Ground Source Heat Pumps

(I found this post in my Drafts folder.  It includes a reference to a device that is no longer in production: the Daikin Altherma.  But everything else still holds.)

Ground source heat pumps, also known as geothermal heat pumps, take energy from ground water, and through the miracle of heat pumps, use it to deliver heat to your home.  As with all heat pumps, the electricity that goes into the system is used to move heat from one place to another, and you can move two- to four-times more energy through a heat pump than is contained in the electricity itself.  For that reason, heat pumps of all kinds are more than 100% efficient.

Ground source heat pumps (GSHP’s) need a few other components to make a system that can heat your house: the “ground” part, which is usually a well, but can be a bore hole, pipes laid out horizontally underground, or a pond; the pump or circulator to get the water to the heat pump; a distribution system in the house to move the heat around; and the pump or blower that pushes the heat through pipes or ducts.  The whole heating system is a lot more than the heat pump itself.

As an electric heat system, a GSHP can be a good match for a zero energy home.  However, the cost for a GSHP system can quickly balloon, and with poor design a GSHP system can have disappointingly low performance (low heat output and higher than expected electricity consumption).  Within our Zero Energy Home pilot we’re looking for the path that is more economical than buying energy, so we need both a low cost heat solution and great efficiency if the zero energy home proposition will prevail.  Can a GSHP do it?

Before we get to that, there are a few other things to consider.  Why a GSHP instead of mini-split heat pumps?  The great advantage of a GSHP is that it gets hooked up to a distribution system that delivers heat all over the house.  If your house is too big, or does not have an open plan, or you can’t improve the insulation enough, moving heat around is required.  A distributed heat system can provide comfort in homes where mini splits won’t work.

Furthermore, the well (or other ground-coupling system) is very site-specific.  The biggest cost unknown with a GSHP system is the well.  Some sites will not have an economical ground energy resource.

Finally, your home’s existing heat distribution system is not designed for the lower temperatures that a GSHP generates.  Because a zero energy home package includes a greatly improved building enclosure, it’s possible that a distribution system designed for 140 degree air could keep the house comfortable when delivering 80 degree air, but that’s something that needs careful review and it underscores the importance of getting the building enclosure right, even if you’re buying an awesome heating system.

So, can a GSHP heat a zero energy home economically?  There are two budgets to consider: the financial budget and the energy budget.  On the financial side, the cost of a GSHP can vary widely, and as with all heating systems, the lower you can drive your energy demands, the less expense there will be in the hardware.  Maybe a GSHP can heat a zero energy home economically: it depends.

$/ton residential heat pumps

GSHP’s use a well to pull heat out of the earth. Well cost can vary a lot, and adds cost to the heating system. Air source heat pumps, represented by the two bars on the right, have a compressor outside, but no expensive ground-coupling.

For the energy budget, the GSHP’s have great performance and high efficiency.  For each energy unit of electricity used, a GSHP can deliver 4 energy units to the house, and unlike the air source heat pumps (mini-splits), a GSHP does not lose heating capacity when it’s cold out.  But a GSHP’s ability to perform at this high level is very sensitive to good design and installation.  Maybe a GSHP can heat your home within your energy budget if your ducts or pipes are suitable and if the system design is ideal: it depends.

While more expensive than other zero-energy-ready heating systems, GSHP's also deliver with good performance and high efficiency.  But poor system design can hurt efficiency, so work through Water Energy Distributors to make sure your system will be designed right.

While more expensive than other zero-energy-ready heating systems, GSHP’s also deliver with good performance and high efficiency. But poor system design can hurt efficiency, so work through Water Energy Distributors to make sure your system will be designed right.

A GSHP is right for you…

  • Your house is too big or compartmentalized to be heated with point source heaters
  • You are limited in how much you can improve your building
  • You have a good water resource
  • You’re willing to pay for good system design and analysis of your existing heat distribution system

A GSHP is not right for you…

  • You have a small house with an open floor plan that can be heated with a point source heater
  • Your existing heat distribution system cannot deliver enough heat with low temperature water or air.

If you want to pursue a GSHP, I recommend working through Water Energy Distributors.  They do all the design work at the distributor, and have a network of contractors who are trained and supported by engineers.  Don’t fall prey to the idea that a GSHP is a miracle device that will solve all your problems.  A GSHP system requires good design.  Otherwise you’re just spending a lot of money on something that will perform little better than electric resistance heat.

About Li Ling Young

Energy Consultant and Urban Homesteader, Northeast USA
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