The Great Debate: Pressurized vs. Non-Pressurized Geothermal Ground Loops

When walking into another contractor’s installation, it never ceases to amaze me how differently installers approach things. This can be attributed to the lack of serious construction standards for ground source heat pump systems, the lack of qualified contractors willing to get proper accreditation and an ever-changing product landscape. Once we determined that closed ground loops were the best alternative for our equipment and our local environment (see this link), one of our other dilemmas as installers was whether to install pressurized or non-pressurized ground loops and flow centers (pumps). Most of the non-pressurized systems we had seen were installed by contractors trying to shave installation costs by using PVC for their indoor circuits. Most of these that I have seen were leaking after a year or two. Nothing beats good old HDPE fused piping to avoid closed loop leaks.

Ultimately, we decided that pressurized loops and flow centers were the best alternative for to meet our needs and those of our customers. However, there are varying opinions depending upon your perspective. I did some research and surveyed several other accredited installers and here is a summary of the potential advantages and disadvantages of each:
Non-Pressurized Loop Advantages:
• Lower installed cost than pressurized loops in HDPE or copper.
• All loops can be manifolded inside the mechanical room to aid in isolating loop blockages by using shut-off valves and the equipment to purge entrained air.
• Open loops allow the customer to check and add fluid himself (not sure if this is an advantage)
• Allows for contraction and expansion.
Non-Pressurized Loop Disadvantages:
• Potential leak of fluid if done in PVC or if customer constantly removing cap to check fluid level.
• Safety concern for customers exposed to methanol or antifreeze and its potential flammability.
• Higher potential for bacterial growth.
• Potential for the dilution of anti-freeze concentration by customers adding water to systems.
• Potential for system damage during flushing or purging if done incorrectly.
• Aesthetically not as pleasing as to piping configuration.
• Vertical lift limitations for any compresserized unit mounted above the flow center.
Pressurized Loop Advantages:
• Neater installation
• Less chance of anti-freeze dilution.
• No exposure for customer to flammable anti-freeze or fumes.
• Two flow center connections vs. three
• Easier to flush out dirt with flush cart at 6 ft. per second
• Less potential for damage to flow center
Pressurized Loop Disadvantages:
• Higher installed cost
• Potential for system loops to “go flat” over time due to temperature change and loop expansion and contraction that requires simple re-pressurization
• More difficult to isolate individual loop problems without interior manifold

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5 comments on “The Great Debate: Pressurized vs. Non-Pressurized Geothermal Ground Loops

  1. David Huston says:

    I have a closed loop pressurized system. The loop system has 5, 100′ loops which are connected to a manifold. The manifold has two pipes coming into the house under the basement floor and then up to the pump station. The system is starting it’s 5th year. Since the installation the piping coming into the house through the floor has sunk nearly 3 inches. The contractor has moved the pump station downward three times to take the strain off the piping and fittings. Both the installer and the contractor who installed the loop piping have never had this problem. They aren’t sure what is causing it thus aren’t able to provide a solution. I’m not sure how big a problem this may be.

    Has anyone else had this happen? If so what if any solution was used.


    • airideal says:

      David, keep in mind that any answer I give you here is pure conjecture as I know nothing about the parameters of your installation. That being said, we make a point of bringing all ground loops through the foundation wall for manifold and not under slab. I am assuming that this was a new home 5 years ago and the decision was made to install the ground loops before the foundation was poured. This is done in many areas, but it is our opinion that loops run horizontally under a foundation may leave themselves prone to the effects of foundation and footings settling. Earth is constantly in motion, albeit very slowly over time. Should the slab or foundation walls settle, as they may do, this could cause unwanted pressure on the ground loop. Naturally, if your foundation is settling, the horizontal loop below will do so as well. Depending upon how slack the horizontal loop is between the vertical bore and the manifold, this may or may not be a problem. HDPE piping has elasticity and permeability, to a point. I would be concerned if your system has shown significant pressure fluctuations in the process. Has your contractor had to add significant brine or increase or decrease system pressure on a regular basis? Or has the system pressure remained constant?

      • David Huston says:

        Thanks for the prompt reply.
        This geothermal installation replaced the original HVAC system.
        This house is 22 years old and shows no signs of settling etc. There are a few hairline cracks in the floor but none in basement walls or interior walls either.
        The manifold is in the front yard and the distance from there to the entry point in the basement floor is roughly 150′.
        The installer has had to purge the system twice for entrained air. The last time they installed a pressure accumulator (this may not be the actual term) to help maintain pressure. He hasn’t had to add brine. The system has gotten low on pressure but never high. The there was a valve near the pump station which may have been a source of air or loss of pressure. This was removed when the accumulator was installed.

        As I mentioned this has occurred over the last 4 years. If it continues at some point the shut off valves will be too close to the floor.

        Maybe they will just have to add some additional piping as time goes on? That providing the system doesn’t exhibit any problems.

        Thanks again for your response. Any ideas are more than welcome.

        Dave H.

  2. sz says:

    Hey sorry to necro this old comment thread, but I have a question you might be able to answer:

    I had two closed-loop, pressurized systems installed in 2006. Each is 2.5 ton attached to it’s own ~300’ well (600’ total, each). They are not manifolded. One of them seems to have air in the loop (I hear cavitation every 30 or 60 seconds or so when it’s running). I purchased RAM-1, which allowed me to use the test port to take the pressure and add water to the loop (I was careful to purge the air from my hose before doing this). The pressure was zero, and I pushed it up to 40 psi–I purged the air from the pump until I didn’t get any bubbles. No more cavitation.

    By the next day it was down to 20, and the next it was back at zero. Since then (a month) I’ve been hearing cavitation as usual.

    I have reason to suspect there is a leak in this loop. Long story short, my installer went to jail for fraud. A second installer I had come to fix it found that the first installer had installed 1” not 1.25” line, and that the loop in question had cut marks in it–this was after he had just cemented in the wells (the first installer went to jail before he got to that part, ha-ha). This second installer was shut down by the city for not pulling a permit and digging without a trench box. Had to fire that guy and I hired a third guy to re-weld the loop together and purge and re-fill it. So now I’m stuck with a very questionable loop that I have no way of replacing.

    BUT, it’s still working 8 years later! With zero pressure! And the other one works fine. So I’m wondering what to do:

    – leave it alone until the questionable unit inevitably dies. Replace with something else (I’ll never do groundloop geothermal again since getting the drilling rig into my driveways was a nightmare–I’m in the city).

    – replace the pressurized loop with non-pressurized. I believe these can be set up to auto-purge, and as long as I make sure there’s fluid and antifreeze in the system, this might actually work with what I’m assuming is an incredibly tiny leak somewhere in the field.

    Your advice would be very welcome. Also, your recommendation on a professional in the Boston area that will do the work and is not (and will not be) in jail for fraud is also welcome. The third installer was actually awesome, but his business shut down and I can’t reach him, so… yeah.

    Thanks and have a great day.

    • airideal says:

      SZ, I don’t know anything about the thermal conductivity of the ground heat exchanger in your area or the type of soil (rock, sand, silt, clay, etc.), so I cannot really comment on the sizing of your vertical bores. Also, anything I recommend here is pure conjecture and not to be relied upon. In our area, 600 feet would be entirely inadequate for 5 tons. Here we would need between 900 and 1000 vertical feet for adequate heat transfer. You say cement was used, but I am hoping that a bentonite geothermal grout was used. Yes, it does sound like there is a leak if pressure is lost within a couple of days. Loss of some pressure over a long time is common, since HDPE piping is permeable (assuming that he did use HDPE). Economically speaking, your best bet might be to switch over to a non-pressurized system where you can add water and antifreeze solution while purging air. This, however, is not good environmentally if that leak is causing a plume of methanol, ethanol, glycol or some other environmentally unfriendly solution to grow in the aquifer. I would suggest removing all the solution and introducing one with an inert antifreeze like Environol. The bigger issue will be to measure the antifreeze level properly via specific gravity and having a premix to add to the system later so that you are maintaining the correct mix for proper antifreeze and heat exchange as recommended by the manufacturer. I would look for a professional geothermal contractor with IGSHPA certification through, The New England Geothermal Professional Association. Good luck.

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