Jordan Garrow is getting ready to build a new house in New York State, on the cusp between Climate Zones 5 and 6, and he’s planning to heat and cool it with a ground-source heat pump. His contractor wants to install a horizontal “slinky loop” heat exchanger, one of several possible options, and Garrow is seeking a second opinion.A heat load calculation for the house specifies a 4-ton system (one with a capacity of 48,000 Btu/hour), but the contractor wants the heat exchange loops designed as if they were serving a 6-ton system.“His plan is to do a 6-ton horizontal slinky loop field with a 4-ton heat pump,” Garrow writes in Q&A post at GreenBuildingAdvisor. “He claims that an oversized slinky loop field performs the best. However, I’ve heard bad things about slinky loops, and slinky is the only kind of loop they do.”It doesn’t seem either efficient or cost-effective to overbuild the field, since that represents a major part of the system’s cost. He’s sought another quote for a horizontal loop using straight lines, as well as a vertical closed-loop heat exchanger, which would be placed in a well. But the local geology is something of a concern.“I’m worried about the vertical [method] since there is a lot of shale in my area,” he adds, “and I’m not sure how deep the wells could go.” Heat Pumps: The BasicsIs a Ground-Source Heat Pump a Renewable Energy System?Are Affordable Ground-Source Heat Pumps On the Horizon?Air-Source of Ground-Source Heat Pump?Ground-Source Heat Pumps, Part 1: The BasicsGround-Source Heat Pumps, Part 2Ground-Source Heat Pumps, Part 3 Our expert’s opinionHere’s what GBA technical director Peter Yost had to say:You all know that I am not nearly as comfortable taking the lead on mechanical systems as I am with questions about building enclosures. So I went to one of my go-to mechanical system experts, Dan Cautley at SeventhWave. Here is a summary of his perspective based on his personal experience with GSHP systems over the years, plus discussions with some key colleagues, and some web research conducted by Dan and me:Oversizing the slinky heat exchange loop: General agreement here that this is not a bad idea.Design load of the system: Also, pretty good agreement that sure seems as though more effort and resources should be directed to a better enclosure.System guidance: There is a good resource specifically on slinky GSHP systems produced by the International Ground Source Heat Pump Association (IGSHPA) as well as a useful YouTube video about the importance of the quality of the loop installation.Dan found and looked over this resource: An Experimental Performance Comparison between Different Shallow Ground Heat Exchangers and found it useful and reasonable. Note that the slinky systems performed better than the “snail” configuration. And one of Dan’s colleagues added that there are now horizontal boring capabilities that eliminate trenching. (See an image from the report below.)Consider other systems: Dan and his colleagues at SeventhWave agreed that it is just good practice to consider other high-performance systems as well as GSHP. For cold climates, the new fleet of cold climate air-source minisplit heat pump systems has really changed this comparison. Consider another type of heating systemDick Russell selected a ground-source heat pump for his house five years ago and, all things considered, likes it. He not only gets air conditioning from the same system in the summer, but maintenance costs have been very low.“I still feel that it was the right choice for my house,” he says, “but that is because the house is superinsulated and the unit is slightly oversized at 2 tons, so that the two-stage unit never has to upstage past first.”A well drilled for the domestic water supply also was adequate for the heat pump’s heat exchanger, which helped control costs. But Russell thinks that Garrow is facing a more difficult problem.“If your house needs a 4-ton unit, and it can’t use a well drilled for domestic water use, then GSHP would be a harder sell, and you might well be better off with the lower installed cost of minisplit [air-source heat pump],” Russell says.Also, Russell adds, Garrow really ought to put more money into the shell of his new house. “Four tons for a house suggests a house just built to code,” he says. “If my house of close to 4,000 square feet is heated well with a 2-ton unit running in just first stage, you perhaps can cut your heat loss in half, and wind up with a more comfortable house that needs a much smaller heating system. Put your first money into the shell design, rather than try to find a cheap source of heat to dump into it.” As a point of reference, the 2,500-square-foot house will have 2×6 walls with fiberglass batt insulation plus R-10 of rigid foam insulation installed over that. In the attic, he’s planning on 18 inches of blown-in cellulose.How should Garrow proceed? That’s the topic for this Q&A Spotlight. Oversized slinky option may not be a bad ideaThe contractor may have a good reason for recommending an oversized field, says Andrew Bater. Over the course of a winter, a slinky system might have trouble keeping up with demand as the ground around the tubing gets colder and colder.“Anecdotally I understand that some slinky systems underperform as the winter goes on,” he says. “The ground around your slinky (or my well) gets colder and colder and the geothermal unit works harder and harder come the end of February or so. I have even heard tales of systems that quit working altogether after a particularly cold winter.“Surprise, the ground is not always the nice 50Ëš that the marketing literature would imply,” Bater continues. “Look for discussion on the anticipated ‘Seasonal Performance Factor’ for your system as a key to understanding this. Upsizing from a 4-ton slinky to a 6-ton slinky may be a way your contractor hopes to avoid this problem.”Further, says Mel Tillyard, putting in a larger field may not cost a lot more money. Tillyard installed a slinky system because it was cheaper than the alternatives, and the property was already torn up.“They installed the slinky field and rough graded the land in a day,” he says. “I’m not not sure an oversized field will really add much to the cost.”Slinky loops require more power for pumping than do other options, says Charlie Sullivan, although plumbing sections in parallel instead of in series can help. “Oversizing the loop is a better idea than oversizing the heat pump,” he adds. “Oversizing the heat pump will lower your efficiency, but oversizing the loop will improve the efficiency. If they are used to oversizing the heat pump, and matching the loop to the heat pump rather than the real load, you might actually do well to have a loop sized for, say, 3 tons even if your load is only 2 tons.“Despite all that,” he continues. “it’s probably more economical to the money into the envelope and PV and use minisplits.” RELATED ARTICLES The numbers don’t add upDana Dorsett thinks the heating load has been miscalculated. “A better-than-code 2,500 square foot house would not have anywhere near 4 tons of heating load,” he writes. “I’ll bet it’s closer to 2 tons. My 1920s 2×4 antique with 2,400 square feet of conditioned space above grade plus 1,500 square feet of insulated conditioned basement doesn’t even have a 4 ton load until it’s -15ËšF outside.“Your load is clearly going to come in quite a bit lower than mine even if you have 2,500 square feet of basement. (Is this a 1-story?),” he continues. “The calculated heat load is complete junk — even code-minimum houses that size won’t have a 4-ton load at -5ËšF or whatever your outside design temperature.”Russell also thinks that the heat load calculation looks fishy. Before putting in his heating system, Russell did his own heat-loss calculations and then solicited proposals from a number of heating contractors to see what they had to say. Their proposals included a number of errors, including erroneous assumptions about the existence of a fireplace and underestimating the amount of insulation he planned to use.“I could only conclude that too many assumptions and incorrect numbers had been used in the installer’s calculations,” Russell said. “Perhaps that approach would work well with older houses or even one just built to barely meet code. I am convinced that a heat loss calculation for a high-performance house must be highly detailed, taking a considerable amount of time, probably more than a contractor can afford to sink into a quote, considering he may not get the job.”In the end, Russell ordered a 2-ton unit rather than the 5-ton unit the installers were recommending. Build a better boxDorsett is with Russell in recommending that Garrow put more money into insulation.For starters, the R-29 that Garrow thinks he’ll get from his planned wall assembly is wishful thinking. Actual performance will be lower because of the thermal bridging through the wood framing, plus a slight loss in R-value from compressing the fiberglass batts. In reality, the R-19 batts plus 2 inches of extruded polystyrene (XPS) foam will yield a whole-wall R-value of R-24.5, not R-29. Using rock wool batts instead of fiberglass, as well as advanced framing techniques, might get the wall to R-28, he adds, “but that’s about it.”If Garrow is really interested in hitting net-zero energy performance in that climate zone, he’ll need whole-wall R-values of R-30 to R-35, requiring up to 3 inches of polyiso insulation on the exterior. He refers Garrow to a Building Science Corporation research paper on high R-value enclosures for high-performance houses.An insulated concrete form foundation made with expanded polystyrene (EPS), combined with an extra layer of EPS, would make a more attractive option. If nothing else, EPS and polyio use a blowing agent that’s much more environmentally benign than what’s used to manufacture XPS, Dorsett says.