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Can I stop my oversized boiler from short cycling?

HelpInAK Member Posts: 41
I guess my question is 2 part:
1. Anything I can do to stop my combi boiler from short cycling when moving between DHW and space heat calls?
2. Why does this only seem to happen when outside temps hit -25F and colder?

Issue: my Lochinvar Noble combi is oversized for the house. Space heat set point is 135F, DWH is 120F. If we use any hot water while there is a call for heat, the boiler cannot transition back to the space heat call without going into a short cycle. A new ignition flame will ramp up the 120 water right up to 145 and the boiler can’t modulate down quickly enough, and takes a 1 minute nap before repeating the cycle. We’ve had to resort to planning out any warm water use by ramping down our thermostats to ensure a call for heat won’t come around the same time we are using DHW. This mainly seems to happen during very cold spells of about -25F and colder, which alaska is experiencing a pretty long stretch of right now. However, I’m wondering if this has always been a problem and am just really noticing it now since the boiler is running for most of the day. Any easy solution to this? Thank you!


  • Leon82
    Leon82 Member Posts: 684
    edited January 2020
    Does the combo have the ramp settings like the regular boilers? You could set it to stay at 20% for a good chunk of time if so.

    I guess it would only work if it considers the transition a new cycle though
    HVACNUT Member Posts: 5,480
    Not sure about combi's but a mod con with an indirect on priority wired into the boiler (not a zone panel), the boiler must revert back to space heating. Meaning the boiler will end the DHW call, go into post purge, then start again for space heat.
    EBEBRATT-Ed Member Posts: 14,560
    buffer tank
  • atrufant73
    atrufant73 Member Posts: 5
    Add a thermal storage tank and use it as a hydraulic seperator to seperate your primary & secondary piping
  • hot_rod
    hot_rod Member Posts: 20,827
    Maybe a time delay for the heat call after a DHW? Or a post purge on the shuttle valve, burner off, to bring the temperature down before the heat switch. There is an anti cycling function on that control, not sure how it works on DHW priority? Worth a try.
    I have heard of this before, Lochinvar may have a work around.

    You can limit that boiler on heat call 20- 100%. firing rate. That may help cycling on the heat calls.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream