Ground Source Heat Pumps, Part 1: The Basics

If you've done any amount of research on ground source heat pumps, chances are that you've heard from people who say that you'd be insane to consider them as a viable system for your house—AND you've heard from others who say you'd be insane NOT to use them.

Where insanity and green architecture meet, you shall find Phil and me mixing a Dark and Stormy and turning on the mike to act as your good-natured guides. For this episode, we will attempt to demystify this polarizing heating and cooling system.

In Part One of the podcast, we cover the basics and discuss:

Be sure to tune in later for Parts Two and Three, when we'll talk about the advantages of each of the types of ground source heat pump and why the costs for these systems can vary tremendously depending on various circumstances. We'll also share some rules of thumb for designing a ground source heat pump system. Phil will share a track from Here We Go Magic called "Collector," and then we'll play "Five Questions" with two professional ground source heat pump installers, Jeff Gagnon and Jim Godbout.

Enjoy the show.

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RELATED MULTIMEDIA

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OFFICIAL TRANSCRIPT

Hello, everybody and welcome to the Green Architects Lounge. I’m your host, Chris Briley.

And I’m Phil Kaplan. Today, we’re talking about ground source heat pumps.

Chris: That’s right. Ground source heat pumps: What are they good for? This is a very controversial subject, don’t you think?

Phil: Somewhat controversial. I think people get excited about it, and they don’t have all the facts. Like just about everything we talk about, there are so many different technologies, but under the right circumstances, only one is the right one for you.

Chris: And I’m back to choosing the cocktail. This is one of my favorites for the summertime: the Dark and Stormy. It’s basically one part rum and two parts ginger brew. Not ginger ale; you don’t want to put Canada Dry in this. You want a Jamaican-style ginger beer; I’m using Maine Root. And I’m using Gosling’s Black Seal rum. OK, now, are you ready to delve into geothermal?

Phil: I’d love to, but I have to preface this…a lot of people say “geothermal…”

Chris: And they’re wrong!

Phil: Yeah. But we understand why; it’s a common mistake. When people say “geothermal,” it means that you’re going deep, deep, deep into the magma.

Chris: Yeah, but what we really mean is “ground source heat pump.” It’s heat pump technology coupled with the heat of the earth to make that heat pump more efficient. In a heat pump, we’re not creating heat; we’re moving heat, OK? So the heat pump cycle is you have refrigerant in a loop, and on one side you’re pressurizing it—it’s hot. Then it goes through an expansion valve and expands into a gas—and it gets cold again. Take your refrigerator: a classic heat pump. The expansion happens inside the box and it gets compressed outside, so the heat is pumped out of the box. You’re actually heating your house with your refrigerator, but you’re making it cool inside the refrigerator.

OK, your normal heat pump—before we bring in the “ground”—is basically treating your house like the inside or outside of your refrigerator, whichever way the heat pump is running. You’re either shoving heat into the house or pumping heat out of the house, just by whatever your distribution method is. So, the thing about the heat pump is, that hot is really hot and that cold is really cold. What if you could always have the temperature consistent in the compression mode? It’s always 50 degrees, let’s say.

Phil: Like it came from the ground, for instance.

Chris: And you’re making the heat pump more efficient. A ground-source heat pumpHome heating and cooling system that relies on the mass of the earth as the heat source and heat sink. Temperatures underground are relatively constant. Using a ground-source heat pump, heat from fluid circulated through an underground loop is transferred to and/or from the home through a heat exchanger. The energy performance of ground-source heat pumps is usually better than that of air-source heat pumps; ground-source heat pumps also perform better over a wider range of above-ground temperatures. is using the inertia of the temperature in the earth to make the heat pump more efficient. That, in essence, is how a heat pump works. One of the great things about a heat pump is it can do either heating or cooling.

Phil: There’s a term that the geeks kick around: COP, coefficient of performance. Essentially, it’s one unit of energy in and one unit of energy out. What if you could change that equation? I’m putting one unit of energy in, but I’m getting two out or I’m getting three out.

Chris: Maybe this is a system that can really do something.

Phil: Air-to-air heat pump, many people say three. I haven’t seen it; I’ve seen two or high twos.

Chris: The literature says three.

Phil: Maybe under ideal conditions.

Chris: Ground source heat pump—what are we looking at?

Phil: Four to five. That’s one of the bigger advantages of a ground source heat pump; the COP is higher.

Chris: The COP is higher, and you’re going to be a lot more efficient with your electrical use.

Phil: Another factor is where you are in the country. Here we are in Maine, and we’re heating most of the time. The ground temperature is in the high-40s, low-50s, depending on how deep you go.

Chris: The frost depth is 4 feet.

Phil: So, it’s a little harder to extract the heat from cold temperatures—but you can do it. If you go down to Virginia where the ground temperature is like 62 degrees, and you need both cooling and heating, it’s an ideal system.

Chris: Another bit of alphabet soup that you’ll see on labels is the EER, which is the energy-efficiency rating. If you’re doing an Energy StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners. house, they’d be looking for an EER of 16.2 if you’re doing an open loop system, a 14.1 for a closed loop, and a 15 for a direct exchange. The higher the EER is, the better. It’s the output of the energy versus the amount of energy consumed. Divide the btuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. per hour by the watts per hour, and that’s the EER.

So, there are three types, Phil: open loop, closed loop and direct exchange. Let’s start with open loop. You have a heat pump and on the outside is a heat exchangerDevice that transfers heat from one material or medium to another. An air-to-air heat exchanger, or heat-recovery ventilator, transfers heat from one airstream to another. A copper-pipe heat exchanger in a solar water-heater tank transfers heat from the heat-transfer fluid circulating through a solar collector to the potable water in the storage tank. of some kind. It’s going to have an earth coupling to get some heat from the earth. Usually we’re using water; an open loop system up here needs to use a well, a pond or a lake. That makes people nervous.

Phil: It makes me nervous!

Chris: You’re drawing this water from the earth, running it through the heat exchanger, and putting it back. If you’re cooling the inside of your house, you’re pumping out hot water and putting it back wherever you got it—

Phil: Into the ecosystem somewhere.

Chris: If you’re cooling your house, you’re warming up the water in your well. If you’re heating your house, you’re cooling off the water in your well with an open loop system. You’re just sipping it off the top. The pump doesn’t have to work nearly as hard, which is why an open loop system is almost always cheaper from an installation standpoint. However, you’re using your well that you drink from, you’re running the water through the mechanical system and putting it back into the well. Which is just fine; it’s just a heat exchanger, like the plumbing you use in your house.

Phil: It’s not as icky as you’re picturing it in your head right now.

Chris: Exactly. I’m running it through a machine, it gets all greasy, and pumps back into the well…. In a closed loop system, you’re not letting that water you pump in go. Instead you’re going to send it out into tubes or deep into a well, and you’re going to bring that same water or glycol mixture back into your heat pump. You’re not mixing it with anything else. So what you need is more infrastructure. You need a greater heat sinkWhere heat is dumped by an air conditioner or by a heat pump used in cooling mode; usually the outdoor air or ground. See air-source heat pump and ground-source heat pump. to disseminate this; you’re not letting nature come in and out of the system. More expensive…

Phil: Less risk.

Chris: Less risk, much more stability, and easier to model. The direct exchange is a bit spookier. I don’t understand it that well. Instead of having a heat exchanger, you have the refrigerant you’re using in that heat pump. You’re sending that refrigerant into the earth to get cooled down or warmed up, and then back into the loop. So, basically you’re cutting out that 15% to 30% inefficiency you’d typically have with a heat exchanger of some kind. It’s like a closed loop system, only it’s the exact same refrigerant; it’s usually not in PEXCross-linked polyethylene. Specialized type of polyethylene plastic that is strengthened by chemical bonds formed in addition to the usual bonds in the polymerization process. PEX is used primarily as tubing for hot- and cold-water distribution and radiant-floor heating. tubing, but in copper tubing—it is part of the machinery. That makes some environmentalists nervous because you’re using not just a propylene glycol but an actual refrigerant that you’re putting in your well or in the earth.

Phil: I smell a little BP potential!

Voiceover: So that’s it for this part of the episode. Tune in for more from the Green Architects’ Lounge podcast. A quick reminder, our music is “Zelda's Theme” by Perez Prado. And our views and our drinking habits do not necessarily reflect those of Green Building Advisor. Thanks for tuning in everyone, and keep up the good work.