Hom much can flow vary?

Discussion in 'Maintenance and Troubleshooting' started by mtrentw, Jul 6, 2012.

  1. mtrentw

    mtrentw Active Member Industry Professional Forum Leader

    I just finished install on two heat pumps. Comfort-Aire 2 ton and 3 Ton units.
    I have a shared loop field. Once inside the house, the two branches run in parallel, each with its own Grundfos UP26-150 three speed pump. I've got both pumps set on middle speed.

    My flow rates are:
    2 ton alone - 7.5
    3 ton alone - 10.5
    Both units running -
    2 ton - 4.5....... 3 ton - 7.5

    I've seen 3 GPM per ton as a target flow rate which would call for 6 and 9 GPM respectively. With current setup, I am getting those numbers minus 1.5 with both running or +1.5 with either running alone.

    Anyone have concerns with these numbers, think I should play with pump speeds any?

    When I've checked Delta T is running from 9-13 degrees in any case.

    Thanks,
    Trent
     
  2. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Pump speed

    The spec's on the unit will indicate required flow.

    Note, most of those 3-speed pumps are the speed of a fixed speed pump at the high setting and slower at the other settings.
     
  3. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    giggles

    Look at the lower end of the performance charts for gpm, in the I/O manuals. I like the delta T, and the temps will come down when the back fill settles in. You need more rain.

    Please play with the pump speeds I would like to know what changes if anything. The goal was 100% heating load so I will be more attentive as we more into winter and heating season, if we ever do this weird weather year.

    Your insulation is being picked up in the AM and should ship Monday or Tuesday. Got caught in traffic while the Pres had a beer, yesterday. Spent today in an attic at 140* F, could have used your help.

    Just track all of the changes not just flow, remember that looking at a heat pump's performance after keeping you comfortable is a moving target. Change only one thing at a time so you know what the change does to the operation. With the fancy flow meters we installed drop the two ton unit to low speed and see what that does to the overall operation. Watch your delta P on both systems, and watch the static pressure on the loops. Let me know if you think we need to add to the loops before doing same. We will be letting air out for a few weeks yet.

    Had a great time, thanks,

    Mark

    ^^^^ ducks as the flow center folks see this.
     
    Last edited: Jul 6, 2012
  4. mtrentw

    mtrentw Active Member Industry Professional Forum Leader

    Addendum. Just got off the caleffi website. All my previous flows should be 0.9 correction factor for glycol load.
     
  5. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    You might devise a control scheme to put the pumps in low when just one is running and high when both are running, but it probably wouldn't be worth the trouble, complexity and expense.

    3 GPM per ton is more of a guideline than an absolute requirement.

    Mark obviously has a whole lot more experience with this system than I, but I agree with him about the delta T and rain helping to consolidate backfill.
     
  6. mtrentw

    mtrentw Active Member Industry Professional Forum Leader

    After a week of 100* days, loops are looking good at 92* return and 77* supply. Hopefully getting a thunderstorm tonight to help with consolidation of the trenches.
     
  7. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Correcting flow rates for antifreeze

    I'm not sure if this is clear, but I'll see if I can make it so.

    Flow rates are exactly what is measured in the field. Doesn't matter what the antifreeze %'s are.

    When designing a system you need to make corrections for antifreeze type/%. Both for flow (head loss) and heat transfer purposes.

    Your system wants that flow spec'd by the manufacturer. Usually 3gpm/ton for closed loop units. How to get that involves a bit of math (with those antifreeze corrections you are noting). What it is in the real world, is just some pressure drop measuring.
     
  8. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    77 / 92 are nice low temps for a loop in July, but the 15 degree delta suggests insufficient flow.

    Closed loop delta T should be sub 10*F. 3 GPM per ton should yield at or below 10*F delta T
     

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