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Honeywell 'The Round' CT87K battery question

Bob Vennerbeck
Bob Vennerbeck Member Posts: 102
During a bit of home renovation, one of the thermostats got removed for painting, then dropped and flew apart....

After realizing it wasn't the mercury model I thought it was, I relaxed a bit - I plan to replace it 'just because', but it does seem to work after snapping all the bits back together.



My real question is that I now know by direct observation that the Honeywell 'The Round' non-mercury bulb CT87K thermostat has a CR2450 3 volt coin cell in it - and it doesn't seem to be mentioned in any of the literature, nor is it set up to be readily replaced unless you bounce the enclosure on a hardwood floor and break one of the plastic clips holding the circuit board....



Does it have a life-span, or shelf life, that we might like to know about? What does it do? It was in use on a highly traditional 24v AC RA832A circulator relay box.



Jus' curious, mostly.



Vbob

Comments

  • HyrdoAirJoe
    HyrdoAirJoe Member Posts: 7
    Re: Honeywell 'The Round' CT87K battery question

    The CT87K is a power stealing stat (it powers it self from the r Terminal power) so I doubt the battery has anything to do with it's operation other than to keep the settings in case of a power interruption.
  • HyrdoAirJoe
    HyrdoAirJoe Member Posts: 7
    Correction on my prior post

    I mixed this up with the T8775 (Digital Round),





    According to HW web site ...



    Power Method: Battery assisted power stealing



    So if this is indeed a power stealing stat, I don't understand the function of the battery.
  • GBriggs
    GBriggs Member Posts: 15
    T87 battery question

    The battery is used when the stat is first powered and when there is a call for heat that last for over 4 hours. Then sort of goes to sleep. It should last over 10 years and is not field replaceable. It takes around 4 minutes to charge the capacitor and the capacitor will run the stat after that. There are two circuits that charge this stat, one when the stat calls for heat and one when the stat is not calling. Please keep the stat powered at all times and the capacitor will remain charged.
  • Bob Vennerbeck
    Bob Vennerbeck Member Posts: 102
    thank you & follow up

    Thanks! - May I ask the source of your information?

    I'm a bit disappointed that Honeywell doesn't tell consumers or installers that the t-stat may/shall require replacement after ten years - I grew up watching my dad use a can of (probably Freon!) Dust-Off on our round mercury thermostats every fall - it got the cat hair & dust bunnies out AND chilled the bimetal loop so you could see it cycle and hear the oil burner rumble into action. He made every bit of home maintenance either a physics lesson or an experiment....

    Those thermostats - since recycled for fancy 'lectronic setback units by new owner - worked fine for what was probably at least 50 years....



    The new round t-stats are in a house that was foreclosed and unpowered for 18 months - probably NOT a scenario that Honeywell imagined - so I will replace both of them, not just the one I dropped.....



    Thanks again



    Vbob
  • GBriggs
    GBriggs Member Posts: 15
    T87 battery question

    This thermostat will work fine if the battery does fail (in 10 plus years) but needs to charge the capacitor first. By keeping the stat powered you ensure this. I would not replace the other stat just give it a try. Yes we are in a different era than the old mercury controls...and a better one too. As for my information I worked for Honeywell for 31 years.
  • Bob Vennerbeck
    Bob Vennerbeck Member Posts: 102
    Thanks again

    GBriggs - didn't mean to sound like I doubted your information - I was just surprised in today's Googly world that I couldn't find any official mention. It's good to know that they aren't designed/destined for the scrap heap after all.



    Vbob
  • GBriggs
    GBriggs Member Posts: 15
    T87 battery question

    HI Bob, Yes it is nice to find out some information the is NOT ON Google!!

    Thanks, Gary
  • Tim McElwain
    Tim McElwain Member Posts: 4,445
    edited December 2009
    Power Stealing thermostats

    may have to be isolated from some electronic controls due to the affect they have on causing vent dampers to attempt to start then shut off or combustion air blowers coming on then shutting off.



    I am sure Gary can give you some good info on that.
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