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Short Cycling in hydronic system

We are recent owners of a 1900sq ft home built on two levels that has
hydronic radiant floor heating on both levels, embedded in concrete
floors. The system runs with a natural gas fired Viessmann Vitogas 50
cast-iron atmospheric boiler and a Tekmar 360 digital mixing control with
associated pumps and sensors. We have only been in this home less than one
year so it is our first experience with high mass radiant floor heating
this winter. (Our winter is southern Vnacouver Island, temperatures
normally above freezing at their coldest and now getting up to about 8 deg
C during the day).

Before I call in the experts I wonder if there are any on this newsgroup
who might give me some advice on our system. The boiler seems to me to be
cycling too often. During the night the boiler will fire for about 3
minutes every 8 minutes, sometimes even more frequently. This seems to me
to be very rapid cycling given that the water in the radiant floor pipes
is unlikely to be cooling down that rapidly??

Is this sort of cycling of the boiler normal? If not, what adjustments
can be made to the mixing control to reduce the on-off cycle without
affecting system efficiency?

Any ideas greatly appreciated.

Larry Gagnon
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Comments

  • Jay_12Jay_12 Member Posts: 46
    Short Cycling

    Hello Larry,

    This could be a case where the boiler operates the boiler pump, thereby trapping the hot water in the boiler. The tekmar 360 Mixing Control system pump contact must operate both the boiler pump and the mix system pump as per attached drawing A360-1.

    Please call tekmar customer service at 250-545-7749.

    Jay www.tekmarcontrols.com 250-545-7749
  • hrhr Member Posts: 6,106
    provide us some more info

    Was a heatloss calc done for the home? If so what is the total load?

    How closly is the boiler output matched to that number?

    How many zones, and does the heatloss calc sheet breakdown the load for the individual zones?

    Typically boilers are sized for the load on a "worse case" or design day. Most of the heating season the boiler output is greater than the building heatload. Multiple small zones make it hard for a boiler to adjust to small load days.

    A few small things can help. Spread the boilers aquastat differental. Most of those boilers I have installed have a 15° diff. An aquastat that allows a 20 or 25° spread helps some. Fire the boiler at 155 and run to 180 for example.

    Some tekmar controls offer this feature to adjust that boiler diff. operation.

    Another option, more expensive, is to add some buffer capacity. Often an additional insulated tank can be plumbed in to store some capacity and lenghten the on/off boiler cycles.

    Of course a modulating boiler really helps as they adjust output to the varying load. Too late :)

    Opinions vary on how to define a short cycle. I like to see a minimun 10 minute on cycle regardless of the load. Some manufactures claim the equipment is built with 100,000 cycle life and short cycles are not a problem.

    To me it is annoying to hear all those components click on and off as frequently as yours are.

    Short burner on cycles can prevent the HX and flue from getting up to temperatures adequate to dry the condensation and this will create problems down the road also.

    Collect some more data and lets see where you stand. It could be some as simple as some control adjustments or you may have an oversized boiler that needs some other ideas.

    hot rod



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  • PlumbobPlumbob Member Posts: 183


    > Some

    > manufacturers claim the equipment is built with

    > 100,000 cycle life and short cycles are not a

    > problem.


    Even if there is only 1 cycle per hour (24/day), 100,000 cycles is 4000 days or 11 years. At 3 cyles/hr it would be <4 years.
  • hrhr Member Posts: 6,106
    Guess I got that number wrong

    maybe it should be 100,000 cycles per minute :)

    hot rod

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