Is a Ground-Source Heat Pump the Right Choice?
To bring the cost of a system down, this GBA reader suggests placing heat exchange tubing beneath the basement slab
Ben Rush likes the idea of 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., despite their reputation for higher cost than other heating and cooling alternatives.
A ground-source heat pump (GSHPs) requires heat-exchange tubing buried in the ground or inserted in a well or pond. The excavation required to bury the lines (or drill an extra well or two) helps to make GSHPs more expensive than air-source units. In addition, the equipment itself tends to be more costly. In all, GSHPs suffer a significant disadvantage when it comes to cost.
Even so, Rush thinks they make sense, and he wonders if he's put his finger on a way to bring down the cost of installing a new system.
"To address the first issue, could the ground loop go under a basement floor?" he asks in a Q&A post at GreenBuildingAdvisor. "Would any additional excavation be required? How much? Would 2 or 3 inches of sub-slab foam insulation be enough to separate the conditioned basement from the year-round +/-55F (Climate Zone 5) soil? Bottom line: would this be an inexpensive — yet effective — way to install the ground loop?
"I really like the idea of GSHPs," he continues, "for two theoretical reasons, and one practical one: A) In zone 5, the soil is cooler than the air in summer — and warmer than the air in winter. Why would I want to put heat into 90° F. air or take heat out of 10° F. air? B) The volumetric heat capacity of soil is about 1000 times that of air; and C) in Chicago, occasionally it might be too cold to heat with a minisplit (not sure if that's three times a year — or once every three years — but it could happen), but it will never be too cold to heat with a GSHP."
Rush's questions are the starting point for this Q&A Spotlight.
Sub-slab tubing will not work
There are three problems with Rush's proposal, replies GBAGreenBuildingAdvisor.com senior editor Martin Holladay. First, heat-exchange tubing needs to be placed in deep trenches, with lots of soil around it. "Digging deep trenches under a slab undermines the footings, and it's expensive," he says. Second, there isn't enough area under a basement slab for the amount of tubing that would be required. Third, he says, a ground loop lowers the temperature of the soil around it, and Rush's proposed placement risks freezing the soil under his house, "an undesirable outcome."
On the relatively high cost of GSHP equipment, Holladay adds, "some theorists" believe the 30% federal tax credit is a contributor. "In other words, manufacturers keep prices high because they know they can," he says. "The equipment cost is subsidized (and, arguably, artificially increased) by the federal government."
Tubing laid in the ground like a Slinky typically needs five to ten times the footprint of the house, adds Dana Dorsett, but it's sometimes possible to drill wells under the slab for the tubing.
"In the Netherlands (where the water table is high everywhere, where drilling through layers of peat, sand, and clay is cheap and easy, and where the outside design temperatures are modest), single-well systems taking only a few square meters of real estate have been used to heat and cool high-R row houses," Dorsett says. "It may or may not be cheaper than minisplits, but the European preference for hydronic heating systems with low temperature panel radiators or radiant floors makes GSHPs a reasonable choice when the work can be done cheaply."
That's more challenging in a code-minimum house in Chicago, he says, because the cooling loads are higher, the outside design temperatures are lower, and the geology is different.
Chicago is not too cold for a minisplit
Holladay and Dorsett both discount Rush's concerns that Chicago winters might be too cold for a minisplit (a type of air-source heat pumpHeat pump that relies on outside air as the heat source and heat sink; not as effective in cold climates as ground-source heat pumps.).
Holladay points out that Vermont homeowners have been heating their homes with Mitsubishi and Fujitsu minisplits even when the air temperature outside drops to -20°F. "So," he says, "I think your concern is misplaced."
Dorsett agrees that there are many houses heated with minisplits in colder areas than Chicago.
"The primary differences are in net efficiency, upfront cost, and heat distribution," Dorsett says. "A typical GSHP will deliver an annual COPEnergy-efficiency measurement of heating, cooling, and refrigeration appliances. COP is the ratio of useful energy output (heating or cooling) to the amount of energy put in, e.g., a heat pump with a COP of 10 puts out 10 times more energy than it uses. A higher COP indicates a more efficient device . COP is equal to the energy efficiency ratio (EER) divided by 3.415. [coefficient of performance] of about 3.5; best-in-class systems will be a bit north of 4.5. Five years ago a right-sized ductless system would tip just north of an average COP of 3, but typical systems would run between 2.5 and 3.
"The cool climate minisplit technology has improved in the past five years, and if done right it's possible to hit a COP north of 3.5 with ductless systems, or north of 3 with best-in-class mini-ducted minisplits," Dorsett continues. "From a net cost point of view, it's often cheaper to go with minisplit and a slightly larger rooftop PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow. [photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow.] array to cover the additional power use of the lower efficiency than it is to go with better-class GSHP, but pricing varies (a lot) from location to location."
He recalled a retrofit in which a three-story house was heated with a single Mitsubishi minisplit head per floor. Although the units had no specified output capacity when the outdoor temperature was -13°F or colder, the minisplits had "no trouble" keeping up when the temperature hit -16°F and didn't break out of positive single digits for a few days.
"The total cost for the three minisplits was about $13,000," Dorsett says, "about one-third what it would have cost to install a single GSHP system big enough to handle the load in my area."
Minisplits are still a better deal
Heat-exchange tubing is one reason GSHPs are more expensive, Dorsett says, and there are a number of other reasons as well: the equipment is made in lower volumes, while minisplits are a "mass-produced commodity in a very competitive market"; it takes more labor to install a GSHP; and there is a greater risk the system won't work as planned. Minisplits, on the other hand, are a "system in a can" with fewer things to screw up.
There are a few trends that could narrow the cost gap, adds Charlie Sullivan.
"For a while, minisplits all had modern variable-speed compressors while GSHPs have single or two-speed compressors," he writes. "Now some GSPHs with variable-speed compressors are available. This allows higher efficiency in part-load conditions, i.e., most of the time. With variable-speed compressors in both, the efficiency advantage of the GSHP becomes stronger."
In addition, directional drilling to make boreholes for GSHPs is starting to become available. Directional drilling, in which boreholes are made diagonally instead of straight down, can be less expensive: "Whether you'll really be able to get a cost reduction from this approach depends on the capabilities and pricing structure of local drilling companies, but what I've read indicates that you can achieve a substantial price reduction."
That said, Sullivan adds, minisplits are still a better deal.
Our expert's opinion
Here's GBA technical director Peter Yost:
There is certainly no shortage of GBA information on ground source heat pumps (GSHPs). I would pay particular attention to Henry Gifford's article, "Ground-Source Heat Pumps Don't Save Energy." The comments section of that post also contain valuable information.
I remain unconvinced that for single-family detached homes the best investment for high performance is a GSHP. There are well-designed GSHP systems, for sure, but they tend to be better suited to larger projects and remain difficult to install properly. Finding qualified installers remains challenging.
There is “new” (2014) guidance on the design of GSHPs from ASHRAEAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). International organization dedicated to the advancement of heating, ventilation, air conditioning, and refrigeration through research, standards writing, publishing, and continuing education. Membership is open to anyone in the HVAC&R field; the organization has about 50,000 members. : Geothermal Heating and Cooling: Design of Ground Source Heat Pump Systems. This version updates the 2007 guide, which was focused on commercial and institutional buildings, and it includes case studies as well as a new section on site characterization. The new section contains a hydrogeological chapter, an area that when not well understood has led to installation issues. This ASHRAE publication is not inexpensive — but then, neither are GSHPs.
Finally, there are a number of papers from the latest (2015) International Ground Source Heat Pump Association conference that may prove useful to those investigating GSHPs or who are involved in their design and installation.
- National Renewable Energy Laboratory
Oct 24, 2016 9:09 AM ET
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