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Weil McLain CG-t5 boiler Water temp stops at 180 , L-Aqua set @140, H set@ 210

MDX Member Posts: 2
Replaced Boiler aquastat , applied contact cleaner to all cont. Bd, terminals, tried new 3way valve as a test -same results , tried 3 diff. mix valve positions -same results- Heating not being used at this time

Weil McLain CG-t5 boiler Water temp stops at 180 , L-Aqua set @140, H set@ 210 0 votes

As a test , turned on upstairs thermostat , 3 way valve working , then stops at 180
what else limits boiler temp to 180 max
even if boiler aquastat is raised to 240 -same results


  • MDX
    MDX Member Posts: 2
    Help appreciated
  • hot_rod
    hot_rod Member Posts: 22,476
    When boilers fill with sediment, hard water deposits, they can overshoot temperature a bit.
    Does it make a banging, rumbling, percolating sound as it heats up? That is another sign of a boiler block sediment problem.
    You may hear the same sound in old gas fired water heaters as they fill with sludge.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,167
    edited May 2023
    The boiler need not be above 200°F in order to provide heat for the tankless coil in most cases. The low limit will operate the burner in order to maintain a minimum of 140°F boiler water for the tankless coil. This is accomplished with the diverter valve terminal #1 closing to #4. This sends 24 volts to the control module in the same way that the space heating thermostat would send 24 volts to the control module. Changing the setting on the high limit will have no affect on the water temperature setting made by the low limit. The high limit setting will only change the temperature that the burner will STOP operating when there is a call for heat from the space heating thermostat.

    By your description, it appears that the low limit is set to maintain 180° boiler temperature. Here is the diagram from the boiler door.

    See the red circles on the 9 pun connector. (larger illustration in file below) If they are connected by the thermostat, the end switch on the diverter valve or with a jumper wire, the burner will ignite. When that connection opens, then the burner will stop. When there is no call for heat from the thermostat, the only other automatic connection is from the end switch of the diverter valve actuator. See terminal 1 and 4 on the ladder diagram.

    So put the limit back to 180° or what ever temperature you need for space heating. If you want to maintain a higher temperature in the boiler for Domestic hot water coil, then increase the low limit. but that is gonna cost ya $$$ in fuel.

    Edward Young Retired

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