Fail Of The Week: Thermostat Almost Causes A House Fire

Fair warning: any homeowners who have thermostats similar to the one that nearly burned down [Kerry Wong]’s house might be in store for a sleepless night or two, at least until they inspect and perhaps replace any units that are even remotely as sketchy as what he found when he did the postmortem analysis in the brief video below.

The story begins back in the 1980s, when the Southern New England area where [Kerry] lives enjoyed a housing boom. Contractors rushed to turn rural farmland into subdivisions, and new suburbs crawled across the landscape. Corners were inevitably cut during construction, and one common place to save money was the home’s heating system. Rather than engage an HVAC subcontractor to install a complicated heating system, many builders opted instead to have the electricians install electric baseboards. They were already on the job anyway, and at the time, both copper and electricity were cheap.

Fast forward 40 years or so, and [Kerry] finds himself living in one such house. The other night, upon catching the acrid scent of burning insulation, he followed his nose to the source: a wall-mounted thermostat for his electric baseboard. His teardown revealed burned insulation, bare conductors, and scorched plastic on the not-so-old unit; bearing a 2008 date code, the thermostat must have replaced one of the originals. [Kerry] poked at the nearly combusted unit and found the root cause: the spot welds holding the wires to the thermostat terminal had become loose, increasing the resistance of the connection. As [Kerry] points out, even a tenth of an ohm increase in resistance in a 15 amp circuit would dissipate 20 watts of heat, and from the toasty look of the thermostat it had been a lot more than that.

The corner-cutting of the 1980s was nothing new, of course – remember the aluminum wiring debacle? Electrical fires are no joke, and we’re glad [Kerry] was quick to locate the problem and prevent it from spreading.

63 thoughts on “Fail Of The Week: Thermostat Almost Causes A House Fire

  1. An alternate cause for your failure – a bug, the 6 leg variety.

    Years ago when I was a licensed electrician I replaced a similar thermostat that had failed the same way. But when I took a close look around the inside of the housing I saw pieces of a wing and other parts. I put the thermostat in a ziplock bag and took it over to COOKS pest control. The guy said “That’s roach parts, we see that all the time.” Then he explained that the adults are too large to get between the contacts but if a baby is in there when they close he gets zapped with 220V and the leftovers can burn the contacts.

    And BTW, the other contacts aren’t neutral, they are the other side of the 220V circuit. NEC requires you to disconnect both sides of the load when switching off a 220V circuit and you need 2 contact sets to switch off both sides. Light switches are single pole because you only switch one side of a 110V circuit. The other side is tied to ground and does not need to be switched.

  2. If you “bug sweep” your house every so often, using the most primitive lash-ups of RF meters and coils, or even the slightly off-tune transistor radio trick used for microwave leakage detection, any of those should detect the RF getting farted out by bad connections anywhere in your walls, particularly within a couple of feet of them. So it’s a good peace of mind check for actual hardware bugs happening in your house, as opposed to listening bugs planted by your arch nemesis.

    1. I’d be interested in trying this out, but have little/no experience with RF – any links or resources you cou point us to, especially regarding the off-tune radio trick, would be appreciated. Thanks.

      1. Okay, was hoping to point you to a good article, but google thinks the only way you’d want to test your microwave would be with your phone nowadays.

        Anyway, basic principle, get an AM radio, preferably small enough to wave around, preferably manual analog tuning, the more basic the better. Now a real basic one, all you need to do is tune it off station completely somewhere where you’ve just got hiss, and when it’s near or pointed at RF interference whether from contacts sparking or microwaves leaking it will sound like a harsh buzz, like a gearbox clearing it’s throat or something. IF you’re in doubt it’s picking anything up, take it to the oldest light switch in the house and turn it on and off, you should hear a clicking.. or turn it on anf off in slow motion and hear more of a buzz, not too much of that though you don’t wanna burn out your switches prematurely. If you’re hearing buzzing all of the time it may be that your fridge or furnace motor is particularly noisy and you could take a stroll to the bottom of the yard to see if it gets fainter. Or it could be you’ve found your first really bad fault which is arcing up a storm. For more info on direction finding with the AM radio to help you zero in, click link in my username.

        If you’ve got a more advanced AM radio with AGC AVC or some built in squelch, you might not hear much off station, so you may need to tune to the edge of a station so it’s barely coming through, then sparks and arcs will disrupt it. This may also be necessary if you’ve got nothing but a digital tuned one to play with that still lets you have some manual tuner stepping. If you’re stuck with a digital one that automatically tunes station up or down, find a really weak one, and possibly you’ll hear it drown out the station.

        Anyway, generally, having figured out the nulls on your radio (see link in username) point the “lobe” (90 degrees to null, should be) side of the antenna at the walls you wish to scan and methodically go over any areas you think has wiring in it. This will only pick up faults when wiring has current flowing, so you may want to switch things on and plug things in to check.

        RF detectors can be very simple, like an NE-2 neon connected to antenna and ground, but since in some circumstances this can pick up over the horizon lightning storms, it’s not terribly selective. Many circuit collections have simple RF meters/detectors in, usually antenna is left up to reader though so grab an non-active indoor TV antenna or something if that’s what you have handy. However, many of these depend on junkbox stuff you may not have, and use older diodes and meters which are hard to find. Hence, here’s a more modern approach..

        1. Oh yes, you might come across some different noises. If you’re getting a growl or less random buzz or tone, within a foot or so of a CRT, running motor, dimmer switch or PSU, that is probably magnetic inductive coupling, not RF, that the coil is picking up from the coils in the device. It’s not something to worry about, unless you’re gonna stick something else with a coil in right next to it, in which case you might want magnetic shielding.

    2. Have done this and can confirm. Broadband has killing the resident’s TV reception very effectively.
      Turned out to be arcing contacts in the gas burner controller. As it didn’t cycle on too often, and not every cycle caused the issue, it was a bear to find.. gas company came out the next day and replaced the controller at no charge, leading to a very happy resident.

    1. Yea, that’s what I was wondering. What’s the part number on that?

      When I looked there were very few (as in, one) retail electronic units that handled more than a paltry 15A. Which on a 120 circuit was around half a baseboard (I kid.. it was probably a full small baseboard) of load.

    1. I’ve got a new, portable electric heater here where you can see through the vent, a simple thermostat switches 2KW at 240V. It’s fine. A bimetal strip is probably chunkier than the contacts in a similarly rated relay anyway. A relay is another part to go wrong, for no benefit.

    1. I can personally verify that it wasn’t all gone by the 80’s. Had a condo built in ’82 full of aluminum wire. The laundry and kitchen got copper and earth, the rest was aluminum and only the bathrooms had earth connected. Some outlets even had line and return backwards. Also had walls so bowed and crooked it was impossible to make floor tiles look right even at first glance. Bare wood posts going into dirt just outside the foundation for the outer walls too. The inspector must have been able to buy a yacht with all his bribes, useful for fleeing prosecution I guess.

      1. Reminds me of the Mafia building an elaborate system of access ramps to Bart Simpson’s school, out of breadsticks.

        Surely those posts would rot after a while? And the damp would capillary up into the whole house. After all the thing trees are good at, is soaking up water, all those xylems and phloems. You might have been able to sue the contructor or person who sold you it, if they had a responsibility to know that. Unless you were only renting of course. Unless it was a “Family” business!

      2. Aluminum wiring is STILL used (and legal) in the US for circuits that feed stoves, washers, and other high-load appliances, when used in conjunction with breakers, connectors, and receptacles designed for use with AL wiring…

      1. Arc Fault breakers have microcontrollers inside that analyse the waveform of the current flowing through it. Arcing typically produces high-frequency components in that waveform, which causes the breaker to trip even when an RCD or a regular breaker wouldn’t.

        1. AFCIs have become law in Canada for some parts of the house (bedrooms in particular) but they are prone to nuisance tripping. Construction grade light switches, hair dryers, and plugging in loads with crappy SMPS can produce enough of a pop that the AFCI picks it up and leaves you in the dark.

          1. All British buildings are wired like that. So if the kettle blows the circuit breaker, you’ve at least got the lights on to go investigate. They’re not allowed to use “wire nuts” either. American wiring really seems hilariously bad to me. It’s like a third world country.

            We also have ring mains, where the wires to all the sockets loop back into the distribution box at both ends. It’s more reliable, and also allows cheaper wire to be used, since everything’s effectively wired twice in parallel. Of course you need longer wire. Then there’s the 240V which makes kettles and toasters more effective. It doesn’t seem to kill a lot of people any more than 120V does, the law in a lot of places says 60V is the threshold for “high voltage” for safety reasons. I’ve had a couple of zaps and I’m still mostly in one piece.

  3. 15A through a thermostat?!
    We have 230V thermostats in the U.K., but there just a control signal to a controller (these days a microcontroller, but historically something simpler) which turns on the pump, sets the valves, lights the gas, etc.

    Serious question: How on earth are US HVAC systems wired?

    1. The heating system in the article is a very low installation cost system that uses 240V heating elements installed at the base of one or more walls in each room, with a thermostat that directly switches power of 5 to 15 amps or so, depending on the size of the heater.
      Most houses have better systems that work as you described or use a low voltage signal to turn on a forced air gas furnace or a heat pump.

      1. Ah! It’s electric heating. That makes complete sense now.

        Built-in electric heating isn’t something I’ve seen in the U.K. (aside from storage heaters, which are completely different in operation) – almost all heating in the U.K. is gas-fires boiler to radiators (or storage heaters in flats without gas). I’ve lived one place with hot air heating, but it was gas-fired and ducted from the boiler.

    2. Some UK room thermostats as fitted in the 90’s and before are a heated bimetallic strip. Same for DHW tank stats – I’ve had one passing a constant 100V rather than simply going open circuit when triggered.
      Then you have the Y plan CH/HW pump which is constantly powered whilst the heating is even off.

      Our systems from the same era are still a shit show.

      Backing off of screw terminals in mains wiring is also a thing. Then the fire starts at the receptacle.
      Nothing in regs for inspecting UK houses but plenty for inspecting industrial electrics.
      I’ve had a cooker switch untouched for a decade or more smoke the wiring due to a lose connection – caused by mains frequency I was told.

      1. Yes, 240v thermostats were installed in the U.K. well into 2000 or later, but not carrying significant current, only a signal – that’s what confused me. My old one was 240v even though it was digital.
        Wireless now seems to be the norm.

      2. Not convinced that “mains frequency” caused a problem with the wiring… that sounds like nonsense a sparky made up.
        Aging plastic, vibration, or mice are more likely I’d think?

        1. It probably IS nonsense but you can definitely feel and hear wires move around in pipes sometimes. Like when a big load gets turned on and off. The fluctuating magnetic field produced can be enough to move wires, but usually the correctly sized wire is too chunky to be moved. So you hear them move when you turn on something that has a large startup current.

      3. I think thermal cycling and vibration (perhaps including that induced by mains frequency) can cause some screwed connections to back off. This is why I (and most component manufactures) insist on screw connections being torqued correctly (during manufacturing and installation) and adequate packaging helps with vibrations created during transport. Granted torque screw drivers are not cheap, but at the very least spot checks should be preformed. A visual inspection including the inspector pulling on all wired connections can help a bit but is not a replacement for torque checks. One can get a ‘feel’ for the ‘correct’ tightness with experience, however I have found this tends to vary drastically from person to person.

        That said, it doesn’t help if the person installing the device doesn’t tighten the connection correctly or strips the thread and is too lazy to replace the terminal or part. I normally double check solid and large multi stranded wires (4/5 strands) with screw clamped connections as I have noticed that they tend to ‘relax’ after some time resulting in a borderline loose connection. This seems to be less of an issue with finer multi stranded wires (+10 strands) as they tend to conform better when compressed in the terminal.

        I am a bit OCD about crimping on bootlace ferules where ever possible, although I would imagine they can introduce a tiny bit of extra resistance to the connection, depending on the ferrule material (perhaps problematic in high power systems). However this is offset by the strands being compressed together to make a better connection and current sharing in the wire end. It’s cleaner and ensures that no errand strands are left dangling in the breeze if done properly.

    3. US HVAC systems use 24 VAC routed through the thermostat to control everything. Then AC contractors (relays) use the 24 VAC to switch high current components on and off.

      The thermostat in this article controls individual room controlled baseboard heaters. They are very efficient because you only provide heat to the rooms you are in most often. And as long as the thermostat is designed correctly it works very well.

  4. I’m a bit confused. The article states that the homes built in the 80’s were garbage (insinuating that the thermostat was a crappy one from the 80’s) then goes on to explain the failed thermostat was from 2008…was the whole beginning of the article complete fluff? It seems to have nothing to do with the fact that the thermostat, the part that failed, was built over 20 years later! Two completely different stories here…

    1. It’s the shitty baseboard heater (whatever that is) that’s from the 80s. Although yes, any other sort of electric heater could’ve used the same thermostat. Maybe the author is an embittered former domestic electrician.

    2. The introduction is meant to explain why many houses built in the 1980s in the US had electric heat, which explains the presence of the thermostat in the house in the first place. The fact that it wasn’t an original thermostat that failed is irrelevant – the one made in 2008 was what failed, and it was there because of construction decisions made in the 1980s.

  5. Here’s an example of this type of heat:
    This one has the thermostat located on the heater itself. it also has an electrical outlet directly above it, which is no longer legal in the US. We also use similar looking units that heat via hot water from a central boiler, but most houses use forced air systems using gas furnaces or heat pumps. Older houses may have cast iron radiators provided with steam or hot water from a central boiler. Oil (similar to diesel fuel) may also be used as a fuel.

  6. Just this weekend I found a power strip near my fish tank that got some water in it had been undergoing a lot of heating and had partially melted. Can’t figure out where the water came from, the tank isn’t leaking. Best guess is that the last time I removed the pump and filter a few drops got out and into the surge supressing power strip.

    It had tripped the breaker, a few days earlier, but we didn’t smell anything and couldn’t find the culprit at that time.

    Long story short, I now use a GCFI inside of a plastic container to prevent water intrusion.

    Electricity isn’t always your friend.

  7. RW, thank you very much for such a detailed and informative reply! The tutorial you provided is excellent and easy to follow – I have an old portable AM radio and will give it a try all around my house this weekend :).

    Thanks too for the direction finding link – much appreciated! I’ll read the Popular Electronics magazine too (from link), I love those classic mags!

  8. No, His House Didn’t Almost Burn Down: A Few Points About Line-Voltage ‘Stats:

    Every switch, whether automatic, like a thermostat or pressure switch, or a manual toggle, will create a small electric arc between the contact points every time the switch opens or closes.

    Except in those which encapsulate the switch points inside a glass bulb filled with an inert gas where no oxygen is present, the heat of that arc in the presence of air (oxygen) will cause oxidation of the surfaces of the contact points unless they are gold.

    That oxidation will build up over time and gradually contribute resistance between the contact points. As that resistance increases, the contact points will heat whenever the switch is “made” and that heat itself will contribute to, and increase the rate of formation of, further oxidation and further resistance until the switch fails.

    Every switch and thermostat is rated for the voltage and amps the contacts are designed to switch. The number of times it can be expected to switch the rated load before failure, from the hundreds of thousands of times and more, is less readily available but can be looked up.

    So no matter how (or how well) the wires are connected to the contact points, if used long enough, failure will occur eventually.

    When a line-voltage device fails through this build-up of oxidation (rather than by a mechanical failure) the wires immediately connected to those points will show thermal damage, anywhere from discolored insulation, to melted wires. For this reason, line-voltage controls must always be enclosed in grounded and/or fire resistant enclosures.

    To summarize, line-voltage controls make sense in many applications but must be selected for the voltage and load for which they are rated, and installed in compliance to the codes.

    And a Word For Those Who Like to Pull Plugs:

    This arcing is minimized by the snap-acting design of the switch mechanism. This is why it is a very poor idea to repeatedly disconnect any load by pulling the plug, which causes unseen, but much stronger arcing, since there is no way you can manually separate contacts faster than a mechanical switch can. If you find a plug with prongs discolored or partially melted, it is not enough to replace the plug, since the receptacle it has been habitually pulled from will be just as bad and will likely itself damage the new plug.

    And a Word About Relays:

    Relays make sense wherever they can spare a more expensive control the full load of the controlled device. They multiply the lifetime of the control by many times, can usually both be inspected for wire discoloration and also changed out more easily than an automatic control. They are easier to stock and carry, and if over-rated, last a very long time.

    1. I should have emphasized the point that you don’t need to be afraid of line-voltage switches if they are properly rated and properly installed. Most switches in a residence are line-voltage, oxidize their contacts since they are exposed to air, and due to their long rated duty-cycles, last decades. A thermostat has to cycle many times an hour during the heating season, normally, but should still last a decade or two. Even with the failure shown above, there was nothing to ignite around the ‘stat because it was properly installed, so only a bad smell and an unresponsive heater. Low voltage controls of all kinds are a fine idea but rarely installed for most residences due to the extra cost.

      1. (Except HVAC, of course, which are much less expensive to install since a transformer and relays for full voltage are all included in the unit and a small, low-voltage cable for all the control circuits, heating, cooling, fan, etc, can be run without conduit or a fire-proof enclosure for the ‘stat.)

        1. Kudos to Roger G. I had a line voltage thermostat connector wires fry after the thermostat had been happily functioning for 3 years. I assumed maybe I had not made a proper connection with the wire nuts. A second alternative is oxidation that I was unaware of. Recently I have had two more thermostats start reading higher temps than the room temperature. The thermostats have a slightly warmer feel than the nearby wall. I will be re-setting the connections to see if it is due to oxidation. I’ll report back if the thermostats go back to normal operation.

Leave a Reply

Please be kind and respectful to help make the comments section excellent. (Comment Policy)

This site uses Akismet to reduce spam. Learn how your comment data is processed.