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is slower circ pump better?

R. Kalia
R. Kalia Member Posts: 349
This fall I will have a condensing boiler with outdoor reset and constant circulation. As in many old houses, I have excess radiation; I don't need 180F water with my current cast-iron boiler, 150F is more than enough all winter.

But suppose I get a much slower circulator with the new system. Now hotter water is needed to get the same heat output from the radiators, but the return water will be cooler (since the average temperature should remain roughly the same for the same heat output). This is good for efficiency.

So it seems to me that it is a good thing to slow down the circulator until one actually needs 180F supply (boiler output water) on a design day. In other words, one should design for higher supply water temperatures rather than lower temperatures---sounds nonsensical when I say it that way.

Does this make sense? What are the disadvantages?


  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928


    1) You've increased heat loss from the boiler jacket by producing a higher temperature water.

    2) You've increased transmission losses in the piping by producing a higher temperature water.

    3) With velocity low enough to get a great enough delta-t to keep the boiler condensing, the velocity will probably be too low for the boiler and it would likely kick out on high temp safety.

    4) With such low velocity any balance problems (particularly in gravity conversions) will be magnified and you probably won't get sufficent flow at all of the rads.

    5) With such low velocity, the iron rads will no longer heat evenly.

    6) It doesn't take much energy to circulate those old systems--in fact you typically have to intentionally introduce restriction to give the circulator something to "work against". If you don't it's electrical efficiency drops, it overheats and its life is considerably shortened.
  • Dave_22
    Dave_22 Member Posts: 232
    Slower pump...

    That might work, but what I would do is to just adjust the setpoint temp of the boiler. You should be able to do that to any new condensing boiler. If you pump less, you might not get enough flow through the boiler and short cycle like mad. It depends on what pump your talking about and how the system is piped. Need more info......
  • R. Kalia
    R. Kalia Member Posts: 349

    Oh. Well, never mind!
  • Steamhead
    Steamhead Member Posts: 16,719
    Well, don't quit just yet...

    How much radiation do you have, and what circ did you plan on using?

    Then go to


    to find out why this is important, especially on an old gravity system.

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  • R. Kalia
    R. Kalia Member Posts: 349

    Thanks for the link to the article! It was very useful. It told me that the existing circulator is in just the right ballpark (within maybe 20% at most).

    If you want the details, We do have a converted gravity system, with a BG series 100 pump. The system could put out maybe 150KBH (I though K=kilo was thousands, M=mega was millions!!) if it ran all the time at 180F; the house doesn't need that much heat and we won't be using 180F water but that's irrelevant for estimating KBH (MBH).

    The considerations in the article don't change based on whether it is an old cast-iron boiler or a new condensing boiler. So I guess we need the same or similar circulator in the new system. The fact that the series 100 curve is relatively flat is good because we'll be putting TRVs on all the radiators.
  • ScottMP
    ScottMP Member Posts: 5,884
    Now I am confused

    Low velocity on a gravity conversion and a low velocity pump wold cause balance problems ? Wold'nt a slow pump speed mimic more closely the original flow of the system design ?

    Injection might do they trick and allow constant circ, in the system. How slow can he get it ? I can't imagine a off the shelv pump being low enough to coause problems. What am I missing.


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  • hr
    hr Member Posts: 6,106
    Most of the small condensing boilers

    need a lot of flow to prevent the small HX'ers from overheating and flashing. You may be best with a PS piping to get the correct pump on the boiler and a smaller low flow pump for the gravity conversion piping.

    I'm not sure any of the condensing boilers would get along with a B&G 100 as their only pump. Except possibly a tank style condensor Polaris, Voyager, Cyclone etc)

    Check the manufactures requirements for flow and pressure drop through the boiler. Pump it accordingly to prevent HX problems.

    hot rod

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  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    It seems the idea was to "bump up" the reset curve to high temp (say 180°) on a constantly circulating system with standing iron rads--a system that is most likely oversized and only needs considerably lower temperature.

    In such a case delta-t would have to be extremely high and flow extremely low--even lower than under gravity circulation. Like you said you probably couldn't find an off-the-shelf circulator that could do this to begin with but if you did (or modified one), I don't believe it would mimic gravity flow.

    Gravity flow is based on different densities of water between the radiation and the boiler. The greater the elevation difference the stronger the potential for flow.

    If you try to mimic this low level of flow with a circulator that works by changing pressure, your flow will most likely take the "easy" route through the lowest, closest rads.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    With the TRVs you definitely don't want to be supplying them with supply temps significantly higher than needed. The TRVs will strive to keep the average radiator surface temperature at just what is needed to maintain space temperature. If supply temperature is excessive, the TRVs will have extremely low flow. The radiator may well heat unevenly, wear will be increased and you may even hear noises.

    p.s. The "M" is the Roman numeral for 1,000. It's also used in the printing industry when indicating quantity.
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