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Ground source heat pumps

Nils Bird | Posted in Green Building Techniques on

I am planning a bi-generational house (approx 3000 sq ft) in Québec on an insulated slab with hydronic heating.  I want to install piping for the ground source heat pump in the trenches needed for the water pipe from the well, the septic system and more significantly around, under and possibly even in the septic tank.  That way I figure I could take advantage of drain water heat as well as the heat generated by the fermentation in the septic tank.  Liquid to liquid heat pumps seem to be a rarity.  Right now I have a two ton air to air heat pump coupled with an oil fired hot air furnace.  The noise of the fan going on is driving me up the wall, even though I purchased the hot air furnace (20 years ago) touted to be the quietest.  Air to air or water to air is out of the question.  Off the shelf  systems don’t seem to exist and my research on the  engineering questions I need answered haven’t turned up many answers or people who are willing to consider the idea.  If the only added costs to the mechanicals of such a system are forethought, elbowgrease, and pex piping, for COP of 4-5, it should be available.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Nils,
    A ground-source heat pump pulls heat from the soil. By April, the soil around the heat-exchange tubing can get quite cold -- in some cases, the soil can freeze. So I doubt if you want to install these heat-exchange pipes in the same trench as your water pipes connecting your well to your house.

    For more information, see "Are Affordable Ground-Source Heat Pumps On the Horizon?"

    1. Nils Bird | | #4

      One of two cautionary provisos I expected. Depending on the size of the digger there will be room to separate the hoses. also well water for the house should contribute to maintaining the temperature, the question is; how much. The septic tank idea might lower the tank temperature so that it slows down fermentation and inhibits the treatment of wastes, but again; by how much? Too much to heat a passive house of 3000 sq. ft.

      1. Expert Member
        Dana Dorsett | | #5

        Chilling effects limiting the biological activity in the septic tank is indeed an issue well understood by designers of septic systems in colder climates.

        A few years ago I encountered a guy living in MN who was even nervous about the idea of installing a drainwater heat recovery heat exchanger on the shower until he ran the numbers. In some situations they even insulate the tops of septic tanks out to some to ensure they don't get cold over the course of a winter.

      2. Aedi | | #6

        Nils,

        If you are really set on geothermal and want the best performance possible, you are better off having the well-driller drill a second hole and installing a vertical ground loop. Accessing the additional heat stored deeper underground would be much safer than extracting heat from your septic system or the ground near your pipes, as the heat in those areas helps maintain those systems. Additionally, exchanging heat with the ground water will boost the overall efficiency of your system.

        There might be ways to integrate a geothermal system into the well itself, but that is beyond my area of expertise.

        1. Expert Member
          Dana Dorsett | | #7

          That can work in a "pump and dump" in Quebec, if the flow of the water well is sufficient, provided there is a secondary well or pond where the super-chilled water coming out of the heat pump can be disposed of. Often that requires a second well located some distance away from the source well.

          It's complicated- every ground source heat pump system is a custom design that comes with design risks. You're at the mercy of the local designer regarding it's actual capacity and efficiency. Modulating air source systems have many fewer factors to consider, and takes less expertise to get right, and can now deliver decent efficiency even at -20C (your average winter temperature is probably a bit warmer than that), and sufficient capacity at -25C for most homes.

          1. Aedi | | #9

            Dana,

            To clarify, I was thinking of a closed loop system -- having ground water flow around the ground loop increases its heat exchange rate, even without instituting an open loop system as you describe. I recall hearing that open loop systems in general are not advisable, but I don't recall why. I presume constant exposure of the heat pump to the dissolved solids in the groundwater has something to do with it.

            You are correct in that it is still a complicated system, and air source heat pumps are preferable in almost every way.

    2. Nils Bird | | #8

      I liked your grandfather's article and the 2013 article but it is 2019 and I was hoping for an update which told me exactly what I wanted to hear. Two things I have read are very encouraging, the variable speed scroll pumps that can achieve a COP almost double that of air to air heat pumps, and the increased availability of hydronic heating supplies. Also I've noticed that in swampy areas in the woods I am able to find open water that has almost no flow and is only an inch or two deep even in midwinter at temperatures like today (minus 22C this morning). I imagine that the decomposition of organic matter under the water produces enough heat to melt the snow and keep the water from freezing

  2. Aedi | | #2

    Ground source heat pumps rarely make sense. While having a well on site does somewhat mitigate the costs, geothermal systems are expensive, and seem to only be getting more expensive, unlike other green energy technology. If you are set on a heat pump powered hydronic system, you should look into air-to-water heat pumps. Sanden offers a unit that operates effectively in very low temperatures.

    While I too prefer hydronic systems, I would be remiss not to note that modern, properly sized and installed air handling systems are very quiet -- though it can be hard to believe, as such systems are admittedly very rare. Ductless mini splits, the preferred heating and air conditioning units of most on this site, are also very quiet, with the quietest units clocking in under 20 decibels (about the volume of a whisper). If you intend to both heat and cool your house, they are probably your best option.

  3. Expert Member
    Dana Dorsett | | #3

    >Air to air or water to air is out of the question. Off the shelf systems don’t seem to exist.

    Sure they exist. A 5 ton Arctic reversible chiller air to water can deliver quite a bit of heat at radiant-slab type water temperatures even when it's -25C outside.

    https://www.arcticheatpumps.com/?gclid=EAIaIQobChMIprjBlqT13wIVE4vICh0hUQYSEAAYASAAEgIPcPD_BwE

    Two to four Mitsubishi MVZ air handlers on a 4 ton ...48NAHZ compressor have pretty good capacity at -25C too, if sized correctly for their zones, and have auxiliary heat strip options.

    http://meus1.mylinkdrive.com/files/MVZ-A36AA4_For_MXZ_MULTI-ZONE_SYSTEMS_Product_Data_Sheet.pdf (<their biggest "full size" ducted air handler version)

    http://meus1.mylinkdrive.com/files/MVZ-A12AA4_For_MXZ_MULTI-ZONE_SYSTEMS_Product_Data_Sheet.pdf (<their smallest)

    https://meus.mylinkdrive.com/files/MVZ_A-AA4_Install_PA79D200H03_03-15.pdf

    http://meus1.mylinkdrive.com/files/MXZ-8C48NAHZ_Submittal.pdf

    http://meus1.mylinkdrive.com/M-Series/R410A+Systems/Multi-Position+Air+Handler/index.html#MVZ-A-AA4+Models

    The inverter drive scroll compressors & right air handlers are WAY quieter than the traditional 1-2 stage split systems with reciprocating compressors sharing an air handler with a ridiculously oversized fossil-burning furnace.

    1. Nils Bird | | #13

      Encouraging news. I'll explore all your links. I thought that a well sealed and well insulated house (R60-40-30-20)would require less heat than a septic tank, field, could provide. In summer a heat exchanger could warm the above sufficiently to offset the heat deficit in the winter. That being said maybe an off the shelf electric resistance heater for the hydronic heating would be cheaper.

  4. Steve Grinwis | | #10

    My friend and I built energy efficient homes around the same time. He focused on the mechanicals and got himself a 2 stage geothermal heat pump, 5 ton.

    I focused on the envelope and reduced my design heat load to ~18k BTU, and installed an air-to-air heat pump.

    We recently had a day where we hit -23C, and compared notes. His system used 80 kWh. Mine used 65 kWh. The difference is that his geothermal system cost him $35k installed, whereas mine was under $16k. I put some of the extra into insulation.

    I came out with a better performing house for less money... Geothermal is dead.

    1. GBA Editor
      Martin Holladay | | #11

      Steve,
      Correct conclusion. I'll have to add it to my collection of stock phrases -- along with "solar thermal is dead."

    2. Dick Russell | | #20

      I think it's a mistake to generalize when it comes to whether or not a geothermal system is a reasonable option. Sometimes it is, while often it is not. However, I do think that the first objective in building any new house in a heating climate should be to make it superinsulated class (tight, very well insulated, very low heat loss). It is a mistake to build ordinary "to code," then look for a cheap source of heat for the end result that squanders the heat thrown into it. If your friend's house needed a five-ton heat pump, vs your two-ton, then either his system was grossly oversized or the house was not really all that "energy efficient." I suspect both, since his system's capacity is 2.5 times yours, while the power used for that day was only 23% more.

      If a new house needs a heating system of more than two tons (24 KBTU/hr), either the shell design is far short of what it could have been, or else the house is absurdly huge. I should note that my own house, of 4,000 sq.ft (in CZ6), has a two-ton GSHP, which handles the load in just first stage. But the area distributor and two of their "approved installers" all wanted to put in a five-ton system. That, indeed would have been a huge price jump, for a grossly oversized system.

      1. Expert Member
        Dana Dorsett | | #21

        >"But the area distributor and two of their "approved installers" all wanted to put in a five-ton system. That, indeed would have been a huge price jump, for a grossly oversized system."

        That is exactly the sort of lack of competence/professionalism that has pretty much destroyed the industry, despite an ample policy support subsidy runway and a high growth rate in the early 2000s.

        GSHP sales peaked in the US in 2008/2009 at fewer than 500,000 units per year, and has been in decline ever since oil prices dropped back to a "new normal" around that time. When competing against $5/gallon #2 heating oil or propane it was a lot easier to sell hastily designed oversized systems than it is at $2.50-$3/gallon.

        And, in the decade since 2008/2009 the incremental improvements in cold climate air source heat pump efficiency & capacity have continued apace, as has public awareness of that technology.

        In the same time frame residential rooftop PV has dropped in price by about 60%, making an investment in PV more valuable than risking it on the potentially marginally higher (if done right) efficiency of a ground source heat pump:

        https://www.solarreviews.com/content/page/25907/media/sunshotgoals.png

        Between those factors it a much tougher market for GSHP now that a decade ago. Yet properly running credible load numbers before specifying the system still seems to be the exception rather than the rule.

        The Alphabet spinout Dandelion selects their target customer and regions carefully using big data crunches to figure out the approximate size and type of heating systems and the regional density thereof in their market locations. They began with a somewhat standardized range of air output GSHP systems to be retrofitted into formerly resistance electricity, oil, or propane fired ducted air heating, but are now including some hydronic output retrofits. By limiting their scope to a handful of cookie-cutter designs and seeking only target-rich regions they can sell for less by saving on design and marketing (and financing) costs. Currently they are only targeting a few Hudson Valley NY counties, but that may change.

        But if they can't get the national volumes up for further reductions in cost it's probably all for naught.

  5. Charlie Sullivan | | #12

    I have a water-water ground-source heat pump that I installed just before the mini-split revolution. It was expensive to install, but it works very well, no pun intended, and my annual electric consumption, on a retrofitted 1970 house, is lower than mini-split-heated passive houses in the area I have seen. However, it took a lot of work to find contractors willing and capable of working on such an unusual system and to correct their errors. So I don't really recommend it to others.

    However, I expect a resurrection of GSHPs eventually, because of these signs:

    1. It seems that directional drilling rigs sized for GHSP installations can achieve lower drilling costs.

    2. Higher performance variable speed compressors are now available in GSHPs, including a few water-water models from Hydron Module.

    3. When consumers start seeing in some way the higher cost of electricity on a cold day in January, the high COP that a mini-split gets in April afternoon doesn't balance out its low COP on a January night as well as it does now.

    So GSHPs may be dead but I have faith in their eventual second coming.

  6. Nils Bird | | #14

    Ground source heating doesn't have to be the be all to end all of low cost heating. If the pipes installed when trenches have been dug, (for well,and septic system) at the cost of piping and elbow grease provide a significant part of the heating load, reliably, at a fraction of the cost, then it should be worth it. Redundancy in the heating system has its uses. Recently guests who were at our house when the temperature was -15C left after we did. When the back door was closed the front one (which hadn't latched properly) was pulled completely open when they closed the back door upon leaving. The house was a just bit chilly ten hours later when we came home, thanks to our oil fired forced air furnace. Other considerations when designing heating systems are those members of the household who like to leave all doors open so they won't have the onerous chore of opening them when they eventually want to go out or come in again. A combination of solar and ground source heating where neither one could supply maximum heating load, but either one could keep the house safe from freezing and keep the house warm 90% of the time would be my ideal.

    1. Aedi | | #15

      I am having difficulty understanding your arguments here. It would be nice if the only additional costs of geothermal energy was the installation of the ground loop, but as I am sure you have seen in your research (and in the articles Martin linked), that is not the case. Ground source heat pumps command a significant premium over air source heat pumps, and there are few (none, as far as I know) on the market with and appropriate capacity for a well insulated 3000 sq ft house in your climate. All of these factors mean that an air source heat pump is far more economical than a ground source heat pump before even considering the cost of a ground loop. The extra money is better spent on improving the building envelope.

      If you prize redundancy in your system, it makes sense for your backup heating option to be as cheap, mechanically simple, and reliable as possible. In almost any application, that means electric resistance heat or a wood stove. And in any case, purchasing door closers from the hardware store to counteract non-latching doors or lazy occupants is much less expensive than designing a separate heating system.

      Don't get me wrong, I'd love it if GSHPs were reasonably priced and competitive with their air source cousins, but it is simply not the case. There is no rational reason for spending a huge amount of extra money on a GSHP. The most rational hydronic choice, given Quebec's cheap and reliable electricity, is an air source heat pump (air-to-water) with backup electric resistance heat in the form of an extra ER water tank or some baseboards. The most economical choice is ductless mini-splits and some electric resistance floor panels for rooms you want to have heated floors. You can still get a geothermal system if you'd like, but there is no reason to cut corners on the ground loop installation and endanger the other systems in your house -- If you really want Geothermal, it will be expensive, and you'll have to deal with that.

  7. Nils Bird | | #16

    I am waiting on information on Canadian sources of ground source heat pump supplies, (thank you Dana for the links). I persist in thinking that a heat pump is a heat pump whether it is air to air or water to water and should be similar in cost, and the big difference between the two is the engineering required. All of my queries so far have met with the same response; ie its' expensive. Not so long ago air to air heat pumps were also very expensive and rare and inappropriate for our climate.
    An alternative approach to supplying calories for radiant floor heating that I am pursuing is a wood fired mass heater which has the advantage of fulfilling another design criteria. Namely how can I design a nice warm low maintenance house which keeps me warm into my dotage yet encourages me to go outside for fresh air and exercise. Also if the chimney sucks condition yet stale air up the chimney all I have to do then is supply warmed fresh air to replace it

    1. Expert Member
      Dana Dorsett | | #17

      > "I persist in thinking that a heat pump is a heat pump whether it is air to air or water to water and should be similar in cost, and the big difference between the two is the engineering required."

      The ground source heat pumps aren't significantly more expensive than air source heat pumps, but the system design work (not the pump engineering) and implementation ARE significantly more expensive. An air source heat pump is at one level "a system in a can"- size it right and use a can-opener and it works.

      By way of comparison, ground source heat pump is a component in a system that has to be designed by someone more competent than your sister's best friend from high school, but doesn't take an engineering degree. Without the core competence in system design there's no guarantee that it will perform as efficiently or with as much capacity as an air source heat pump after factoring in all pumping power, etc. Air is air- and works pretty much the same everywhere (with some adjustments for altitude/density). Like politics, soil and sub-soil conditions are local.

      1. Nils Bird | | #18

        Engineers refer to texts which give specific heat transmission rates for materials and calories needed to raise or lower temperatures of different materials. Then they make educated guesses. With enough research and most significantly a heat pump which makes abject failure affordable I would take the risk. I have, in the past been offered a job as civil engineer based on the results of government engineering exams which I passed with only my high school physics to back me up so I know it is not rocket science and engineers always build in safety factors to hedge their bets. I imagine the heat transmission of soil varies most with reference to water saturation; and density which would be similar at similar depths. Humidity, dust and wind will significantly affect air to air heat pump performance and downtime for defrosting. All this to say; I haven't given up yet.

        1. Expert Member
          Dana Dorsett | | #19

          The key is to find the competent system designer (degreed or otherwise) with a verifiable track record.

          Weather factors affecting air source heat pumps are minor compared to the variability in soil temperature & conductivity, and comparatively simple to design for.

          The easiest ground source systems to design around are pond loops or reliably high volume wells for a pump & dump. Once you get into earth coupling heat exchangers it gets a lot fuzzier-bigger error bars. There is a natural tendency on the part of designers to over-design for capacity and overpump those systems rather than charge somebody tens of thousands for something that doesn't adequately heat the place, and have to hear about it forever after, which adds to the up front cost, and lowers the as-used efficiency. That's not to say that designers/specifiers of air source systems don't have similar tendencies, but the up front bite it the wallet isn't nearly as severe.

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