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How to limit a short-cycling boiler

My boiler seems to be short-cycling, at least a little bit. I'm curious if there are any steps I can take to limit it. On average over the last 24 hours, it's running for about 7 minutes then off for 14 minutes.

The details:
Boiler - Rheem RCBH199DVLN (120K BTU)
3 Zones (all in concrete slab) - Garage (1800 sq ft), basement garage (420 sq ft), basement (1906 sq ft).
Temps - 120 degree designed input with a 20-degree delta. Boiler uses an outdoor reset and supplies lower water temperature based on outside temp. I'm using Taco variable speed pumps that speed up/down to maintain a 20-degree delta.
Basement garage is set at 50 degrees. Garage and basement are both set at 70 degrees. It's 15 degrees outside today (central Iowa). The period between about 1am and 5am in the chart below when it was running near continuously it was between 0 and -5 outside.

Here is a 24 hr chart of my temps coming out of the boiler. This is what I'm using to determine when it is coming on and shutting off.

In your opinion, is this short-cycling? I've heard it's ideal to have the boiler running for at least 15 minutes per cycle. Are there any changes you guys think I should make?


  • Jamie Hall
    Jamie Hall Member Posts: 21,112
    That almost looks like your thermostat is defeating your boiler's outdoor reset. Ideally a boiler with outdoor reset, modulating, should simply sit there at the water temperature the outdoor reset has determined and just simply run... and run... and run. That's your best efficiency, and if the return water is below 140 you can really get good results.

    But. What I'm seeing in the graph looks an awful lot to me like a "smart" thermostat set for hot air heat cycling the boiler on and off.

    So... what do you have for a thermostat, and how is it set up?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • bphillips921
    bphillips921 Member Posts: 22
    I have three Honeywell T6 Pro Hydronic thermostats. The garage thermostat uses a slab sensor as the control temp, the basement garage and basement use the built-in air temp sensor. The thermostats are set at a constant temp.

    The thermostat has a heating cycle rate setting to limit the cycles per hour. I left those at the default and recommended, 3CPH for the garage with a slab sensor and 1CPH for the two zones with an air sensor (page 8 of manual - https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Honeywell-TH6100AF2004-Install-Instructions.pdf)

    I also have the Minimum on time set to "off". I actually just now saw that setting. Maybe setting that to "15 minutes" on each zone will solve my issue.
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,278
    Does it have a boost function? That could be working against you here and could be disabled or delayed for a longer time.

    A minimum output of 18,000 is pretty high for underground spaces, so constant operation during the coldest days is probably possible but doubtful you can run for months at a time.

    Also, you might be able to electronically limit the output of the boiler (ie turn it from a 18-120kBtu boiler into a 18-40kBtu boiler).

    Last, 120 degrees is really high, especially if a space is being kept at 50. Try 100 just as an experiment.

    There's not much of a downside to increasing runtimes and it does add efficiency so give some of the above a shot.
  • bphillips921
    bphillips921 Member Posts: 22
    Yes, the boiler has a boost setting, and it's enabled. But it doesn't look like it's being used as the supply temp has never gone over the set 120 degrees. From the manual:

    The setting is to increase the set temperature of the unit on cold start ups if the actual room temperature doesn’t reach the thermostat set temperature quick enough, the Boost time function will increase the set temperature of the Combi Boiler 10°F (5.5°C) after the selected Boost time setting has passed.

    I think my garage is the biggest BTU hog (1,800 sq ft) and it is set at 70 degrees. So I figured the boiler is always needing at least the 18Kbtu to keep up with tha space. Maybe I'm wrong on that.

    I don't see an obvious way to limit the boiler output, unless it's this combustion rate setting. Is that it?

    I'll drop the input water tonight to see if that helps.

    Thanks for the tips!
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,278
    That's the one, you can drop the capacity there.
  • bphillips921
    bphillips921 Member Posts: 22
    Nice! I'll drop the temp and limit the boiler to see what I get.

    Is the goal really to have the boiler always run? Or should I just try to increase the run times for each cycle?
  • nosirra1Arrison
    nosirra1Arrison Member Posts: 53
    You have a PM (message)
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,278
    Yup always running while meeting the temperature is just fine. It’s more pumping energy, which may or may not matter (some mod cons have high head loss, some don’t)