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Return Line - Grundfos Alpha 2

All, I'm having problems with a Grundfos Alpha 2 15-55F only showing 3 GPM max on a dedicated hot water return line. This is a new install that is returning the hot water to the cold inlet of a Rennai RU199iN Tankless Water Heater. The return line is approximately 50 Ft of 1/2" copper coming off the end of the 3/4" hot water trunk line at the furthest fixture in a small 3 bedroom single level home. Any ideas of what might be the issue?


  • hot_rod
    hot_rod Member Posts: 14,748
    What is the return temperature? I would guess you would need less than 1 gpm on a loop that small
    The pump doesn’t move the flow to the fixtures, it just returns enough to keep that loop temperature adequate at the far fixture.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • ScottSecor
    ScottSecor Member Posts: 385
    I'm trying to picture your setup, please take a photo and share it with us, so we can visualize. I agree with @hot_rod you should only need about 1gpm for a system that small. You did remember to install a check valve near the circulator pump correct?

    What is the issue, not enough hot water or that it takes too long to get to the faucet?
  • AMG63
    AMG63 Member Posts: 14
    Not that it would have anything to do with performance but if this is for domestic hot water it should be the stainless steel model.
  • GroundUp
    GroundUp Member Posts: 1,170
    edited February 2020
    What's the problem? Odds are you're not moving 3 GPM with that setup, and cast iron circs like the 15-55F do not belong in domestic water systems. Aside from the circ being wrong, it sounds like you're moving recirc water so what's the issue?
  • hot_rod
    hot_rod Member Posts: 14,748
    The Alpha has a S in the model to indicate stainless a 15-55 can be stainless flange or Union
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • MarketCO
    MarketCO Member Posts: 2
    The problem is that the pump when wired into the Circ-Logic of the Tankless will show error code 63 and signals the pump to shut off. According to Rennai, the flow rate to the tankless needs to be 6 gpm or higher. To start troubleshooting I wired the pump separate from the tankless and it's flowing a max of 3 gpm. With a pump capable of flowing 21 gpm I wouldn't think it would be an issue even after factoring in the loss for fittings.

    ScottSecor there is a check valve installed on the cold supply line. The issue was it was taking too long to get hot water to the farthest fixture.

    As far as "cast iron circs like the 15-55F do not belong in domestic water systems" I'm not sure what you are trying to reference. The pump has an epoxy coating that resists tuberculation, which is common for non-coated cast iron systems. Plus, Grundfos advertises the Alpha 2 15-55F as suitable for "domestic hot-water recirculation".
  • GroundUp
    GroundUp Member Posts: 1,170
    What you're not factoring in is the way-too-small pipe size for flowing 6 GPM. Any circulator pump has what they call a "curve" which dictates flow rate at a given pressure drop, and the only way an Alpha2 15-55 will flow 21 GPM is with zero resistance. 6 GPM through a 1/2" Type L copper pipe is 8.3 feet per second of velocity, versus 4.1 FPS at 3 GPM which is already impressive. AT 8.3 it would be whining like a blender in the piping. Even at 3 GPM (the readout on an Alpha is not always accurate), you've got 8 ft of pressure drop in 50 ft of 1/2" tubing and that's with no fittings or valves, which is about all the grunt the 15-55 will give you. I'm no professional with domestic water but if you want to get 6 GPM through that recirc line, you need a larger pipe size. Odd they'd require a minimum flow of 6 GPM when most DHW loads are less than 2. Domestic water wear out even a stainless impeller and housing, so don't be fooled into thinking that thin coating is going to stop the deterioration of the cast iron. It can be used, obviously, but is not the proper choice.
  • BillyO
    BillyO Member Posts: 263
    does the boiler have a schematic for HWR piping? is it piped as drawing shows?
  • Zman
    Zman Member Posts: 6,535
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • BillyO
    BillyO Member Posts: 263
    Minimum activation flow rate is .4 GPM for your unit
  • ScottSecor
    ScottSecor Member Posts: 385
    On page 5 of the link below the manufacturer suggests a minimum of 3/4" return pipe. Could this be the problem? I think the manufacturer is assuming that the return line is up to two hundred feet long. Thankfully in your case I think you mentioned the return line is only about fifty feet. Rev D Rinnai Circ Logic with Grundfos Kit.pdf
    EBEBRATT-Ed Member Posts: 8,823
    If you need to flow 6 gpm (which sounds like a lot) the 1/2" tubing is too small, your restricting the circulator
  • ch4man
    ch4man Member Posts: 244
    plus your using an Alpha, not even on Rinnai's radar
  • Jamie Hall
    Jamie Hall Member Posts: 15,724
    At the risk of pointing out the obvious... the most flow you could possibly need is 1 gpm. You get hot water eventually at the bathroom from a faucet? OK, all you're trying to do it mimic that flow all the time, so the hot water is always there. In fact, you could likely get by with a good bit less flow.

    You do have to have the check valve on the return line, of course, so that when you do open a faucet the water comes from the hot water heater.

    Now. Do you have those hot water lines insulated? If not, why not? Both main line and return. Without insulation, a recirculating system like that is incredibly wasteful.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Henry
    Henry Member Posts: 986
    Check B & G System Syser! Normal 1/2 inch copper pipe flow rate is .5 GPM to 1.5GPM!
  • icy78
    icy78 Member Posts: 397
    If there was 0 heatloss in the hot water piping you would need 0 gpm thru the recirc loop, right?
    So on an insulated loop, especially a short one, as yours is, there is minimal heatloss and all you need is to move enough water to overcome that heatloss.
    I cant remember the figure but 5 btu per foot for 3/4 maybe.
    You only need to figure for the return piping.
    So 50 ft x 5 btu is 250 btu.
    At say a 10 delta T you need a trickle.
    For pump health say .5-1 gpm is more than enough.
  • GroundUp
    GroundUp Member Posts: 1,170
    K bye!
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