Circulator Sizing for Indirect

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

  1. mattk22359

    mattk22359 New Member

    Feb 14, 2010
    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:

    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 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: and here:

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

    Dana In the trades

    Jan 14, 2009
    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. :)
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  4. cattledog

    cattledog New Member

    Jan 29, 2006
    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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