Cathleen Dalmeida is budgeting for a heating and cooling system as part of an energy retrofit and is wondering whether a heat pump is part of her future. An obvious question: How much do they cost?
“Is there a general rule of thumb for pricing of a ground-source heat pump and air-to-water heat pump for a medium sized installation?” she asks in a Q&A post at Green Building Advisor.
She wants to know whether anyone can offer an estimate in dollars per ton of heating and cooling, and whether there would be any drawbacks to using a ground-source heat pump in a climate that doesn’t require much cooling.
That’s the topic for this Q&A Spotlight.
Ground-source heat pumps are not the cheapest option
Dalmeida has provided no details about the house, or the climate zone where it’s located, and as a result, it would be impossible to provide any specifics on a heat pump. But, GBA senior editor Martin Holladay points out, “In general, ground-source heat pumps are quite expensive, and aren’t the most cost-effective way to heat or cool a home.”
In an energy retrofit, work typically starts with air-sealing, followed by an insulation upgrade and possibly improvements to existing windows, Holladay says, and heating and cooling loads should be determined with a Manual J calculation.
“In most cases,” he says, “you won’t be using a ground-source heat pump for this type or project.”
Cathleen then asks a follow-up question: “If someone already has radiant floor heating, or wants to install radiant floor heating, what type heating technology would you use?”
Holladay answers that these systems usually include a boiler, which can run on one of several types of fuel. It’s also possible for a radiant-floor system to be built around a water heater, he says, noting, “some of these installations [radiant floor heating systems that operate off a water heater] are successful, while others are glitchy and problematic.”
Radiant system are unfairly maligned
Not all radiant system have to “suffer the doomsday scenario,” writes Richard McGrath. When properly designed, and when it takes solar heat gain into consideration, “it is the finest system available.”
There are a couple of caveats, he says. Rooms on the south side of the house, where solar heat gain is strongest, should never be zoned with rooms on the north side. In a tight house, water temperatures circulating in the floor should be well below any temperature the floor could reach from radiant solar energy.
But given some care, these systems don’t have the shortcomings that Holladay suggests in his article “All About Radiant Floors.”
“The title of Martin’s paper should have been ‘All About Cheap, Poorly Designed Radiant Floors on a Budget,’ ” McGrath writes. “I say this because every time this paper is referenced and I read it just to remind myself how bad a name creeps without proper knowledge have given radiant heat.
“Research ‘radiant ceilings,’ ” he adds. “It’s been done for decades successfully and is not just for commercial applications any more, as if it should only have been there to begin with.”
As to coming up with a ballpark price, McGrath says, forget it. “I don’t believe there is a set per-ton price and I would even say that if someone is willing to give you that price without first performing a proper heat loss and having a good idea about the [airtightness] of the home, you should not even consider him a candidate to perform the work,” McGrath says.
But consider solar thermal as the source of heat
One problem with conventional ground-source heat pumps, McGrath says, is the amount of energy it takes to pump fluid through heat-exchange tubing buried in the earth or submerged in a well or pond. Energy consumption for pumps, he says, is “huge” and not included in many estimates of system efficiency.
Instead, McGrath promotes a system built around a solar thermal system. He argues that a heat pump getting 70-degree fluid rather than fluid at 50 degrees or less (from the ground) is far more efficient. Tanks for holding hot water can be buried.
“When there is no sun for an extended period, it can take up to a week before the stored fluid drops 10 degrees and enters the heat pump at 60 degrees, at which point you’re still far better off than at 50 degrees with a ground-source heat pump,” he writes.
Is solar thermal really dead? Not according to McGrath. “I cannot still understand what aversion there is on a website such as this to technologies that meet the definition of passive by many published standards,” he says. “Long ago this movement started because of things that were bad for the environment. Now it seems common practice to put those bad things like refrigerants right in the house with the folks we began to save the planet for.
“What exactly is an environmentally friendly refrigerant, and is there really such a thing?”
Ground-source heat pumps aren’t always too expensive
While ground-source heat pumps are often one of the more expensive heating and cooling options, the difference is apparently not always dramatic.
C. Maglio recently priced a ground-source heat pump, a conventional heat pump, and minisplits for his (or her) new house. After applying the 30 percent federal tax credit, the “high end” ground-source heat pump installed by a reputable company was about $1,000 more than the conventional heat pump, Maglio says.
After the tax credit, the system would cost about $21,400, or $5,355 per ton of capacity. “There is a good argument that I could put half that money in equipment and the other half in shell (insulation, etc.), thus reducing my load and making minisplits a better option, but frankly I’m not comfortable risking it,” Maglio says. “I’m already getting everything I can from our local trades in terms of shell. Mention exterior foam insulation around here and people look at you like you have two heads.”
In southern New England, Dana Dorsett says, ground-source heat pumps typically run between 2 and 6 tons, with a 3-to-4-ton system being typical. The cost in the region is about $9,000 per ton.
“I’ve never seen a quote for a 2-ton system less than about $28,000, and that was a handful of years ago,” he said. “Even after the 30 percent income tax credit, it’s still $20K. By contrast, modulating ductless minisplit systems run $3,500 to $4,000 per ton. The amount of design and engineering required to do a 2-ton GSHP system right is about the same as doing a 10-ton system, and every GSHP system is a highly customized design.”
When retrofitting an older home with a 50,000 to 60,000 Btu/hour design heat load, Dorsett says, it’s usually more effective to reduce the heat load to less than 40,000 Btu/hour with air sealing and insulation, and possibly window upgrades, rather than pay for the extra 1 or 2 tons of heat pump capacity.
“As a general rule, building envelope efficiency provides more comfort than mechanical systems efficiency,” he adds. “And the life cycle of building envelope efficiency measures is 2 to 4 times longer than any HVAC system.
“When in doubt, spending the real money on lowering the load is usually going to be a better long-term deal than highest efficiency systems to support that load.”
Our expert’s opinion
Here’s what GBA technical director Peter Yost had to add:
I followed up with Dalmeida to get more details about her situation and learned that her home is located in Coupeville, Washington (not far from Seattle). It’s about 1,500 square feet, and indeed she has a radiant floor heating system that includes a propane boiler. Checking weather sources, we learn that Coupeville typically has about 5,500 annual heating degree days and just 22 cooling degree days.
In general, it makes the most sense to consider ground-source heat pumps (GSHPs) when the space conditioning loads — heating and cooling — are both significant; that’s hardly the case in Coupeville. And when the two loads are this different for any closed-loop system, there is the risk of progressive cooling of the soils in the vicinity of the underground piping, robbing the system over time of some of its efficiency. Of particular importance is not undersizing the ground loop; it can hurt the performance of any system, but in the case of a heating-only system, it can be disastrous.
Another generality for GSHP systems is they better serve larger rather than smaller buildings, particularly given the sunk costs of the underground loop work. Researchers from Seventhwave (formerly Energy Center of Wisconsin) suggested that going with a GSHP (and particularly with a more complex ground source-to-water rather than the more familiar ground source-to-air) is likely to add up to as much as $10,000 to a system for a basic single-family home; that’s a pretty steep starting point to work from.
How does solar thermal figure in this?
We learned, too, that Dalmeida is interested in adding a solar thermal system. I figured I really needed “adult supervision” on this one, since integrating, or not, a GSHP system with solar thermal for domestic hot water can get pretty complex. Here is what engineer Dan Cautley of Seventhwave had to say:
“From a purely thermal point of view, adding solar thermal to boost the input of a GSHP isn’t wrong-headed, since a) the solar panels will be heating cool ground water, and the cooler the water, the better they perform, and b) any temperature increase on the GSHP intake will increase heat pump performance. (Side note: it wouldn’t be a good idea to run the ground water loop through solar panels at all times, since they’d just dump energy back to the environment under colder nighttime conditions.)
“Economically, however, it is probably not a good idea; can a several thousand dollar investment in added solar really pay for itself? This is a thermally complex system, and a good quantitative answer would require field study and/or some pretty sophisticated modeling (maybe TRNSYS, for example). But my intuition is it wouldn’t pay off.
“A separate solar domestic hot water system might be a fit, but again I’m skeptical. Most GSHPs have either a desuperheater, which heats water when the unit is in cooling mode, or a dedicated domestic water heating cycle. The desuperheater might not get them much if they don’t use cooling, but a water heating cycle could heat all of their domestic hot water (assuming there are no unusual loads). I’m guessing that the added cost of a water heating cycle to a GSHP would be much less, and would deliver more energy, than a separate solar water heating system. The electric energy input works against the GSHP, though — figure a COP of maybe 4 for water heating by GSHP, while a solar system has only a little pumping power and a COP of maybe 10 to 20.”