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  1. #1
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    head loss math through field loops

    Hi
    I wanted to check in about this:
    There is a home with a geothermal system with 3- 500 ft loops in the side yard. Pipe is 1.25" polpy pipe.
    I wanted to make sure the circulator pumps are not too small.
    Calculation for 1-1/4" pipe moving 20 gpm (both heat pumps with both stages on)
    through approx. 1600'= 5.6' per 100'= 5.6' X 16= 89.6' just from the field

    Seems kind of high. Is this correct?

  2. #2
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    Not hard to figure but need a little more info. Are loops 500' totqal length, piped in parrallel? What is pipe size in loop? How far from house to loops, and what pumps are you using? What is tonnage of units? If loop field is piped end to end you are in trouble. 20 gpm generally covers about 6-7 tons total.

  3. #3
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    Thread Starter
    Quote Originally Posted by Earth Power AC View Post
    Not hard to figure but need a little more info. Are loops 500' totqal length, piped in parrallel? What is pipe size in loop? How far from house to loops, and what pumps are you using? What is tonnage of units? If loop field is piped end to end you are in trouble. 20 gpm generally covers about 6-7 tons total.
    Name:  Geothermal system as built.jpg
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  4. #4
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    Nice diagram. The shunt pipes between all three loops concerns me. Are these vertical loops or horizontal? (It does not affect calculations, just curious. 510' loop are usually vertical made loops). All said, I'd have to get back to the office to run the loop calculations through software. Do you have fittings at the units to measure pressure drop across the unit and calulate flow? If those shunt pipes are in as shown the whole thing could be bypassing the loops.

  5. #5
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    Thread Starter
    The loops are vertical. I was not there for the outside installation work, but The installer of the vertical loops, a major drilling company, is reputable company.
    There are fittings, pete's plug to measure pressure drop.
    My math is incorrect at the top of the post. I found out later the ground loop was installed in parallel
    Because loops are in parallel, I found out the pressure drop is in the range of 40 to 50 feet through the ground loop in the field.

  6. #6
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    Your head pressure is probably less than that. Probably closer to 25-30. Are you sure the loops are 1 1/4". Usually the main pipe will be that size and the loops 1" or 3/4". I'll run the numbers Monday for the fun of it. You should be able to get the charts on the units from Hydron, and measure pressure drop across the units to determine actual flow. Typically 3-5 psi pressure drops is normal.

  7. #7
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    Thread Starter
    Quote Originally Posted by Earth Power AC View Post
    Your head pressure is probably less than that. Probably closer to 25-30. Are you sure the loops are 1 1/4". Usually the main pipe will be that size and the loops 1" or 3/4". I'll run the numbers Monday for the fun of it. You should be able to get the charts on the units from Hydron, and measure pressure drop across the units to determine actual flow. Typically 3-5 psi pressure drops is normal.
    The designer told me that the circuit was 1- 1/4". Don't know how the circuit was actually set up outside.
    Correct the hydron unit is about that 4 to 6.5 depending on the stage. I thought the chart in the hydron manual was in feet of head but it might be psi. I am getting about 10 psi of static pressure near the supply manifold with pumps off.
    I have about 7 psi at the inlet supply of one of the heat pump with system on and about zero psi on the return circuit that going down to the basement.

  8. #8
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    Thread Starter
    Earth Ace,
    I think my biggest issue is being able to read and know the correct pressure drop through out the system.
    I do a lot of radiant heat and boiler systems. Basically this Geo system is a hydronic system that doesn't expand and contract
    as say a boiler in the basement with baseboard circuits on the upper floors.
    Seems like I need to get pressure readings then figure out if those readings are normal for this particular hydronic system.

  9. #9
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    I ran the pipe sizing through my software and got a curve on your system. It looks like about 27' of head at 22 gpm. I had to guess at the flow resistance through the units but used a comparable unit so it should be close.

  10. #10
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    Quote Originally Posted by Earth Power AC View Post
    I ran the pipe sizing through my software and got a curve on your system. It looks like about 27' of head at 22 gpm. I had to guess at the flow resistance through the units but used a comparable unit so it should be close.
    So that being said, the two Wilo pump model # 1.25 X 3.30 seem to be able to handle the head and the GPM for this system.
    The pumps are set up in a push/pull arrangement.

  11. #11
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    I did not look up the pump curves but calculated it with 2 Grundfos 1/6 HP pumps and they were just right.

  12. #12
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    Thread Starter
    I am probably getting 27' at 22 gpm through the ground loop.
    There are two flow meters in the supply pipes that go to each heat pump. The highest reading when both heat pumps are working is 8.5 gpm
    each. I believe that the head must be higher than 27' because I am getting a total of 17 gpm and not 22 gpm of flow through the interior piping anyway. It seems like I need more head through heat pump supply circuit to overcome the friction losses in that portion of the system.

  13. #13
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    That is a little low for correct operation. I will try to look up the pump curves on these pumps. I'm pretty sure my calculations are close.

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