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Q&A Spotlight

Backing Up a Minisplit Heating System

A homeowner considers ceiling resistance heaters to supplement his minisplits

Image 1 of 2
A wall-mounted electric heater is one option for providing a little extra punch on the coldest of winter days when a minisplit heat pump might have a hard time keeping up. (Photo: A. Davey via .)
Image Credit: Image #1: A. Davey / Flickr
A wall-mounted electric heater is one option for providing a little extra punch on the coldest of winter days when a minisplit heat pump might have a hard time keeping up. (Photo: A. Davey via .)
Image Credit: Image #1: A. Davey / Flickr

Low temperatures where David Gadbois lives aren’t Siberian, but he’s still interested in supplementing his ductless minisplit heating system with electric resistance heaters, something to provide a boost on just the coldest days of the year.

“I was thinking about slightly undersizing my ductless minisplit system and using a handful of electric resistant heaters to supplement on, say, the 5% coldest days,” Gadbois writes in a Q&A post at GBA. “These heaters could also help smooth out temperature gradients throughout the house, and finally can also serve as backup heat if the minisplit fails altogether.”

Although he isn’t specific about where he’s writing from, Gadbois notes the coldest days of the year are going to bottom out at about 5°F.

What he has in mind are some “low-temp, low wattage” ceiling panel heaters that run on 120-volt power.

Does that plan make sense? That’s the topic for this Q&A Spotlight.

Consider other electric options

Gadbois likes a couple of things about ceiling-mounted panels: they don’t make any noise, they don’t protrude into the living space, and they don’t require fans to distribute heat. Still, several GBA readers offer some alternatives.

Michael Maines, for example, writes that he had wanted to accomplish much the same thing at his own house and ended up with a . Although these units project slightly from the wall, they do not incorporate a fan and have what Maines says is a “sleek and low profile.”

“They distribute heat via passive convection, so you get better air circulation that you would from a true radiant heater,” he writes. “In many ways they work like electric baseboard heaters, but look better. They also cost more, but are still very affordable.” [According to the company’s website, the units cost $140 and draw 475 watts of electricity.]

As to ceiling panels, Maines has no direct experience, other than running into a few old ones on renovations. is another wall-mounted option available both with and without a fan.

Another suggestion comes from Charlie Sullivan, who writes that an air-to-water heat pump such as a might be worth looking into.

“You can put minipslit-like fan coil units where you’d put the minisplit heads (with more options for small low-profile units), and supplement with small panel radiators where you want smaller amounts of heat delivered locally,” Sullivan says. “Radiant ceiling panels are also an option.”

One more electric option comes from Lance Peters, who notes that 4×8 hydronic sheets that are joined and taped just like drywall are under development. They would, he says, turn an entire ceiling into a radiant panel. He offers to radiant ceiling heating from Uponor.

Heaters as safety concerns

Wall-mounted heaters do take some floor space, Gadbois points out, and should someone make contact with a heater they might get burned. “Also worth considering that wall units that get hot are not kid-friendly,” he says. “Heck, even adults that aren’t paying attention…”

That’s not the case with the Envi heater that Maines installed. The surface of the heater never gets hot enough to be dangerous, he says. The wall above the heater gets “pretty warm,” but not warm enough to burn anyone.

“I’m not trying to convince you away from ceiling panels; it looks like there is a wide variety to choose from,” Maines says. “Just trying to clear up any misconceptions about wall heaters.”

Brian P is another GBA reader who has some experience with wall heaters as well as minisplits, and he suggests Gadbois not intentionally undersize the heat pumps.

“I would suggest going with a correctly sized minisplit system,” Brian says. “If you want some electric resistance backup and consistent temperatures (may not be an issue), I can understand planning for that. We primarily heat with a minisplit and have 500-watt Stiebel Eltron wall heaters (240 volts, hard-wired) in a few areas (bedrooms, bathrooms). We only use them when temperatures are forecast to drop below -10°F, but they aren’t necessary.”

If Brian had it to do over again, he’d simply wire in some extra 120-volt outlets where he could plug in wall heaters as they were needed. “See how the minisplit works out,” he suggests. “If if meets your needs, you’re all set. If you need more comfort due to temperature gradients, buy a few wall heaters.”

What about cove heaters?

Malcolm Taylor says he’s used cove heaters, a type of radiant panel mounted on the wall just below ceiling height, but probably would not suggest them again.

“I specced radiant cove heaters on two projects and one homeowner went ahead and installed them,” he writes. “To me the ‘radiant’ aspect of the heat seems a bit overplayed. I don’t notice any appreciable difference between the way the heat is delivered from any other resistance heater. It is nice that they are out of the way of furniture (and kids), but they are a lot more intrusive architecturally perched up near the ceiling.”

Given the premium in price over baseboard heaters and the lack of a real difference in performance, he adds, Taylor would be unlikely to recommend cove heaters again.

If there’s someone to make the case for cove heaters it appears to be Roger Berry, who suffered through coal and wood in parts of his house, single-pane windows that always seemed cold, and a 1960s renovation that brought with it some radiant ceiling panels that were, in a word, “awful.”

“After growing up and spending 40 years with all the dust, noise, uneven distribution of forced-air heat homes, I chose to go all in with cove heaters for our ‘new pretty good house,’ ” he writes — heaters from Radiant Systems Inc. Because the house is so well insulated, and equipped with high-quality, triple-pane windows, the cost of heating with electricity has not been overwhelming.

“I will point out that the sensation of radiant cove heaters is quite different from forced air (or minisplits I suspect),” Berry writes. “The air temperature is never more than a degree off from thermostat settings.”

That’s unlike a room heated with a minisplit or a forced-air system, which provide warm air well above the thermostat’s setting, he adds. For men who are losing their hair, as Berry says he is, the air might feel cool enough to don a cap inside. “However,” he adds, “neither of us miss the whooshing noises and redepositing of dog and cat hair onto every surface. Filters not withstanding, the dust bunnies were always on the move. Minisplits might amplify the warm head effect due to the higher point of air distribution. Dust bunnies might stay put under the sofa.”

Berry adds that cove heaters aren’t any less attractive than the indoor units that come with minisplits or wall-mounted electric heaters. “They have just become part of the ‘wallpaper’ of awareness,” he says, “much like any vent or light switch does. We don’t worry about blocking vents with furniture or rugs, we don’t have mobile dust bunnies, and best of all each room has their thermostat, so everyone gets to sleep at whatever temperature they like. The whole house, heater elements and wiring was done for less than a quarter of a small forced air system.”

Other points to consider

Cove heaters are available in either 120-volt or 240-volt models, Berry continues, so one issue is having enough room in the main panel for the wire and the breakers that will be required. He suggests planning on about 150 watts per foot of heater.

Heat from the units is noticeable 2 or 3 feet away, but after that it is not. The heaters can’t be painted, and the special paint on the front of the heats should not be touched, at least until the heaters have run for a while, Berry says. “They are not meant for rapid warmup of even small spaces,” he adds. “Think of them as slow cookers, not microwave ovens.

“All that said, they are still resistance heaters and consume electricity accordingly,” he says. ” If I had had an economical option I would probably have done minisplits.”

With electricity consumption in mind, Dana Dorsett suggests resistance heaters should not be oversized.

“If anything, undersize it by at least 25% from the full design load, or even half,” he says. “It’s too easy to become complacent and leave the resistance heaters running, and they will [be] carrying the bulk of the load rather than letting the heat pump do the heavy lifting.”

Using occupancy switches or timers is one way around that potential problem.

Our expert’s opinion

Here’s what GBA technical director Peter Yost had to say:

One of my first research projects at the NAHB Research Center (now called the Home Innovation Center) was to evaluate Solid State Heating Corp. (SSHC) radiant ceiling panels in comparison to a conventional forced-air heating system. (You can read a one-page project summary or the full project report .)

The idea is that the appropriate number and size of ceiling heating panels, controlled in each room by a thermostat that sensed operative temperature (operative temperature being the air temperature plus the mean radiant temperature, divided by 2), can be operated like lights. You turn the panels on when you enter a room and turn them off when you leave. The panels achieve their operating temperature of about 165°F in about 90 seconds. Their location — either centered in a room or favored towards areas of greater heat loss (such as large windows or patio sliders) — gives the best viewing angles from the panels to room occupants.

The panels look a bit like ceiling tiles, are about 1 inch thick, and are mounted to the ceiling, run by either 110 volt or 220 volt electrical circuits. A radiant panel provides most of the heat for my office at home (see Image #2 below).

Do they work? In a nutshell, yes. Their advantage is room-by-room zoning and very fast on-and-off. How much you can depress air temperature because of the elevated mean radiant temperature the panels deliver is a function of how air-leaky your building is and how long you stay in one location. Leakier buildings tend to make the radiant panels shine a bit more compared to forced-air and the more you move about, the greater the advantage of nearly instant on-off of the panels (Over time, the air and mean radiant temperature equalize/reach equilibrium, eroding the MRT advantage).

One cool result of the study (cited above) was adding modulation to the panels; they now are split into quarters so that a panel can be 0, 25%, 50%, 75%, or 100% energized. This eliminates the large and rapid drop in MRT and hence thermal discomfort when the original panels, which were only on or off, shut down on signal from the thermostat.

4 Comments

  1. Steve Knapp CZ 3A Georgia | | #1

    Makes me remember
    Decades ago I lived in an apartment building that had radiant baseboard heaters (a rarity in Georgia). They were likely original to the building an installed before the owners decided to add central air. I really liked how quiet they were. Plus, each room was its own zone.

    If I could live without air conditioning, which I can't, it would be a nice way to stay warm, especially in a really tight house.

  2. Robert Swinburne | | #2

    seconding the Envi heaters
    I have use and recommend Envi Heaters. They have a dial and 475 watts is the highest setting. the lowest setting is more than enough to heat my kids's 130 s.f room with two poorly insulated exterior walls here in Vermont. We use a second one in the larger main room when we leave for a few days in the winter as our only other heat source is a woodstove. They can be hardwired or plug into a standard outlet.

  3. User avater
    Michael Maines | | #3

    Robert, I purchased mine
    Robert, I purchased mine (noted in the article) based on your recommendation. Ours heats a 6' x 8', poorly insulated pantry to between 50° and 60°, with the dial turned almost as low as it can go. If I put more effort into air sealing and insulating I'm sure the lowest possible setting would be plenty.

  4. Irene3 | | #4

    Another option, which I
    Another option, which I expect the OP has already considered, is to get a minisplit system that handles lower temps, as some of the Mitsubishi and Daikin models do.

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