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GB142 Will not ignite for heat only

GB142 Member Posts: 32
edited October 2022 in THE MAIN WALL
So today I noticed a peculiar behavior of my GB142. The AM10 is registering 34 degrees outside, but my boiler will not ignite based on the outdoor temp. I have all registers on in the house and pumps running but the boiler is not igniting.

I shut it down and then restarted it. When I start it, the GB142 turns on, and the fan starts but the boiler does not ignite. If I turn domestic hot water to a higher temperature, the boiler does ignite.

I have my AM10 set at 130 for design temp (14F). I have never understood the other design temp setting at warmer temps. Currently, at (80F) Could I have it set incorrectly and it is affecting the unit from starting at these colder temps?

The RC10 is new in 2020. Could it be bad? The older one didn't go bad, it just had a broken dial.

I just received a brand new igniter and thermister, but have not installed them. However, it the boiler starts when domestic hot water is called, I would think they are fine.

Outdoor temp control is sending proper outside temp.

Any thoughts?


  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 5,374
    edited October 2022
    According to you photograph, the ignition or flame indicator is lit and the DHW is indicting the DHW pump is operating. Also the boiler water temperature is 192°F somewhere. So if the heat circulator is not moving that water you may have a bad circ pump for space heating or you may have an air blockage in the radiator piping. If the space heating circulator is operating and the boiler flame is not on, then you may be off by the high limit or by the operating limit. Look for a water circulation problem first.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • GB142
    GB142 Member Posts: 32
    edited October 2022
    Thanks Ed, however, that pic was taken when I have the domestic hot water "on". If I turn the domestic hot water off, the boiler will not ignite to heat the radiant system.

    To avoid any misinformation, the Boiler is only igniting when I hae the domestic hot water dial in heating mode and "not" off.

    My goal was to direct all boiler resources to the "heating system" mode, since we had no need to heat domestic hot water, for at least another 24 hours. I have always been perplexed why my GB142 seems to spend so much time in domestic hot water heating mode.

    To avoid any issues, I turned the domestic hot water on last night and the unit worked fine.

    So after 16 years of use, am I just learning that the heating mode of a GB142 will only work when both systems are in working mode?

    Does that make sense?
  • Big Ed_4
    Big Ed_4 Member Posts: 2,136
    edited October 2022
    Jump out the on/off terminals on the AM10 and see if it fires up . The on/off terminals should be connected to a end switch , like on a pump relay ... if it does , you may have a bad wire or end switch . Assuming the outdoor set point is set around 65*

    There was an error rendering this rich post.

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 5,374
    edited October 2022
    That GB142 should work fine in heat only mode. It has been some time since I worked on one of them. I remember installing that boiler for at least 3 different customers. The idea is to have very low temperature water going thru the heating radiators during this time of year. As the outdoor temperature drops, the radiator temperature increases. This way the flame never actually goes off during the colder months of the winter. The flame just gets bigger and smaller as the outside temperature changes. The colder it is outside the more heat you need to add inside.

    That all changes when the hot water is needed. In order to keep up with DHW demand, the burner goes into full high flame to make as much hot water as it can so you don't run out of hot water when you are taking a shower. That is called "DHW Priority"

    Once you stop using hot water and the DHW tank is up to temperature, then the boiler goes back to space heating. Since the water in the boiler might be very hot from making Domestic Hot Water, the burner will go off even during a call for heat from the thermostat(s). The circulator pump for the radiators should be operating to take that hot water and put it in the radiators. This will continue until:

    1. The water temperature in the boiler gets low enough to where the burner must come on the heat the water going to the radiators


    2. The thermostat is satisfied and all circulator pumps stop.

    In you original explanation you said that the burner does not light in heat mode. That may be a normal condition as I explained above. What is unclear is "Are you having a no heat or not enough heat condition?" That is more to the point and i believe that is probably why you were looking at this in the first place.

    So... Is there a no heat situation?

    If so, then the DHW priority may be part of the problem.

    If the DHW tank thermostat is defective, and it does not shut off, then the priority for the DHW will keep the space heating circulator off. You may not realize that if your water heater is equipped with a mixing valve. All you notice is that you are not getting enough heat. when you turn off the water heater manually, then the DHW priority is off and the space heat circulator will be able to heat the home.

    Does this make sense to you? I know that my explanations can be difficult to follow sometimes.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics