Circulator Sizing for Indirect

Discussion in 'Water Heater Forum, Tanks' started by mattk22359, Mar 6, 2010.

  1. mattk22359

    mattk22359 New Member

    Messages:
    5
    Location:
    NH
    I'm trying to finish specing out the components I'll need to convert my tankless coil to an indirect water heater. At the moment I am trying to properly size the circulator pump. I was hoping someone could check my work. All calculations were taken from the article on circulator sizing on this web site located here: http://www.comfort-calc.net/circulator_sizing.html.

    I first calculated Equivelant Feet Pipe as follows: (assuming 1" copper):

    16' Copper (est.) x .042 = .672 EFP
    1 - Tee (side port) = 4.5 EFP
    4 - 90* Elbows = 10 EFP
    2 - unions = 1.5 EFP
    -----------------------------------
    16.67 EFP

    16.67 EFP x .04 = .67 Feet Head

    So assuming a Superstor Ultra SSU45 which has 7.9 Feet Head @ 10gpm (recommended flow rate)

    If I assume a 1.5 Feet Head for the boiler that would be:

    Boiler = 1.5
    Plumbing = .67
    SSU45 = 7.9
    ---------------
    10.07 Total Feet Head @ 10gpm

    If this is correct then using the the Taco Pump curves as shown here http://www.comfort-calc.net/Piping/Taco_pump_curves.JPG in order to maintain the Superstor's 10gpm recommended flow rate I would need at
    least Taco 0012 assuming it is better to have a little more flow rather than less then spec as indicated on Taco's own web site here: http://www.taco-hvac.com/uploads/FileLibrary/curves003-008.pdf and here: http://www.taco-hvac.com/uploads/FileLibrary/curves009-0014.pdf

    Thanks in advance for any additional insights/help/guidance you'd be willing to share.
  2. Dana

    Dana In the trades

    Messages:
    3,028
    Location:
    01609
    You're probably going for overkill here- unless you have enough boiler output that you need higher flow to protect the boiler there's no advantage to higher flow rate. Install enough pump to keep the boiler's delta-T down to the 20-25F range and you'll be getting about as much output out of the system as you would at higher flow, which would only result in a lower delta-T on the boiler, not more first-hour gallons out of the system.

    If your boiler's output is, say, 108,000BTU/hr and you're designing for a 20F delta-T, that means you need 108K/20=5400lbs/hr of water pump rate. A 8.34lbs/gallon that's 5400/8.34=647gallons/hr which is 10.8gpm. For a 25F delta-T you only need 8.6gpm, and for a 30F delta-T, 7.2gpm.

    As the flow rate drops the head also falls, and it's faster than linear- at 7gpm the head is roughly only about half what it is at 10gpm. At 8.5gpm it's about 70-75% of what it is at 10gpm, so a Taco 007 would be more than enough pump to keep the delta- under 30F, and it would run close to 25F. A Taco 0010 would lower the delta to close to a bit over 20F. A Taco-0012 would be overkill, raising the flow to something like 14-15gpm, and the delta-T down to ~15F for no benefit.

    Methinks a Taco-007 would be fine here. You'd be giving up at most a few percent of first-hour gallon performance due to lower-than-recommended flow on the SS. If you're concerned that the boiler is ultra-sensitive to high delta-T, go with a Taco-0010, then kick yourself when it turns out your head calc ran a bit high. :)
  3. cattledog

    cattledog New Member

    Messages:
    42
    Location:
    Portland, Oregon
    take a look at Grundfos 15-58

    If you are trying to span between a taco 007 and 010 take a look at the small Grundfos three speed. At 10 feet of head you can get 6 gpm on medium and 10 gpm on high with a 15-58.

    You will have some adjustment possibilities to tune the delta T across the boiler if the equivalent feet of pipe calculation is not exactly matching the real system.
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