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Large difference in target temperature and actual home temperature

mhylas Member Posts: 2

I recently replaced a very old thermostat is a new nest version 3. The reason for the replacement was that the boiler/baseboard heating would constantly kick on even if I explicitly set it to a lower temp. Making the house as hotter and hotter.

I decided to refresh the thermostat first because it was over 20 years old and figured the smart thermostat would benefit us in the long run anyway. The problem still seems to persist after the successful nest setup. See target temp and actual temp pic link below, it has been set like this for a half an hour. Its about a 20 degree difference.


I included a pic of the pc board (honey well) that the thermostat is connected to. I have not changed any settings on here. Does this look correct or is it worth adjusting anything?

Apologies that I could not include any other information, but I will be happy to include more if I know what to find. As of right now I literally need to manually switch off the boiler from a switch to make sure the house comes back down in temperature. Any help is greatly appreciated!


  • Zman
    Zman Member Posts: 7,546
    Is the boiler firing and circulating water when there is no call for heat? You should be able to feel the pipes to be sure. If so, does it cool off if you remove the wires from the nest?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • JakeCK
    JakeCK Member Posts: 1,356
    edited October 2021
    This sounds potentially dangerous. When you say shutting it off at a switch, do you mean the switch by the boiler? And when you say it constantly kicks on even when set to a lower temperature do you mean that it is cycling on and off by some means other than the t-stat or do you mean it is constantly on until you flip that switch? I hope it is cycling off the aquastat. Please tell us the boiler does have a working pressure relief valve?

    Also post good detailed pictures of the boiler and its near boiler piping and any wiring directly on the site, instead of links.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,590
    edited October 2021
    I have a feeling that you have a problem that is not associated with the thermostat. The Honeywell control in your picture is designed to offer priority to domestic hot water. In order to accomplish this you need to maintain a minimum boiler temperature in order to provide hot water in the tankless coil located inside the boiler. You must also have an operating "Flow Control Valve" or "Flo-Check" valve (usually located in the supply pipe leaving the boiler) in order to prevent gravity glow of heat from the boiler to the radiators.

    I can think of at least 3 possabilities. The flow control valve is defective, or the flow control valve is manually opened (for service) and was never closed after the service was completed, and finally the circulator contacts on the Honeywell Control are fused together causing the circulator to operate even if the thermostat is not calling for heat.

    To check the third, you will set the thermostat to the lowest setting (or disconnect the thermostat wire completely), then check to see if the circulator terminals have power when the burner is not running. C1 and C2 should never have power if the thermostat is not calling for heat. FYI the C1 & C2 terminals will not have power when the burner is operating when there is no call for heat. You must check for power when the burner is not running

    Yours Truly,
    Mr. Ed

    PS... I just noticed that this is my 2,222nd post on this site. Maybe I'll play the lottery tomorrow. What do you think of the numbers 22-8-1-24-11-02 (That is leaving out the L and the C in the Honeywell part number)
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • mhylas
    mhylas Member Posts: 2
    I meant to directly reply to everyone, I only see the option to post a comment below.
    @Mr. Ed , I believe its option number 3 (honeywell control aquastat.) I say this because I sense a slight bit of a burn electronics smell near the boiler when I turned it back on. Even with the new thermostat off and disconnected, the heat was still going.
    Do you think its worth just replacing the entire aquastat? If so other than remembering where the wires went and the configuration of the dials, is there anything else I would need to do before turning it back on after replacement?
    Thank you very much for your help!

    I am going to play these number for the mega millions this week. If I win I will be reaching out to you!

    @JackCK, duly noted regarding about attaching the photos, will do this going forward. Also confirming yes, the switch by the boiler. It has a red faceplate stating "burner emergency switch." When I mean cycling on and off. The boiler will kick on while the floorboards are giving off heat still. stop for say 3-4 minutes and go back on, to keep the floorboards hot. This continues until I manually shut it off. Also, yes, I am concerned about a safety issue as well. I feel like it can be the aquastat malfunctioning after reading the feedback about the same make and model boiler (slantedfin liberty 2.)

    If this is the cause, I hope I can replace it easily myself.