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steam boiler with automatic vent damper

Does anyone know exactly why you cant use a power stealing Tstat such as a nest on a steam boiler with an automatic vent damper? i ran into this problem with a customer that had a existing nest which we all know is a power stealing tstat. we had no common to tsat. it worked with old weil mclan but when we put a new steam boiler in (williamson thermoflo) the tsat kept dying. we measured 13volts at tstat on wall. as well at boiler as soon as we used the switch on automatic damper to keep damper open we got 24volts in both places. we called the tech line of manufacture but they werent helpful at all in helping me understand why? all they said is you can not use a power stealing stat on their boiler with no explaination. Now im the kind of guy who has to know why , i study the electric diagram of boiler and the manual to automatic damper and neither was any help. So i was just wondering if anyone could explain as to why?. (End result was well left damper in manual open and explained to customer either he gets a new non power stealing tstat or leave the damper open and loose some efficency to new boiler.


  • Kjmass1Kjmass1 Posts: 200Member
    Get customer to run a common or if you have the wires just add a 24v transformer. Nests definitely need constant 24v.
  • John732John732 Posts: 3Member
    Kjmass1 thank you i appreciate your feed back. I know how to make it work but my question is why is that happening, what’s in the design of the electric circuit that doesn’t allow the nest to work without battery dying. I found that when damper is closed it only allows 13volt to stat which is why it can’t recharge because it needs 24v . Only happens with boilers with automatic dampers.Yes one option is to get a common to stat at the time customer didn’t want to break open ceiling.
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