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Boiler had stuck firing continuously; any things need checking?

System: oil-fired two pipe steam, weil Mclain boiler, burnham burner

While we were out of town, the thermostat failed in the open position. The cat sitter found the house at 90F, turned the thermostat to off, and verified that the boiler stopped running. The boiler would have been operating independently for up to 18 hours. I called the oil company and about 8 hours later, house temp at 75F, the technician showed up. Found that the thermostat batteries were corroded (somehow I missed replacing them this season) and changed them. Verified that the thermostat still operated the boiler and left. We got home a day later and things "seem OK".

I'm wondering what else he might/should have done? For example, I found the water in the sight glass was very high and while the system was off/cold, I drained it down to about half way.

He told the cat sitter that the plastic holder for the batteries was very brittle. I assume I ought to replace my nice old thermostat (Honeywell ChronoTherm III) that always held the room at exactly the set point. Is there anything that works as well?

Thanks for any help.


  • NY_Rob
    NY_Rob Member Posts: 1,370
    edited December 2018
    If you don't need automated setback, the Honeywell T87F is even more reliable then your previous t-stat... and no batteries required. You can find New Old Stock T87F's on ebay for $40 or so.

    If you do need/want automated setback, the Honeywell VisionPro t-stats are well regarded by many of the pros here. You'd need a "C" wire to power the VisionPro or most any modern electronic t-stat though.
    Modern electronic t-stats will default to "off" on prolonged power outages to prevent the very dangerous condition your old t-stat presented with a permanent "on" state.
  • GW
    GW Member Posts: 4,304
    Stuck closed is extremely rare, but a lottery ticket 😀
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    [email protected]
  • janwalker47
    janwalker47 Member Posts: 24
    Apparently the guy said you could get thermostats now that default to some preset temperature instead of just Off. Or in my winning ticket case, just On.

    The specs I've read so far don't mention this kind of possibility. In fact, they don't seem to mention "failure" much at all. LOL.
  • GW
    GW Member Posts: 4,304
    I suspect there may be a wire that is shorting. But hard to know for sure. I’ve never seen a stat stuck closed
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    [email protected]
  • Fred
    Fred Member Posts: 8,518
    Yes, check for shorted wire. I had this happen once, at a different home. Turned out to be a finish nail in a door casing that nicked the thermostat wiring. If you walked across the floor a certain way and at a certain spot, it would short and fire the furnace. My son would call me at work and tell me he just got home from school and the house was almost 100 degrees. Happened a couple times before I decided to run new wiring and found the problem.