I mainly talk about air source heat pumps here. But ground-source heat pumps are an important type of heat pump, too. So today, let’s look at the issue of open-loop vs. closed-loop design when installing this type of heat pump.
What is a ground source heat pump?
First, I use the term ground source heat pump, but they’re also called ground-coupled or geothermal heat pumps. They do exactly the same thing as air source heat pumps: They move heat between indoors and outdoors. The equipment is really just a heat exchanger with a lot of interesting physics that keeps your house cool in summer and warm in winter.
The only difference is that in a ground source heat pump, the outdoor exchange happens with the ground, groundwater, or surface water. The diagram below shows four different ways that outdoor heat exchange can occur.
The main advantage of ground source heat pumps is that it’s easier to draw heat from the ground in winter and dump it into the ground in summer. Why, you ask? A couple of reasons. First, the ground temperature doesn’t vary like the air temperature does. Second, the ground or water that you’re drawing heat from or dumping it into can have a higher heat capacity than air. (See the comment by RoyC below for a better understanding of that second issue. It’s the one that begins with “I have to disagree with Allison…”)
Open-loop vs. closed-loop
Anytime you’re in the position of choosing a heating or cooling system, you have to make decisions. One that comes up with ground source heat pumps is deciding between the open-loop vs. closed-loop types for the outdoor heat source or sink. The diagram above shows four closed-loop systems.
In that type, the fluid that exchanges heat with the ground is a mixture of antifreeze and water. As the name suggests, it’s contained in a closed system that circulates the fluid through the pipes. That same fluid going through the pipes over and over.
One type of open-loop system is sometimes referred to as a pump-and-dump system. That pretty much tells you how it operates. You pump water from the ground or a pond, run it through the heat exchanger, and dump it onto the surface or into a storm sewer. That type is widely illegal (see next section).
But there are three other types of open-loop systems that handle the water responsibly after the heat exchange. The diagram above shows one of them, the doublet. It has water being pumped out of one well and then sent back into a different well. The other two open-loop types are the standing column well and the dynamic closed loop. The latter is really an open-loop/closed-loop hybrid. See Jay Egg’s article on this topic for more detail.
Pretty simple concept, right?
Which type is better?
As with most things, which type is better depends on whom you ask. But there is a pretty good agreement among a lot of people in the industry that one type is better than the other. Before I tell you which it is, though, let’s list the important issues.
- Water quality
- Groundwater contamination
- Aquifer depletion
Let’s start at the end because the last three are connected. In some places, you’re not allowed to install an open-loop system (or certain types of open-loop systems anyway). In dry areas out West, water is scarce and highly regulated. Aquifer depletion and groundwater contamination can make that problem worse.
An open-loop system can pump A LOT of water from the ground. In a large, inefficient house, that could be many tens of thousands of gallons per year. Groundwater contamination happens when the used water just gets dumped onto the ground. There it can carry surface contaminants down into the ground.
That leaves the issues of efficiency and water quality from the list above.
On the surface, open-loop systems seem to be more efficient than closed-loop systems. One reason for that is that water transfers heat more readily than the closed-loop antifreeze mixture because it’s more conductive. Also, if you have a large supply of groundwater, you’ll have a more constant temperature over the course of the heating or cooling season. The area around a closed-loop system can heat up in summer and cool down in winter, making it harder to cool or heat the house.
But there’s more to it than that. I spoke with John-Paul Kiesel of Water Furnace, one of the biggest names in ground source heat pumps. He told me that when you look up the efficiency of the two types of systems, you’re not getting the full picture. It’s like comparing apples to oranges. That’s because a significant source of energy use is absent from the open-loop efficiency.
The closed-loop system efficiency includes the pump energy whereas the open-loop efficiency does not. And pump energy can be much higher in an open-loop system.
Let’s say you have matching systems that are of the vertical well type. The open-loop system works against gravity the whole time as it pulls water from the ground. The closed-loop system, however, is pulling the antifreeze mixture up one pipe while the fluid is going down the other side. The fluid going down helps push the other side up, resulting in less pump energy use.
Kiesel said about the efficiency question, “If you have a properly designed closed loop, it shouldn’t make any difference.”
The quality of the water in an open-loop system is what can make a huge difference. You’re putting groundwater or pond water in contact with the heat exchanger. That can lead to pitting, corrosion, scale, or other problems. The acidity and mineral content of the water as well as the amount of silt and even iron bacteria can shorten the life of the equipment.
Kiesel told me that the expected lifetime of a closed-loop system is 25 to 30 years. With poor water quality, an open-loop ground source heat pump may last only 10 to 15 years. He also told me he doesn’t have a preference for open-loop or closed-loop. As with just about everything in the world of building science, both types can work well when designed, installed, and commissioned properly.
Comments from industry pros
I posted a question on LinkedIn about this topic to see what others had to say. I especially wanted to hear from those with direct experience with either or both types. Here are some of the comments I got there:
Bradford White (engineer): “I’m definitely in the closed-loop camp as a default. Control your water, know your water.”
Ty Branaman (HVAC instructor): “Closed loops require purging but less contamination and less energy usage in the pump since what is pushed is pulled in the loop.”
Rob Brown (former contractor, now working for a manufacturer): “Open loops are FAR more susceptible to failure.”
John-Paul Kiesel (manufacturer’s rep): “Open loop works great as long as the water quality is in range and flow rates are adequate.”
Dan Nall (architect): “I’ve had bad experiences with the 2 open-loop systems with which I’ve been involved. An open-loop system is sort of like having unprotected s3x with a relative stranger.”
Ross Trethewey (engineer): “We have designed 250+ geothermal heat pumps systems in the Northeast over the last 15 years and every single one is a closed-loop system.”
The sum and substance
There’s your quick overview of open-loop vs. closed-loop ground source heat pumps. One thing I didn’t mention is cost or ease of installation. For both of those, the word on the street is that the open-loop system wins. It’s just easier and cheaper to install a pump-and-dump system. But the drawbacks, especially the water quality issue, can eat into any savings you get upfront if the equipment lasts only half as long. As always, do your homework, get a full design upfront, find a good contractor to install it, and then get it commissioned.
My description of open-loop ground source heat pumps above just scratches the surface. To understand the intricacies of it better, see this article by Jay Egg: When Does Aquifer Thermal Energy Transfer Work Best?
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can subscribe to our newsletter and follow him on LinkedIn. Images courtesy of author, except where noted.
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