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cga boiler overrides high limit

brosek Member Posts: 5
I have a Weil McClain Gold CGa boiler. The high limit ("Boiler Temp") is set to 190. But every few days it starts heating only to 140 then shutting off. If I adjust the high limit slightly, say up to 191 and back to 190, it will heat up to 190 and stay there for a while. Then eventually it goes back to heating only to 140. I've tried adjusting the high limit (e.g. to 185) but this doesn't seem to make a difference - it still switches to heating to 140.

Low limit seems to always be 20 degrees below high limit so if it's heating to 190 it kicks on at 170 and if it's heating to 140 it kicks on at 120, provided there's at least one zone calling for heat.

The indirect hot water tank starts calling when the tank drops below 150 and kicks off once it reaches 155. So once the boiler switches to heating only to 140 then hot water tank calls continuously and the circulator runs continuously.

Other info:
- Taco ZVC404-4 zone controller, all 4 zones wired, priority switch on.
- Nothing connected to the pins labelled "OUTDOOR" on the main board.
- System is about 3.5 years old, problem first noticed this winter.
- Tried fiddling with "Economy Adjust," made no difference.

Any ideas appreciated.


  • Jamie Hall
    Jamie Hall Member Posts: 22,180
    My first thought would be a bad aquastat...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Alan Welch
    Alan Welch Member Posts: 261
    The high limit is just what it's called---- a limit. It should stop the burner when its set temp is reached. The water temperature in the boiler is maintained at the low limit temperature, and will range between the low and high only when there is a call for heat or hot water. As the weather warms up, the boiler temperature should be lower most of the time. You probably don't have a problem.
    HVACNUT Member Posts: 5,480
    What is the economy setting?

    Check the Boiler Temp Sensor.
    There's a resistance chart in the manual. Page 47 in the Series 2 manual.
  • brosek
    brosek Member Posts: 5
    I think I figured it out. Manual says:

    "In accordance with Section 325(f)(3) of the Energy Policy and ConservationAct, this boiler is equipped with a feature that saves energy by reducing the boiler water temperature as the heating load decreases. This feature is equipped with an override
    which is provided."

    And section 325(f)(3) of that act says:

    "When there is no inferred heat load with respect to a hot water boiler, the automatic means described in clauses (i) and (ii) shall limit the temperature of the water in the boiler to not more than 140 degrees Fahrenheit."

    Which is exactly the behavior I'm seeing. Except I don't want it to do that, because then the circulator just runs forever, and the water isn't really as hot as it should be.
    HVACNUT Member Posts: 5,480
    If the Economy setting is at minimum, then there's no time delay on ignition. If the limit is closed.
    With Economy at minimum, the boiler should reach its limit setting above 140°.
    Check the sensors.
  • brosek
    brosek Member Posts: 5
    It seems to me that if the LIMIT light goes off when the displayed temperature reaches exactly 140, it's not a problem with the temperature sensor.
  • Zman
    Zman Member Posts: 7,496
    Can you post a picture of your piping? I wonder if you are getting ghost flow through the indirect which is causing your problems. Outdoor reset and indirects don't play well together unless you are set up for DHW priority.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • SuperTech
    SuperTech Member Posts: 2,032
    edited May 2020
    I'm assuming you have the hydrolevel aquastat. Do you have the optional outdoor temperature sensor connected or is the unit varying the high limit using the thermal targeting?
  • brosek
    brosek Member Posts: 5
    The optional outdoor temperature sensor is not connected, so I think it must be doing it using the thermal targeting.
    HVACNUT Member Posts: 5,480
    What Series CGa and what operating control?
  • brosek
    brosek Member Posts: 5
    Looked up the CP# and it says "CGa-5 PIDN T007 S2 Blr" manufactured 06/19/17.

    Not sure what you mean by operating control here. Did mention the zone controller in the original post.
    HVACNUT Member Posts: 5,480
    > @brosek said:
    > Looked up the CP# and it says "CGa-5 PIDN T007 S2 Blr" manufactured 06/19/17.
    > Not sure what you mean by operating control here. Did mention the zone controller in the original post.

    The operating control is the board with the temp and economy dials. And it's a Series 2 (S2).

    It is a cold start boiler, but with the economy set to OFF, the burners should remain on until the temperature reaches it's set limit, or the heat demand has satisfied. No way is the boiler locked in to a low temp (140°) limit. Something is wrong.