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Thermostat Fail-safe?

jmob77 Member Posts: 39
I have a one-pipe steam system, controlled by a basic digital Honeywell programmable thermostat; which I installed about two years ago to replace the ancient, noisy, analog programmable that was there when I bought my house.

This morning, I awoke to a blank screen on the thermostat; the display suddenly came back on and showed that the room temperature was nine degrees above the 68-degree set point. It seems the battery conked out while the boiler was in the middle of a cycle and the flame kicked off as soon as the thermostat came back to life (the flame was off by the time I got downstairs to inspect the boiler). I replaced the two AAAs and everything seems fine.

What I want to know, is this a safe setup? I know my low-water cutoff is functioning properly, as is the pressuretrol. It seems shocking to me that a $25 piece of battery-operated electronics is, in a way, the primary fail-safe in my heating system. Why is it that the system will keep heating if the thermostat loses power mid-cycle? Shouldn't it be that any loss of power at the thermostat causes the whole system to power down? Is there something I should do besides be more vigilant about my batteries?

Many Thanks!

Brooklyn, NY


  • Jamie Hall
    Jamie Hall Member Posts: 22,952
    Interesting that it failed on rather than off on low voltage. Be interesting to look at the circuitry involved. That said, can you really truly expect a cheap bit of consumer grade electronics to be fail safe, never mind fail operational?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Mark N
    Mark N Member Posts: 1,115
    edited January 2018
    What model it? Batteries might have just been a bit low. Best to change at the beginning of the heating season. One would think that if the batteries in the thermostat die the boiler would turn off. Next time the boiler is running pull the batteries and see what happens. You might have another problem.
  • ratio
    ratio Member Posts: 3,605
    What model of Honeywell stat, & where purchased? Are "clicks" audible when it calls, or is it silent?

    In any case, I'd immediately replace it with something I trust a little better, not purchased from a big box store.

  • jmob77
    jmob77 Member Posts: 39
    Not home; would have to check model. Makes audible click while turning on or off. The manual warns that you should shut off the system switch on the stat before changing battery, because it may allow system to keep running. Following this logic, it seems that any temporary battery loss could create the same situation. Seems a poor, possibly irresponsible design to me.
  • Jamie Hall
    Jamie Hall Member Posts: 22,952
    Interesting. As I say, it would be enlightening to look at the circuits...

    I would point out, though, that failure of a thermostat to turn off cannot be considered a safety hazard in most applications (there are some where it would be, but home heating isn't one of them -- and most of those you will find have backups). A nuisance, yes. An expense, yes. A safety hazard? No.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • jmob77
    jmob77 Member Posts: 39
    In my case though, it seems like a thermostat failure would likely result in the low water cutoff being the next line of resistance; seems like a big jump in the severity of the situation, no?
  • Jamie Hall
    Jamie Hall Member Posts: 22,952
    Well, if you don't have auto-feed you could say that -- but your boiler shouldn't shut off on low water just because it's running all the time. If it does, you have other problems.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    delta TCanucker
  • BobC
    BobC Member Posts: 5,473
    Your out of town for a week and the batteries fail. It's zero degrees out and your not going to home for many days. Would you rather pay for the fuel it takes to run the boiler for a few days OR pay for the damage caused by freezing pipes?

    This why the old mercury Honeywell thermostats were so good - no batteries and no chance of contact failure. As long as it doesn't fall off the wall, as reliable as anything made by man.

    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
    JUGHNE Member Posts: 11,021
    This is a reason to run the common wire to the tstat.
    Requires at least 3 conductors for heating only.
    I always run the common if at all possible.
    Also install the batteries.

    Even the simple HW Focus PRO 5000 claims to give you about 2 months warning flashing. (with the disclaimer to change batteries before leaving home for more than a month.)

    It is embarrassing to explain to a customer that their newly installed $6-8K system failed because of 3 dollars worth of batteries.
  • jmob77
    jmob77 Member Posts: 39
    I have two C wires tucked away in the wall since changing to this stat. I guess it's possible I can connect AC power to this new one? I'll check the literature...