Can loop flow rates be too high?

Discussion in 'Maintenance and Troubleshooting' started by newgeohp, Jun 28, 2010.

  1. newgeohp

    newgeohp New Member

    Hello All
    Overall I think my system is working well (Its saving me money), But I think the loop flow rates may be high causing low loop delta T. The Installer is trying to find his notes from the install and hasn’t got back to me with the measured flow rates. My crude calculation come up with 14 GPM and may explain the low delta T measurements.

    Is there a value that is too high? Everything I see listed from WF says 2-3 GPM per ton and the performance data tables only goes up to 8 GPM for my system. Should flow be set (restricted) for a certain delta T?

    I have a Water Furnace Envision NVD026B111CTL 2 Ton, 2 Stage, ECM blower with Desuperheater. I have attached a PDF file that I created this weekend to document my system performance for my future reference and it has all of my system particulars that I have access to.

    Tonight I will turn off my desuperheater and take EWT and LWT measurements and try and calculate flow rates. I have also thought about playing with the valves to reduce flow to get a delta T in the range of 5-8°F and see what effect it has on system performance.

    Any advice or insight would be appreciated, Thanks Richard.

    Attached Files:

  2. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    14 GPM is far more than an 026 needs. 6 GPM would suffice. Off the top of my head I'm not sure what flow is needed through 1.25" loop pipe to ensure turbulent flow, but 14 seems high for that as well.

    I don't think it would be useful to add head loss by creating a restriction. About the only thing I can think of to do would be to retrofit a smaller pump. That might save a hundred watts or so in pump power, resulting in saving a couple Jacksons per year in power cost - it would take many years for that to pay off the cost of the pump and labor to retrofit it and repurge the system.

    Does the calculated GPM and head loss match the curve for the pump you have?
  3. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    You can not get accurate measures with DSH on.
    That said, you misunderstand, low delta t with high flow is not necessarily bad, as long as ground couple is still able to transfer heat.
    If you think of units of heat energy vs degrees, a 2 delta t at 14 gal/min is not necessarily better or worse than a higher dt at lower flow.
    The biggest (though modest penalty) is extra pumping cost.
    I did have a 3 speed pump shipped to me inadvertantly once and found "high" (obvious choice to installer) did not give best performance.....seemed EWT dropped too rapidly.
  4. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    It looks like your system is running pretty good.

    Using 1.25" pipe combined with a relatively short loop (570 ft of pipe plus header) can results in very little pressure drop, and thus a high flow rate. Using only one 26-99 can result in 14 gpm flow, but 26-99 is already pretty efficient pumping.
    I am not sure if I would change anything.

    You are only rejecting 17KBTU/h, the rest might go in your hot water tank.
    Last edited: Jun 28, 2010
  5. newgeohp

    newgeohp New Member

    More information

    Thanks for the Input, Here some more information for what it is worth.
    I use frictional loss of the pipe, fitting, and Delta P on the HP and did an iterative process using the pump curve to come up with the 14 GPM (head loss matched flow on the curve). Tonight when I got home I turned the desuperheater off and my delta T went from 1.5 to 2.5 in stage 1 and stage 2 went from 2.5 to 3.5. I then adjusted one of my valves to increase my delta T and took some measurements over several hours. I did notice as I restricted flow my delta T went up but my EWT went down (better heat transfer?). In stage 2 with Delta T of 7°F my EWT went down 2°F. Lower EWT = Lower Compressor currents (about 200mA or 42.9W).
    Now with the desuperheater turned off and the valve opened back up I used the WF calculation LWT = (EWT + (HR/(GPM*500))) and calculated 19.5 GPM in stage 1 and 18 GPM in stage 2. So alls this tells me is I really need to get the measured flow rate with my calculations coming between 14 and 19 GPM. As a note I did have to turn the 90° ball valve about 70° to get a delta T of 7°F.
    With 790’ of 1 ¼ pipe I only have 42 gallons in the loop which mean I’m doing a change over every 2-3 minutes.
    I’m not too worried about this I was just trying to check the performance of the system this weekend to make sure it was within specification. I knew the flow rates would be higher with a small loop but I couldn’t find anything that said high flow rates are good or bad. The only thing I found talk about the problems with low flow rates. I wasn’t sure what the effect would be on the compressor, loop heat transfer, erosion of the piping.
    Flow center is a B&G 26-99F 1 pump. Most all the information about my system is in the attachment on the original post.
  6. Looby

    Looby Member Forum Leader

    I suspect the 14 GPM guesstimate is way too high. Head loss increases
    almost as GPM-squared, so flow tends to be self-regulating. I'd be very
    surprised if you're getting more than 10-12 GPM. That's more than you
    need, but nothing to worry about. You could live with a smaller pump --
    but the (Grundfos?) 26-99F consumes only 245 watts.

    Low delta-T (resulting from a higher-than-necessary flow rate) actually
    increases efficiency -- but not enough to justify intentionally oversizing
    the loop pump (that's why WF doesn't recommend more than 8 gpm).
    But since you already have about the smallest pump in common use,
    it's doubtful that a smaller one would save enough to justify a swap.

    Adding a restriction with the existing pump would save little or nothing.
    Pumping power is proportional to (gpm x delta-P). A restrictor valve
    will reduce gpm, but will also increase delta-P.

    Source-side delta-T, in itself, isn't important -- what you want to verify
    are the heats of extraction/rejection = GPM x delta-T x brine_factor. If
    they're within spec and the load-side delta-Ts (across the air coil) are
    also within spec, the system is working correctly.
  7. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Well said.
  8. Looby

    Looby Member Forum Leader

    My bet would be that the drop in EWT indicates less heat transferred
    from your house to the loop. According to the spec sheet, any reduction
    in flow (at constant EWT) decreases BTU/hr, kW, and EER. Of course,
    reducing BTU/hr into the loop field must eventually lower EWT.

    It may seem counterintuitive at first glance, but the whole point of geo A/C
    is to raise EWT -- 'cause that's the only way to lower indoor air temp.
    Last edited: Jun 29, 2010
  9. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    again possible looby.
    main thing is to calculate heat of rejection, which will be the best measure.
  10. Looby

    Looby Member Forum Leader

    I concur 100%. Good flow and delta-T measurements are required.
    Back-calculating GPM from a presumed HR verifies nothing.

    Last edited: Jun 29, 2010
  11. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    I am not familiar with wf at all. Don't they publish pressure drop numbers through the coil like everyone else to use as a indicator of flow rate?
  12. Looby

    Looby Member Forum Leader

  13. WF_Inc

    WF_Inc Member


    WaterFurnace’s service and installation training is a 3-4 day course, and much too extensive to go through in a forum. Unfortunately, we do not have enough information to answer your questions. We trust that you will continue to work with your installing contractor to determine if your unit is running to manufacturer’s specifications. If your contractor is in need of technical assistance, they can contact our technical support staff.
  14. newgeohp

    newgeohp New Member

    OK, lets slow this down a bit.

    Mr. Water Furnace your post looks to be a generic reply but don’t worry, I love my Water Furnace system and I’m happy with my installer. I think the system is working great, the house is comfortable and it will have saved me 31% on my first electric bill. After all is said and done that’s the only two measurements that matter.

    With that being said the original question “Can loop flow rates be too high? What I got from the replies (Thanks for your input) is Theoretically it shouldn’t matter and other than some small efficiency losses from using a pump that is slightly larger than required, but is OK because my pump is the smallest standard pump typical used.

    My end result is to check the system performance and calculate rejected heat (HC) based off of measured EWT, LWT, and loop flow rate. I knew my flow rates would be higher than normal with my small loop, so for fun I was trying to determine my flow rates by working backwards (method 1)and by calculating frictional losses (method 2). I started with assumption my rejected heat numbers match the published data based on the measurement #1 (it’s saving me money). This is probably not a good idea since assumptions are like rule of thumbs. But both methods did confirm the idea that my flow rate is going to be high (>10 GPM). This is what brought me to the question "can too much of a good thing be bad". I also know if my measured flow rates are not very hi then my rejected heat numbers will be low and I may need to investigate more.

    Thanks for your input, and I will let you know what the actual flow rates are when I get them. Hopefully he we get back to me soon but I don’t want to bug him in his busy season. We can even take bets to see who is the closest

    Looby 10-12 GPM
    Richard 15 GPM
  15. Looby

    Looby Member Forum Leader

    Good summary.

    The only major consideration I didn't see mentioned was antifreeze
    type and concentration. Since you're in VA, I presume antifreeze is
    essential during the heating season -- and antifreeze (of whatever
    type) has a significant effect on p-drop versus gpm calculations.

    As a WAG, I'll assume that your antifreeze is 20% methanol; and
    based on that, I'll change my flow rate prediction to 9 GPM @ 60°F.

    Last edited: Jun 29, 2010
  16. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Looby invested considerably more detail into his analysis than did I.

    Since the present pump is already the smallest in common use, leave everything well enough alone (don't restrict flow by partially closing valves) and enjoy the economical operation of the system.

    The extra flow builds in a capacity reserve for extreme weather - be happy.
  17. newgeohp

    newgeohp New Member

    Results are in.

    Well, the results are in and the flow rate based off of the pressure measurement during the installation was between 9-10GPM. They have since measured it with the B&D flow meter tool and got 6GPM.
    As it turns out the low delta T on the water side was due to the low delta T on the air side. When they check it they got 14°F and it should have been 21°F for the given operating conditions. They took all the pressure and temperature measurements on the refrigerant side of the unit and got some unexpected values that Water furnace is figuring out now.
    But the house stayed cool and only lost a few degrees when it was 105 outside and saved me $85 more dollars for a total of $134 in two months. Hopefully I will save even more when the bugs are worked out.
  18. geome

    geome Member Forum Leader

    Please let us know the reason for the low air delta-T when you get an answer. Thanks!

    Was the air delta-T measured at the coil?
  19. newgeohp

    newgeohp New Member

    Yes, Air measurements were taken right at the unit.
  20. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    The low airside Delta-T could be caused by too much airflow or any of charge, TXV or duct problems. If in fact the low airside Delta-T is causing low waterside Delta-T, I wouldn't expect the cause to be high airflow.

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