The clever feature that makes cheap heaters safe — and why they're actually dangerous
You’ve probably heard from all sorts of people that electric space heaters are dangerous things which are likely to cause a fire. And it is true: they can be dangerous and they can start fires. However, likely not in the ways you imagine they can.
Even this cheap plastic heater, which cost less than $20, has multiple safety devices which make many of the dangerous scenarios you might imagine nearly impossible. And in this video I’ll show you the clever bit of engineering and logic that went into making these things safe despite their bargain basement cost. Now, before I continue, I need you to know that I’m going to be demonstrating some very stupid things which nobody should ever repeat. The written warnings on a space heater (or really any product) should never be ignored - they were written in blood. However, I don’t think we do ourselves any favors when all we know is “this is dangerous” but we don’t understand why and how it’s dangerous.
That’s a kind of ignorance which leads to unfounded fears and paranoia. I quite happily use these bargain basement heaters whenever I need a source of supplemental heat - and not simply because I’m a Midwestern cheapskate and that's the one would buy anyway, but because I understand the ways these might fail and the protections they have against those failures. I love nothing better than when more people understand more things more completely so let’s take a peek inside. There really isn't a whole lot going on in these things. We have two nichrome wire heating elements sitting behind a fan driven by an electric motor. And, well that’s it! AC power from the wall gets sent through those wire elements and they start getting really hot.
And the spinning fan forces air movement through those hot wires to transfer their heat output into the air and then spread it around the room. And thus explains the entirety of its basic principle of operation. The controls and how power is routed through the device are where things get more interesting. If we follow the power cord we’ll see that incoming power is first sent through this tip-over switch.
That switch is actuated by a little plastic foot sticking out of the heater’s bottom and it must be depressed by the weight of the heater in order for the switch to close and the heater to function. That's the first safety device and it ensures the heater will shut itself off if for some reason it gets knocked over. Next to that switch are a few steel plates which simply serve to make the heater more bottom heavy and less likely to be tipped over in the first place - this is a pretty recent addition to these cheap heaters and it’s a very welcome one. It makes them far more stable. After making its way through the tip-over switch, power then finds itself at the thermostat. Even these cheap heaters have actual thermostats which react to the room temperature and will shut the heater off once it's warm enough, allowing it to maintain a consistent temperature by itself.
These are basic things, just a bimetallic switch that the control knob applies varying pressure to in order to affect its trip point, so they’re not accurate enough for the manufacturer to print actual temperature values on the control but they are more accurate than you may imagine: if you run the heater on its maximum setting until the room is as warm as you want it to be and then you slowly turn this back down until it clicks off [click] it’s gonna maintain that room temperature fairly well. You might even put a little mark with a sharpie on where it's pointing. That way you can put it there again! Who needs fancy digital thermostats when you've got a brain at your disposal? Once it’s through the thermostat, power then finds itself at the main selector switch.
This thing is a little complicated because the heater has two heating elements of different sizes to allow for three levels of heat output. The 600W or the 900W element can be run individually as a low and medium setting (called eco here for annoying reasons), or they can be run together on the high setting to produce 1,500W of total heat output. The switch selects between powering one, the other, both, or neither on its fan setting, a setting which turns this into either a pretty crappy desk fan or a decent white noise machine depending on how you wanna think of it.
Anyway, power gets sent out of this switch through these wires which land on this terminal block on the heating element assembly. The heating elements and the fan receive their power from here. These two terminals are for the heating elements themselves. As you can see each element is simply a wire which is wound around a circular, slotted temperature-resistant support structure a whole bunch of times until it’s back at the start of the circle and on the other side of the terminal block. And on that other side, you’ll notice there’s a little something extra going on.
We still haven’t completed a circuit here - everything we’ve looked at so far has come from the incoming hot wire and we have to connect it to the outgoing neutral wire for the heater to actually function. And this terminal here is where the neutral wire of the power cord attaches to. But the heating elements (as well as the fan and the neon power indicator) don’t connect directly to neutral: first they go through this switch. This is a high-temperature limit switch.
It’s an unassuming little fella but it’s the reason why this plastic heater can be made cheaply yet also safely. This switch is normally closed and so will allow the heating elements to become powered. And under ordinary circumstances, it will stay closed forever. But if anything should happen which causes that switch to get hotter than 75 degrees Celsius (that’s 167 Fahrenheit) a bimetallic snap disc will force the switch to open, breaking the circuit and shutting off the heater. Say, for example, the fan motor had failed.
The fan in these things is critical not only to help move the room’s air around to distribute the heat they produce more evenly but also simply to get that heat out of its plastic body. The 1,500 watts coming from this is a lot of heat! And even with the open front and back, without that fan spinning this thing will quickly start melting and perhaps catch fire. But this limit switch will intervene if the fan stops turning for any reason.
To show you this, I completely removed the fan from this heater. It’s gone. So when I switched it on, it started drawing full power yet couldn't move any air through itself. That could be a disaster, but not even 15 seconds later this switch opened and the heater shut off.
Here’s a close-up. And for the record - I cut the fan motor out of the circuit, here. So that arcing is entirely down to the switch breaking the current from the heating elements. The heater was pretty quick to shut itself down, wasn’t it? That’s due to its design. Look at where the terminal block is. It’s at roughly the 10 o’clock position of the circular heating element assembly as installed in the heater, and that places the limit switch directly above the heating elements.
With the fan functioning, enough air is pulled through to keep the limit switch cool. But if the fan stops, or just isn't working, the air next to the heating elements will quickly heat up and rise right into the limit switch. That’s why it reacted so quickly. But 75 degrees C is cool enough to open that switch in other scenarios, too. For instance, say the heater becomes blocked by something. Like this random griddle insert I have lying around.
While the fan is still functioning normally, it’s not able to get that much air out of the heater with the griddle in front of it. And that means the temperature inside is climbing. Some air is still getting around the griddle so the temperature doesn’t climb that rapidly but about 90 seconds after placing it there, the limit switch opened and shut off the heater. Good job. Wanna see something even more dangerous? Every heater out there has a very explicit “do not cover” printed on it somewhere but what if, oh, a very flammable beach towel were to be thrown on top of it? Again - don’t ever do this. I had a fire extinguisher at the ready just in case.
But, in just under a minute, the air inside the heater had exceeded 75 degrees Celsius so that limit switch opened and the heater shut itself down. The towel wasn’t even that hot when I took it off. But perhaps you noticed something a little strange in the clips I’ve just shown you.
When the heater shut itself down, its power consumption didn’t drop to zero. It was drawing about 1 watt despite that limit switch being open and thus the circuit being broken. What’s consuming that power? Well, and here’s the twist - the limit switch itself.
Take a look in the thermal imaging camera. That limit switch is still hot despite the rest of the heater having cooled off. And that’s because this limit switch actually has a small PTC heater embedded within itself and connected across the switch from the incoming hot to a second neutral connection down below the main switch. You don’t need to know what PTC means - it’s a kind of self-regulating heating element and they can be made very small. The upshot is, with the limit switch closed that PTC heater is bypassed and doesn’t do anything.
But if that switch on top opens, suddenly there’s 120V across the heater so it starts functioning and it gets warm. The purpose of this is to keep the heater disabled indefinitely. The moment that switch opens, because of that PTC heater kicking in, the limit switch will remain above its trip temperature so it cannot rest.
Therefore it prevents the heater from working again so long as it has power going to it. And that means a single instance of overheating will effectively lock out the heater from functioning until someone tends to it. Resetting that limit switch is as easy as unplugging the heater for a while, shutting off that PTC heater and allowing it to cool off and close again. And it has written instructions on the heater to explain this procedure. But until you actually do that, the heater will remain locked out and it cannot turn itself back on. This is fascinating and I only discovered this behavior when I filmed this test! I fully expected the heater to reset on its own, start functioning, and then just trip again - which is a reasonable approach to safety.
But a lockout is even safer. Whatever might have caused one of these things to overheat - say someone tossing and turning in bed and throwing a blanket on top of it in their sleep - still presents a potential danger if the heater can start back up on its own. So, by ensuring it can’t until the user takes positive action to reset that lockout, it lowers the risk of fire even further. This is such an elegant solution made using ridiculously simple components and that’s the sort of problem solving and ingenuity that really floats my bread. Butters my fancy.
Tickles my boat. Now, this lockout mechanism isn’t quite perfect. For one, if while it’s locked out there was a power interruption for about a minute or longer then the switch will reset and the heater will start working again when the power comes back. But also, while the reset instructions imply that you need to unplug the heater to reset it, you actually don’t and that presents one scenario where it could potentially reset on its own even with a steady source of power keeping the lockout functional. If, while it was in the locked-out condition, the room temperature were to rise enough to trip the heater’s thermostatic switch, the opening of that switch will remove power from the PTC heater in the limit switch and before long it will cool down and close again.
With the thermostat tripped it's not gonna start heating again, but if the room temperature then cools enough to close the thermostatic switch, the heater will resume functioning. But, in case it’s not apparent, that’s not a very likely scenario. Speaking of the thermostat, though - I’ve uncovered another surprise. Back when I started writing this script I said there were multiple safety devices. And there are two: the limit switch and the tip-over switch.
And I was going to include the thermostat as a third line of defense against dangerous situations. See, I figured at its highest setting it might maintain a room temperature of... I dunno, 90 degrees? 32 Celsius? So even if the limit switch somehow didn’t trip in an overheating situation the thermostat should shut it off eventually and at least limit its heat output. But I wanted to test that, so I stuck the heater in a closed highly flammable cardboard box, set it to max, and poked a thermometer in the side. Again - don’t do that. Uh, turns out it got hot real fast which tripped the limit switch.
Which, I mean, good! But that doesn’t answer my question. Thinking that was just a case of things getting too hot too quickly I tried it again but with the heater set to its low heat setting and the thermostat still set to the max. And the limit switch still tripped on me. So I put a different heat producing device with its own ability to regulate the temperature inside the box along with the space heater and kept the inside of the box near 150 degrees.
The thermostat still functions as normal when the heater is in fan mode so I was watching the kill-a-watt to see when the thermostat would switch it off. But it just… didn’t! After more than 10 minutes of being in a 150 degree hot box (that’s 65 Celsius) it was still happily running. And when I checked to see just how much I’d have to move the thermostat dial back from max to make it shut off… it was a lot farther than I would have expected. I knew these thermostats aren’t calibrated very well but that’s ridiculous. So count the thermostat out as a safety device. If it's set to its highest setting it’s just never gonna intervene.
That may very well be unique to this model of heater, and the limit switch in the heating element assembly is much more important anyway, but yeah. So now, let’s circle back to the start where I said space heaters are dangerous and can cause fires. I hope by now you can see that the design of the heater makes it extremely unlikely that the thing itself will catch fire but that is absolutely not the only way things can go wrong. The simplest and most straightforward fire hazard to explain is... uh... flammable items near a hot thing. The warnings about keeping flammable stuff at least 3 feet away from the heater are all about minimizing that risk (and by the way, 3 feet is a yard which, fun fact, is pretty close to a meter - keep that in your back pocket).
However, that risk is situational and depends a lot on the kind of space heater you’re using. If potentially setting fire to objects in the room is your concern, you might be surprised but these cheap heaters are actually among the safest options. The device itself doesn't get very hot at all because the fan does a very good job moving air through it, and that also means the air that's leaving isn’t actually all that hot.
In a room-temperature environment, the highest temperature reading I observed with a thermometer right in front of this was 155 degrees, not quite 70 Celsius. So unless something highly flammable manages to get through the grilles and actually touch the heating elements, this isn't likely to set fire to anything. That doesn’t mean you should tempt fate, but it should hopefully put your mind at ease. However, other kinds of heaters have much hotter surfaces, particularly those with heating elements that glow, so again that risk is very situational. But it’s easy to understand and to mitigate.
The larger reason space heaters are known to be a fire hazard actually has nothing to do with the heaters themselves and everything to do with your home’s electrical wiring. Most of these things which are on the market, regardless of type, will put out 1,500 watts of heat and that requires 12 amps of current at 120V. That is the maximum allowed to be drawn continuously from a typical 15A circuit, and that means plugging in a space heater becomes an instant stress test for whatever circuit it’s on, and that can reveal electrical issues you didn’t know about and potentially with awful consequences. For instance, if there’s a very slightly loose wire nut in a junction box somewhere in your walls, or maybe there’s a connection point somewhere that's got a little corrosion on it, that’s usually not a problem. While it might introduce a little bit of electrical resistance at that point in the circuit, if all you’re using that circuit for is charging a laptop or phone, not much current will go through that connection so the issue won’t ever present itself.
But if you plug a space heater in, that can quickly change. That marginal connection point will start heating up because of the large current the heater is drawing through the circuit. And if it heats up too much, that can start a fire.
This is made worse by the fact that technically a space heater should be the only thing on a 15 amp circuit if it’s gonna be drawing 12 amps. But in real life, it’s going to be sharing that circuit with some other stuff: perhaps a TV and a sound system in the living room or a desktop computer in a bedroom. That means when the heater’s running at its full power you’re really pushing that circuit and so if there are any problems with the wiring they’re more likely to become an issue. But, I don’t wish to fearmonger.
That scenario is realistically quite rare. The wiring that's in your walls mostly doesn’t get touched unless you’re doing remodeling or something. So spontaneous development of issues, while that can happen, is quite rare.
However, the receptacles that you plug stuff into (including a space heater) those things get touched all the time. And each time a plug is inserted and then removed from a receptacle a teeny tiny bit of wear occurs. The springiness of the contacts inside there can weaken and if the outlet is really worn it might barely be able to hold on to a plug. Receptacles like that are primed to become a major issue when you plug a 12 amp load into them and let it rip, and they are a major reason electrical fires start when people use space heaters. A poor connection between the plug and receptacle can introduce enough electrical resistance to make the plug extremely hot - to the point of melting wires and burning insulation. If you have loose receptacles in your home, they really should be replaced.
Which, by the way, is quite easy to do but you’re gonna have to look for your own information and guidance there. But then, even with a perfectly cromulent electrical system with everything in hunky-dory tip-top shape, there’s the problem of extension cords and power strips. Although it is sometimes inconvenient, a space heater should always be plugged directly into a wall outlet. No splitters, no power taps - directly into a wall outlet. Extension cords, power strips, and all those adapters and whatnot are not built to the same standards that actual receptacles are. So although those products may technically be rated to pass more current than a space heater needs, their often poor construction means each individual connection might not be so great and it introduces an element of risk which you might as well just not introduce.
In fact, the voltage drop introduced by the extra connections between the space heater and the wall outlet on the other end of that power tap is part of the reason that the heaters were only drawing 1,200 to 1,300 watts in my testing. Plenty of power strips have ended up very melted because someone plugged a space heater into them, so just don’t. I hope you can see, though, that the space heater itself is not scary. Even these really cheap ones are designed and built to be extremely safe. The danger in using them is actually just because most people don’t have a very thorough understanding of electrical safety.
Most of the scenarios where a space heater can become dangerous are quite easy to spot long before that occurs but you need to know what a poor electrical connection is and how to detect it. If you plug a space heater into an outlet which is holding it firmly, you’re probably fine. But if that outlet can barely hold onto the plug, find a different one. And if you want to be extra safe, feel how warm the plug is getting after the heater’s been running for 10 minutes or so. It’s completely normal for it to be a little warm to the touch, but if it’s hot - something’s very wrong and you need to shut off the heater and unplug it. As a closing message, if you live in a place which is cold enough that a failure of your heating system could freeze your pipes, I think it’s very wise to have some space heaters on hand for emergencies.
$60 can get you three of these things and while they won’t be able to match your main system’s heat output, they can probably keep your home significantly above freezing and at least one room can stay comfortable. And if you’re on the heat pump train but you don’t have a dedicated resistive heat backup, such as if you went with a multi-head mini-split system, these can do the trick just fine. The heat strips in a centrally-ducted heat pump system are just a bigger version of this thing, so a few of these heaters can absolutely be used to fill in the gap when and if the outside temp is so cold that the heat pump can’t quite keep up on its own. If you are ever in a scenario where you need to use multiple space heaters at the same time, you’re gonna need to identify which outlets are on what circuits because two space heaters running on the same circuit will quickly trip the circuit breaker. Although - another fun benefit of these cheap fan-forced heaters is that two of them actually can share a circuit so long as they’re both set to low or one's on low and the other’s on medium.
Strategic placement of these fellas can probably be your only heat source in a pinch - as those of you who saw my video on HVAC system sizing already know. That would be a very expensive way to heat your home in most cases but hey, the actual heaters are quite cheap so it’s a compelling backup option. However, as much as I like these cheap fan-forced heaters and think they’re unquestionably the most reasonable option to keep on hand as a backup, this baseboard-style of heater has become my new favorite when I actually want supplemental heat somewhere. This places the heating elements in open air and relies solely on the convection currents which form as the air inside heats up, expands, and then floats out the top. These things are completely silent but they can also sustain their full 1,500 watts of output - something those oil-filled heaters simply cannot do.
Because of how this works it has much hotter surfaces and that makes them more dangerous in some ways, but it also don’t have any moving parts which can fail. Still, sure enough, take a peek inside and we find the same exact limit switch that we do in the fan-forced heater. So if someone were to, I dunno, throw a beach towel over one of these things, it will eventually lock itself out just like the fan heater does. Actually, I better test that. Again, don’t ever do this yourself.
The limit switch did kick in when it was completely covered by the towel but… I wondered what would happen if it was mostly covered but the area above the limit switch wasn’t. Yeah here things aren’t so rosy. Because the limit switch is below the heating elements (which it has to be otherwise it would trip in normal operation) so long as there’s enough of the heater uncovered to supply sufficient airflow through those vents down there the limit switch stays cool enough that it doesn't trip. I stuck a thermometer under the towel to see how hot things were getting and I kept creeping the towel further and further to the right to block more and more of the heater's venting. By the end we had exceeded the thermometer’s maximum temperature reading of 200 Celsius, 392 Fahrenheit, and the heater was still running.
For the icing on the cake - I set a timer at that point for 5 minutes to end the test before things got out of hand and while it did shut off before that timer ran out, it wasn’t the limit switch that shut it off. It was the thermostat! That’s why power consumption went to zero and not a watt or two. Once I removed the towel it quickly turned back on.
Not so great. And the adhesive on that little “hot” sticker sure was destroyed. But on the other hand, some of you have kitchen stoves which emit open flames.
That might seem like a non sequitur but my point there is that we aren’t so scared of objectively dangerous things when we understand what those dangers actually are and how we can avoid them. In the case of this heater - sure, it might start a fire if someone throws their jacket on top of it. But so long as you’re careful enough to not do that, the fact that there are no moving parts in this thing means it can’t really fail in a way which presents a danger on its own.
So, as with so many things in life, there are trade-offs. And those are up to you to figure out. Once you’ve done that, you can make informed decisions for your own benefit.
And that’s a kind of empowerment that we all deserve. ♫ manageably smooth jazz ♫ And not simply because I’m midwub AAAGGH - I’m a midwestern. That’s hard. The springingnyess of the cont… spring-ee-ness. Springiness. The springingest… springingess? Springiness.
Oh. That’s… s[bleep] I picked the wrong one. You mean I have to put this back apart right now? Back apart? This is a great use of camera resources - memory. This is… this whole thing is gonna be backed up in two places and it’s just me putting a thing back together that I forgot to do before I started recording. The switch selects between powering one, the other, both or - I’m not even pointing at it. Things do seem to be heating up.
But the mere possibility of something occurring doesn't need to be scary. Learn about those possibilities and you can get ahead of them. Just a reminder. Useful in lots of ways.
2025-02-05 21:04
