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Sizing circulators for old gravity-hot-water heating systems

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HeatingHelp
HeatingHelp Administrator Posts: 650
edited January 2018 in THE MAIN WALL
Sizing circulators for old gravity-hot-water heating systems

When servicing old hot-water heating systems that originally circulated by gravity, but now use circulating pumps, I began to notice a lot of oversized circulators. Here's how to correctly size circulators for old gravity-hot-water heating systems.

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  • Sebra_wells
    Sebra_wells Member Posts: 1
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    When I replace older boilers that are converted gravity system I use the system syzer app from bell and gossett to calculate head pressure for system and add head pressure of boiler and near boiler piping. Then choose your pump based on thses requirements
  • Steamhead
    Steamhead Member Posts: 16,901
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    Thanks, Erin B)
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Consulting
  • Erin Holohan Haskell
    Erin Holohan Haskell Member, Moderator, Administrator Posts: 2,326
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    Steamhead said:

    Thanks, Erin B)

    Thank you!

    President
    HeatingHelp.com

  • jumper
    jumper Member Posts: 2,289
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    Do we want flow through system (not necessarily through boiler) to be same as when it was gravity?
  • Paul48
    Paul48 Member Posts: 4,469
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    I've always been curious as to what that was. Typically
  • Steamhead
    Steamhead Member Posts: 16,901
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    Yes. If we over-pump these systems, the water short-circuits through the radiators.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Consulting
  • Alan (California Radiant) Forbes
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    Thank you, Frank! Very helpful!
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • GW
    GW Member Posts: 4,702
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    I’m curious, if the Water flowed with no circ in the old days, I don’t understand why the pump needs to be upsized. Doesn’t the convection process aid the circulation process?

    I rolled the dice with an Alpha pump on an old gravity job we did a couple of months ago (old boiler 210k input, new boiler 151k input). The Alpha moves about 11 gpm. Considering the giant rads, I thought the tab-bit lower flow would be fine. And it is, went though the wicked cold snap just fine.

    It seems like giant rads can allow for a slightly lower flow. Just wondering
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    gary@wilsonph.com
  • Paul48
    Paul48 Member Posts: 4,469
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    Without the circ, each supply riser could only accept so much hot water, always leaving some for the next radiator in line. That's why the mains are so large. When you pump it, the water want to find the path of least resistance....5,4,3,2,1,..........drop the sixth radiator. That's typically what happens. The most remote radiator gets dropped. By over-sizing, you're trying to create head in the full length of the main.
    GW
  • Alan (California Radiant) Forbes
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    Gary: The newer boilers with smaller heat exchangers wouldn't tolerate gravity flow; too slow to pick up all the heat.

    Interesting about the Alpha, but why use one where the ΔP never changes?
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • Paul48
    Paul48 Member Posts: 4,469
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    I often wondered about using a DT circ with the sensors on the most remote supply and return.
    GW
  • GW
    GW Member Posts: 4,702
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    Alan here on Mass the utilities have a $100 instant rebate on ecm pumps so all the heating guys use these, they’re cheaper!

    If I didn’t get at least 10 gpm I would have put a bigger pump in. Ball valved flanges, all good
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    gary@wilsonph.com
  • GW
    GW Member Posts: 4,702
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    Alan yes this was a Bosch KBR, has its own boiler pump. Alpha was just for space heat
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    gary@wilsonph.com