It’s been a great 31 years, but the single-zone HVAC system has finally reached the end of the road, and gtmsmith has narrowed the search for a replacement to two possibilities. Neither involves fossil fuels.
Gtmsmith describes his options in this Q&A post:
- A 4-ton ground-source heat pump made by ClimateMaster that will require two 320-foot-deep wells. The full package, including a humidifier and air-purification equipment, will cost $21,500 after a tax credit, including the $11,680 charge for digging two heat-exchange wells.
- A Daikin 4-ton air-source heat pump, with the same extras, for $18,000.
Gtmsmith says the 2700-square-foot house is well insulated for a building of its age. It has new doors and will be getting new windows and is located in southeast Pennsylvania on the border between Climate Zones 4A and 5A.
“We are easy going and happy with 66-68ºF in the winter and 74-76°F in the summer on the T-stat,” gtmsmith writes. “We have been happy with single stage heat pump heat in previous winter winters in a much tighter and smaller home prior to this.”
He plans to add a small wood stove for supplemental heat, although not for the entire house.
Gtmsmith is leaning toward the air-source heat pump option for a number of reasons, including lower cost and better financial terms, a longer warranty and less disruptive installation.
“Did I just make up my mind?” he asks. “Help!”
That’s where we start this Q&A Spotlight.
Air-source is a better bet
Josh Durston thinks the air-source option is, on balance, the better route for gtmsmith. First, the air-source system uses “inverter technology,” a means of varying DC power to ramp output up and down so it matches heating and cooling loads more precisely than a one- or two-stage system powered by AC would.
“The GSHP [ground-source heat pump] is on off with maybe an unloaded first stage on some models, the ASHP [air-source heat pumps] will run longer and modulate to the load for better comfort,” Durston says.
Durston’s parents have had a ground-source heat pump for nearly 25 years with “essentially no issues or maintenance costs” as well as low utility bills. “But,” he adds, “the ASHP are so good these days it’s hard to justify invasiveness of drilling and the higher capital cost. Farms with the land for big simple horizontal loops are the best applications IMHO.”
He advises gtmsmith not to install a humidifier or an air scrubber. Instead, he says, put the money toward air sealing, which will increase relative humidity in the winter. Balanced ventilation with heat or energy recovery, plus an air filter with a high MERV (Minimum Efficiency Reporting Value) rating would be a good idea.
Durston, however, does have a nit to pick with the Daikin air-source heat pump—it has what he considers a relatively low HSPF (Heating Season Performance Factor), a measure of how efficient the unit is over the course of a heating season.
“It seems to have a relatively low HSPF of up to 10,” he says. “I kinda see 10 as the minimum acceptable HSPF in 2020 ideally 12 or higher in heating dominated areas.” He suggests that gtmsmith look at some other heat pump brands (one source of information for comparing equipment can be found here).
Systems sound oversized
Both the air-source and ground-source systems are sized at 4 tons of capacity (48,000 Btu/hour), which to more than one GBA reader sounds too big.
Durston’s parents, with their 25-year-old ground-source system, have lived comfortably in southern Ontario with a 3-ton heat pump and never needed supplemental electric heat. “House doesn’t drop below 70°F on the coldest nights,” he says, “but the heat pump runs at basically 100% duty cycle.”
Trevor Lambert comes to the same conclusion. “4 ton sounds way too big for the climate you’re in and the house you’ve described,” he says. “I’d expect a 4 ton unit could heat an uninsulated house with single pane windows in climate zone 4.”
And if the 4-ton system can become a 3-ton system when sized correctly, Lambert adds, that’s one more reason to choose an air-source heat pump over a ground-source model.
“Ordinarily I’d agree with you,” Durston replies, “but with just a $3000 gap between the two (which might shrink to $0 with right sizing) I don’t think you can outright dismiss the GSHP. Done right the GSHP could easily beat the ASHP operational costs, and with a similar install cost it might be worth considering.”
If 4 tons of capacity sounds too high, gtmsmith says, consider that a Manual J calculation was completed, and that the contemporary house has lots of big, leaky windows.
There’s a way to check the design heating load, says Dana Dorsett, because gtmsmith should have a record of how much heating oil he’s used. Dorsett explained that approach in this GBA post. Gtmsmith also can estimate cooling loads using this methodology from a post by Allison Bailes.
“Don’t count on modulation to save you from the comfort & efficiency deficiencies of oversized equipment in either heating or cooling mode,” Dorsett says. “For large air handler ducted systems with limited modulation range (or single speed geo systems) it can be more comfortable to even undersize it—have the heat pump cover the load at the 95th or 93rd percentile temperature bin, and use strip heat to cover the difference.”
Dorsett suggests that gtmsmith consider the “turndown ratio” —the ratio of maximum capacity to minimum capacity—when he considers what equipment to buy. The idea is that it will be easier to tolerate oversized equipment if it has a higher turndown ratio because it will able to operate over a wider range.
“Oversizing by more than 1.2x can sometimes be a comfort & efficiency issue for systems with only a 2:1 turn down,” he writes. “Oversizing by 1.5x is usually find if the turn down ratio is 5:1 or higher.
“Turn down ratios are not infinite,” Dorsett continues. “If you oversize by very much the lower efficiency of cycling on/off during the shoulder seasons can eat up more than the amount of ‘extra’ heat -strip power use of a slightly undersized system. The oversized system will cycle more, modulate less which provides lower comfort too.”
Plan ahead for building upgrades
Something else that gtmsmith should consider, Dorsett adds, is how much heating and cooling capacity the house will need once some of his planned upgrades have been carried out.
When a house is as leaky as gtmsmith says his is, reducing the heating or cooling load by a ton can be pretty cheap by addressing some of the deficiencies in the building enclosure.
“Running a Manual-J on the ‘after building upgrades’ version of the house and going with the smallest equipment reasonable for that load may cost you a bit in extra strip heat use in the near-term, before upgrades if it’s going to take you a year or three to finish it,” Dorsett says, “but that cost adder is offset by the lower equipment cost.
“It’s almost always the case with HVAC equipment that replacing like-for-like capacity-wise is a mistake, even when not planning to upgrade the building envelope,” he continues. “A 4 ton air conditioner for a 2700′ house is almost always sub-optimally oversized, even for the ‘before upgrades’ version of the house.”
He recommends that gtmsmith have a look at a series of “Nate the House Whisperer” posts that cover this topic.
Our expert’s opinion
Peter Yost adds these thoughts:
Tim has done quite a bit of both ground-source heat pump (GSHP) and air-source heat pump (ASHP) systems and had this to say:
- It’s good to see that the GSHP is two-stage but still not good that there are essentially no modulating GSHP systems. This really limits the turndown ratio.
- Even with the subsidy, the GSHP system costs more than an ASHP.
- When sizing a system based on future building enclosure improvements, he states in writing that optimal sizing and subsequent system performance requires that the improvements be carried out.
- He is finding that many heat-pump systems may be sized optimally but installers are “hedging their bets” by installing oversized resistance backup.
Mark added these perspectives:
- Even with federal tax subsidies, GSHP systems come in with final installation costs significantly greater than new ASHP systems. For that reasons, he usually steers clear of ground-source systems.
- Installing one 4-ton ASHP with multiple zones has been problematic. If only one zone is calling, and let’s say that zone is 9 kBtu/hr, a turndown of 2 means the outdoor unit running at 24 kBtu/hr is cycling on and off repeatedly. Mark would do either two 2-ton systems or try to convince the client to go with minisplits, with turndown ratios of 5 and greater.
- Mark wishes it was easier to get turndown ratios for all systems.
I am going to close with an overarching question: Why did we ever start offering financial incentives for GSHP in the first place and why do they persist? I could find little to nothing on the history or inception of subsidies for GSHP and I have never understood why they are grouped financially with proven winners: energy improvements and renewable energy technologies.
Maybe when GSHP systems first became available, especially in colder climates, the performance and the novelty drove financial incentives. And maybe the to-this-day confusion over “geothermal” versus “ground-source” systems—with the former including high-temperature earth heat not even requiring a heat pump and the latter always including or needing a heat pump—has led to GSHP subsidies that simply have not delivered the way that energy improvements and renewable energy systems have.
Scott Gibson is a contributing writer at Green Building Advisor and Fine Homebuilding magazine. Peter Yost, our expert, is GBA’s technical director and founder of a consulting company in Brattleboro, Vt. called Building-Wright.