pressure in closed loop systems

Discussion in 'Vertical and Horizontal Loops' started by milkweed, Mar 4, 2016.

  1. milkweed

    milkweed Member

    While shopping for sources of SDR-11 pipe, some manufacturer's only offer 3608,
    a few others offer 4710. I understand that the numbers refer to their resistance
    to their respective categories so 4710 is better than 3608, but is there any
    advantage to having 4710 pipe for geothermal loops?

    The main advantage thrown out for these is that
    3608 has an operating pressure of 160 psi at 73 degree F, and 4710 is 200 psi.

    Do operating pressures in a closed loop system ever come close to 160 psi?

    Some of the explanations for the technical details between 3608 and 4710 mention
    surge pressures, but that all seems to be in reference to irrigation or drainage (open systems).
    Does a closed loop system experience pressure surges in normal operation?

    The manufacturer of my heat pump throws out this in their design guide:
    "NOTE: The expansion and contraction of earth loop piping will cause a 50 to 60 psig
    water pressure change in system between summer to winter."
    I can imagine that polyethylene pipe will contract radially in winter,
    but the loop fluid is not going to compress.
    Would the pressure drop calculations for PE pipe be based on winter temps?
    (so it is already accounted for when selecting a circulator pump)

    Or, do closed loop systems require an expansion tank?
  2. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Well, imagine wells 500ft deep, and you serve a 7 story building (140ft high), so you have 640ft of water pressure on the pipe in the bottom of your borehole. Now you also pressurize your system, and you add the pumping pressure. So yes, pressures in the pipe at the bottom of your borehole can easily exceed 200 psi.

    You also might have standing water in the borehole providing pressure from the outside, might be dry there too. Now your loop is running warm in the summer, so your pressure rating at 73F become much less.....

    Pressure drop tables are for certain temperatures, pressure drop increases with colder and denser water (above 39F). The pipe shrinks more that the water, thus pressures increase in the winter.

    One of the reasons no pressurized systems became popular.
  3. milkweed

    milkweed Member

    Are vertical loops generally limited on the depth then? It seemed to me that most of the residential vertical loops were around 200 feet deep, but I haven't been paying that much attention to vertical designs. Do vertical loops use a lower SDR for greater pressure tolerance?

    In a fairly level horizontal field would the pressures only come from the circulator pump(s) then?
  4. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    We limit ourselves to 500ft, since technically you become a mining operation if you go deeper than 500ft. Yes, some engineers spec SDR-9 pipe for vertical operations. But it also hinders heat transfer due to thicker pipe walls. It come more down to the heavy duty equipment needed to drill deeper quickly, thus the lack of applications.

    Yes,in horizontal systems, pressure come from pressurization and pumps.
  5. milkweed

    milkweed Member

    I apologize if this is a totally idiotic question.
    So do heat pumps (the heat exchanger loop) have less pressure tolerance than say SDR-11 / 3068 (16o psi) piping?
    So that the circulator pump should be connected off of the heat pump's out port?

    hhmmm, well I guess the circulating GPM speed/volume would not be that high, unless you bought too powerful of pump than is needed for the system
  6. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    No issue with pressure in the heat exchanger.
  7. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    If the system is full of fluid, the fluid will not cause a pressure issue.
  8. mrrxtech

    mrrxtech Member

    I didn't have answers to your question but I looked up the number system used and some background on PE. Read the following info then go to the website to learn more using the link below. HDPE Background and Benefits
    For the past 80 years, since its discovery, Polyethylene has established itself as one of the most reliable and versatile thermoplastics on the market. Recently, after the continued success and development of bi-modal HDPE resins throughout Europe, North America has followed suit in its implementation of PE4710 resins (preceded by PE3408 and PE3608 resins). The introduction of these bi-modal resins has demonstrated remarkable benefits, namely in the form of higher pressure ratings, less raw material needed, increased resistance to environmental stress cracks, and longer lifetimes.
    Read more including nomenclature for PE pipe plus other info at this link:

    My horizontal loop uses the 3608 PE, it's 1.5 inches ID which gives me 180 gallons in the loop with 2000 feet running 4 feet below the surface of a field. The pipe is very thick walled which should help prevent a leak underground.

    The gauge on my loop reads 15 psig each time I add water, but it slowly drops back down to 0 psig and stays there. My Trane Unit has a 25 psig limit on the water side heat exchanger so I limit the pressure below that or isolate the Unit when flushing the loop.

    That increase in pressure of a loop in summer to 60 psig must me due to the loop having no surge tank, or bladder expansion tank and with no air vent the loop. As the water heats up it expands raising the pressure if there is no place for the water to go to.

    When the two loop circulating pumps run there is no indication on the pressure gauge. Also I don't see a pressure with heating and cooling mode. The reason is probably due to having a PE Surge tank welded in at the high point, and a 2 gallon bladder tank as used for hot water tanks, piped into the system. I also have an Auto Air vent that came with the surge tank. You can buy the Auto air vent separately and install it on your loop high point.

    You can connect you Loop Circulating pumps on either the in put or output. I have one pump pushing water into the Unit on the supply line and a 2nd pump taking a suction on the Unit Outlet side.

    My loop drops 80 or more feet into a valley over a 600 feet length, runs through a spring fed area in the valley, then returns back up the hill to the house. The pressure reads 0 due to the loop being a complete circle/continuous run.

    As far as apologizing for your questions, most instructors will tell you there are no dumb questions except the ones you don't ask.
  9. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    If a loop is full of fluid there is only head loss for friction of the fluid in the pipe. No pumping up or down penalties.
  10. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    mrrxtech, I'm trying to wrap my head around a system that has a surge tank and an expansion tank in it. Not sure I understand the thinking.
  11. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    First due to the physical property of the HDPE pipe expanding more than water fluid with increasing temperature, you see a pressure drop in the summer, not an increase in pressure.
    Second, you told us that you have a 6 ton system, which nominally needs 18 gpm. Pumping 18 gpm of glycol through 2,000 ft of 1.5 HDPE pipe is resulting in over 100 ft/hd of pressure drop, which would need a huge amount of pumping power and would result in a very inefficient system.
  12. mrrxtech

    mrrxtech Member

    There are various ways to avoid pressure increases or decreases (forming a vacuum) in a closed loop water system when the temperatures are known to change from hot to cold. The surge tank is used in a commercial Pressurized Water Reactor Primary System and is called a Pressurizer, which makes this the first choice for those who work in that industry when doing a DIY geothermal install. The problem was that surge tanks are hard to find made out of PE, but they do exist and I purchased one and had it welded into the high point of my loop.

    As the loop water contracts or expands with the temperature the Surge tank is the best way to avoid pressure changes since the water system has a high point to expand into as it heats up, and contracts out of as the water becomes colder. The surge tank can be used to determine volume in the loop based on level, a place to monitor pressure since it has an installed gauge, a place to add water since there is a ball valve and fitting to hook a water hose to, and an auto air removal Watts float valve.

    For domestic hot water systems or home heating boilers a 2 gallon bladder tank is used as the surge volume. I added the 2 gallon tank as a source for water as the air slowly found its way out of the Watts float. I was influenced by my Brother to add the tank, since he had used one in the past on a loop. Either method would work as a surge volume. I can isolate and fill the bladder tank with additional antifreeze or water then line it up to the loop for a slow make up over time after the initial in surge into the loop due to the bladder tank being pressurized to 20 psig when filling it.

    If you go on ebay and do a search on Geothermal Heat pumps and Components, you'll see flow centers that have a bladder tank in then middle of them, and pump centers built around a return tank that sits on the floor. This supports the use of either surge pressure control method. My loop has both features by my choice.

    If you've seen pictures of my loop on this website you can see the surge tank and bladder tank in the pictures. I can find the company that makes the PE Surge tank with all the features discussed above if you decide to use the high point PE surge tank method in your loop. Only one company I have found makes this PE Surge Tank, I was lucky to find it before completing my loop install.

    Let me know if you have any further questions. Ron
  13. mrrxtech

    mrrxtech Member

  14. mrrxtech

    mrrxtech Member

    I was looking for a device that Trane shipped with my Unit which allows you to pipe the Desuperheater water to and from the water heater by removing the water heater drain valve and inserting a device with a pipe within a pipe plus a drain valve.
    Trane calls it a Water Heater Hook Up Kit.
    Carrier sells their Geothermal Unit with desuperheater without this device to make the hookup simple to perform.

    Anyone seen one of these Water Heater Hook Up Kits for sale?
    Last edited: Jun 8, 2016
  15. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader


    OK makes sense. I had, in my mind, an open to the atmosphere surge tank (atm pressure) and an expansion tank (15psi?), both which would contradict one another. But, I've seen these before.

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