Basics of Ground Source Heat Pump Technology (Part 1)

We’re in the midst of finishing up some feasibility studies for ground source heat pump (GSHP) or geothermal heat pump (GHP) technology, so I figured that now would be good time to give you some basic things to think about when considering GSHPs.  In this post I’m going to cover some important design considerations, and in another post next week I’ll cover some important things about economics and installation.

What is Ground Source Heat Pump Technology?

GSHP technology takes advantage of the fact that the ground temperature stays relatively consistent throughout the year.  For example, as I look outside my window right now it’s about 20°F and just getting ready to snow (brrr!), but the ground itself—once you go down about 20 ft or so—is probably closer to 50°F.  During the summer time it can get up 85°F or higher outside, but once again, the ground is keeping its cool at 50°F.

GSHP systems use electrically powered systems to transfer energy to and from the ground.  During the winter they pull energy out of the ground to heat your building, which means that the ground is acting like a heat source.  During the summer they reject energy to the ground to cool your building, which means that the ground is acting like a heat sink.

10 - GSHP flow chart

Energy transfer to and from the ground occurs using fluid that flows through a series of pipes buried underground.  The ground does a good job at transferring energy to and from the ground loop, but heat pumps within the building can also add or reject additional heat from the pipe loop as needed to meet building temperature requirements.

Just as a side note: don’t confuse GSHP technology with geothermal power, which uses hot water or steam from deep inside the earth to produce electricity.

It’s all About Balance

As you are withdrawing and adding energy (in the form of heat) to the ground over the course of the year, the ground surrounding your GSHP loop may also heat up and cool down a little bit—we’ll call this localized heating and cooling.  As you move further away from the system, the ground is less and less affected.  The ground will dissipate some of this energy (or lack thereof) over time, but for the most part, you are essentially “storing” energy in the ground for use at a later time.

10 - GSHP localized heating and cooling

When you’re looking to install a GSHP system, it’s really important to consider your annual heating and cooling requirements.  In an ideal world, your annual heating and cooling loads would be approximately equal and balance each other out over the course of 12 months, but if you live in an area where your heating loads are dominant (like me!), you’re probably going to be “pulling” more energy from the ground to meet your heating needs throughout the year.  This may or may not cause the ground temperature to go down over time.  Similarly, if you live in an area where the cooling loads are dominant (like the southern part of the US), you are probably going to be rejecting more heat to the ground over the course of a year.  This could cause the ground temperature to go up over time.

So why does this balancing act matter?  Well, heat pumps operate best within a certain range of temperatures.  Once the ground surrounding the loop gets too hot or too cold, so does the liquid within the pipes. Over time the heat pumps simply won’t operate at their peak efficiency anymore.  As a result, they consume a lot more energy trying to get the same job done.

Heating and Cooling Basics

As I mentioned before, you really need to know your heating and cooling loads before you consider installing a GSHP system.  As you might guess these systems come at a premium to traditional technologies.  The last thing you want is to have your system be over-sized (and spend more money up front than you have to) or have the system be undersized (and not perform as promised).  Chances are good that you will need to have your facility’s existing loads modeled in Trane Trace, Carrier HAP, or some other modeling program to get it right.

Things to consider:

  1. Don’t assume that your building’s existing heating and cooling load capacity will be equal to the size of your GSHP system.  Many existing heating and cooling systems these days are slightly-to-moderately over-sized for their application.  Why is this the case?  Well, say the design engineer accidentally sizes the system just a little bit too small.  You can bet that the engineer is going to catch a lot of flak from the customer about how it’s not comfortable in their facility.  But if the design engineer sizes the system just a little bit too big, the customer is going to meet their annual heating and cooling requirements, no problem, even if the trade-off is a slightly higher electricity or gas bill due to non-optimal system operation.  If you take your heating and cooling loads to be equal to your installed heating and cooling capacity, you’re probably going to be over-sizing your GSHP system.
  2. If your facility contains a boiler room right now, that too is contributing its fair share of heat to the building’s envelope.  Once you install a GSHP system, that heat gain isn’t going to be there anymore, so your GSHP system will have to be sized accordingly.
  3. Think about your facility’s process loads.  Certain facilities may have refrigerated areas, contain data centers, or have manufacturing processes that skew “typical” heating and cooling loads.  For example, many hospitals require year-round cooling for their operating rooms even though the outside air temperature dictates that they should be in strictly heating-only modes.

Improperly-sized GSHP will likely operate just fine for a few years until the load imbalances catch up with them.  Then the heat pump efficiencies will suffer and the system probably won’t operate as designed.

GSHP System Factors

Many different things go into designing a GSHP system.  I already mentioned the ever-important heating and cooling loads above, but other significant factors include:

  1. Knowing what kind of soil/bedrock is in your area – Different types of rock/soil mineral and moisture contents conduct heat differently.  Drilling contractors can perform what is called a “conductivity test” to determine this information for you.  Testing usually costs $6,000 – $12,000, which isn’t exactly cheap, but you’ll need to know how the ground is going to react to a GSHP system over time if you want it sized accurately.
  2. Bore field depth and spacing – Cramming your bore field loop into a small area is going to concentrate the energy gains/losses, and could accentuate the localized ground temperature changes that I mentioned above.  Deeper boreholes give the system more surface area to conduct its energy transfer, but 1,000 ft deep bore holes aren’t usually economical (or even technically feasible).  Most boreholes range in depth from 250 – 500 ft deep, but of course there are always exceptions.  It completely depends on what kind of bedrock is in your area (see point #1 above!).
  3. Heat pump efficiency – More efficient heat pumps require less ground loop to get the same job done.  They also use less electricity, which means that your operating costs will be lower.

With all of this in mind, be very, VERY careful of someone who tells you that “based on this rule of thumb” your system is going to need X boreholes or be Y tons in size.  I was talking to a GSHP vendor last week who told me that my system design for a recent feasibility study was “at least two-times oversized” and then he cited some statistics.  Well, it turns out that the statistics he mentioned were developed using information from residential GSHP systems located in the southern state of Alabama.  Residential houses have virtually nothing in common with my commercial facility located in downstate NY because 1) commercial heating and cooling loads are a lot bigger and tend to have a lot more components than residential heating and cooling loads, and 2) our weather patterns up here in NY are completely different than Alabama.

Being able to think critically about your existing heating and cooling loads is essential to the success of a GSHP project like this.  Are you considering GSHP technology?  Performing a feasibility study up front can help you to decide if it’s worth the trouble and expense of conducting a detailed design analysis.  Give us a call to see if we can help you out.

Want some more resources on GSHP technology?  Check out the International Ground Source Heat Pump Association (IGSHPA)’s website FAQ here. Look for my upcoming second post soon!

Update: Part 2 on ground source heat pump technology is posted.

About Sara Aaserud


Sara works as a Renewable Energy Engineer in ANTARES’ Fayetteville, NY office. Her favorite types of projects are solar PV feasibility studies and electricity rate tariff analyses. She can be reached at saaserud@antaresgroupinc.com.

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