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Ground Source Heat Pump Loop Sizing

lance_p | Posted in General Questions on

YES, I am very aware of the requirement to properly calculate the size of ground loops for a given ground source heat pump system, and that there are many factors at play.

BUT, I’m also painfully aware of how easy it is for pros to scare people into thinking they’re the only people with the ability to do something properly, AND I also realize that the solution to many undesirable design variables in GSHP ground loops is simply to make the loops longer.

So here’s my situation.  I’m building a new house and installing a gas furnace, but the bundled AC that comes with this system is an Air Source Heat Pump that can be used for heating as well.  I’m fantasizing about one day swapping the ASHP for a GSHP and since I’m developing the property now, I have a rare opportunity to run the ground loops very affordably compared to digging up a finished yard in the future.

My question is, in CZ6A, roughly what is the average size loop(s) recommended per ton of heat pump output?  My plan is to take that number and add 30%, bury the pipes and hopefully be in a position to make use of them some day.

The design heat load on the house is about two-tons, or 24k btu/hr.  We have a lot with shallow bedrock but it will have several feet of fill over a large area.  From what I gather, laying a ground loop close to bedrock can yield good performance as rock conducts heat better than many soil types.

Also, is there a specific type of pipe to use?  Diameter, wall thickness, special material?  I know the diameter has to support the desired flow rate, but is sizing up beyond that beneficial?  Does laminar vs. turbulent flow become an issue at some point?

Thanks for your input.

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Replies

  1. paul_wiedefeld | | #1

    For a house with that heat loss, consider skipping the geothermal entirely. Doesn’t make much sense to install a ASHP today, spend a bunch of money on ground loops, wait 20 years, then maybe save a trivial electricity amount (if any). Instead, there are upgrades that make more sense today, both enjoyment and energy wise.

    1. lance_p | | #2

      Hi Paul, thanks for your points. This will be a very efficient new build and about 3,500 sqft. Not much left on the upgrade front.

      The idea of using an ASHP is great, but the unit that’s available with this furnace is not capable of two tons heating in our rather cold climate. Our design temperature is -26C (-15F) and we get plenty of nighttime lows beyond that.

      Stabilizing our heat pump output with a ground loop would be more about enabling the hp to do its thing without relying on auxiliary heat sources, and less about saving electricity/money.

      I’m thinking I can get the loops installed for the cost of an excavator for a day, some clean fill above the loops, and the cost of the pipe and my time. Roughly $1000, which is pocket change compared to doing it afterwards.

      1. paul_wiedefeld | | #3

        Electric resistance heat for the handful of hours under the capacity of a cold climate heat pump (like a Mitsubishi hyper heat or equivalent) will be much cheaper than thousands spent on ground loops, especially if you won't be using them for a few decades.

        1. lance_p | | #4

          Understood. There are two points to make here.

          One, the available heat pump is far from being a capable cold-climate unit. I don’t remember the specs off hand, but it’s not going to work in our climate. I don’t know if a suitable cold climate unit can be substituted or retrofitted.

          Two, I’m planning to use the ground loops immediately to pre-condition my incoming ventilation air, and within 5-10 yrs for a GSHP. These loops will obviously be overkill for ventilation, and if that was my only planned use I would go much smaller.

          1. paul_wiedefeld | | #6

            Is there time to just install a GSHP to start?

  2. charlie_sullivan | | #5

    One of the challenges with ground source heat pumps is that sites vary significantly, and so sizing is somewhat guesswork. Generally, in your climate, you would want to have the tubing a lot deeper than several feet, to be well below the frost line. However, the special situation of being able to lay the tubing right against bedrock could make the performance better even with less-than-ideal depth. There are ways to do detailed thermal modeling of a scenario like that but that's way to complex to do on a routine basis. Another thing that is sometimes done is to make a single test loop and measure its performance before deciding the total system needed. But approaches like that are more suited to commercial scale systems, because the study might cost more than the system.

    In addition to your approach of taking a guess but then oversizing, another approach is to mitigate under sizing by combining with an air source heat pump, and using the air source heat pump whenever the outside temperatures are mild and only using the ground source during the coldest hours. But then you have the cost of installing two systems, and the complexity of figuring out how to combine them into one air handler. Some ways to do that include making both systems hydronic so you can easily use valves to direct the water as needed, using minisplits for the air source part, or just stacking heat exchangers and making sure that your fan has enough capacity.

  3. Expert Member
    AKOS TOTH | | #7

    Lance,

    I would talk your contractor and see if they are flexible on the equipment they will install. You don't want a sub par heat pump in cold climate. The COP on lot of the units drops significantly bellow freezing, they might provide heat but it is useless if COP is around 1.

    For a new build, you want a proper hyper heat cold climate unit. If they can't supply one, you can look at something like MrCool Universal DIY. This will provide 100% of nameplate capacity down to -5F, you won't even need resistance backup at all. If you are on the handy side, the equipment install should not be too hard and the HVAC folks can than connect the ducting to it.

    I recently bought 3 tons of cold climate ducted heat pump for less than what you will spend on the pipes, fittings, manifold and pumps for your geo loop. Never mind the cost of trenching or the actual GSHP. As far as I'm concerned in zone 6 or less, GSHP is dead.

  4. walta100 | | #8

    Sounds like you have your heart set on a geo system and we are unlikely to change your mind.

    You will find like mined people on the geo exchange forum.

    https://www.geoexchange.org/forum/

    Walta

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