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pressure/head loss, resistance of baseboard loop

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dpenny
dpenny Member Posts: 16
in Gas Heating

Background: am replacing current baseboard heater/boiler cuz it could not keep house at 70F this past winter. I have 5 quotes/bids that differ GREATLY as to equipment & price. 3 utilize primary/secondary setup, while 2 say primary only is fine for my single loop situation. We have mild winters, one week at 20F. I want to figure the pressure loss in loop to help me evaluate the circs being suggested. Which of the following would you trust more:

  1. First principles: length of baseboard piping (house perimeter) + elbows + up/down into crawl space = 286 equivalent feet. 3/4" copper @ 8gmp ⇒ 25 psi loss or 57 feet.
  2. Use existing pump curve & current flow: Grundfos UP 26-99F @ 4gpm ⇒ 30 feet head.

#2 include loss of boiler, whereas #1 does not.

Are these two close enough to size new circ pump to then compare what is in the several quotes? Please don't wander too far from my Q.

THX.

Comments

  • hot_rod
    hot_rod Member Posts: 25,385
    • 8 gpm is way to high of a flow rate for 3/4 copper, 4 gpm is best practice,
    • 4 gpm in 3/4 copper type M is 2.5 fps velocity.
    • .016 psi/ ft pressure drop. So call it 300 feet of equivalent length gives you 4.8 feet of heat
    • So the circ spec is 4 gpm at 4.8' head. Well within a small residential circulator.
    • Pressure drop through the boiler depends… a basic cast boiler next to nothing. A restrictive mod con would be more restrictive, that is where primary secondary or a hydro sep comes in. Separate the two circuits.
    • 8 gpm in 3/4 copper would be 5 fps (feet per second) velocity, that will be noisy and cause wear in the tubing and ells. 4 fps is about as high as you want for velocity. Rule of thumb 2- 4 fps.
    Screenshot 2025-07-30 at 3.53.42 PM.png
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • HydronicMike
    HydronicMike Member Posts: 88

    Why wouldn’t you figure out the btu requirements first? Then all sizing flows off of that.

  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 18,275
    edited July 31

    @hot_rod

    I am using B & G system sizer and I am getting way different #s. 4 gpm in 3/4 copper is 4.63 ft/100 feet of pipe. @286TEL' that would be 13.24' feet of head not to mention the 8 gpm he wants to get.

    Your chart is psi not head

    Alan (California Radiant) Forbes

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