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Customer looking for thermostat to help get longer boiler run time.

bob eck
bob eck Member Posts: 930
On of my customers installed a condensing gas boiler that is 9 years old. It is an TT PE110 condensing boiler with built in IDWH
It is installed with cast iron radiator system all one zone and is piped P/S.
Everything is good.
Home owner now wants to install an WiFi programmable thermostats. Has old set back Cronotherm II thermostat. Day setting 71 and night setting 69.
I know with condensing boilers you should set the thermostat and forget it but this is what the home owner is doing.
Now they want WiFi thermostat.
Run time of boiler is about 7 minutes. Contractor wants to get longer run time for boiler to about 10-15 minutes when there is a call for heat. Heat up the radiators so longer off cycle.
Is there a Honeywell thermostat that let’s say the thermostat is set for 71 degrees and the boiler turns on and instead of boiler running 7 minutes it would not turn the boiler off until the thermostat hits 72 degrees. Contractor believes he will get longer run time this way.
Boiler is running on outdoor reset set to factory settings.
Any suggestions contractor looking for a Honeywell thermostat


  • SeymourCates
    SeymourCates Member Posts: 162
    Yes, good old TT with a 4:1 turndown and a 30K minimum fire. Sorry, but the 'stat is unlikely to be the problem. The seven minute run time is caused simply by the boiler operating at 30K and the radiation delivering less than 15K. At milder temperatures and lower SWT, the output from the CI rads is minimal.

    Since the system has quite a bit of mass, I'm a bit surprised at the 7 minute run time as it takes significantly longer than 7 minutes just to warm up all that iron.

    Have the contractor determine if the call is still ongoing after the seven minute time period. If it is, the 'stat is confirmed as not the problem.
  • bob eck
    bob eck Member Posts: 930
    The home owner told the contractor he knows when the boiler kicks on because there is a click noise and the yellow light comes on the thermostat and boiler runs for about 7 minutes. Could the old thermostat be bad.
  • SeymourCates
    SeymourCates Member Posts: 162
    The question is not how long the boiler runs but how long the thermostat is calling. A H/O is ill suited to provide such data in most cases as they connect the length of the call with the boiler run time.
  • Zman
    Zman Member Posts: 7,573
    The actual boiler run time is determined first by the length of the call from the t-stat. After that, it is determined by the amount of radiation, minimum output of the boiler, system mass, and boiler on/off temp differential.

    It sounds like the owner would simply like to have a longer call for heat and assumes the boiler is running whenever there is a call. Perhaps some owner education is in order. Just because there is a call for heat, that does not mean the boiler is firing.

    The owner will get a longer call for heat by lowering the boiler temp on the reset curve. The downside to this is that the actual boiler cycles will get shorter as the amount of heat radiated will be less.

    If the boiler has the Trimax control, you could divide the total run time by the number of cycles and show the owner what is actually going on. I would also be sure that the t-stat does not have an anticipator that is set incorrectly.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • NY_Rob
    NY_Rob Member Posts: 1,370
    Robertshaw just released a WiFi module for their 4000/5000/6000 series T-Stats. Those T-stats have user programmable differentials in 0.5deg increments up to 3deg.
    If the customer sets the differential to 1.5 or 2deg his calls for heat will be longer.

    Of course swapping out thermostats will do nothing to address short-cycling if that's the actual issue.
  • Gordy
    Gordy Member Posts: 9,546
    edited December 2017
    The only way to get a longer heat call is with lower SWT. everything else is a fixed constant running with what ever the OAT is at the time.

    The limiting factors are the amount of radiation, minimum modulation, and the TDR of that boiler.

    Has the HO even fine tuned the outdoor reset?
  • Gordy
    Gordy Member Posts: 9,546
    Your dealing with 9 year old technology. Out dated at the pace the industry has changed achieving better control strategy, and smaller boilers. Why do you think mod/con manufacturers have taken these machines to higher tdrs, and lower low end modulation.

    The only other thought is do deeper set backs. There goes comfort.

    We have no idea how well the TT is matched to the heat load of the structure in question.
  • mikeg2015
    mikeg2015 Member Posts: 1,194
    With hot water radiant, it's never that simple.

    1) Get them a Honeywell Visionpro Wifi and set to 1 or 2 cycle per hour.

    2) boiler run time depends on min firing rate, radiator capacity and given water temp. Flow rate can also restrict boiler capacity and cause short cycling.

    Ideally, the boiler should not have been sized more than the capacity of the connected radiant load. Ideally, it's sized somewhere between the connected load and heat loss at design conditions. If zoned, you might size closer to heat loss as you don't need extra capacity for recovery.