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Short Cycling Question

jad3675 Member Posts: 127
edited November 2021 in Radiant Heating
Dumb question is dumb...but with a mod/con hydronic system, what would be considered short cycling? The number of fires per hour, the run time of firing, or a combination of the two?

I aske because I'm seeing this behavior from (I think) my 2nd floor zone - the run at 3:20am can be ignored, that was a call for DHW. Unfortunately, I'm not sure what the thermostat is doing - it's calling for heat (it's set at 3 CPH), but if so it gets satisfied rather quickly, which seems odd. Since the boiler supply temp continues to rise when the firing rate goes to zero, I think the entire system is off at the zone panel (otherwise, if the circ was running the supply temp would drop, not rise - supply and return temps are taken from the boiler's modbus interface).
Main floor heat kicked on around 4:30am, I think. I really need to find a way to monitor the non-smart bits of the heating system.

Here's a link since the attached pic is crummy:


  • hot_rod
    hot_rod Member Posts: 22,251
    Most generally the opinion is at least a 10 minute run cycle every time it fires.
    What type of boiler, many of them have one or more short cycling parameters. Some you can delay the ramp, some you can limit SWT, some can be timed out. Check and see.

    The exception to the short cycle rule of 10 minutes would be a combi boiler. On DHW call they bounce on and off a lot, every time a tap opens, even for a few seconds of run time.

    At some point a properly piped buffer is the only absolute way to simmer them down now.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • jad3675
    jad3675 Member Posts: 127
    Thanks, Hot Rod. It's a TT Instinct - I'll look into the manual if see if there's anything more to tweak for short cycling. Looking at the chart, most fires are in the 12 minute range, so maybe I am thinking about it too much.