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Modulating boilers ALL vary the flame. Vitodens, and perhaps others, also modulate the circulator.

The Munchkin (without Vision 1 that allows outdoor reset) varies the flame to keep delta-t across the HX as close to 20° as possible during a heat call. You set a fixed supply temperature.

When outdoor reset is used, the flame is modulated to achieve the computed "target" temperature. Delta-t will <I>not</I> have an artificial fixed amount.

If indoor feedback is added to outdoor reset, the rate of change in space temperature and/or the difference between desired temp and actual temp will modify the outdoor reset target temperature.

If indoor feedback alone is used, the desire is to use the rate of change and difference between actual/desired to maintain actual=desired as closely as possible. Again, delta-t is not fixed.

Smooth modulation continues as long as the lowest possible firing rate does not exceed the high limit during a heat call (Munchkin without Vision 1), exceed the target tempeature (outdoor reset) or result in actual > desired (indoor reset or feedback). When this happens, the boiler cycles on and off.

Zoning only affects modulation inasmuch as how it affects the load. For example if just one small zone calls, you're extremely unlikely to get modulation until other zone(s) call and increase the load. Proportional control (TRVs) are superior to digital zones (valves or circulators) since nearly all zones will have some load at nearly all times. This allows smooth modulation until the load of the structure as a whole drops below the minimum modulation rate.


  • Joe_55
    Joe_55 Member Posts: 62

    Could anyone explain when does a modulating boiler modulate, heating 3 zones of baseboard emitters with a steady 20* delta T even using ODR. thanks
  • Joe_55
    Joe_55 Member Posts: 62

    Mike: Sincere thanks for your in depth explanation. I will continue to absorb as much of this information as I possible can, so far if I understand correctly the HX delta T keeps the boiler modulating (gained knowledge)without Vision 1 ODR.

    However my munchkin having the older control board could not adapt to Vision 1 so I have installed a Tekmar 260 for out door reset with (1)indoor sensor in the living zone I plan to have this setup functional for this heating season.

    How, in your opinion will this system work are there any down sides or inprovements that could be made ? ( I am just a home owner with no stock in KEYSPAN)

    again many thanks for your valued opinion


  • Sorry, but I can't answer that question. Once you attach an after-market control to a modulating boiler you've sort of entered the "Outer Limits". "We control the horzontal. We control the vertical."

    Provided that the Tekmar is designed to take control of the burner, I would presume that the basic operation would be similar to what I described. If it has not taken control of the burner, I have utterly no idea what will happen. It's quite possible that the modulation ability will be lost. HTP told me a years ago that the original Munchkin could use aftermarket reset at the expense of loosing modulation.
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