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Residential boiler troubleshooting

Toad Member Posts: 14
edited February 26 in Gas Heating

Looking for any thoughts to help troubleshoot issue with 20-25 year old Weil McLain boiler. 

Situation is:

1. Would call for heat from thermostat and pump/circulator would go on (hot water radiator system) but boiler would not fire up to heat the water. 

2. Tried calling for heat for another zone, and boiler fired up. 

3. Then everything worked ok for a bit. 

4. Then issue repeated itself and this time calling for heat from second thermostat did not resolve. 

5. HVAC tech (good guy, good company) came over and was able to replicate. Thought it was that the damper was not getting triggered to open, so no boiler ignition. Didn’t seem to be broader issue with the board. Plan was to replace the damper.

6. Had to wait on part so he left damper in open position in the meantime (said not energy efficient but would allow boiler to fire up while we waited the day or two for the new part). Worked fine in this configuration.

7. Replaced damper and all seemed good. Then same issue happened again. Resolved it by calling for heat from different zone. Good for a week. Now issue repeated again. 

We’ll have them come back to house, but wanted to see if any consensus thoughts here on what might be happening and best way to verify and correct - if for nothing else to help accelerate the troubleshooting. 

Thanks in advance for any thoughts. 


  • Alan (California Radiant) Forbes
    Test for 24 volts at your ignition control module when in fail mode. If you get 24 volts, replace the module. 
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,852
    edited February 25
    There are several things that you mentioned that could be of concern. And you may have more than one problem happening at once. Especially if you have a damp boiler room. Water and electricity don't mix very well, so there may be more than one electrical control failure due to moisture on the circuit boards.

    Can we rule out moisture?

    Assuming that we rule out moisture, other things that come to mind.
    1. When one zone calling for heat fails to start the burner is it the same zone each time?
    2. When the second zone resolves the problem, can to first zone call for heat again later and duplicate the problem every time? (in other words, will the burners always fail if only the problem zone is calling by itself)
    3. What type of thermostats are operating the zone valves. Old mercury switch type? Digital type with battery backup? Smart thermostat connected to WiFi?
    4. Is you heater equipped with a Low Water Cut Off of any type?
    5. Do you have a Zone Valve Control panel or are your zone valves wired directly to the thermostats with a dedicated zone valve transformer. (mostly looks like a bunch of thin wires that are connected to screw terminals or wire nuts like spaghetti)

    Answers to these can often be in the form of a picture posted in your next response.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • Toad
    Toad Member Posts: 14
    Thank you for your responses. Answers to the questions are;

    0. Quite dry and low humidity in that area. Do not think moisture playing a role. 

    1. I have primarily noticed it with the main zone as the second zone is for basement only which we only run when things get to the teens outside and third zone is for a radiant floor we typically don’t use. But after using the second zone’s thermostat to start the burner on one occasion, it didn’t always successfully do so as we tried on some subsequent “experiments”. When it didn’t do the trick, I was then able to use the third zone’s thermostat successfully. 

    2. When second zone’s thermostat resolved it, first zone’s thermostat would then start to work seemingly normally again for days. So no, it doesn’t seem like problem zone consistently fails. 

    3. Thermostats are various Honeywell digital ones  (but not smart). Zone 1 (main zone) is programmable and has a battery). Zone 2 is programmable (no battery). Zone 3 (floor) not programmable and no battery). Not WiFi. 

    4. I believe he mentioned there was high temp/pressue and low water cutoff. Also a low water refill valve leading into it. 

    5. I think the former, but not positive. 

    Thanks again. 
  • 109A_5
    109A_5 Member Posts: 1,385
    Hello @Toad,
    You need a technician that is skilled with troubleshooting electrical and control circuits. When it acts up, finding the point of failure should be fairly easy to locate with voltmeter readings.

    Thermostat closes (call for heat)
    Damper end switch closes
    Power to ignition board

    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System