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Zone Circulator sizing

sgall Member Posts: 37
Hi,  I have been reading and researching as much as I can but I am still confused about zone circulator sizing.  I am converting from oil to NG and having a Burnham Alpine 80 installed into a 3 story/3 zone house with old cast iron radiators.  Each zone is a steel pipe split loop system that loops around in the basement and the radiators branch up from this loop.  The loop piping is 1 1/4" until the split comes back to a common return pipe which appears to be 1 1/2".  The radiator piping coming off the loops measure 7/8" outside diameter measurement using a caliper (1/2" maybe?).  Using  the following and the article below would suggest I need 7 GPM for each zone.

Zone 1 - 433 sq ft radiation, heat loss - 39,224

Zone 2 - 472 sq ft radiation, heat loss - 24,768

Zone 3 - 482 sq ft radiation, heat loss - 15,396

<a href="http://www.masterplumbers.com/plumbviews/2003/sizing_circulators2.asp">http://www.masterplumbers.com/plumbviews/2003/sizing_circulators2.asp</a>

I have read that you calculate the pump head based on the longest run.  This is where I am confused.  I am assuming that when calculating the longest run you have to include the smaller radiator piping that branches off from the main loop - is this correct?   For a split loop do you have to double the longest run measurement or multiply the longest run by 2x?. In terms of flow noise do you only base the maximum GPM allowable on the main loop or the smaller radiator branch piping?  Thanks in advance for any light you can shed on this.


  • Ironman
    Ironman Member Posts: 7,373
    edited September 2011
    Circ. Sizing

    The numbers that you provided (sq. ft of EDR vs. heat loss) are conflicting. Not saying that they're wrong. Someone may have installed more radiation than needed. And tightening the envelope of the house would reduce the load.

    Let me explain: Older systems typically run 170* average water temp. At this temp you multiply the sq. footage of the radiation x 150 to get the btu output. With a total of 1387 sq. ft of EDR, you would have an output of 208K  btu's, but your heat loss numbers total 79k btu's. If your heat loss numbers are correct, then you're considerably over-radiated. Don't panic! This is actually a good thing. It allows you to lower the supply water temp substantially and that will save fuel and give better comfort in return.

    Zone 1 requires the highest output temp per sq. ft of EDR. Based on the btu load vs. s footage of EDR, I come up with multiplier of 90. That translates to 140* average water temp. This means the the hottest water that the boiler will have to supply will be 150*. The outdoor reset curve should be set to supply this on the coldest day of the year. Any other time the control will supply cooler water as the outdoor temp rises.

    Now, to answer your original inquiry on circ sizing:

    Zone 1 needs 4gpm.

    Zone 2 needs 2.5 gpm.

    Zone 3 needs 1.6 gpm.

    Considering that you have c.i. rads with old (and probably large) iron pipes, a Taco 007 or a Grundfos Up15-42 circ would be more than sufficient on any zone. You could also use a Grundfos ups15-58 on low speed. :)

    P.S. If your heat loss numbers are correct, then you'll need to go to the next size up on your boiler. You have to size on boiler output and then subtract any piping losses, not on boiler input.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • sgall
    sgall Member Posts: 37
    Thanks so much for your reply

    Bob,  I realize we are doing things a bit backwards but we spent so much money on oil last season that we had to do something.  So we are switching to gas and replacing the boiler before we improve the house.  I am purposely undersizing the boiler somewhat with the aim of tightening the house up. The article/link I referenced by Frank Steamhead Wilsey had the following:

    "If a heat-loss calculation indicates using a smaller boiler than the amount of radiation would seem to indicate, size the circulator (but not the boiler) to the radiation. Again, even though you're not generating as much heat, the radiators and pipes are still there with all that water in them."

    So the subject is confusing to me in that I know what you calculated is the Industry standard way but if I use the table provided in the article/link I come up with 7 GPM per zone.  The Grundfos 15-58 would probably work for both speeds but then I am presented with the problem of sizing the pipe that is shared by the three zones.  If i go with your calculation I think 1" would work but  if each zone requires 7 gpm then a 1" shared pipe would only allow 8 gpm and if all 3 zones were calling for heat there would be a bottleneck. 

  • Ironman
    Ironman Member Posts: 7,373
    Info Overload?

    You've obviously done a lot of research in seeking to educate yourself about hydronics and you're to be commended for that. I wish more H.O.'s would do so, it would make my job easier.

    I'm looking at your diagram and comments in your other post on the Wall. In it, you're showing prim/sec. parallel. You state that your system is monoflo with 3 zones plus an indirect. The part that you circled in black is the common primary, which you state is 1 1/2in., correct?  And the drops to the boiler are 1"?

    What you may be missing (correct me if I'm wrong) is the principle of hydraulic separation that will take place in the primary (main) header that you've circled. In this section of pipe, two distinct circuits and flow rates can share a common area between the two boiler Tee's and exchange heat without substantially disrupting flow in either circuit. In fact, counter flow can exist under certain circumstances. The 1in. lines are sufficient for the boiler drop loop to carry 80k btu's at 8gpm with a 20* Delta T, or 80k btu's at 5.3gpm with a 30* Delta T. This is what Mark was referring to when he said that the boiler could handle a 30* Delta T and 1in piping.

    Regarding Frank's article on pump sizing for a converted gravity flow, I concur that a slight up-sizing is good and the 15-58 is still more than sufficient. My confusion, and maybe yours, is that you stated that this is a monoflow system, but Frank is dealing with converting a gravity flow system. They are not the same. Could it be that you have O.S. Tee's in the system and that you've mistaken them for monoflow Tee's? I would recommend Dan's article on gravity systems to clarify things. You can find it under the "Systems" tab above. In either case, the 15-58 circs. should work fine for you.

    I would also suggest a slight modification regarding the indirect piping: instead of connecting to the primary main, Tee in between the boiler and it's circ. and also on the boiler supply.Then use the priority feature of the boiler control. you'll still use a circ on the indirect, but the boiler circ will be held off while the indirect is receiving heat.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • sgall
    sgall Member Posts: 37
    Cannot thank you enough

    for your input and you're right I may have overdone the research and am perhaps overthinking this. 

     I don't know what size the common primary pipe is as that is new piping and what I was trying to figure out.  We are going to make it 1 1/4". 

    I have may have misread Frank's article because I thought it addressed both gravity conversions and non gravity conversions.

    I pried off a plate from one of the main loop to radiator tees and it says Monoflo with the B&G logo in the middle and then Up-feed written below. 

    Will the indirect modification you suggest work with a Superstor 60 which requires 10gpm?

    many thanks,

  • Ironman
    Ironman Member Posts: 7,373

    It will work with any brand. That's the standard way to do it.

    Best Wishes on your project.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Steamhead
    Steamhead Member Posts: 16,823
    Actually, the article did address both scenarios

    but emphasized gravity conversions as these are most likely to be over-pumped. Basically I took the info in the old Bell & Gossett Handbook and worked the numbers in the other direction, so it could be used with any brand of circ.

    It sounds like you're on the right track though. 
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
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