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Boiler overheats when it cycles for too long..

adamzelazo Member Posts: 1
I have Burnham boiler model P205bwnvh, it was installed back in the 90’s. Ever since the house was built sometime in the early 50's a honeywell mercury dial thermostat has been controlling the boiler, Though recently I have realized that the boiler is short cycling. Turning on for only about 25 seconds, every 5-6 minutes. At first I thought It was due to the thermostat being old and simply inaccurate, somehow “Thinking the room was cold when it actually isn’t.” So with that thought I went ahead and Installed a programmable touchscreen thermostat, from the brand White - Rodgers. Within minutes after the Install, the baseboard pipes began to bang, and the boilers relief valve went off. I looked throughout the instruction booklet, and found the information about cycles. I switched the cycle from slow to fast hoping it would help, though it didn’t. I called me uncle who Is rather handing with kind of stuff. He did some research for me, and told me that my old mercury thermostat has something called an anticipator, which modern thermostats don't have. Trusting his knowledge I reinstalled my old thermostat for my own sanity, and it continues to short cycle driving me and my gas bill loony. I have asked on serval other sites and people have told me that my aqastat could be to blame. I checked it, and it is right between 180, and 200 degrees, should I turn it down? I really want to get to the bottom of this!


  • BobC
    BobC Member Posts: 5,228
    edited February 2016
    is the pump working and are you sure there isn't air in the system?

    Maybe the boiler is overheating and creating steam, the aquastat might be shot. Try turning the aquastat down to 160 or 170 to see if things change.

    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
  • aircooled81
    aircooled81 Member Posts: 197
    I would first guess the short cycling is not due to the thermostat. i would check to see how much of the firing stages up, do the burners come on, then turn off right away?
    I would suggest checking the loop pressure, do you have 40 psi in the loop before its even going to fire off?
    There are safeties on the boiler that could be shutting it off prematurely, but that aquastat can go bad. If you feel comfortable using a voltage meter, see if the aquastat is staying closed when it fires, btw - turn the main gas valve off while your working on this so you dont get a surprise when the gas fires.
    Very peculiar a new stat kept the boiler running, if your old honeywell is bad, then thats whats been hiding your other problems.
    Dont forget to check the expansion tank, banging might have been from all the air in the loop released by a blown diaphram? And possibly why your boiler blew the pressure relief.