WEBVTT Kind: captions Language: en 00:00:00.160 --> 00:00:02.800 Welp, winter heating season is quickly  approaching 00:00:02.800 --> 00:00:04.211 (in the northern hemisphere anyway) 00:00:04.211 --> 00:00:10.591 which means that many of us are firing up our  heating systems for the first time in many months and hoping for the best. 00:00:10.591 --> 00:00:14.343 Have you ever wondered  why your furnace makes the noises that it does? 00:00:14.343 --> 00:00:17.019 And how come it seems to work in  distinct steps? 00:00:17.019 --> 00:00:18.690 What’s a heat exchanger? 00:00:18.690 --> 00:00:21.684 Why do I need to be worried  about carbon monoxide poisoning? 00:00:21.684 --> 00:00:23.714 Why do we use furnaces at all? 00:00:23.714 --> 00:00:28.258 Great questions!  So great that I’m making a video to answer them. 00:00:28.400 --> 00:00:32.960 Now first, we’re gonna be talking about the  typical forced-air gas-fired North American   00:00:32.960 --> 00:00:38.560 heating system in this video because that’s what’s  common where I live and what I have access to.   00:00:38.560 --> 00:00:42.720 Pretty much every small to medium-sized  building over here constructed in the last   00:00:42.720 --> 00:00:48.250 half century or so in a winter climate has at least one  of these providing it with heat. 00:00:48.250 --> 00:00:53.383 If you’re used to a heating system with a central boiler and  radiators, well this ain’t that. 00:00:53.383 --> 00:00:58.560 Those aren’t exactly rare in North America, but are typically  limited to older buildings that were constructed   00:00:58.560 --> 00:01:01.119 before we became addicted to air conditioning. 00:01:01.119 --> 00:01:05.722 Ever since central air conditioning became common, we’ve settled on ducted systems like this 00:01:05.722 --> 00:01:11.298 where a furnace acts as both the heat source for winter months and air handler for the air conditioner 00:01:11.298 --> 00:01:12.690 in the summer months. 00:01:12.800 --> 00:01:16.747 Let’s start by asking what the basic  job of the furnace is. 00:01:16.747 --> 00:01:20.809 You might say, well obviously it’s to provide heat for a living  space - 00:01:20.809 --> 00:01:22.837 and you’d be more-or-less right! 00:01:22.837 --> 00:01:29.827 But more specifically, its job is to safely release as much of the  heat energy contained in a combustible fuel that it can 00:01:29.827 --> 00:01:34.468 into the living space and distribute  it with the aid of a blower fan and ductwork. 00:01:34.468 --> 00:01:38.013 Nearly everything in the furnace  is designed around safety, 00:01:38.013 --> 00:01:43.486 and its sequence of operations is performed  specifically to prove the integrity and functionality 00:01:43.486 --> 00:01:46.945 of each of its components before it  allows itself to run. 00:01:46.945 --> 00:01:52.860 If it fails its own tests, it locks itself out and you’ll need to call your  local HVAC company. 00:01:52.860 --> 00:01:54.665 Better that than be dead! 00:01:55.120 --> 00:02:00.000 We’ll go over that sequence of operations as well  as the components themselves shortly but first,   00:02:00.000 --> 00:02:02.833 let’s discuss why we use these things. 00:02:02.833 --> 00:02:06.735 These seem to invite a sort of bafflement in those who don’t live in North America. 00:02:06.735 --> 00:02:12.687 If a boiler is what you’re  used to, well that makes sense, but quite often I’ve run across folks who seem to think 00:02:12.687 --> 00:02:16.404 our furnaces are somehow inefficient machines. 00:02:16.404 --> 00:02:19.630 Not at all, these are incredibly efficient. 00:02:19.630 --> 00:02:24.860 This particular furnace is able to capture 96% of the heat energy in a fuel. 00:02:24.860 --> 00:02:28.083 That’s excellent,  but some models get even more than that! 00:02:28.083 --> 00:02:32.043 Hi, it’s me from the future with an embarrassing  correction. 00:02:32.043 --> 00:02:39.581 So we’re about to talk about the AFUE or “A-few” which stands for annual fuel  utilization efficiency. 00:02:39.581 --> 00:02:43.343 And for this furnace It’s 95. 00:02:43.343 --> 00:02:48.354 Not 96. I had countless opportunities  to look at this label before now, 00:02:48.560 --> 00:02:50.432 alas I took none of them. 00:02:50.432 --> 00:02:51.914 And my memory failed me. 00:02:51.914 --> 00:02:54.640 So for the rest of this video, if I say “96” 00:02:55.200 --> 00:02:57.911 I probably meant 95. 00:02:57.911 --> 00:02:59.200 Anyway, back to the AFUE. 00:02:59.920 --> 00:03:04.560 It’s measured as a more-or-less yearly average  to help account for the differences in efficiency   00:03:04.560 --> 00:03:08.720 at start-up and shut-down. During a steady  heating state it’s actually slightly higher.   00:03:09.360 --> 00:03:15.681 Being able to get 96% of the heat energy available  in a fuel into the living space is tremendous, 00:03:15.681 --> 00:03:21.298 but even the worst natural gas furnaces on the market  have at least an AFUE of 80. 00:03:21.298 --> 00:03:26.630 A long time ago I made a video specifically about the rationale  behind burning fuel for space heating, 00:03:26.630 --> 00:03:28.569 but this is essentially it. 00:03:28.569 --> 00:03:33.929 Furnaces excel at burning fuel  and capturing nearly everything we can from it. 00:03:34.160 --> 00:03:40.640 That said, they’re probably (and, hopefully) on  their way out as we move towards electrification.   00:03:40.640 --> 00:03:45.040 Now, we don’t typically use electric resistive  heating to heat entire homes because,   00:03:45.040 --> 00:03:50.720 while electric heat is itself 100% efficient,  electrical generation is not. 00:03:50.720 --> 00:03:55.200 And even if it were, the electrical requirements of resistive  heat aren’t practical on a large scale.   00:03:55.760 --> 00:03:59.268 But heat pumps are able to turn this math on its  head. 00:03:59.268 --> 00:04:06.267 Since heat pumps, which are basically just air conditioners running in reverse, can move much  more energy than they themselves consume, 00:04:06.267 --> 00:04:09.788 they’re undoubtedly the future of home space heating. 00:04:09.788 --> 00:04:15.210 Even right now with our majority fossil-grid, they result in fewer carbon emissions than directly burning fuel 00:04:15.210 --> 00:04:19.520 because they’re efficiency can approach 500% - 00:04:19.520 --> 00:04:22.353 more than accounting for the losses in  generation. 00:04:22.353 --> 00:04:28.480 But we’ll save that topic (and its current challenges) for a video  in the I Promise not-too-distant future. 00:04:29.200 --> 00:04:31.366 For now, though, back to this! 00:04:31.366 --> 00:04:37.840 One of the things we’ve learned over the years as we’ve embarked on this human endeavor is that burning things is unpleasant.   00:04:37.840 --> 00:04:42.800 Burning pretty much any fuel, including  the so-called “clean burning” natural gas   00:04:42.800 --> 00:04:47.760 produces particulate matter which isn’t great  to breathe, and of course there’s the much more   00:04:47.760 --> 00:04:53.360 immediately deadly combustion by-product carbon  monoxide which, through evolutionary bad luck,   00:04:53.360 --> 00:04:56.830 is much more appealing to your red  blood cells than oxygen. 00:04:56.830 --> 00:04:58.249 Which is bad. 00:04:58.640 --> 00:05:02.314 So generally, we shouldn’t burn things indoors. 00:05:02.314 --> 00:05:06.928 And yet, here’s a device which burns things… indoors. 00:05:06.928 --> 00:05:09.790 Ah! But this particular furnace is  a condensing furnace, 00:05:09.790 --> 00:05:16.910 and thanks to these two pipes here, the combustion of the fuel sort of  happens outside - but more on that later. 00:05:16.910 --> 00:05:23.700 You may also know of a certain other device which does  burn fuel indoors without ventilation - the common stovetop and oven, 00:05:23.700 --> 00:05:29.987 and research is beginning to suggest that maybe  this isn’t so great either, but there is the small saving grace 00:05:29.987 --> 00:05:35.390 that the quantities of fuel burned  for cooking are generally small fractions of that which a furnace uses, 00:05:35.390 --> 00:05:39.930 and also there are ways to  mitigate this through ventilation but I digress. 00:05:40.240 --> 00:05:46.160 Anyway, a furnace like this needs  to burn its heating fuel and then it has a bit of an exhausting job ahead of itself.   00:05:46.880 --> 00:05:47.808 Exhausting. 00:05:47.808 --> 00:05:51.936 We want the byproducts of the  combustion process to make their way outside 00:05:51.936 --> 00:05:55.837 where they’ll only be problematic in about… a decade or  two. 00:05:55.837 --> 00:06:02.901 But of course we want to be able to extract the heat generated by the combustion process and release that into the living space. 00:06:02.901 --> 00:06:05.334 We can do that with a heat exchanger. 00:06:05.334 --> 00:06:09.082 In a typical furnace, this takes the form of a series of tubes. 00:06:09.280 --> 00:06:10.325 Actually, though! 00:06:10.325 --> 00:06:17.051 The heat exchanger in a standard 80+ furnace is simply some number of typically U-shaped steel tubes. 00:06:17.051 --> 00:06:18.413 You might call them U-tubes. 00:06:18.413 --> 00:06:23.578 The fuel is burned inside these tubes, which of course makes  them get pretty freaking hot. 00:06:23.840 --> 00:06:29.376 Those tubes, then, are put in the path of air being forced through the furnace with the aid of a blower motor, 00:06:29.376 --> 00:06:33.718 and that air keeps the heat  exchanger from melting by cooling the tubes down, 00:06:33.718 --> 00:06:36.971 which also incidentally heats the air. That’s a  nice bonus. 00:06:36.971 --> 00:06:42.480 The practical upshot of this is that the heat from the fuel gets released into the air flowing through the furnace 00:06:42.480 --> 00:06:49.818 and eventually out of the vents which heat your home, but the combustion  byproducts stay separated inside those tubes. 00:06:50.080 --> 00:06:54.339 And of course, we need those by-products to go  away somehow. 00:06:54.339 --> 00:07:00.902 And also, hang on, you need oxygen to burn fuels and the burning of the fuel consumes that oxygen - 00:07:00.902 --> 00:07:07.101 so you’re gonna need a way to supply those tubes with fresh air in addition to getting  rid of the byproducts. 00:07:07.101 --> 00:07:10.848 Aha! Well now we’re getting into the more practical and safety bits of the furnace. 00:07:10.848 --> 00:07:16.299 If you’ve ever lived with a furnace like this, you’ll likely have noticed that there are two  blower motors. 00:07:16.299 --> 00:07:19.228 There’s the big one that makes air come out of your registers. 00:07:19.228 --> 00:07:22.449 But before that one  comes on, you hear another one. 00:07:22.449 --> 00:07:23.777 What’s that for? 00:07:24.080 --> 00:07:27.926 Well, that’s arguably the most important part  of the furnace. 00:07:27.926 --> 00:07:32.555 This blower is called the draft inducer, and really it’s more of a sucker. 00:07:32.555 --> 00:07:37.602 It pulls air - and later, other fun gasses - through the heat exchanger. 00:07:37.602 --> 00:07:39.846 In other words, it induces a draft. 00:07:39.846 --> 00:07:44.829 That air then gets pushed up through some sort of chimney where it will eventually  exit the home, 00:07:44.829 --> 00:07:48.509 or as is the case here it exits through this PVC pipe. 00:07:48.509 --> 00:07:54.068 The draft inducer lives on the exit side of the heat exchanger, so it pulls air through the tubes, 00:07:54.068 --> 00:07:58.153 and what lives on the other side of those tubes are the flamethrowers. 00:07:58.480 --> 00:08:00.445 Now we’re getting to the burny business. 00:08:00.445 --> 00:08:03.509 If we’re burning a fuel, it’s gotta come from somewhere, 00:08:03.509 --> 00:08:09.084 and where it comes from is deep underground  having been trapped there for millennia before we extracted it. 00:08:09.680 --> 00:08:12.540 But as far as your furnace is concerned it  comes from the burners. 00:08:12.540 --> 00:08:18.640 These are essentially just specialized nozzles which release some  amount of gas into the heat exchanger’s tubes.   00:08:18.640 --> 00:08:22.560 Thanks to the constant supply of fresh  air brought about by the draft inducer,   00:08:22.560 --> 00:08:24.232 it will burn quite nicely. 00:08:24.232 --> 00:08:27.692 The draft inducer ensures not only that there is fresh air for burning 00:08:27.692 --> 00:08:32.603 but also that what comes out the other side of the heat exchanger tubes doesn’t make it into the living space 00:08:32.603 --> 00:08:35.473 and is instead safely expelled outdoors. 00:08:35.760 --> 00:08:42.079 Because this device deals with the, frankly,  dangerous combination of combustible fuels and enclosed spaces, 00:08:42.079 --> 00:08:48.669 it’s designed with a number of safety interlocks and a sequence of operations to ensure things don’t go too terribly wrong. 00:08:48.669 --> 00:08:50.342 But before we get into that, 00:08:50.342 --> 00:08:53.421 I know this is the second "but before we get into that," 00:08:53.421 --> 00:08:54.592 apologies, 00:08:54.592 --> 00:08:57.708 let’s discuss the condensing part of this furnace. 00:08:57.708 --> 00:09:02.449 When you burn a fuel like natural gas, one of the byproducts  is water vapor. 00:09:02.449 --> 00:09:09.018 A standard 80+ furnace doesn't really know what to make of that so sends it out  with the carbon monoxide and other icky stuff. 00:09:09.360 --> 00:09:15.280 But that water vapor is hot, and more importantly  holds energy in the form of latent heat.   00:09:15.280 --> 00:09:18.660 When it condenses into a liquid, as it will  inevitably do - 00:09:18.660 --> 00:09:22.485 that’s all the steam you see coming from the rooftops of homes in the dead  of winter - 00:09:22.485 --> 00:09:24.293 it releases that energy. 00:09:24.293 --> 00:09:27.314 But that’s no good if it’s just happening outside. 00:09:27.314 --> 00:09:32.470 If we could make it condense inside, we’d be able to get more energy out of the heating fuel. 00:09:32.470 --> 00:09:39.032 And so, a condensing furnace has what is essentially a second heat exchanger after the main fire tubes 00:09:39.032 --> 00:09:42.885 with a greater surface area that can cool the exhaust gases further 00:09:42.885 --> 00:09:49.005 which helps get more heat out of the fuel all by itself, but more importantly it gets most of the water vapor 00:09:49.005 --> 00:09:55.920 to condense and release its latent heat inside the furnace rather than  to outside air where it is otherwise wasted. 00:09:56.640 --> 00:10:02.669 That’s how this furnace is able to get 96%  of the energy available in the fuel into the  home. 00:10:02.669 --> 00:10:06.173 And it’s also how it’s able to exhaust  through a PVC pipe. 00:10:06.173 --> 00:10:11.330 I mean, think about this, it’s burning flammable gas, there’s literal fire inside of it, 00:10:11.330 --> 00:10:16.264 and it outputs 70,000 BTUS or about 20 kilowatts of heat. 00:10:16.264 --> 00:10:24.145 And yet, it loses so little of that heat in the exhaust that it can safely exit through a plastic pipe which barely gets warm at all. 00:10:24.145 --> 00:10:29.295 Compare that to an 80+ furnace with a steel exhaust chimney  that gets so hot you can’t even touch it, 00:10:29.295 --> 00:10:33.753 and you’ll understand why condensing  furnaces are a big deal and a great idea. 00:10:34.080 --> 00:10:38.385 A condensing furnace is a little more  complicated, of course, than a conventional one.   00:10:38.385 --> 00:10:43.894 Most of that has to do with the fact that dealing  with the condensate isn’t the easiest task 00:10:43.894 --> 00:10:48.455 for one, well now you have to deal with it when  previously you didn’t. 00:10:48.455 --> 00:10:51.774 This furnace has a drain pipe on its side for that very reason. 00:10:51.774 --> 00:10:57.542 But also that water isn’t just plain water, it’s pretty acidic thanks to other combustion byproducts 00:10:57.542 --> 00:11:03.635 so the secondary heat exchanger needs to be made of materials, such as stainless steel, which resist corrosion. 00:11:03.635 --> 00:11:11.440 This adds somewhat to the cost of the furnace, but getting an extra 10 or 15% of the energy out of your heating fuel makes it, to my mind,   00:11:11.440 --> 00:11:16.320 obviously worth it and we absolutely should  find ways to help subsidize the added cost   00:11:16.320 --> 00:11:22.930 for those who need financial assistance because  in the long run it will always save money and resources. 00:11:22.930 --> 00:11:24.800 It’s frankly a no-brainer at this point. 00:11:25.440 --> 00:11:29.920 Oh, and that other pipe there? Well, here’s  another way to get an increase in efficiency.   00:11:29.920 --> 00:11:33.564 This furnace gets its oxygen supply from outside. 00:11:33.564 --> 00:11:37.258 This pipe is simply supplying the combustion section with outside air, 00:11:37.258 --> 00:11:41.979 in fact you can see that it just opens to this space (and the other end is outside). 00:11:41.979 --> 00:11:43.305 Why do that? 00:11:43.305 --> 00:11:47.682 Well, a conventional furnace gets its combustion air from the room it’s sitting in. 00:11:47.682 --> 00:11:51.867 That means it creates negative pressure whenever the draft inducer is running, 00:11:51.867 --> 00:11:57.403 and that brings in some amount of cold outside  air to replace what leaves through the exhaust. 00:11:57.403 --> 00:12:04.624 In contrast, this supply pipe, along with the seals  in this panel, makes it so that the combustion section of the furnace 00:12:04.624 --> 00:12:10.483 is essentially entirely outdoors, creating no negative pressure at all. 00:12:10.483 --> 00:12:14.641 Of course, particularly right now in the  human experience we’re discovering 00:12:14.641 --> 00:12:18.370 that we really ought to have a little  more exchange of air than we do. 00:12:18.471 --> 00:12:24.114 Most of our efficiency-first building practices  came about thanks to the energy crises of the seventies, 00:12:24.114 --> 00:12:28.194 and we’ve been living in mostly sealed  boxes which it turns out isn’t super great, 00:12:28.194 --> 00:12:31.015 and not just because of respiratory  disease pandemics. 00:12:31.015 --> 00:12:38.442 Simple build-up of carbon dioxide from our exhalation can  be hazardous, so some amount of negative pressure is probably a good thing 00:12:38.442 --> 00:12:41.077 and this topic deserves more attention and study. 00:12:41.077 --> 00:12:42.113 But anyway, 00:12:42.400 --> 00:12:46.560 Let’s finally bring it back to the  start with one of the questions I asked;   00:12:46.560 --> 00:12:50.262 why does the furnace seem to do things in distinct  steps? 00:12:50.262 --> 00:12:52.722 Ah, well that’s all about that safety. 00:12:52.722 --> 00:12:56.277 There really aren’t that many components in a  basic household furnace: 00:12:56.277 --> 00:13:00.421 just the draft inducer, heat exchanger, main blower,  an ignitor, and a gas valve. 00:13:00.421 --> 00:13:07.932 But we absolutely need to make sure everything is  in working order before we go let some flammable gas out of a pipe. 00:13:07.932 --> 00:13:12.160 And to do that, we rely on  a few sensors and a sequence of operations. 00:13:12.800 --> 00:13:17.921 Since this furnace is quite modern we have a  circuit board with a microprocessor handling everything, 00:13:17.921 --> 00:13:22.784 but the same basic things have been  happening in forced-air furnaces for decades. 00:13:22.784 --> 00:13:28.160 When the thermostat calls for heat, the sequence of  operations begins by turning on the draft inducer. 00:13:28.720 --> 00:13:34.720 Now, this component absolutely must be functioning  properly for the safe operation of the furnace.   00:13:35.280 --> 00:13:39.200 If it opened the gas valve without  airflow through the heat exchanger,   00:13:39.200 --> 00:13:40.986 it could be a disaster. 00:13:40.986 --> 00:13:47.840 So, to prove that the inducer is actually functional, the circuit board looks for the output from this pressure switch to change.   00:13:48.400 --> 00:13:52.560 If it did, that means there was a change in  air pressure where the switch is located,   00:13:52.560 --> 00:13:56.041 which would only occur if the draft inducer  is working. 00:13:56.041 --> 00:14:03.440 Looking for a change in switch output also allows the furnace’s logic board to detect a stuck switch and refuse to operate. 00:14:04.080 --> 00:14:08.240 Once the operation of the draft inducer  has been proven, there will be a pre-programmed   00:14:08.240 --> 00:14:12.720 delay period to ensure no unburnt  gasses remain in the heat exchanger.   00:14:12.720 --> 00:14:16.205 Assuming the furnace shut down correctly  there shouldn’t be any, 00:14:16.205 --> 00:14:17.934 but better safe than sorry. 00:14:17.934 --> 00:14:22.620 While that delay is happening, this furnace sends power to the hot surface ignitor. 00:14:22.620 --> 00:14:26.977 This is an ignitor that ignites the  fuel by being a surface which is hot. 00:14:27.200 --> 00:14:32.211 An alternative method of ignition is a sparky  thing, but that seems to have gone out of fashion for some reason.   00:14:32.634 --> 00:14:36.464 Anyway, because it takes a while for the hot surface  ignitor to become hot enough 00:14:36.464 --> 00:14:42.320 that its surface can ignite things, the delay period for clearing the heat exchanger is a perfect time to warm it up. 00:14:42.960 --> 00:14:45.297 Next comes the opening of the gas valve. 00:14:45.297 --> 00:14:52.884 This is a very time-sensitive step because natural gas and propane, which is actually what fuels this furnace  - 00:14:52.884 --> 00:14:54.546 I’m really in the middle of nowhere... 00:14:54.546 --> 00:14:56.046 can be explosive! 00:14:56.046 --> 00:15:00.465 We don’t want that, so we need a way to know that the fuel has actually ignited 00:15:00.465 --> 00:15:06.520 and isn’t just being dumped into the heat  exchanger unburnt where it could potentially go boom. 00:15:06.520 --> 00:15:11.309 And this is actually really easy to do, we just use a thermocouple as a flame sensor 00:15:11.309 --> 00:15:15.345 to detect a rapid  rise in heat brought about by flames. 00:15:15.640 --> 00:15:17.943 And this is done quite cleverly. 00:15:17.943 --> 00:15:21.998 This furnace, like most, has multiple burners arranged linearly. 00:15:21.998 --> 00:15:26.249 The hot surface ignitor lives  here next to the rightmost burner. 00:15:26.249 --> 00:15:30.532 The burners are designed so that a flame will  quickly propagate from each one to the next, 00:15:30.532 --> 00:15:37.415 and assuming nothing’s wrong with them and that the fuel supply is adequate this should occur essentially instantly. 00:15:37.415 --> 00:15:44.415 And so, to determine that ignition happened correctly, the flame sensor is placed at the burner that’s farthest away from the ignitor. 00:15:44.415 --> 00:15:50.111 It will thus only register a flame when all burners have successfully ignited. 00:15:50.111 --> 00:15:57.662 When the furnace opens the gas valve, it looks for a rapid rise in temperature from that sensor and it should see it right away. 00:15:57.662 --> 00:16:03.440 If it doesn’t, usually within just two seconds, it closes  the gas valve and aborts its mission. 00:16:04.080 --> 00:16:09.920 This checking-for-flame step is so quick because  if any of the burners failed to ignite right away,   00:16:09.920 --> 00:16:13.281 they’ll be pumping unburnt fuel into the  heat exchanger. 00:16:13.281 --> 00:16:18.720 If that’s allowed to happen for more than just a couple of seconds, a  dangerous quantity of fuel may now exist in   00:16:18.720 --> 00:16:23.506 the heat exchanger which, if it were to ignite,  would be likely to damage it. 00:16:23.506 --> 00:16:26.789 And that’s very bad and also dangerous. 00:16:26.789 --> 00:16:31.231 So, if it doesn't see flames right away, that gas valve is closed again 00:16:31.231 --> 00:16:36.391 and the draft inducer remains running to clear the  unburnt fuel from the heat exchanger. 00:16:36.391 --> 00:16:41.301 After a pre-programmed period of time, the furnace will  make another ignition attempt. 00:16:41.301 --> 00:16:50.143 And after a certain number of failed attempts - it’s five in the case of this furnace - the system locks itself out and no heat for you. 00:16:50.143 --> 00:16:57.266 At least for about an hour; this furnace, and many others, will try again later because there could be all sorts of reasons for a failed ignition event, 00:16:57.266 --> 00:17:04.320 including temporary loss of gas pressure, so it does self-reset to keep your pipes from freezing if it can help it. 00:17:04.320 --> 00:17:10.293 So long as the flame sensor sensed flames, though,  the logic board then gives the all-clear for the next step. 00:17:10.293 --> 00:17:11.690 Which is to wait a lil’ bit. 00:17:11.690 --> 00:17:16.331 Rather than start the blower motor right away and give you a nice blast of cold air, 00:17:16.331 --> 00:17:21.887 the furnace will simply sit pretty and allow the heat exchanger to get a little hot before it switches on the blower. 00:17:21.887 --> 00:17:26.910 But once it does that, well now you’ve got heat coming from your heat vents. 00:17:26.910 --> 00:17:30.225 When the thermostat  is satisfied and it stops calling for heat, 00:17:30.480 --> 00:17:34.800 the gas valve closes, extinguishing the  flames, and the inducer fan remains running   00:17:34.800 --> 00:17:39.545 for thirty seconds or so to make sure all the  remaining exhaust is replaced by fresh air.   00:17:39.840 --> 00:17:44.240 At the same time, the blower motor keeps  running to cool off the heat exchanger and,   00:17:44.240 --> 00:17:47.631 of course, get the rest of the heat it contains  out of it. 00:17:47.631 --> 00:17:52.800 But after about a minute, it shuts down completely and patiently  waits for the next call for heat. 00:17:53.440 --> 00:17:57.280 Now there’s gonna be at least one other  safety device in the furnace, too,   00:17:57.280 --> 00:17:59.323 and that would be a limit switch. 00:17:59.323 --> 00:18:04.889 This is a switch which trips at a certain temperature, and it’s designed to protect against overheating. 00:18:04.889 --> 00:18:10.708 If for some reason the blower motor stopped working, or airflow was restricted in some other way, 00:18:10.708 --> 00:18:16.720 the heat exchanger will overheat before too long because  it doesn’t have enough airflow to cool itself down.   00:18:16.720 --> 00:18:22.121 So, that limit switch is there to detect such a  scenario and shut down the furnace if it occurs. 00:18:22.560 --> 00:18:28.442 So, there’s clearly a lot of safety built into your  typical furnace, and some clever ingenuity, too.   00:18:28.720 --> 00:18:34.560 But there’s one thing it generally won’t guard  you from, and that’s carbon monoxide poisoning.   00:18:34.560 --> 00:18:38.940 If everything is in working order, that shouldn’t  be possible, but… 00:18:38.940 --> 00:18:43.069 well whenever you burn fuel there’s a risk of carbon monoxide. 00:18:43.069 --> 00:18:48.151 Furnaces can be particularly tricky because the heat exchanger wears out with time. 00:18:48.151 --> 00:18:55.083 Going from room temperature to having fire inside of it and back a dozen times per day for 15 years is hard on it, 00:18:55.083 --> 00:18:58.160 and over time imperfections can form from this thermal stress. 00:18:58.720 --> 00:19:03.947 The most dangerous kind is a crack in the heat  exchanger’s tubes, which can allow exhaust gasses 00:19:03.947 --> 00:19:09.305 - and thus carbon monoxide to exit the  heat exchanger and make it into the living space. 00:19:09.680 --> 00:19:15.165 This is why it is vitally important that you  have carbon monoxide alarms in your living spaces, 00:19:15.165 --> 00:19:17.131 and especially in your bedrooms. 00:19:17.131 --> 00:19:23.388 Carbon monoxide poisoning is a rare thing, but an alarm is the only thing that can guard you against it. 00:19:23.388 --> 00:19:29.014 Annual furnace inspections by a trained technician may be able to catch problems before they become dangerous, 00:19:29.014 --> 00:19:32.753 but accessing the heat exchanger for a thorough  inspection can be difficult 00:19:32.753 --> 00:19:37.648 and might not be performed in your typical “pre-season tune-up special.” 00:19:37.648 --> 00:19:44.302 So please, if your home is heated by combusting fuel - even if it’s not a forced air furnace but instead something like a boiler 00:19:44.302 --> 00:19:49.971 or indeed, even if fuel is burned anywhere in your home for cooking or water heating or anything, 00:19:49.971 --> 00:19:54.223 invest in carbon monoxide alarms and  test them regularly. 00:19:54.223 --> 00:19:55.705 They may just save your life. 00:19:56.240 --> 00:20:01.358 So, that’s how your typical North American gas-fired forced-air furnace works. 00:20:01.358 --> 00:20:06.081 It’s really pretty simple, but thanks to modern technology,  quite efficient, too. 00:20:06.081 --> 00:20:10.945 On that note, condensing heat exchangers aren’t limited by any means to forced-air furnaces. 00:20:10.945 --> 00:20:14.708 Boilers can take advantage of this tech, even water heaters! 00:20:14.708 --> 00:20:20.320 So long as natural gas or other fuel combustion remains a prominent form of space and water heating, 00:20:20.320 --> 00:20:23.106 we really ought to do it as efficiently as we can. 00:20:23.600 --> 00:20:29.552 However, as we move headfirst into this decade,  we’ll undoubtedly start to see natural gas become phased out 00:20:29.552 --> 00:20:31.354 in more and more places. 00:20:31.354 --> 00:20:37.869 In areas with mild heating needs, heat pumps are already viable with little more investment than an air conditioner. 00:20:37.869 --> 00:20:44.699 And for colder climates like mine, geothermal heat pump systems are available and  are dropping in price fast. 00:20:44.699 --> 00:20:47.534 But we’ll get into those weeds maybe in a month or two. 00:20:47.534 --> 00:20:49.043 Perhaps three. 00:20:49.043 --> 00:20:56.447 It will be this heating season. I’ve got a thermal camera, now, and there’s a reversible  minisplit the garage so - no excuses. 00:20:57.314 --> 00:20:58.234 OK bye. 00:20:59.085 --> 00:21:01.663 ♫ efficiently smooth jazz ♫ 00:21:03.023 --> 00:21:05.233 Eugh, this is a clunky line. 00:21:05.233 --> 00:21:10.634 Pretty much ever small-to-medium-sized building built over here geh gu, ugh... 00:21:10.634 --> 00:21:16.426 Nope, that's not the line I wrote! 00:21:16.426 --> 00:21:19.391 ...immediately deadly combina - combustion. 00:21:19.391 --> 00:21:19.891 Dangit! 00:21:19.891 --> 00:21:25.455 And of course, there's the much more immediately deadly combustion byproduct carbon monoxide whi --- ohhh, 00:21:25.455 --> 00:21:27.166 carbon wasn't said right! 00:21:27.166 --> 00:21:28.409 "ca ma ma ma fehhhhr" 00:21:28.409 --> 00:21:32.648 ...path of the air being forced through the furnace with the aid of a blower *motor* 00:21:33.376 --> 00:21:35.448 Ehh... blower *motor*. [clears throat] 00:21:35.448 --> 00:21:37.304 That's... with a *blower* motor. 00:21:37.304 --> 00:21:39.925 Blower *motor* is stressing the wrong thing. 00:21:39.925 --> 00:21:43.794 ...with a greater surface area that can cool the exhaust gasses further 00:21:43.794 --> 00:21:47.034 which helps gets more heat out of the help ef frark! 00:21:48.529 --> 00:21:53.299 Yes, our furnaces don't smelt anything or really have much of a connection to metallurgy at all. 00:21:53.299 --> 00:21:57.009 Such is the terrible realm of regional jargon. 00:21:57.009 --> 00:21:59.780 At least they have something to do with heat! 00:21:59.780 --> 00:22:02.647 I mean, we could call them "dog kennels" so, hey.