Air-Source or Ground-Source Heat Pump?

first_img RELATED ARTICLES Dana is building a tight, well-insulated house in climate zone 6 and now faces a choice between a ground-source heat pump and an air-source heat pump for heating and cooling.“After the 30% tax incentive, there is not much increase in cost for the geo system,” Dana writes in a Q&A post at GreenBuildingAdvisor. “I am being told different stories in regard to system performance and longevity of equipment (depending on what side of the fence you’re on).”It’s no short-range decision, either. Dana hopes to stay in the house “until my kids have to make the choice of where to send me.”Ground-source heat pumps extract heat from the ground, usually via tubing that’s either buried in the ground or encased in vertical wells. Air-source heat pumps, which rely on an air-to-air exchange, are less expensive to install. But conventional systems have had difficulty operating in cold weather, as someone in climate zone 6 can expect to see.There are arguments for both system, but what’s the best choice for Dana? That’s the topic of this month’s Q&A Spotlight. Mitsubishi Mr. SlimHeat Pumps“One caveat: manufacturers of ground-source heat pumps (GSHPs) don’t include the energy used by pumps when calculating efficiency or COP ratings,” Holladay says. “As a result, actual COPs of these systems vary widely; many experts have been called to troubleshoot GSHP systems with grossly oversized pumps. Such pumps can kill the efficiency of the system. Sadly, engineers who understand pump sizing are relatively rare.”Another type of heat pump to put in the mix is the ductless minisplit, a type of air-source heat pump. Unlike a conventional air-source unit, a minisplit doesn’t need a system of air ducts. A single outdoor condenser can be connected to several fan-coil units inside via copper tubing. And these devices can operate efficiently in lower temperatures than conventional air-source heat pumps.“The advantage of ductless minisplit units is that they are packaged systems that are hard to screw up,” Holladay says. “In contrast, GSHP systems are site-built from a variety of components by local contractors with widely varying levels of skill and knowledge.” GREEN PRODUCT GUIDE Go with the ground-source heat pumpJesse Lizer faced the same choice as Dana, and elected to go with ground-source equipment. Not only are these systems becoming more common in Lizer’s area, but prices don’t have to be stratospheric.“There are many installers who can be trusted to install a good system and do not inflate the prices,” he writes. “A 2-ton system installs for under 20k with vertical wells and full ductwork. While the pump reduces COP, it is still fairly higher then ASHP, especially when the temps start to drop as they will in mine and your zone 6.”Lizer thinks that an air-source heat pump is likely to need some kind of backup heat while most ground-source units will be able to meet heating demands even when outdoor temperatures fall well below zero.“While minisplits are getting below 0 for working temps, their efficiency and output drops as well,” he adds. “With the tax credits as well as the local utility rebates, the net price of our WaterFurnace 5.0 COP (less with factoring pump) is the same cost as the minisplit install and less then a typical 16 SEER ASHP with gas furnace backup.“Obviously this is location dependent,” he adds, “so be sure to check with your local electric supplier as well as they may have additional rebates available. Ours offers ones for GSHP as well as efficient home construction (such as Energy Star 3.0).”Brian Knight adds that a ground-source heat pump also will give Dana more control over indoor conditioned air than a minisplit system with two or three fan-coil units. Other issues to consider“There’s no easy answer to your question,” writes GBA senior editor Martin Holladay.Manufacturers of ground-source heat pumps list the unit’s coefficient of performance (its COP), which is the ratio of energy produced to energy consumed, but as Holladay points out the numbers can be somewhat misleading. Are Affordable Ground-Source Heat Pumps On the Horizon?Ground-Source Heat Pumps (2010)Ground-Source Heat Pumps (2009)GBA Encyclopedia: Heat Pumps: The BasicsGBA Encyclopedia: Green Heating OptionsPodcast: Ground Source Heat Pumps, Part 1: The BasicsPodcast: Ground-Source Heat Pumps, Part 2: Rules of ThumbPodcast: Ground-Source Heat Pumps, Part 3: Five QuestionsHeating a Tight, Well-Insulated HouseGround-Source Heat Pumps Don’t Save EnergyGround-Source Heat Pumps Have Low Operating CostsIs a Ground-Source Heat Pump a Renewable Energy System? Building Science Digest: Ground Source Heat Pumps Air-source is the answerBut a number of posts point to the advantages of air-source heat pumps, particularly the minisplits.“I think if you analyze this closely, you’ll find that a minisplit or two will come out on top,” writes Kevin Dickson. “The good ones maintain a high COP right down to 5F outside, which eliminates the old advantage of GSHPs.”There’s also the cost issue, he adds. A ground-source heat pump will cost more than twice as much, and repair and replacement will be more difficult.“When you start with low heating cost, there isn’t much money to be saved with a higher investment in equipment,” he says.If the house has a tight shell, passive solar features and a generous amount of insulation, says James Riggins, the higher cost of a ground-source unit will be hard to justify.“Mitsubishi has 2 models that will provide full COP down to 5 degrees F and continue to heat down to -13 degrees F, all without any resistive heating element,” Riggins says. “I installed one in my last project (my own house) in climate zone 5, 7000 ft. elevation, and it worked extremely well on bitter cold days during construction. But more importantly, after house completion, it has never been turned on because of the Passive House features built into the design. Bottom line: it’s far more cost effective to invest in passive efficiency and passive solar, than in expensive and complex HVAC equipment.”James Wagner has had the same experience in his own zone 6 house outside Boston, where Mitsubishi Mr. Slim units “exceeded all expectations.”“In pricing both options out I found the ASHP to be about 30% the cost of a GSHP system,” Wagner says. “That being said, I found it much more effective to invest in the homes envelope with a portion of the money saved by not choosing the GSHP, ensuring the effectiveness of the minisplit units. I was consistently amazed at my energy costs this past winter and the comfort they provided.” Our expert’s opinionHere’s how GBA technical director Peter Yost sees it:First, make sure you read this Pro/Con on ground source heat pumps before you do anything else.Second, I keep loose tabs on this issue through two top engineers, Marc Rosenbaum of South Mountain Company and Dan Cautley of the Energy Center of Wisconsin. And I keep hearing the same things:1. Ground-source heat pump systems work great if you have an expert installer, for both the above- and the below-grade work, and you stick with the leading manufactured systems. Play with either or both of these and you are playing with fire.2. The new ductless, minisplit air source heat pumps are proving to be efficient, durable, and reliable.If you have the 30% tax incentive for the GSHP, that puts the two on a bit of a more level playing field cost-wise, but unless you have plenty of testimonials for your local installing contractor, I still would give the minisplit ASHP the edge.And I must admit this: ground source or “geothermal” heat pumps have been given quite the green “pass” or “seal of approval” because they are portrayed as using a “renewable” energy source, and that makes me crazy. GSHP and ASHP both use electricity for heat pump operation, and while both use “sinks” that at one time or the other came from the sun, calling either renewable is just too much of a stretch!last_img