1 00:00:00,160 --> 00:00:02,800 Welp, winter heating season is quickly  approaching 2 00:00:02,800 --> 00:00:04,211 (in the northern hemisphere anyway) 3 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. 4 00:00:10,591 --> 00:00:14,343 Have you ever wondered  why your furnace makes the noises that it does? 5 00:00:14,343 --> 00:00:17,019 And how come it seems to work in  distinct steps? 6 00:00:17,019 --> 00:00:18,690 What’s a heat exchanger? 7 00:00:18,690 --> 00:00:21,684 Why do I need to be worried  about carbon monoxide poisoning? 8 00:00:21,684 --> 00:00:23,714 Why do we use furnaces at all? 9 00:00:23,714 --> 00:00:28,258 Great questions!  So great that I’m making a video to answer them. 10 00:00:28,400 --> 00:00:32,960 Now first, we’re gonna be talking about the  typical forced-air gas-fired North American   11 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.   12 00:00:38,560 --> 00:00:42,720 Pretty much every small to medium-sized  building over here constructed in the last   13 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. 14 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. 15 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   16 00:00:58,560 --> 00:01:01,119 before we became addicted to air conditioning. 17 00:01:01,119 --> 00:01:05,722 Ever since central air conditioning became common, we’ve settled on ducted systems like this 18 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 19 00:01:11,298 --> 00:01:12,690 in the summer months. 20 00:01:12,800 --> 00:01:16,747 Let’s start by asking what the basic  job of the furnace is. 21 00:01:16,747 --> 00:01:20,809 You might say, well obviously it’s to provide heat for a living  space - 22 00:01:20,809 --> 00:01:22,837 and you’d be more-or-less right! 23 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 24 00:01:29,827 --> 00:01:34,468 into the living space and distribute  it with the aid of a blower fan and ductwork. 25 00:01:34,468 --> 00:01:38,013 Nearly everything in the furnace  is designed around safety, 26 00:01:38,013 --> 00:01:43,486 and its sequence of operations is performed  specifically to prove the integrity and functionality 27 00:01:43,486 --> 00:01:46,945 of each of its components before it  allows itself to run. 28 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. 29 00:01:52,860 --> 00:01:54,665 Better that than be dead! 30 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,   31 00:02:00,000 --> 00:02:02,833 let’s discuss why we use these things. 32 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. 33 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 34 00:02:12,687 --> 00:02:16,404 our furnaces are somehow inefficient machines. 35 00:02:16,404 --> 00:02:19,630 Not at all, these are incredibly efficient. 36 00:02:19,630 --> 00:02:24,860 This particular furnace is able to capture 96% of the heat energy in a fuel. 37 00:02:24,860 --> 00:02:28,083 That’s excellent,  but some models get even more than that! 38 00:02:28,083 --> 00:02:32,043 Hi, it’s me from the future with an embarrassing  correction. 39 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. 40 00:02:39,581 --> 00:02:43,343 And for this furnace It’s 95. 41 00:02:43,343 --> 00:02:48,354 Not 96. I had countless opportunities  to look at this label before now, 42 00:02:48,560 --> 00:02:50,432 alas I took none of them. 43 00:02:50,432 --> 00:02:51,914 And my memory failed me. 44 00:02:51,914 --> 00:02:54,640 So for the rest of this video, if I say “96” 45 00:02:55,200 --> 00:02:57,911 I probably meant 95. 46 00:02:57,911 --> 00:02:59,200 Anyway, back to the AFUE. 47 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   48 00:03:04,560 --> 00:03:08,720 at start-up and shut-down. During a steady  heating state it’s actually slightly higher.   49 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, 50 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. 51 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, 52 00:03:26,630 --> 00:03:28,569 but this is essentially it. 53 00:03:28,569 --> 00:03:33,929 Furnaces excel at burning fuel  and capturing nearly everything we can from it. 54 00:03:34,160 --> 00:03:40,640 That said, they’re probably (and, hopefully) on  their way out as we move towards electrification.   55 00:03:40,640 --> 00:03:45,040 Now, we don’t typically use electric resistive  heating to heat entire homes because,   56 00:03:45,040 --> 00:03:50,720 while electric heat is itself 100% efficient,  electrical generation is not. 57 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.   58 00:03:55,760 --> 00:03:59,268 But heat pumps are able to turn this math on its  head. 59 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, 60 00:04:06,267 --> 00:04:09,788 they’re undoubtedly the future of home space heating. 61 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 62 00:04:15,210 --> 00:04:19,520 because they’re efficiency can approach 500% - 63 00:04:19,520 --> 00:04:22,353 more than accounting for the losses in  generation. 64 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. 65 00:04:29,200 --> 00:04:31,366 For now, though, back to this! 66 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.   67 00:04:37,840 --> 00:04:42,800 Burning pretty much any fuel, including  the so-called “clean burning” natural gas   68 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   69 00:04:47,760 --> 00:04:53,360 immediately deadly combustion by-product carbon  monoxide which, through evolutionary bad luck,   70 00:04:53,360 --> 00:04:56,830 is much more appealing to your red  blood cells than oxygen. 71 00:04:56,830 --> 00:04:58,249 Which is bad. 72 00:04:58,640 --> 00:05:02,314 So generally, we shouldn’t burn things indoors. 73 00:05:02,314 --> 00:05:06,928 And yet, here’s a device which burns things… indoors. 74 00:05:06,928 --> 00:05:09,790 Ah! But this particular furnace is  a condensing furnace, 75 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. 76 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, 77 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 78 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, 79 00:05:35,390 --> 00:05:39,930 and also there are ways to  mitigate this through ventilation but I digress. 80 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.   81 00:05:46,880 --> 00:05:47,808 Exhausting. 82 00:05:47,808 --> 00:05:51,936 We want the byproducts of the  combustion process to make their way outside 83 00:05:51,936 --> 00:05:55,837 where they’ll only be problematic in about… a decade or  two. 84 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. 85 00:06:02,901 --> 00:06:05,334 We can do that with a heat exchanger. 86 00:06:05,334 --> 00:06:09,082 In a typical furnace, this takes the form of a series of tubes. 87 00:06:09,280 --> 00:06:10,325 Actually, though! 88 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. 89 00:06:17,051 --> 00:06:18,413 You might call them U-tubes. 90 00:06:18,413 --> 00:06:23,578 The fuel is burned inside these tubes, which of course makes  them get pretty freaking hot. 91 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, 92 00:06:29,376 --> 00:06:33,718 and that air keeps the heat  exchanger from melting by cooling the tubes down, 93 00:06:33,718 --> 00:06:36,971 which also incidentally heats the air. That’s a  nice bonus. 94 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 95 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. 96 00:06:50,080 --> 00:06:54,339 And of course, we need those by-products to go  away somehow. 97 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 - 98 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. 99 00:07:07,101 --> 00:07:10,848 Aha! Well now we’re getting into the more practical and safety bits of the furnace. 100 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. 101 00:07:16,299 --> 00:07:19,228 There’s the big one that makes air come out of your registers. 102 00:07:19,228 --> 00:07:22,449 But before that one  comes on, you hear another one. 103 00:07:22,449 --> 00:07:23,777 What’s that for? 104 00:07:24,080 --> 00:07:27,926 Well, that’s arguably the most important part  of the furnace. 105 00:07:27,926 --> 00:07:32,555 This blower is called the draft inducer, and really it’s more of a sucker. 106 00:07:32,555 --> 00:07:37,602 It pulls air - and later, other fun gasses - through the heat exchanger. 107 00:07:37,602 --> 00:07:39,846 In other words, it induces a draft. 108 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, 109 00:07:44,829 --> 00:07:48,509 or as is the case here it exits through this PVC pipe. 110 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, 111 00:07:54,068 --> 00:07:58,153 and what lives on the other side of those tubes are the flamethrowers. 112 00:07:58,480 --> 00:08:00,445 Now we’re getting to the burny business. 113 00:08:00,445 --> 00:08:03,509 If we’re burning a fuel, it’s gotta come from somewhere, 114 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. 115 00:08:09,680 --> 00:08:12,540 But as far as your furnace is concerned it  comes from the burners. 116 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.   117 00:08:18,640 --> 00:08:22,560 Thanks to the constant supply of fresh  air brought about by the draft inducer,   118 00:08:22,560 --> 00:08:24,232 it will burn quite nicely. 119 00:08:24,232 --> 00:08:27,692 The draft inducer ensures not only that there is fresh air for burning 120 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 121 00:08:32,603 --> 00:08:35,473 and is instead safely expelled outdoors. 122 00:08:35,760 --> 00:08:42,079 Because this device deals with the, frankly,  dangerous combination of combustible fuels and enclosed spaces, 123 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. 124 00:08:48,669 --> 00:08:50,342 But before we get into that, 125 00:08:50,342 --> 00:08:53,421 I know this is the second "but before we get into that," 126 00:08:53,421 --> 00:08:54,592 apologies, 127 00:08:54,592 --> 00:08:57,708 let’s discuss the condensing part of this furnace. 128 00:08:57,708 --> 00:09:02,449 When you burn a fuel like natural gas, one of the byproducts  is water vapor. 129 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. 130 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.   131 00:09:15,280 --> 00:09:18,660 When it condenses into a liquid, as it will  inevitably do - 132 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 - 133 00:09:22,485 --> 00:09:24,293 it releases that energy. 134 00:09:24,293 --> 00:09:27,314 But that’s no good if it’s just happening outside. 135 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. 136 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 137 00:09:39,032 --> 00:09:42,885 with a greater surface area that can cool the exhaust gases further 138 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 139 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. 140 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. 141 00:10:02,669 --> 00:10:06,173 And it’s also how it’s able to exhaust  through a PVC pipe. 142 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, 143 00:10:11,330 --> 00:10:16,264 and it outputs 70,000 BTUS or about 20 kilowatts of heat. 144 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. 145 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, 146 00:10:29,295 --> 00:10:33,753 and you’ll understand why condensing  furnaces are a big deal and a great idea. 147 00:10:34,080 --> 00:10:38,385 A condensing furnace is a little more  complicated, of course, than a conventional one.   148 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 149 00:10:43,894 --> 00:10:48,455 for one, well now you have to deal with it when  previously you didn’t. 150 00:10:48,455 --> 00:10:51,774 This furnace has a drain pipe on its side for that very reason. 151 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 152 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. 153 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,   154 00:11:11,440 --> 00:11:16,320 obviously worth it and we absolutely should  find ways to help subsidize the added cost   155 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. 156 00:11:22,930 --> 00:11:24,800 It’s frankly a no-brainer at this point. 157 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.   158 00:11:29,920 --> 00:11:33,564 This furnace gets its oxygen supply from outside. 159 00:11:33,564 --> 00:11:37,258 This pipe is simply supplying the combustion section with outside air, 160 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). 161 00:11:41,979 --> 00:11:43,305 Why do that? 162 00:11:43,305 --> 00:11:47,682 Well, a conventional furnace gets its combustion air from the room it’s sitting in. 163 00:11:47,682 --> 00:11:51,867 That means it creates negative pressure whenever the draft inducer is running, 164 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. 165 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 166 00:12:04,624 --> 00:12:10,483 is essentially entirely outdoors, creating no negative pressure at all. 167 00:12:10,483 --> 00:12:14,641 Of course, particularly right now in the  human experience we’re discovering 168 00:12:14,641 --> 00:12:18,370 that we really ought to have a little  more exchange of air than we do. 169 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, 170 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, 171 00:12:28,194 --> 00:12:31,015 and not just because of respiratory  disease pandemics. 172 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 173 00:12:38,442 --> 00:12:41,077 and this topic deserves more attention and study. 174 00:12:41,077 --> 00:12:42,113 But anyway, 175 00:12:42,400 --> 00:12:46,560 Let’s finally bring it back to the  start with one of the questions I asked;   176 00:12:46,560 --> 00:12:50,262 why does the furnace seem to do things in distinct  steps? 177 00:12:50,262 --> 00:12:52,722 Ah, well that’s all about that safety. 178 00:12:52,722 --> 00:12:56,277 There really aren’t that many components in a  basic household furnace: 179 00:12:56,277 --> 00:13:00,421 just the draft inducer, heat exchanger, main blower,  an ignitor, and a gas valve. 180 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. 181 00:13:07,932 --> 00:13:12,160 And to do that, we rely on  a few sensors and a sequence of operations. 182 00:13:12,800 --> 00:13:17,921 Since this furnace is quite modern we have a  circuit board with a microprocessor handling everything, 183 00:13:17,921 --> 00:13:22,784 but the same basic things have been  happening in forced-air furnaces for decades. 184 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. 185 00:13:28,720 --> 00:13:34,720 Now, this component absolutely must be functioning  properly for the safe operation of the furnace.   186 00:13:35,280 --> 00:13:39,200 If it opened the gas valve without  airflow through the heat exchanger,   187 00:13:39,200 --> 00:13:40,986 it could be a disaster. 188 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.   189 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,   190 00:13:52,560 --> 00:13:56,041 which would only occur if the draft inducer  is working. 191 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. 192 00:14:04,080 --> 00:14:08,240 Once the operation of the draft inducer  has been proven, there will be a pre-programmed   193 00:14:08,240 --> 00:14:12,720 delay period to ensure no unburnt  gasses remain in the heat exchanger.   194 00:14:12,720 --> 00:14:16,205 Assuming the furnace shut down correctly  there shouldn’t be any, 195 00:14:16,205 --> 00:14:17,934 but better safe than sorry. 196 00:14:17,934 --> 00:14:22,620 While that delay is happening, this furnace sends power to the hot surface ignitor. 197 00:14:22,620 --> 00:14:26,977 This is an ignitor that ignites the  fuel by being a surface which is hot. 198 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.   199 00:14:32,634 --> 00:14:36,464 Anyway, because it takes a while for the hot surface  ignitor to become hot enough 200 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. 201 00:14:42,960 --> 00:14:45,297 Next comes the opening of the gas valve. 202 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  - 203 00:14:52,884 --> 00:14:54,546 I’m really in the middle of nowhere... 204 00:14:54,546 --> 00:14:56,046 can be explosive! 205 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 206 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. 207 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 208 00:15:11,309 --> 00:15:15,345 to detect a rapid  rise in heat brought about by flames. 209 00:15:15,640 --> 00:15:17,943 And this is done quite cleverly. 210 00:15:17,943 --> 00:15:21,998 This furnace, like most, has multiple burners arranged linearly. 211 00:15:21,998 --> 00:15:26,249 The hot surface ignitor lives  here next to the rightmost burner. 212 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, 213 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. 214 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. 215 00:15:44,415 --> 00:15:50,111 It will thus only register a flame when all burners have successfully ignited. 216 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. 217 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. 218 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,   219 00:16:09,920 --> 00:16:13,281 they’ll be pumping unburnt fuel into the  heat exchanger. 220 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   221 00:16:18,720 --> 00:16:23,506 the heat exchanger which, if it were to ignite,  would be likely to damage it. 222 00:16:23,506 --> 00:16:26,789 And that’s very bad and also dangerous. 223 00:16:26,789 --> 00:16:31,231 So, if it doesn't see flames right away, that gas valve is closed again 224 00:16:31,231 --> 00:16:36,391 and the draft inducer remains running to clear the  unburnt fuel from the heat exchanger. 225 00:16:36,391 --> 00:16:41,301 After a pre-programmed period of time, the furnace will  make another ignition attempt. 226 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. 227 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, 228 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. 229 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. 230 00:17:10,293 --> 00:17:11,690 Which is to wait a lil’ bit. 231 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, 232 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. 233 00:17:21,887 --> 00:17:26,910 But once it does that, well now you’ve got heat coming from your heat vents. 234 00:17:26,910 --> 00:17:30,225 When the thermostat  is satisfied and it stops calling for heat, 235 00:17:30,480 --> 00:17:34,800 the gas valve closes, extinguishing the  flames, and the inducer fan remains running   236 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.   237 00:17:39,840 --> 00:17:44,240 At the same time, the blower motor keeps  running to cool off the heat exchanger and,   238 00:17:44,240 --> 00:17:47,631 of course, get the rest of the heat it contains  out of it. 239 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. 240 00:17:53,440 --> 00:17:57,280 Now there’s gonna be at least one other  safety device in the furnace, too,   241 00:17:57,280 --> 00:17:59,323 and that would be a limit switch. 242 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. 243 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, 244 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.   245 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. 246 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.   247 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.   248 00:18:34,560 --> 00:18:38,940 If everything is in working order, that shouldn’t  be possible, but… 249 00:18:38,940 --> 00:18:43,069 well whenever you burn fuel there’s a risk of carbon monoxide. 250 00:18:43,069 --> 00:18:48,151 Furnaces can be particularly tricky because the heat exchanger wears out with time. 251 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, 252 00:18:55,083 --> 00:18:58,160 and over time imperfections can form from this thermal stress. 253 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 254 00:19:03,947 --> 00:19:09,305 - and thus carbon monoxide to exit the  heat exchanger and make it into the living space. 255 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, 256 00:19:15,165 --> 00:19:17,131 and especially in your bedrooms. 257 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. 258 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, 259 00:19:29,014 --> 00:19:32,753 but accessing the heat exchanger for a thorough  inspection can be difficult 260 00:19:32,753 --> 00:19:37,648 and might not be performed in your typical “pre-season tune-up special.” 261 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 262 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, 263 00:19:49,971 --> 00:19:54,223 invest in carbon monoxide alarms and  test them regularly. 264 00:19:54,223 --> 00:19:55,705 They may just save your life. 265 00:19:56,240 --> 00:20:01,358 So, that’s how your typical North American gas-fired forced-air furnace works. 266 00:20:01,358 --> 00:20:06,081 It’s really pretty simple, but thanks to modern technology,  quite efficient, too. 267 00:20:06,081 --> 00:20:10,945 On that note, condensing heat exchangers aren’t limited by any means to forced-air furnaces. 268 00:20:10,945 --> 00:20:14,708 Boilers can take advantage of this tech, even water heaters! 269 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, 270 00:20:20,320 --> 00:20:23,106 we really ought to do it as efficiently as we can. 271 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 272 00:20:29,552 --> 00:20:31,354 in more and more places. 273 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. 274 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. 275 00:20:44,699 --> 00:20:47,534 But we’ll get into those weeds maybe in a month or two. 276 00:20:47,534 --> 00:20:49,043 Perhaps three. 277 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. 278 00:20:57,314 --> 00:20:58,234 OK bye. 279 00:20:59,085 --> 00:21:01,663 ♫ efficiently smooth jazz ♫ 280 00:21:03,023 --> 00:21:05,233 Eugh, this is a clunky line. 281 00:21:05,233 --> 00:21:10,634 Pretty much ever small-to-medium-sized building built over here geh gu, ugh... 282 00:21:10,634 --> 00:21:16,426 Nope, that's not the line I wrote! 283 00:21:16,426 --> 00:21:19,391 ...immediately deadly combina - combustion. 284 00:21:19,391 --> 00:21:19,891 Dangit! 285 00:21:19,891 --> 00:21:25,455 And of course, there's the much more immediately deadly combustion byproduct carbon monoxide whi --- ohhh, 286 00:21:25,455 --> 00:21:27,166 carbon wasn't said right! 287 00:21:27,166 --> 00:21:28,409 "ca ma ma ma fehhhhr" 288 00:21:28,409 --> 00:21:32,648 ...path of the air being forced through the furnace with the aid of a blower *motor* 289 00:21:33,376 --> 00:21:35,448 Ehh... blower *motor*. [clears throat] 290 00:21:35,448 --> 00:21:37,304 That's... with a *blower* motor. 291 00:21:37,304 --> 00:21:39,925 Blower *motor* is stressing the wrong thing. 292 00:21:39,925 --> 00:21:43,794 ...with a greater surface area that can cool the exhaust gasses further 293 00:21:43,794 --> 00:21:47,034 which helps gets more heat out of the help ef frark! 294 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. 295 00:21:53,299 --> 00:21:57,009 Such is the terrible realm of regional jargon. 296 00:21:57,009 --> 00:21:59,780 At least they have something to do with heat! 297 00:21:59,780 --> 00:22:02,647 I mean, we could call them "dog kennels" so, hey.