Ground-Source Heat Pumps, Part 2: Rules of Thumb
Deciding to use a ground-source heat pump is a big step; bigger still is dealing with all of the many variables that affect the design, scale, and cost of the system
In Part One of this episode from the Green Architects' Lounge, we only scratched the surface. Now it's time to really dig in and decide if 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. system is right for you, and if so, to start planning for it.
In Part Two of the podcast, we discuss:
55-ft. depth per ton at 30% bleed
85-ft. depth per ton at 10% bleed
150-ft. depth per ton at 5% bleed or less
175-ft. depth per ton
Be sure to check out Part One, where we discuss the difference between the various ground source heat pump options and share the recipe for a Dark and Stormy. Also, be sure to tune in later for Part Three, when we'll play "Five Questions" with two professional ground source heat pump installers, Jeff Gagnon and Jim Godbout, and then address the elephant in the room: cost.
Enjoy the show.
Chris Briley: Want to talk about the advantages of open loop versus closed?
Phil Kaplan: Well, we talked about it briefly. There’s a higher risk when you do the open loop, and big cost ramifications.
Chris: Imagine this. This is the reason why if you go to Google “geothermal” or “ground source heat pumps” or talk to different people, you’re going to get zealots on each side saying, “I would never do ground source heat pumps; they make no sense whatsoever.” And then you turn right around and someone says, “They’re the best. I always do that because they’re so efficient and great.” I did a project—a 2500-sq.-ft. house with an energy-efficient shell and some passive solar design—and we did an open loop system. The whole infrastructure cost about $28,000, which is not bad for a ground source heat pump. I don’t think I could hit that number today; that was in 2006.
Phil: It’s not a cheap system.
Chris: It’s not. The owners of this house had one utility bill from June 2007 to June 2008—electric, because the pump runs on electricity—and it was just over $1,000. That’s pretty good. You spend $28,000 and your bill is $1,000 from now until whenever. You show that to somebody and they say, “Wow! I’m doing that!”
Now, I had another house of 2600 sq. ft. that we were going to do geothermal for. I got the quote for the well: $72,000. What? I asked if it was $72,000 for the pump and the well and the distribution, and he said no; the well and the earth coupling—getting water into the house—72 grand. I called up Jeff Gagnon—he and another mechanical contractor Jim Godbout will be on playing Five Questions. These guys are professionals who have installed this stuff with lots of success. So, Jeff gets on the phone, and he’s like “That’s insane!” Fast-forward a couple of weeks later, and the earth coupling is $32,000. So, I’ve got 30 grand inside the building and 30 grand outside. Uh-uh, we’re done. Good night, Irene. For a relatively efficient 2600-sq.-ft. house, there’s no way.
Why was that the case with one house and not the other? It comes down to a lot of the variables that have to do with an open-loop system. For instance, let’s talk about some rules of thumb for geothermal and an open loop system. For your ground source heat pump, you’re going to want to plan for about 3 gallons per minute of flow from your well—3 gallons a minute per ton to heat and cool. For a house with a 60,000 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. demand, you’ll want about 5 tons of heat; a ton of heat is 12,000 Btu. I want 3 gallons a minute to go through that 5-ton system, so I need 15 gallons a minute to flow through that thing. That doesn’t necessarily mean the well has to produce 15 gallons a minute; I just have to have it available. So, there might be storage tanks or some other mechanism. The heat pump won’t always be on. You need flow out of the well. If you have a low-flow well, you have to dig deeper so that you have a higher recharge rate. With well depth in an open loop system, there’s a thing called bleed. I can draw water out, and I can put back up to 30% in a different location. If the house is near a body of water in Maine, the system has to have at least 5% bleed. What this means is I can pull the water out of the well, but I have to put it all back in except for 5% of it.
Phil: That still creeps me out a little, Chris.
Chris: It’s OK that it creeps you out, because there are some ramifications to this. Here’s a rule of thumb: If you’re doing an open loop system and you’re allowed to have 30% bleed, then it’s basically 55 ft. per ton of cooling. If the demand of the house is lower than the well is shallow…if you’ve got a high-demand house, then you need more water, you need more mass and you need more flow.
Phil: And it’s more expensive. So, smaller, more efficient house, and everything else gets cheaper.
Chris: If you can get down to 10% bleed, then you’re 85 ft. per ton. This is a standard 8- or 6-in. well casing. If you’re at zero bleed, then you’re at 150 ft. per ton. So, if you need 5 tons of heat, then you’re in a really deep well. And if you have an even higher-demand house with zero bleed, then you’re digging even deeper.
Now, back to that example of the house that didn’t work out—we were getting low yield off the well. We were probably getting 2 gallons a minute, and we had dug down 250 ft. That means we would have had to dig down quite a bit deeper, and in this region, we probably would have hit salt. If we hit salt water, everything has to be stainless steel, and it gets crazy. And we’re not allowed much bleed. So, we’d need one well to dump our bleed in, and we’d need two wells to feed the house. We’d be digging three wells for this house to make a geothermal system work. That’s insane.
All of this would be great to know up front, but since a lot of people are learning this as they slog their way through, they run into these variables.
Phil: I’m curious where in the country open loop is allowed and where it’s not allowed.
Chris: At the first house, that well almost went artesian. We were getting almost 100 gallons a minute and we were only 150 ft. deep. The well guy was like, “Guys, we are done here. If we go any further, we’re not going to be able to cap this thing. It will overflow, and we’ll have a stream in the back. Then we’ll have to plan for that.” I was thinking, “Awesome! We could put a turbine on it!” That was dumb, but I couldn’t help myself. Anyway, we did a classic pump and dump on that—which is frowned upon by people like you, and probably me, if I knew what was going on back then. Ninety percent of the water that comes out and goes through the pump gets discharged overboard. However, it’s only being used in the dead of winter. We were pulling out perfectly potable water at 150 ft. deep—not very deep in an aquifer—probably very similar water quality to what’s on top, and heck, they’re drinking it, so it’d better be good. We were just discharging that overboard, and it was a very wet environment where it was, so it was probably all OK. So, it was freezing cold outside, and freezing cold water was being dumped—who cares.
However, there are situations commercially … in fact, there’s a lawsuit at an inn that put in a significant geothermal system, an open loop system, and what happened was a lot of people around there started getting salt in their wells—which tends to happen coastally, but when it happens to an entire neighborhood after something just changed, which was the installation of this thing, they have a point. They have a class-action lawsuit, which is happening right now and may trigger some regulations in your state—which happened to us, which is why there are a lot of bleed regulations, especially around bodies of water.
Phil: So, if you want an open loop system, get it now while it’s still legal.
Chris: Well, what you want is to make sure it’s done responsibly. It may take two wells. Make sure you check all the regulations and your installer know what the heck he’s doing. Otherwise, you’re going to turn around and be spending 70 grand on a well they started a month ago that’s now three wells, because they’re learning on the job.
Phil: Closed loop—I love these rules of thumb.
Chris: In a closed loop system, you can do horizontal or you can do vertical. If you’re doing horizontal, that means coils of 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. You’ll dig down 6 to 8 ft. deep.
Phil: Oh, is it that much?
Chris: Well, if you’re in Virgina, 4 or 5 ft. deep. If you’re in Maine, 8 or 10 ft. deep—which is why we don’t do it here. There are only a few installations, and they’re not doing well. The closed loop horizontal is not so good when your frost depth is 48 in. That means the temperature is 32. Here in Maine we go vertical rather than horizontal.
Phil: So, unless you own a backhoe—which most of us do up here in Maine…
Chris: In a vertical closed loop system, instead of burying the tubes, you’re shoving them down a well—so the well has to be quite a bit bigger, because you’re not using the well water; you’re using water that you put into the well and draw back out again. That would typically be 175 ft. deep per ton. It’s not unheard of to be 1,000 ft. deep to do a closed loop vertical system.
Phil: I thought you were talking me into this.
Chris: No! Because a closed loop system—while more stable and more controlled and not dealing with bleed issues; you already know how deep you’re going to go and you’re not concerned about the flow of the well—that’s a lot of infrastructure.
Phil: Yeah, it is, but stay tuned. There’s a reason why these things are still around and people get excited about them. I’m a naysayer, but I’m wrong often. So, why don’t we take a little break, and when we come back we’re going to hear from two guys who are experts in this area. They’re going to tell us when a house is a good candidate for a ground source heat pump and when it’s not.
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