annual maintenance, you wouldn't have had to waste time on Thanksgiving playing with it.

    You need to prime the trap to aid in draining, and to prevent flue gasses from expelling from the drain, into living space. 
  • lchh1975
    lchh1975 Member Posts: 3
    Thanks everyone for the suggestions. Looking closer at the recuperator design I see how backed up drain could prevent the airflow. FWIW the boiler's still working without any problems.

    I guess my question is it normal/expected to have that much rust accumulating in the condensate drain? Here's another pic from today, it's been less than 3 days since I cleaned it.

    The boiler is 7 years old and it's my second season with it. Previous service/maintenance history unknown, but overall my impression is that previous owner was taking reasonably good care of everything in the house.

  • lchh1975
    lchh1975 Member Posts: 3
    HVACNUT said:

    You say you have no trap but in your pic you labeled it a "clogged". Where I'm from it's called a trap.

    The picture in the manual makes shows a bigger rectangular reservoir (Item 1 in the attached image), so I meant that I didn't have anything like that. But yes I agree this p-trap is... a trap.

  • PRR
    PRR Member Posts: 69
    > picture in the manual makes shows a bigger rectangular reservoir

    That IS big. And it looks to me like a silt trap, any crud coming down from above will lay in the square part for many years. Yet your actual installation has a for-purpose P-trap which does not fit with that picture.

    Sometimes the factory changes things and doesn't tell anybody.

    Now you know, clean that P-trap, at least with annual service.

    I do not know anything about the rust.