Randy Bunney is building a new house in a challenging environment — north central Minnesota, where overnight temperatures plunge well below zero and heating-degree days over the last three years have averaged more than 8,600 annually.
The high-performance, passive-solar home will be a relatively small 1,100-square feet with two bedrooms, an open living-kitchen-dining area, 1 1/2 baths, a mudroom and a mechanical room. Bunney is planning on exterior walls insulated to R-40, the roof to R-60, and “near airtight” construction.
The main source of heat will be an air-source heat pump able to function at low temperatures. The question is exactly how the technology will be put to work. Bunney and his advisors are considering two possible alternatives.
The first is to pair a Mitsubishi air-source heat pump with an electric furnace that would kick in when the heat pump hits its cold-weather limit of 13 degrees below zero.
“Our design team concludes the cabin’s small footprint does not warrant a heat pump with multi-indoor units for zone heating,” Bunney writes in a post at GreenBuildingAdvisor’s Q&A forum.
“Instead, the design calls for running a single heat pump coil into [a] plenum for [an] electric resistance coil. The plan offers initial cost savings over multi-splits. Duct work will transport heat to individual rooms. We will use an ultra small marine wood stove for backup in case of power failure.”
Plan B seems to be installing a number of minisplit heads in the house, their number and location to be determined. An electric furnace would still provide supplemental heat when outside temperatures were too cold for the heat pump.
“We have a good design team open to suggestions,” Bunney says as he invites questions and comments. That’s the focus for this Q&A Spotlight.
Keep it simple
Jin Kazama points out that heating loads in a tight, well-constructed house will be very low, and that Bunney might be better off designing a simple heating and cooling system rather than one that requires a plenum.
“You should keep it as simple as possible,” Kazama writes. “Look at in-floor electrical heating instead of baseboards if you do not want anything to show. You will not need high power versions[s] and [it] should be pretty cheap to include in your concrete floors.”
When it comes to deploying heat-pumps, Kazama adds, it would be better to buy two small units rather than a single compressor and several indoor units. “It usually ends up the same price,” he says. “You always have a backup if one fails, and the efficiency is higher on most individual models.”
“I think that we would experience potentially uncomfortable variations in room temps with only two indoor units,” Bunney replies. And the builder and architect are concerned about a wood stove overheating the building, which helps explain his choice of a stove that measures only 11 inches by 13 inches by 18 inches.
A single ductless minisplit will work
Despite his reservations, Bunney shouldn’t be overly concerned that a single ductless minisplit heat pump won’t work, suggests GBA senior editor Martin Holladay.
“If your house has an excellent thermal envelope — above-code levels of insulation, as few air leaks as possible, and high-performance windows — you can certainly heat your house with a single ductless minisplit heat pump,” he writes.
There are just two caveats:
- Family members must be willing to leave their bedroom doors open during the day.
- Family members must be willing to tolerate room-to-room temperature differences of about 5 degrees on some nights.
“If you are worried about room-to-room temperature differences, you should consider installing a ducted minisplit unit,” Holladay says.
As to Bunney’s question about the need for either a heat-recovery or energy-recovery ventilator, Holladay says that while every tight house needs a ventilation system, exhaust-only and supply-only systems are other alternatives he might consider.
Combining heat pumps with electric backup
In Kazama’s opinion, Bunney will face minimal comfort issues if he heats the house with two ductless minisplit heads, one at each end of the building, and pairs that with individual thermostats in closed rooms for electric backup.
“I don’t see how you could end up with more than 2°-3°F of difference all over the place,” he writes. “You then set up individual thermostats in closed rooms with a desired minimum set point (let’s say 1°-3°F lower than the minis setup) and be done. The backup will only start up on the cold nights and you will never experience more than ~2°F of difference all around.”
Kazama has heated two floors, each measuring 1600 square feet, with minisplits located at each end of each floor. “Setting the temperature 1°C higher than usual makes a great impact on comfort and the energy usage ends up nearly identical as the resistance backup heating kicks in further in the night,” he adds.
That suggestion makes sense to Bunney, who sees it as a way of getting the comforts of zoned heating he’s been hoping for.
“Electric heat in slab as backup to [the] heat pump will give us a more comfortable radiant heat than forced air,” Bunney continues. “The normal downside of cost to operate electric radiant heat will be a minimal factor, as we will only use in cases of extreme cold as backup to heat pump.”
Consider cove heating (and a bigger wood stove)
Malcolm Taylor wonders whether Bunney has considered as an alternative to electric resistance heat in the concrete slab.
Cove heaters are cheaper, and the money Bunney saved could be invested in more insulation for the house.
At the same time, Taylor offers a word of caution about the minuscule wood stove Bunney is planning to use. Would its 4-inch flue meet code requirements?
The stove is designed for small, wooden ships, Bunney replies, “not a place where one wants to deal with hazards from a malfunctioning wood burning stove.” And besides, the rural cabin will never see a building inspector, and code requirements are few and far between.
Still, adds Ven Sonata, leave the tiny stoves to boats and tents and get yourself a moderately sized wood stove that will throw some real heat.
“The few days you will need it will be 30° below zero,” Sonata says. “Luxuriate, crack a window if it gets too hot. The Btu loss is so trivial with wood heat you can’t measure it. You won’t use half a cord a year. But if you do have a serious outage you are laughing.
“By the way, I agree with the others,” Sonata adds. “I live in a superinsulated house exclusively heated with a wood stove and no ducting. The heat evens out within a few degrees. Put a transom over your bedroom doors if you are worried, then you have privacy and air flow. Transoms were the 19th century solution…no electricity required. They work just fine with single source heat pumps.”
Our expert’s opinion
Here’s how GBA technical director Peter Yost sees it:
We have a pretty good network of high-performance, cold climate builders and projects here in Vermont, so I decided to just ask around a bit and collect insights on cold climate heating systems for high performance homes. Here is the collective wisdom I could corral (with overnight lows in the last couple of weeks ranging from about -10° to -20°F for the folks I talked to):
- Keep it simple and robust. By and large, in Vermont, that means trust the cold-climate heat pump to perform to spec, but back it up with a wood stove. No plenum or ducts. No radiant in-floor systems. Just follow the practices Martin Holladay outlines above.
- Don’t install the thermostat for the minisplit too close to the inside unit. This comes up a lot: if the heat pump thermostat is at or near the head of the interior unit, it results in less than peak performance of the whole system, especially when it gets really cold.
- Shut down your ERV/HRV during really cold snaps. It seems to be a pretty common practice here in Vermont to turn off mechanical ventilation systems during really cold spells (below, say, 0°F) to reduce the load on the heat pump and to avoid frequent defrosting cycles that jump systems from drawing 100 watts or less to 800 watts or more. I am unaware of any air quality studies of this practice, but in homes with open floor plans and occupants who leave sleeping space doors (or transoms) open, I’ve not not heard of folks sleeping through their alarms because of elevated CO2 levels.
- Don’t undersize your wood stove. Since the grid seems to go down often when it is really cold, it seems wiser to have that extra capacity in your stove than not.
- Actively operate your passive home. Quite often when it gets really cold at night, you have sunny day(s) before or after, so that the solar gain balances out minimal interior temperature erosion during the nights. With really good window coverings (quilts, cellular shades, etc.) operated accordingly, this becomes part of your peak-shaving strategy.