I Have the Coldest Bedroom. But This Fancy Space Heater Finally Did the Trick. | Reviews by Wirecutter

By Thom Dunn

Thom Dunn is a writer focusing on home heating and cooling. He once blew up a power strip with a space heater and a Marshall half-stack.

“Big black mirror on the wall, can you warm this room at all?”

That was the question on my mind when I first stumbled across an ad for the Boldr Kelvin.

I’ve tested dozens of space heaters over the years, but I’d never seen anything quite like this: a rectangular sheet of black glass hanging ominously on the wall behind the couch of a sterile upscale apartment like a turned-off sideways TV screen on the set of some dystopian sci-fi show.

Naturally, I was intrigued.

It wasn’t just the sleek design (which is also available in white). The Kelvin was also one of the few commercially available space heaters I’d found that uses far infrared heat, which penetrates the skin more deeply and directly than the heat from the space heaters we recommend.

Even more curious was that the standard-size Kelvin claimed to use only 450 watts of energy—much, much less than our space heater picks, which use 1,500 watts on high or 750 watts on low. And yet somehow the Kelvin still costs roughly five to 11 times more than one of those other, ostensibly more powerful heaters.

So why would anyone pay $400 for something that puts out only half as much heat as the low setting on a $60 Vornado heater? Was that sleek modernist Instagram aesthetic really worth the investment? Was the Kelvin all a marketing scheme, or was I missing something?

There was only one way to know for sure: I had to install a Kelvin in my home. And to my surprise, it solved a long-standing problem I had otherwise learned to live with.

This sleek glass panel uses far infrared light to warm your skin directly. But the cost and the semipermanent installation could be dealbreakers for some people.

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The Kelvin is seriously great at maintaining a comfortable temperature. This isn’t the kind of heater that generates a ton of raw heat. Instead, it keeps you comfortable while using less energy.

“Most space heaters are built to just throw in a room whenever needed, so they’re designed to pack as much of a punch as they could need,” Madi A, Boldr’s CEO and co-founder, told me over a video call. “We’re building a solution that is designed to be sized correctly for your home.”

And that’s exactly what I found in my initial tests.

The mini-split heat pump that serves as the central HVAC system in my home can’t quite reach the bedroom. But the Kelvin on the wall helps to supplement the warmth before we go to bed or when we wake up in the morning. Using the app, I set minimum temperature levels for different times of day, and the heater kicks on only when the temperature drops. We’ve tried other space heaters in the past, but they’ve always been too loud, too energy-intensive, or too big for our floor space, making it dangerous for us to walk in the dark. For my home, using the Kelvin is a much cheaper alternative than spending $10,000 on another indoor mini-split.

Two other Kelvin owners I spoke with had similar experiences. (I found one of them on Reddit, and the other one I connected with through an introduction from the company.) They weren’t relying on these high-tech mirrors as their primary heating sources; they just needed something to help them maintain a comfortable baseline temperature in a few specific rooms. In fact, both of these homeowners—one in Delaware, the other in upstate New York—also used solar panels as their primary energy source, and they told me that whatever electricity the Kelvin used was essentially negligible, or at least low enough that it wasn’t a strain on their systems.

The far infrared heat feels surprisingly nice and helps keep energy costs down. Unlike other space heaters I’ve tested, the Kelvin uses far infrared heat, which is generally better at heating things than warming the overall air in the room. Think of it like the sensation you get when you feel sunshine on your skin. The Kelvin might not increase the ambient room temperature as well as an oil-filled radiator does, but you can still feel the warmth deep within your skin.

I reached out to Juejun Hu, an associate professor of materials science and engineering in the Photonic Materials Group at the Massachusetts Institute of Technology, for help in better understanding the effect of the far infrared light. “Normally the heating element passes through the air, and the air passes the heat onto you,” he explained. “But in this case, the light sort of skips all the middle with the air and directly reaches your body or your clothing and turns into heat.”

If we’re being technical, the Kelvin isn’t actually any more efficient than any other electric space heater you can find. But the infrared mechanism causes the warmth to feel more comfortable and immediate—even at a lower wattage—because it’s more direct than mechanisms that transfer the heat through the air.

The glass finish helps disperse heat around the room. Most infrared heaters come with a catch: You have to be standing directly in front of them to feel the effects of the light.

But the Kelvin gets around that with the help of its strange mirrored finish. Glass is very heat absorbent, Hu explained, and once it heats up, it acts as an isotropic radiator, which means the heat goes “essentially everywhere in all directions with more or less the same intensity.” Hu continued, “So that means if you’re standing here, here, or there, you’re going to feel it the same.”

As a result, the heat from the Kelvin radiates at a surprisingly wide angle. It doesn’t matter if you’re 3 feet away from the heater, 10 feet away, or standing against the wall beside it—the infrared light spreads pretty evenly around the room.

It comes with its own remote temperature sensor. Although the glass does a great job of diffusing the infrared heat around the room, the included temperature sensor can help to even out the heat more so that it raises the temperature in all the right corners. The ping-pong-ball-sized sensor connects to the central Kelvin unit via Bluetooth, so you can place it anywhere within a roughly 30-foot radius to ensure that the temperature readings are accurate across the room.

The Kelvin is completely silent, with no fans or other moving parts to bother you. Most of the space heaters we recommend use convection heating, which means they need a fan to move the heated air around the room. Some also rely on an oscillating mechanism, which rotates back and forth to help with that same distribution process.

But the Kelvin’s far-infrared-by-way-of-glass heating method has no need for noisy parts or fans. In fact, it produces no air movement at all, so you don’t have to worry about the Kelvin kicking up dust, say, or blowing in your face while you try to watch TV. It barely even emits an electrical hum.

It doesn’t take up any floor space. Most space heaters aren’t particularly large; our top pick from Vornado, for example, occupies only about 1 square foot of space on the floor. But even that could be intrusive in a small bedroom like mine. By mounting the Kelvin on the wall, you don’t have to worry about pointing your heater in the right direction, or moving it out of the way when your robot vacuum comes around, or keeping it away from curious kids or cats.

And let’s be real: It does look fancy. I joke about the ominous black mirror in the bedroom, but it’s honestly not that different from a turned-off television. The company also sells a version with a white glass panel, which may blend in better on standard white walls. The option to position it horizontally or vertically is another nice perk.

The Kelvin isn’t great at warming up the air. At one point during my tests, I decided to crank the Kelvin as high as it could go and see how long it would take to increase the overall temperature in the room. Over the course of three hours, the air itself warmed up by only about 5 degrees Fahrenheit. In contrast, any of our space heater picks could have done that in roughly 30 minutes through convection heating.

But I must reiterate that this is by design: Far infrared is great at heating objects—but not air—at a lower wattage. And that’s exactly what the Kelvin does. As long as you’re somewhere within the Kelvin’s broad line of sight, the light waves will keep you feeling warm and radiant. But if you’re hiding around the corner of a wall, or if you’re expecting the air itself to feel warm and toasty, you may be disappointed.

The glass surface gets uncomfortably hot to the touch. When I first plugged the Kelvin in, my insatiably curious 4-year-old did what he does best: He impulsively grabbed at this shiny object. Then he yelped in pain.

Don’t worry; he was ultimately fine. But he hasn’t tried to smear his fingers on that glass again.

Later on, when he wasn’t around, I measured the surface temperature of the Kelvin at around 175 °F. It isn’t the absolute hottest space heater I’ve ever tested (that distinction belongs to the wall-mountable but not infrared De’Longhi HMP1500 Mica Panel Heater, which reached a whopping 304 °F), but it is still pretty high up on the list.

It’s also a lot hotter than any other glass object in the house, and it has no obvious visual indicator to caution you when the heater is on. The product manual does warn about this, noting that a “key benefit” of the panels is that you can mount them higher on the wall, thus “reducing the likelihood of accidentally touching them or falling against them.”

The remote sensor is also not great for kids (or pets). As helpful as the sensor is, its ping-pong-ball-like appearance can be another dangerous temptation. My 4-year-old thought it was a toy and kept trying to steal it, and another Kelvin owner I spoke with said they had a similar experience. I can imagine that some pets may be tempted to chew on it, as well. Best to keep the sensor in a place where curious creatures can’t reach it.

The Kelvin takes up a good amount of wall space, which can make it difficult to place. Unless your home aesthetic is staunchly minimalist, finding a place to hang the Kelvin that isn’t already occupied can be difficult.

I was fortunate to find a place in our bedroom for the standard-size Kelvin, which measures a little over 3 feet wide and about 1.5 feet tall (the company also sells larger panels that are about 4 feet by 2 feet). Our 1,200-square-foot home had only one other spot where the panel could have fit, unless we decided to rearrange all of our artwork and other decorations.

Even after you find a space for the Kelvin, you still have to deal with the glare of the glass. “It blends in nicely, but there’s still that refraction on it,” said Mike Finnegan, another Kelvin owner. “Like if you’ve got it near a window, you can see the reflection of the outdoors on it. So it always kind of sticks out in some way.”

The semipermanent installation requires putting some holes in the wall. Normally when I test space heaters, I just place them on the floor in the middle of a room and then put them away when I’m done. But the Kelvin requires a little more planning: You need to use a jig to drill anchors into the wall and then hoist a 20- to 30-pound sheet of glass up onto the screws.

That may sound like a reasonable feat if you, like me, are handy and able-bodied. But it could be a genuine challenge for some people. As Kelvin owner Mike Finnegan said to me: “If you can’t mount a TV by yourself, you probably can’t mount one of these.” Or at the very least, you’ll need some help.

The unsightly power cord hanging from the heater is less than ideal. As with most space heaters, the plug on the Kelvin comes with about 6 feet of slack on the cord. That might work for something standing on the floor, but it starts to get complicated when you’re hanging something 3 or 4 feet up on the wall.

Ideally, you’d be able to install your Kelvin right above an existing wall outlet. But even then, you’d still need to deal with the dangling cord. You could try to hide it with a wall-mounted cord cover, for a slightly less conspicuous result.

As with a lot of smart-home devices, the app is still a work in progress. The Kelvin may resemble a giant iPhone, but you’ll find no touchscreen display here. If you want to do anything more than turn it on or off, you have to download the Boldr app. Like a lot of other smart-home apps, it has a nice user interface and offers basic functionality, but it’s still prone to glitching and other frustrations.

The scheduling feature, for example, allows you to set your preferred temperature at different times of the day—but you have to go through and manually enter each individual day separately. The app also has a built-in “tariff” feature that allows you to track or limit your energy use; unfortunately, this feature crashed every time I entered my local electricity rate. (The app has an optional AI component that’s supposed to be able to read your electrical bill, but that has never worked for me, either.)

Boldr acknowledged that the app is still a work in progress—something that isn’t uncommon among smart-home startups. However, the two customers I spoke with said that the company was helpful and responsive when they reached out for tech support, and that the app had come a long way from when they started using it.

Did I mention that the Kelvin is expensive? The Kelvin’s $400 price tag makes it much more expensive than most other space heaters out there—though its lower energy usage could help you save some money in the long run.

Within the broader category of infrared-heating panels, however, the price of the Kelvin isn’t that much of a standout. Herschel infrared panels are similarly priced, and while they don’t have the same glass finish, they do come with a generous 10-year warranty, in contrast to the Kelvin’s two-year warranty. Some people really like the Herschel panels, while others have complained that, well, they don’t heat the air itself enough or that you feel the heat only when you’re directly in front of them.

You can also find some slightly smaller 400-watt panels at retailers such as Home Depot for about $100 less than the Kelvin, though again, these models don’t have the same glass finish to help with the heat dispersal. And you might consider saving another $100 or more on top of that with any of the various cheap infrared-heating panels available on Amazon—but we didn’t find any models there that looked convincingly effective, let alone safe.

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The Boldr Kelvin isn’t for everyone. But as skeptical as I was, this heater still won me over in the end, because it helped me find a solution to a problem that I had otherwise learned to live with: a cold bedroom.

If you just want something small to help you spot-heat in the living room sometimes, you might be better off with a Vornado model or any of our other standard space heater picks.

But if you—like me—have that one room that just won’t heat like the rest of the house, the Kelvin could be your answer. Assuming you have the wall space for it, of course.

This article was edited by Megan Beauchamp and Harry Sawyers.

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Thom Dunn

I’m a writer with a focus on heating, cooling, and humidification devices, as well as gadgets such as Christmas lights and karaoke machines.

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