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Steam/Thermosta/24V AC question

BB Member Posts: 8
Howdy.  recently converted my old steam boiler from oil to natural gas w/ a new carlin burner.  the ignition unit pushes 24v AC on a two wire run to the thermostat.  as part of these changes to my heating system, i've been trying to use a new nest thermostat and am having issues.

the thermostat will work for a while and then fail in a replicable manner.  doesn't generate formal errors, just starts acting erratically and then fails to make heat calls to the boiler after 12-24hrs.  after lengthy troubleshooting w/ nest i returned one and the replacement unit behaved in identical fashion.  worked well for a period, then showed escalating and remarkably similar irregularities and eventually failed to make appropriate thermostatic boiler fire calls.

im consistently reading 29.47 on the line, rather than 24.  nest claims that this is just barely outside the tolerance of their device for power variance and is the likely culprit; not enough for it to trip device burnout or linepower codes, but enough to create wonky behavior.  given that i've been able to replicate the behavior in predictable fashion, and working in tech with an appreciation for how potentially complex/persnickety the internal integrations within the device are, i'm inclined to think this is at least a possibility. 

that said, my questions are:

-has anyone encountered a similar situation

-my boiler tech tells me that a 29V read is actually pretty standard and that significant swings in the output from a 24V ignition control box are pretty standard.  this sound accurate?

-given that it's AC, my assumption is that there's no simple rectifier or other inline solution to step-down or govern the power coming to the thermostat.  any suggestions?

-anyone know how this device actually works on a two line system? as it has an internal battery, wouldn't it be converting the power and thus conditioning it for itself anyway. 

-how does the power/ boiler call loop work on two wire?

assuming that my boiler guy is dead on and that AC power is typically rather dirty and with great variance, i have a hard time believing that the nest engineers wouldn't have taken that into account and designed greater tolerance for variance into their platform.

thanks much in advance for your time and experience.

Nest 2nd Generation

Carlin 60200-02FR Ignition Gas primary


  • Jamie HallJamie Hall Member Posts: 6,653

    In my humble opinion, to begin with, the Nest is singularly ill-suited to a steam system.  Steam systems do not like setbacks of more than a few degrees, if that.  The Nest is a very nice thermostat, though (again, IMHO) wildly overpriced -- for scorched air.  Marginal for hydronic.  And wholly unsuited to gravity hot water, steam, or radiant.

    That said... I can't think of a good reason why it shouldn't work under your conditions, at least as strictly a thermostat, the only function of which is to close that 24 v (nominal) AC circuit on a call for heat.  My conclusion would be that something is messing with its little mind regarding where it is set, the way the space is occupied, and -- for all I know -- the phase of the moon.

    Have you tried a good conventional programmable thermostat?  There are, I believe, even models which can be controlled from the internet, if for some reason you need that capability.

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • ZmanZman Member Posts: 3,476
    Solar flares?

    Just kidding! Jamie makes an excellent point.

    As far as your malfunction. I would suspect electrical interference. It could be the t-stat wire running along line voltage wires within the framing or maybe a nearby magnetic dimmer. Try wiring directly to the boiler and seeing if the problem persists.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • BB Member Posts: 8
    edited February 2013
    thanks + more thoughts

    thanks much for the initial feedback.  i definitely can appreciate your points re: nests value in this application, as well as its likely overall fragility.  that said remote control is of use to me and i'm interested in evaluating the effectiveness of its learning algorithms over time in standard residential use.

    ZMan - when you say try wiring directly to the boiler, can you be more specific?  it has been wired directly to the ignition on a new wire i pulled to a new thermostat location.  the changes we've recently made are significant: swap from oil to gas allowed us to sneak a second liner in the chimney and add a woodstove.  which requires relocation of the thermostat out of the wood heat-source room. 

    Jamie.  I haven't tried a conventional programmable...i just keep falling back on the in-place old mercury rotary as i try to solve the bug.  additional relocation seems like a good test.  i can easily enough pull the new pair to another room to see if i experience the same, or can isolate to an issue with its current placement.

    do either of you happen to know if a a 24v AC ignition reading 29v can be considered reasonably expected behavior?
  • ZmanZman Member Posts: 3,476

    If you already ran a new wire along a clean path, it seems unlikely that is the problem. Are their any potential sources of interference near the nest?

    I have no first hand experience with nest t-stats. I think your assumption that since it has it's own battery, it likely is just acting as a switch is correct.

    I have found 24 volt circuits usually read about 27-28 volts

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • GusHerbGusHerb Member Posts: 3
    edited February 2013
    I take it

    That you don't have a common attached to the C terminal on the Nest? If not that's probably your issue. Nest claims to work fine without a common but in reality that's not true. I've been hearing many complaints around this with behavior just like your describing.

    Also I'd disable the "adaptive programming" and just set it and forget it. Otherwise that Nest will drive your system crazy.
  • BB Member Posts: 8
    re: common

    you're right, no common, just two wires currently wired into the W1 & RH ports.  Is there an easy way to add a third common wire to a 2 wire system?  The ancient rotary mercury therm  that i'm trying to replace seems to have two leads spliced to the red/hot line coming from the ignition, yielding some manner of three wire situation.

  • Jamie HallJamie Hall Member Posts: 6,653

    but to no surprise, I have been unable to find detailed installation instructions or wiring diagrams for the Nest.  Good advertising; real short on information.

    That said, a Common connection might help.  Might not, too.

    If you need remote connectivity, may I suggest the either the Honeywell RTH8580F or RTH6580F?  Both are wi-fi/internet controllable and programmable -- and do work on steam systems, which the Nest does not even claim to be able to do.  Both of them do require a C connection (live 24 vac).

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • BB Member Posts: 8
    possible to use a jumper for C?

    is it possible to wire a jumper to add a C termination on my two wire set-up?  or do I run a three wire from the ignition box?  I assume it can support it and appears to have two unused termination screws.  all input appreciated - pic attached.
  • Jamie HallJamie Hall Member Posts: 6,653
    As I understand it

    C is hot 24vac.  So it needs to come from the hot side of the 24vac transformer, not the ignition box.  I don't see how you could make up for it with a jumper...

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • astoria2013astoria2013 Member Posts: 15
    Carlin Gas relay

    In my experience, Carlins TT terminals are not the best to use as a 24 volt power source.  First of all , like most systems like that, if the boiler shuts off on the pressure control or low water cutoff or any other switch, that TT power is disrupted (shut off).  You need an uninterupted 24 volt supply for that thermostat. 

    Second, I have monitored those carlin relays and I have come across a few that have a surge in power for no reason up to 36 v.  Most of those jobs with such different voltage the thermostats needed to be replaced. 

    Lastly,  the TT terminals are technically not a true 24 volt circuit.  a 24 volt circuit is a R-load and C-common.  That TT terminal is just a 24 volt switch with no true ground-common.  thats why if you want to start that boiler, take a pair of jumpers and jump the TT and boiler will start.   On the other hand, take a true 24 volt transformer and jump C-common and R-Load and you burn out transformer and maybe a bunch of wires as well as a puff of smoke.  (do not try this). 

    If I were you I would run a dedicated 24 volt circuit just for thermostat power.  Or better yet have an electrician with knowledge of boiler circuits.
  • NY_HammerNY_Hammer Member Posts: 65
    Here it is

    but not much info of course.

    Even a monkey can replace existing t-stat, bit no wiring diagram. And C terminal is not mentioned.
  • Tim McElwainTim McElwain Member Posts: 3,893
    I suggest getting in

    touch with Carlin to see if Nest is compatible with Carlin system. I have found that Nest is not compatible with a number of different applications
This discussion has been closed.


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