John Ball faces the usual questions as he decides on a heating system for his new home: What system will deliver the best results at the lowest price? What will keep Ball and his wife comfortable in their Canadian locale in Climate Zone 7?
But there’s something else that Ball has to consider: Their new retirement home will be empty during the winter when they’re in Florida escaping the snow and the cold. As they get older, and health care becomes more expensive, they expect to be returning to Canada on a year-round basis.
As a result, Ball explains in Q&A post at GreenBuildingAdvisor, he has to plan on two scenarios: one for now, and one for later.
He’s been given a long list of options so far, including a ground-source heat pump with a radiant-floor distribution system, an air-to-water heat pump, an electric resistance water heater or boiler, and an air-source heat pump.
“We are totally confused as to what type of system to install,” he writes. “We are on a restricted budget so solutions like a ground-source heat pump are out, and we do not have access to natural gas. We are looking for a balance between initial system cost and efficiency. Heating is not critical now as we spend our winters in Florida, but as Canadians we will eventually find health care costs prohibitive and need to stay home in winter.”
One particular concern is whether the concrete slab for his slab-on-grade home will feel cold in the winter. Winter usually brings a few days of 35 below zero weather, so he’s considering doubling the amount of rigid foam beneath the slab, from 2 inches to 4 inches.
“We are open to any solution,” he says. “What would you suggest?”
Consider performance in very cold weather
One of the options is an air-to-water heat pump, such as the Daikin Altherma. However, Dana Dorsett points out that the Daikin is somewhat expensive and probably wouldn’t be able to keep up with the coldest days in Climate Zone 7.
“An electric boiler is very cheap to install,” he adds, “but more expensive to operate, and you may need one to cover the shortfall on the Altherma when it’s -20°F outside.” The Altherma has no specified output below -4°F, he says.
If Ball won’t be around during the winter, Dorsett says, he might want to consider the Fujitsu air-to-air heat pump (the RSL3H product line), which has a specified heat output at -15°F but will continue to run in lower temperatures.
“You can use an electric boiler slaved to a floor thermostat to keep the slab from feeling cold (but only when you’re there), and use the minisplit to maintain the room temperature (all the time),” Dorsett suggests. “You can buy a lot of RLS3H for the price of an Altherma — they’re nice and quiet, too.”
With an electric boiler heating the slab, setting the floor temperature a couple of degrees above the room temperature “guarantees barefoot comfort,” he adds.
“To make sure that the minisplits are pulling the bulk of the load, you may need to buy a hard-wired wall thermostat, since the setpoints will have an offset when it’s just sensing the temp of the incoming air at the head,” Dorsett says.
Skip the radiant floor
Stephen Sheehy’s advice is not to choose a radiant-floor distribution system. “With a tight, well-insulated house,” he says, “you’ll find the heating system often not running enough to keep the floor warm.”
He also disagrees with Dorsett’s suggestion for setting the floor temperature slightly above the room temperature. “If the floor is 25°F below body temperature, you may not get warm feet,” Sheehy says. “A basic question is this: In winter, do you usually walk around in bare feet?”
No bare feet, Ball replies, but the flooring will consist of ceramic tile and engineered wood (no carpet), so the idea is not to have any really cold surfaces under foot.
Skipping the in-floor heat would be Charlie Sullivan’s vote as well. “If you have the slab and the building well insulated, the slab will reach equilibrium with the room temperature,” he writes. “Whether the slab is unheated and at 19° or 20°C (66° to 68°F), or heated to 21°C (70°F), or even 23°C (73°F), makes surprisingly little difference in how it feels to your bare feet… In all cases, heat is conducted away from your feet into the floor, and how cold it feels has more to do with the thermal conductivity of the floor surface material than a few degrees of temperature difference.
“If you insulate well and have a flooring material with moderate to low thermal conductivity, such as wood or carpet, you won’t have a problem with the floor being too cold,” Sullivan says.
Air-to-air heat pump can be the heart of the system
Ball can now add some price estimates to the discussion.
An air-to-air heat pump would be the least expensive option. A ductless minisplit (air-source heat pump) paired with an electric boiler is next, costing an additional $5,000, and the air-to-water heat pump with an electric boiler is $15,000 more than a simple air-to-air heat pump.
A consensus seems to be emerging, says GBA senior editor Martin Holladay: Skip the floor heat and insulate the slab.
If Ball ends up choosing an air-to-air heat pump, Holladay adds, he should make sure the system is rated for below-zero operation. “Equipment from either Mitsubishi or Fujitsu would work; you may even want to consider using Mitsubishi’s new MVZ air handler paired with a Mitsubishi HyperHeat outdoor unit (although this approach isn’t the most efficient),” Holladay says. “If you’re worried about keeping the house warm when the temperature drops to -30°F, a couple of electric-resistance space heaters are all you need.”
Neither the Mitsubishi nor Fujitsu units have the option of built-in electric-resistance heat for very cold weather. Holladay suggests that Ball could buy one or two electric-resistance heaters at any hardware store, although Ball doubts that arrangement would work in a 2,000-sq. ft. house when the temperature falls to 30 below zero for several days.
“For the ductless minisplit approach to work, you need the pay attention to the thermal envelope of your new home, making sure that insulation levels are higher than minimum code requirements; that air leakage rates are as low as possible; and that high-performance windows are specified,” Holladay says. “Installing enough linear feet of electric-resistance baseboard heat to help your home ride through the occasional cold snap is not particularly difficult or expensive. That said, if the approach makes you nervous, you can install a propane-fired furnace or an oil-fired furnace if you want.”
Our expert’s opinion
Here’s what GBA technical director Peter Yost had to say:
It’s hard to give good guidance without full information about the climate, the design of the home, and details on patterns of occupancy. It turns out that John Ball’s home is located in northern New Brunswick in a rural area with very reliable grid electric power. The floor plan is pretty open and they routinely do a fairly aggressive nighttime setback of 5°C (9°F). Someone does check their house on a regular basis while they are away.
In the past, Ball has had a high-efficiency wood stove (which could have handled the peak load for Ball when conditions max out any heat pump, at least when someone is there to feed the stove). But this time around, Ball is fine with backup/peak heat coming from any and all of these approaches: propane fireplace, electric fireplace, surface-mounted radiant electric heat, or baseboard electric heat.
In terms of cooling, Ball says that few homes in their town have central AC systems because of great maritime breezes, but he is pretty concerned about heat gain in their master bedroom and great room, both with lots of glass to the southwest picking up their great view.
(Side note: It has always amused me that historically, the peak electric load in Canada is generally the hottest day in summer, and the peak electric load in Florida is the coldest day in winter. The former is because space heating is largely non-electric in Canada, and the latter because all that strip heat in Florida is kicking in at the same time.)
Ball also is quite open to photovoltaics over time, and although New Brunswick reports fewer than 50 homes with grid-connected PV systems, that is likely to change quite a bit in the near future. The province has set a goal of 40% renewable energy generation by 2020.
So where does all this new information leave us? I’d make the following points:
- A minisplit cold-climate heat pump seems like a good fit, given Ball’s open floor plan. With a minisplit system, a nighttime setback is usually a mistake, since these heat pumps operate most efficiently without a setback. But if Ball wants the nighttime setback, that means either a “smart” thermostat that will ramp up based on weather info, and/or quick-acting spot heat for comfort such as the radiant surface-mount panels (while the outdoor temperature rises in the morning and the heat pump moves away from deep overnight lows).
- Ball is open to exterior shading devices to manage that southwest summer sun. Adjustable or fixed-but-seasonally-installed awnings can be both functional and attractive. And if this approach eliminates the need for summer space conditioning, maybe Ball would be open to a high-efficiency propane heating system, which could provide domestic hot water as well.
- We can all agree that putting as many dollars as Ball can into the performance of his building enclosure is a sure bet, regardless of the heating system(s) he chooses.
- Integrating grid-connected PV is becoming less expensive and more practical at an amazing pace; every project should be looking at how this fits into the picture.