Clay Whitenack, planning a new home in central Kentucky, had assumed that a ground-source heat pump would be a “no-brainer” for heating and cooling. Then he began reading about minisplit air-source heat pumps, and suddenly the situation didn’t seem so simple.
He’s intrigued with the possibilities for minisplits, but he’s not certain he’ll have a floor plan that would be compatible with this type of system, he writes in Q&A post at Green Building Advisor.
“My question is this: Aside from getting your house as well insulated and tight as possible, do you need a certain floor plan or construction feature to make minisplits work?” he asks. “Does the foor plan need to be open without many walls and closed rooms? Will this only work with smaller square footages? In other words, can I take a traditional, 2-story, ‘normal’ house plan (3,000 to 4,000 square feet), wrap it up with R-30 walls and R-60 attic, and get good results with some minisplit units, or do I need to build a house with a specific floor plan?”
Further, Whitenack wonders whether he needs to build in a contingency plan in the event a minisplit system would leave some rooms too hot and humid in the summer or too cold in the winter. “I would be a major pain at that point to try and go back and install ductwork,” he says.
Whitenack’s questions are the topic for this Q&A Spotlight.
First things first: Good planning
It will be impossible to design an efficient heating and cooling system without performing room-by-room heating and cooling load calculations, GBA senior editor Martin Holladay tells him, and those calculations won’t be possible unless you have a floor plan.
“Any heating and cooling system needs to be designed,” Holladay writes. “That principle applies to a conventional forced-air system hooked up to a furnace and a split-system air conditioner, and it also applies to a minisplit system.
“Start by performing a room-by-room heating and cooling load calculation,” he adds. “Once you know your loads, you can start designing your system, using a combination of ducted minisplits and ductless minisplits.”
In addition to recommending a number of articles at GBA (these are listed in “Related Articles” at left), Holladay suggests that Whitenack hire an architect or designer who’s familiar with green building principles so that he gets a compact house with as few bump-outs and ells as possible.
Weighing ducted minisplit designs
Minisplits don’t have to be ductless, Nate G points out.
“If you’re worried about the floor plan or door status interfering with the units’ ability to heat or cool the house, there’s always a ducted minisplit heat pump, which is still going to be more efficient than a conventional ducted heat pump,” Nate G says. “And you’ll likely only need one, which may save you money compared to a multi-split system with several indoor heads.”
Jerry Liebler, who also lives in Kentucky, suggests that minisplits will indeed need help to heat and cool rooms where doors are typically closed. Conventional ducted heat pump systems can easily keep up with closed rooms, he says, but they have “abysmal energy efficiency” when temperatures fall below 20°F.
His solution would be to combine a conventional air handler with a minisplit designed for low-temperature operation.
“I’ll dedicate a closet to the minisplit and air handler,” Liebler says, “Return air will flow through the ceilings to the top of the closet, the air handler output will go to ducts below the floor. The air handler will be ‘slaved’ to the minisplit with its power controlled by sensing the current drawn by the minisplit. The minisplit will have a remote thermostat located in a hallway.”
One zone probably won’t be enough
A two-story house will probably need at least two heating and cooling zones, one upstairs and one down, Dana Dorsett says.
In Climate Zone 4A, where Whitenack’s house will be built, a correctly sized minisplit system will “nearly meet or sometimes beat” the efficiencies of a typical ground-source heat pump (GSHP), he adds. Cold-weather minisplits will have a Coefficient of Performance of no better than 2 at subzero temperatures, but will see COPs of 3 as temperature rise above 15°F, and will hit 4 or more when outdoor temperatures hit 40°F.
“If you spend the difference in upfront cost between a GSHP and a couple of minisplits on rooftop solar, it’s often a better investment,” Dorsett says. “GSHP costs vary all over the place, but so does the quality of GSHP system designs. A minisplit is a pre-engineered ‘system in a can,’ with far fewer ways to go wrong (though truly creative installers are able to invent new ways to screw it up every day!).”
Minisplits also are more realistically sized for the type of house Whitenack has in mind. Most HVAC equipment is “ridiculously oversized” for the loads they’ll encounter, Dorsett says, while a typical 1.5-ton cold-climate minisplit can deliver about 20,000 Btu per hour at 15°F, “which covers the heating load of most ‘pretty good’ houses at that temperature,” he says.
“A tight 3,000 to 4,000 square foot code-minimum house might have a heat load north of 35,000 BTU/hour, but if you pick a shape that’s easy to air-seal that doesn’t have a lot of thermally bridging corners and bump-outs, slapping 2 to 4 inches of foam on the exterior of that house, an energy heel roof truss that allows 18 inches of fluff in the attic, and using U-0.25 or lower windows, gets you down to the 20,000 range, at which point a couple of 3/4 tonners (one per floor) will have plenty of capacity at your likely outside design temperature,” Dorsett advises.
Foundation insulation will be key
Whitenack likes the Federal and Georgian architecture of historic homes in Kentucky, and he’d like to replicate this aesthetic in his new house. That means a simple square or rectangular footprint, and uncomplicated roof geometry. It also means thick masonry walls.
“However, even though we want the house to look like it has been there for 200+ years, we realize that we no longer live in the 1800s, and building a house that is viable in the present as well as the future is something equally, if not more, important,” Whitenack says. “We want a modern house on the inside with a historic house veneer. If we can make this work, we will get the best of both worlds. If it won’t work, however, I want to know early in the design stages so we can figure out what has to be compromised.”
It’s the masonry exterior that could bedevil his hopes for energy efficiency.
“With brick veneers there is a huge thermal bridge to contend with at the top of the foundation, with no really great solution,” Dorsett writes, “Think carefully how you are going to deal with that, since it will undercut the performance of a high-R house. One approach is to build a 12-in. step-down on the exterior side of the foundation with the brick and foam extending a foot below the foundation sill, or installing a foot of aerated autoclaved concrete (AAC) insulating block between the top of the concrete and the foundation sill, with interior-side rigid insulation going all the way up to the top of the AAC.
“If you do nothing and just plant it all on top of the foundation, you’ll have an R-1 to R-2 stripe of concrete and brick at the bottom of your R-30 wall, and above your R-15 to R-20 foundation,” he adds. “With thick foam and brick veneers there is also the issue of finding appropriate brick-ties that can handle the length, which can be a real cost adder. Deeper commercial ties are probably what you’d need to manage 4 inches of foam.”
In that instance, Nate G adds, Foamglas insulation might be better than ACC. It’s marketed as a thermal break for brick ledges and has twice the R-value as a typical ACC block. If the building inspector isn’t sure about the load-bearing capabilities of Foamglas, insulating fire bricks probably would work.
Alternately, says Holladay, insulating the basement walls on the inside might be simpler.
But insulating the floor between the basement and the rest of the house probably isn’t the best option, even when the house won’t have any heating and cooling ducts in the basement, Holladay says.
“First of all, ducts are not the only issue,” he writes. “Most basements also including plumbing, including hot water pipes — and it’s good to have plumbing inside of the conditioned space of your building. (In Vermont, if you insulate the basement ceiling but not the basement walls, it’s pretty easy for your pipes to freeze. That’s probably not true in Kentucky — but it’s still a good idea to keep your pipes indoors.)
“Second, many basements have a water heater. Water heaters belong indoors, not outdoors.
“Third, it’s difficult to do a good job insulating a basement ceiling, because all of the penetrations make the insulation work awkward. It’s usually easier to insulate the basement walls.
“Fourth, most homeowners (these days) occasionally visit their basement or use it for storage — and they don’t really want the basement to be cold in the winter.”
Our expert’s opinion
Peter Yost, GBA’s technical director, had this to say:
It seems as though there are two main issues in this one:
- Running brick veneer below-grade to give a more realistic Federal brick building appearance without creating a thermal bridge at the transition from the below-grade to above-grade wall assemblies.
- Using minisplit heat pump(s) in spaces or homes with a less-than-open floor plan.
First, I’d recommend reading this GBA article. You don’t need to read all of it. Scroll down to the boldface subhead that begins, “A systems integration example,” and start there.
As luck would have it, GreenBuildingAdvisor architect Steve Baczek, is a great resource for both of these issues.
On the question of running brick veneer below grade, Steve suggests taking a look at the GBA detail shown at left. (GBA Prime subscribers can download the image from this web page: Brick veneer detail.)
Steve says, “This details shows a great overlap between the continuous exterior rigid insulation and the top of the foundation wall. The exterior insulation runs down to the brick ledge step in the foundation and the basement can be insulated on the interior. You could carefully run the finished grade right to the first course of the brick veneer if you want a Federal appearance, or you could move the flashing and weeps up a course or two and cover the first course with loose finished grade material.”
Steve does a lot of high-performance and Passivhaus projects and routinely uses both ducted and ductless minisplits.
Here are his comments on that: “Will a minisplit(s) work? The short answer is yes! But the long answer is where the rubber meets the road. As with any system, success depends on a well designed relationship of all the parts.
“In this case — a new home designed to look historic — issues of house orientation, glazing, insulation levels, airtightness, ventilation, and heating/cooling distribution are all inextricably linked to each other. I always use the abacus analogy: All of these issues are not to the right, nor are they to the left, they are all somewhere in the middle — and that somewhere depends on placement of the other parts of the system.
“For example, a lot of glazing in a dining room facing due south would require a distribution different than that of a master bedroom of the same size with one third of the glazing on the north side of the building. Having a project team already experienced in these relationships, or willing to master them, is the key.
“Once these relationships are understood, and HVAC loads are determined, the next set of relationships are imperative. These relationships are the number of units, size of units, location of units, and whether ducted or single-source. Depending on the overall performance of the house, these decisions will either be increasingly difficult or increasingly easy to support.
“For example, a code-built house yields both the problem of “load” and of “distribution.” As I move the home’s overall performance toward Passivhaus performance, the problem of load diminishes. While that still leaves the issue of distribution, this problem will become much easier due to the lower load.
“Because I’m not a mechanical engineer, I make the concept of energy (or heating and cooling) easy on myself. In a house (relative to energy) I need to do two simple things: convert/supply energy as inexpensively and efficiently as possible; and hold on to that energy as long as I can.
“In this framework, duration is directly linked to load. If I build a good house and can hold onto the conditions a long time, then I don’t need much heat or cooling.”
Steve cautions that minisplit heat pumps deliver heat in a different way than the higher pressure and higher air volumes of conventional forced-air centralized systems. “Duct runs can’t be long and can’t provide much resistance, and you need dedicated return ducts,” he says “And a good general rule of thumb on high-performance space conditioning and ventilation: Don’t try and use your ventilation system for distribution; ventilation systems simply don’t move enough air for adequate distribution and mixing.”