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Replace High Limit Aquastat?

I have a Burnham Series 2 50K boiler heating the first floor apartment in my house. It's about 7 y.o. Tenants have complained that the heat was slow to come up since the day it was installed, but the installer came back to check and ensured that everything work working as expected. While working on another boiler in the basement I recently decided to give the Burnham system a checkout and discovered that the boiler shut off (during a heat call) at a supply temp of about 145 deg., as measured by my clamp-on thermometer about 2 feet from the boiler, and cycled back on at around 125 deg. These numbers were roughly similar to the boilers internal themo/pressure gauge numbers.

This boiler doesn't have any low limit wiring, etc, so it just has a high limit aquastat. It was set to 180 deg. per its dial. I bumped the high limit up gradually and now have it set at about 210 deg to achieve a measured max supply temp of 180 deg. My question is, is this dial just badly mis-calibrated, or is the high limit aquastat broken and should be replaced? I measured the output over several boiler cycles and it seems to shut off consistently at ~180 deg. now.

The heat it working much, much better, recovering from overnight setback in less than an hour, when it was taking 2.5 hours.


  • STEVEusaPA
    STEVEusaPA Member Posts: 6,388
    edited December 2015
    Most likely the sensor in the well is not making good contact with the metal. I would check that first.
    They make a paste you put in the well (please dont use caulk) to make sure you are getting proper contact.
    I would also make sure your clamp-on is reading correct temperature. I usually have no luck with long term usage of k-type clamp-ons. How does the boiler gauge compare to your clamp-on?
    You do want it sensing the right temperature, because if it's moved and starts making the right contact, your gonna end up with boiling water in your boiler...not good.
    So either make sure its sensing the proper temperature in the well, or you should replace it.
  • dtrani
    dtrani Member Posts: 25
    edited December 2015
    Thanks, I'll take it out and see what it looks like in the well.

    The boiler's dial thermostat is reading slightly less than 180 when the boiler shuts off.
  • damitdave
    damitdave Member Posts: 2
    up dating older boiler safty switchs and valves ???
  • Paul48
    Paul48 Member Posts: 4,469
    Maybe I'm not thinking correctly, but if the sensor doesn't make good contact, it can only overshoot?
  • Big Ed_4
    Big Ed_4 Member Posts: 2,542
    correct it will bump up higher if mot properly in the well ... Sounds like the capillary which uses an oil base has an leak .Replace the control or if it's an thermistor type replace the thermistor
    I have enough experience to know , that I dont know it all