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Temperature drops too low before boiler fires again

boobird Member Posts: 36
edited December 2017 in Gas Heating
There are periods of time where the system seems to work fine and maintains a nice temperature range between 73-74.

I have an intermittent issue where the room temperature drops too low before the boiler fires again. My thermostat is set to 74 and the radiators will turn cold. Eventually the boiler will fire up again, but sometimes the delay is significant (1 hour?). During this time, the room temperature definitely drops below 74.

  • I have a high limit aquastat set to 190
  • My thermostat is set to 74 permanently
  • Cycles/hour = 3
  • My circulator pump runs only when the boiler is firing. When the boiler stops, the circulator pump also stops.
My understanding is that in most systems,
  • Aquastat turns on the boiler
  • Thermostat turns on the circulator
I don't think my system works this way.
  • My thermostat seems to control the boiler
  • The boiler controls the circulator
Since the circulator pump does not work during the boiler's off times, the stagnant boiler water is much hotter than the radiators. The aquastat measures this hot water as greater than 190 and therefore does not fire even when the thermostat calls for heat. Is this a crazy theory?


  • Paul48
    Paul48 Member Posts: 4,470
    Does your thermostat require batteries? If so, try replacing them.
  • boobird
    boobird Member Posts: 36
    edited December 2017
    My thermostat is hardwired.
    I have a wireless remote sensor and the batteries are 10 months old. The sensor seems to read fine (if they somehow go offline intermittently, i am not aware of it)..
  • NY_Rob
    NY_Rob Member Posts: 1,370
    Is the thermostat in the "problem" room or is it in a different room?

    What is the role of the "remote sensor"?
  • John Mills_5
    John Mills_5 Member Posts: 935
    How old is the system? Oldies usually stayed hot all the time and the stat cycled the circulator. Newer boilers are cold start. Fire & circ would come on with call for heat. When the boiler hit high limit, circ stayed running to keep putting heat out until the stat satisfied. Something isn't right somewhere but have to know for sure how yours is setup to guide further.
  • mikeg2015
    mikeg2015 Member Posts: 1,183
    Not sure why there’s a 1 hour delay if you are at 3 cph. Maybe it’s reacting too slow and overshooting.

    You might need a Honeywell visionpro thermostat with PID loop control. They manage tighter temp control.

    This is also why outdoor reset works great. Longer circulator run times and boiler cycles to
    Hold a lower temp but emitters stay close to heat loss
  • unclejohn
    unclejohn Member Posts: 1,736
    Is this a new problem or has it been like this since install. The pump needs to run when ever there is a call for heat unless you have a low limit stat on the boiler to keep return temp up.
    HVACNUT Member Posts: 4,085
    I guess you started a new thread with the same topic.
    I posted on the other one. Sorry.
  • EzzyT
    EzzyT Member Posts: 1,166
    edited December 2017
    Gentlemen I went to look at this house today and the original system was a two pipe revers return hot water system. Some hack attempted to separate the system from a single zone to a 3 zone system. Boiler way oversized and of course piped incorrectly. I will be giving the homeowner @boobird a quote to do a complete system analysis a room by room heatloss of the entire house and to replace with a proper sized piped boiler.