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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

Posted on Oct 24 2016 by Scott Gibson

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 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.

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Image Credits:

  1. National Renewable Energy Laboratory

Oct 24, 2016 9:09 AM ET

In the process of installing. Tax credit was the difference
by dean sandbo

I am currently having a geothermal system installed in North Carolina. We are using the waterfurnace 7 series. This was a full remodel, with all new ductwork as well. We used the horizontal boring method, as we had room on the property. It was amazing how they bored horizontally and combined the pipes in a header pit, and then brought them to the variable speed pumps and the units. The federal tax credit made the difference. I had 4 real world quotes for top of the line air source heat pump systems, and net of the tax credit the geothermal was actually cheaper to install. So if the ground source is cheaper to operate, I am ahead of the game. Also nice not to have noisy units outside, and trying to figure out the benefit of the desuperheaters for hot water as well.

Oct 24, 2016 9:38 AM ET

Thanks to Dean Sandbo
by Charlie Sullivan

Thanks for your report. I am pleased to hear that you have what should be an excellent system at a reasonable net cost. If you are willing to share details of the cost and size, that would be interesting to many here.

Oct 24, 2016 10:52 AM ET

air seal and insulate
by Erich Riesenberg

Our midwest home, directly west of Chicago, with hot humid summers and snowy winters, is well insulated and heated with a portable air source heat pump. A Friedrich bought on sale under $300.

It is amazing the gyrations people will go through for bragging rights.

Air seal and insulate. Eat plants.

Oct 24, 2016 11:49 AM ET

Geo in Chicago
by Jason LaFleur

We are leading energy consultants in Chicago, and I want to say thank you for laying out and supporting a repeated discussion I have with clients. On historic residential retrofits, geothermal may make sense. On commercial buildings, maybe worth evaluating. But if you are doing a gut rehab or new construction single home, we find it often isn't worth it.

It will be great to now have this resource to point single-family homeowners to, and save my breath for discussing high-performance enclosures. Thanks again!

Oct 24, 2016 4:40 PM ET

The importance of load calculations.
by Dana Dorsett

The importance of accurate load calculations goes WAY up when considering a mechanical solution as expensive as a ground source heat pump (GSHP). There is usually quite a bit of low(er) hanging fruit on thermal retrofitting of existing houses that are more cost effective too.

In dean sandbo's North Carolina, a state of the art right-sized modulating air source heat pump can hit pretty close to average GSHP system efficiency, with lower system design risk.

The 30% Federal Income Tax subsidy that was extended for many efficiency & renewable energy measures late last year was NOT extended for GSHP systems. That subsidy will end at midnight on 31 December 2016. After that the industry has to fly on it's own (or state & local subsidies), but I'm not too hopeful that it will be as cost effective as air source heat pumps + rooftop solar by the time the solar subsidies also dry up completely in 2022. Solar is becoming dramatically cheaper year-on-year, and air source heat pumps continue to improve incrementally with every model release.

In my area (southern New England) GSHP systems cost ~ $9000/ton (before subsidy) installed, high efficiency ductless costs ~$3.5-4K/ ton installed. At the current local pricing of $3-3.25/watt for PV the $6-6.5K difference buys about 2000 watts of rooftop PV, that delivers ~2300-2500 kwh of AC electricity every year. A typical 2500' house would take 2- 2.5 tons of ductless to heat & cool the place, and even assuming the total output of the PV is only 2000 kwh/year due to unfavorable shading factors, the 4000-5000kwh of electricity would cover well over half of the heating power use, even at a paltry HPSF of 10, , far more than the difference in power use between GSHP and ductless. (The best in class ductless units test in the HSPF 13-14 range, and would pretty much cover all of the power use.)

Before 2020 the installed price of solar will be under $1.50/watt, unless there is severe canges in Federal and state policy support. It's already that cheap in Australia (~USD$1.26 /watt in a 2016 survey of small scale rooftop systems), using the same panels, inverters, & racking systems used here. There is no reason it won't hit those price points in the US soon, even though the national average is more like $3.50/watt. At $1.50/watt the price difference buys 4000 watts of PV per ton, which is way more than needed to cover the entire heating & cooling energy use of the ductless heat pumps.

But there are plenty of local markets where GSHP is cheaper than it is in my neighborhood. Only time will tell if it's going to be able to successfully surf the tsunami of cheap PV in the GSHP cost markets.

Oct 26, 2016 11:21 AM ET

IBACOS Research
by Kohta Ueno

For what it's worth, it looks like IBACOS researched this under Building America. If my memory is correct, they have researched both GSHP tubing below the basement slab, and in the foundation excavation around the exterior. I have only read the executive summary, not the full report.

This report presents a cold-climate project that examines an alternative approach to ground source heat pump (GSHP) ground loop design. The innovative ground loop design is an attempt to reduce the installed cost of the ground loop heat exchange portion of the system by containing the entire ground loop within the excavated location beneath the basement slab. The horizontal sub-slab approach presented in this report will reduce installation costs by approximately
$2,500/ton (Oberg 2010; Appendix A) compared to those of a conventional vertical bore well.
For the 1.5-ton system installed in the cold-climate, 2,772-ft2 , two-story unoccupied test house in
Pittsburgh, Pennsylvania (hereinafter referred to as the Pittsburgh Lab Home), a traditional vertical well system is expected to cost $17,800, compared to $14,000 for the horizontal sub-slab coils.

Based on the measured data, the research team determined that the ground loop heat exchanger
met the heating and cooling needs of the Pittsburgh Lab Home. The analysis indicates that a subslab GSHP is an effective strategy for capturing most of the advantages of a GSHP without the expense of drilling vertical wells adjacent to a low-load house in a suitable climate region. Because of the low margin for error in system design and the greater risk of an undersized system causing negative and compounding side effects, the sub-slab heat exchanger design requires more verification and stress testing before it will be ready for a production home environment.

Oct 26, 2016 1:47 PM ET

Under slab? Bad idea
by Mike Nelson Pedde

Ben: Martin lists several excellent reasons why installing GSHP looping under the slab of your home is a bad idea. I'm no expert, but I can add one more, which is that the lifespan of the piping is usually set at about 25 years. I'm hoping your house will stand longer than that, and it would not be fun to bust up the basement to replace the piping.


Oct 26, 2016 2:46 PM ET

Thanks for that example, Kohta!
by Dana Dorsett

"For the 1.5-ton system installed in the cold-climate, 2,772-ft2 , two-story unoccupied test house in Pittsburgh, Pennsylvania (hereinafter referred to as the Pittsburgh Lab Home), a traditional vertical well system is expected to cost $17,800, compared to $14,000 for the horizontal sub-slab coils."

The $14K for a 1.5 ton system is $9333/ton, which is typical of recent years' per-ton costs even for systems 2-3x that size in my neighborhood.

Compared to a 1.5 ton ducted mini-split system that might costs $7000, even the cheaper version leaves $7000 on the table for rooftop solar, which at this year's unsubsidized $3.50/watt national average buys 2kw of PV, which in Pittsburgh would deliver about 2300 kwh of annual output, more than covering any difference in system efficiency.

If a 1.5 ton system fully covers the heat load at Pittsburgh's ~+5F 99% outside design temp the design load would be something like 18
,000 BTU/hr. Assuming a 65F heating/cooling balance point that's 60F heating degrees, so the house would need about 300 BTU per hour for every degree-F below the balance point. Pittsburgh average about 5700 base 65F heating degree days per year, so the average annual heating energy use wold be about 41 MMBTU.

At a COP of 1 (electric baseboards) that would take about 12,000 kwh of power use, but with a best in class GSHP with an average COP of 4.5 it would only take about 2675 kwh, and a more likely pretty-good GSHP system averaging a COP of 4 it would use about 3000 kwh.

A fairly modest efficiency ducted mini-split might only deliver an annualized COP of 2.5, which would burn through 4800 kwh, and a better class ducted system would average a COP of 3, only using 4000 kwh.

The subtracting out 2300 kwh reaped by the PV the net use of the CRUMMIER minisplit + PV is only 2500 kwh that has to be purchased, which is already less purchased power than the better class GSHP. Subtracting it from the better class mini-split yields 1700 kwh/year purchased power, which is well under that of the better class GSHP.

When rooftop PV is half the price that it currently is (which will happen well before 2025, maybe even before 2020, since it's already cheaper than that in someother first-world nations) it becomes a no-brainer, since the the difference in GSHP and mini-split heat pump cost buys enough solar to cover more than 100% of the power used by a by a better class mini-split, and over 95% of the power used by the crummier mini-split.

In short, GSHP has to get a LOT cheaper going forward to compete against mini-splits + PV (on either an annual operating cost basis, or lifecycle cost basis. ) To get there with additional subsidy would require a major increase above the 30% income tax subsidy, spending far more in just subsidy for the GSHP than it would cost to install a mini-split (unsubsidized.) Without a dramatic reduction in price/performance GSHP has no larger scale future, and will be relegated a boutique market of retrofits or commercial buildings.

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