Loop static pressure mystery

Discussion in 'General Discussions' started by JeffInCO, Jan 5, 2020.

  1. JeffInCO

    JeffInCO New Member

    After a recent upgrade (going from a single 4-ton WF Synergy to the same Synergy plus an additional 3-ton 504W11 W2W unit), I'm hearing a lot more noise from my now multiple loop pumps than I used to hear. The loops were originally designed to accommodate 8 tons, so the loop field is unchanged but we now have two flow centers with three pumps total vs the one single-pump flow center that we had before.

    The noise is clattering from newly added flow-check valves, and a "bubbly" sound in the pumps which is either air in the loops or (more likely) cavitation in the pumps, which I know is bad news.

    Due to the noise, and concerns about adequate flow, I've only allowed one unit at a time to run, aside from some brief testing of both units running simultaneously.

    I purchased a Dwyer digital pressure gauge, and a PT needle so that I could measure pressure at the PT ports on the units. (Got a temp probe as well... thanks to those on this forum for providing info in earlier threads on models for the pressure gauge and temp probes).

    On the new W2W unit, I've found with the unit running that source out pressure can go as low as -1 PSI (yes, that's negative one). With the suction side of the loop pump 2.5 feet above the PT port, the pressure is even lower there... hence my concern that we have cavitation.

    If the entire system sits idle for an hour or more, loop static pressure seems to settle at around 15 PSI. But strangely, if I run either unit for 10 minutes or so, then shut off, I see a static pressure of only around 5 PSI. If both units are left off, the pressure gradually increases back to 15 over the course of about an hour.

    After sitting idle for the hour, then starting one of the units, I see "source in" temps of around 50 F. After running for the 10 minutes, source-in temps drop to about 47-48 F and settle there as the unit continues to run.

    Is it normal for static pressure to drop so much after the unit runs for a while (as cooler water circulates in the loops)? I saw another thread on this forum where it was mentioned that loop pressures are typically lower in the summer, with higher temp water in the loops because the HDPE pipe has a higher coefficient of expansion than does water. Based on this, I would expect pressure to be higher as the loop temp decreases.

    Could this drop in pressure as the loop temps go down be caused by a lot of air in my loops? The contractor replaced one of the loop pumps and fixed a few leaks on the new flow center after he had hauled the flush cart back up to his truck. He increased the pressure a bit (not sure how much) with water injected through a PT port, but that obviously does not purge air from the system.

    I think if the static pressure remained around 15 PSI, I would not be seeing these low to negative pressures at the source out port.


  2. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    Hi and welcome!
    a lot going on here. There are check valves designed to not clatter or chatter. They were designed for vfd use. Have you tried goosing your pressure up to 15 psi via the pt ports while the units are running? This may eliminate or lesson bubbling noise which could be cavitation, air bubbles in loop, or both together.

    I would try goosing it and move forward based on results of goosing.
    hope this helps
  3. JeffInCO

    JeffInCO New Member

    Thanks, Eric. I was contemplating increasing the loop pressure anyway. I'll give it a try.

    BTW, I have four vertical wells, each 250 feet deep. Looking at pressures for the first time since the system was originally constructed in 2010, it just occurred to me that the pressure in the pipe at the bottom of the wells is 250 feet of head ( 108 PSI), plus any additional static pressure with which the system is charged at ground level. In the mechanical room, I can see that I have 1-1/4" SDR11 pipe . The older 1-1/4" is rated 160 PSI, the newly-added 1-1/4" is 200 PSI. I know the wells are 1" HDPE 3408 from the well permit, but have no idea what SDR of pipe was used or its pressure rating. What is typically used for vertical geo loops? I saw other threads where static pressures of 50+ PSI were mentioned; this seems dangerous if my vertical pipe is rated at 160 PSI also... the pressure at the bottom would be almost exactly the rated pressure of the pipe.

    Thanks again,

  4. JeffInCO

    JeffInCO New Member

    I increased the static pressure to 24 PSI (aimed for 25, but it settled at around 24 after I had disconnected the hose).

    The loop pump is definitely quieter, though I still hear some bubble or cavitation noise. Now the pressure is dropping to around 8 PSI on the Source Out port of the W2W unit while running. After the unit shuts off following a 10-minute run, I see the static pressure immediately increase to 14 PSI, then very slowly ramp back to 24. I'm still wondering what the cause is of the decreased static pressure after cooling the loops.

    A few other questions:
    • The Synergy flow center originally had its pump on the Source In side (it now has two pumps, one on each side). The W2W flow center has its pump on the Source Out side. Is there any reason to go with the pump on one side or another?
    • Do the pressure drop tables in the WF specifications take into account the 1 foot (Synergy) to 2 foot (504W11) difference in height between the source in and out port? The tables specify tenths of a PSI, while two feet of head is 0.9 PSI, so is significant.

  5. mtrentw

    mtrentw Active Member Industry Professional Forum Leader

    1- I wouldn't be concerned about pressure rating of pipes at depth. While the absolute pressure in the pipes may be higher 250' down, the relative pressure is what the pipes are rated for. You could have 10,000 psi of pressure in those pipes if there is 9,950 psi of pressure outside the loop.
    2- The fact that pressure drops down significantly and then rises again when off seems to point to air or gasses in your loop. Gas expansion and contraction with temperature change is much more notable than fluid expansion and contraction. My first inclination would be to eliminate air/gasses in the loop. Maybe a purge/flush.
  6. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

  7. mtrentw

    mtrentw Active Member Industry Professional Forum Leader

    Without knowing how accessible your system is and how easy it would be to modify, you may want to consider adding an air purge/bleed valve. I have one of these setups in my system to allow any dissolved gasses to find their way out.


    That being said, it may or may not work with large volumes of gasses, just dissolved gasses that are circulating by it. You did say you hear bubbles, so that probably means you're circulating the air, so it could work. As air comes out, you would need to have some method to bump pressure back up with antifreeze solution. I've recently posted on another thread my solution for goosing the loop pressure.
  8. JeffInCO

    JeffInCO New Member

    A followup... My contractor brought back the purge cart and re-flushed the loops. We definitely saw some air; no big "burps", but some moments where the water in the flush cart became rather bubbly.

    Static pressure seems to be much more stable now.



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