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Need Help Troubleshooting High Limit Switch on Gas Boiler

I'm having a problem with my boiler not getting the water temperature hot enough.  I'm thinking it's a bad high limit switch, but before I replace it I'd like to know if there is anything else I should look at.

My symptoms are as follows with my questions below. 

Thanks in advance for any help.

Weil-Mclain CGa-5 140,000 BTUH Input

Installed Dec. 2005

Standing pilot.  Gas / hot water

High Limit Switch Honeywell L4080D1036

Transformer Relay R8285D-1000

Limit switch was set at 180F.  Pressure/temp gauge on boiler showed boiler turning on at 105F off at 125F.

I now have switch set all the way to 240F.  Boiler on at 130F off at 145F.

I don't think the pressure/temp gauge is wrong for a couple of reasons:

- Temp gauge on return line concurs with the low temp cut in reading.

- My house is only getting to 60F (I live in PA and it's in the 20's right now).  This is what made me go check the boiler in the first place.

I don't remember if it was this forum or not, but someone with a similar issue wondered if the well that you screw the switch into might have scale covering it on the inside of the boiler that was affecting the reading.  Could this be possible?  I'd rather not drain the boiler right now in order to unscrew the well (it's in the 20's).  But if that's something worth doing, I will. I certainly don't want to buy a new switch to find out I needed to clean off the well.

Any suggestions are appreciated.

Thank you very much.



  • Paul48
    Paul48 Member Posts: 4,470
    Re: Scale

    Scale would prevent the transfer of heat to the probe, causing the temp. to overshoot the hi-limit.
  • 98chaos
    98chaos Member Posts: 9

    Paul48 -

    Yes, of course. I hadn't thought about that. It's always the simple things that get overlooked when it's late, you're tired, and it's cold.

    Thank you.
  • Tim McElwain
    Tim McElwain Member Posts: 4,480
    Remove the

    high limit aquastat from the boiler insert the capillary bulb under the cold water faucet and connect a ohm meter to the wire contacts on the aquastat  with the switch set at 180 it should show continuity. Now put a kettle on the stove stick the capillary bulb in the water and turn on the burner, if you have a cooking thermometer stick in the kettle next to the capillary, at around 180 you should loose continuity on the meter if you do the limit is okay and I would then suspect the well.
  • 98chaos
    98chaos Member Posts: 9
    Problem Solved - Not the Limit Switch

    Well after a day's worth of isolating components and troubleshooting, I found a wire corroded off of one of my zone's switching relays.

    The downstairs zone would call for heat and the circulator would run, but the boiler wouldn't fire.  The only time the boiler fired was if the upstairs zone called for heat.  So the downstairs circulator was constantly running, and the boiler couldn't keep up with the heat loss (because it wasn't firing).  I originally thought there was a problem with the high limit switch.

    In any case, thanks for the input.  I may have spent a few days on this, but I'm much wiser as to how my system operates and how to track down problems with it.
This discussion has been closed.