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Forced-air Furnaces: The What, Why, and How

Jun 07, 2021
Well, the winter heating season is quickly approaching (at least in the Northern Hemisphere), which means many of us are turning on our heating systems for the first time in many months and hoping for the best. Have you ever wondered why your oven makes those noises? And why does it seem to work in different steps? What is a heat exchanger? Why should I worry about carbon monoxide poisoning? Why do we use ovens? Great questions! It's so cool that I'm making a video to answer them. Now, first, in this video we're going to talk about the typical North American gas and

forced

air heating system, because that's

what

's common where I live and have access to.
forced air furnaces the what why and how
Virtually every small and medium-sized building built here in the last half century in a winter climate has at least one of these providing heat. If you are used to a heating system with a central boiler and radiators, this is not the case. These aren't exactly rare in North America, but they are generally limited to older buildings that were built before we became addicted to air conditioning. Since central air conditioning became common, we've turned to ducted systems like this one, where a boiler acts as a heat source during the winter months and as an air handler for the air conditioning in the summer months.
forced air furnaces the what why and how

More Interesting Facts About,

forced air furnaces the what why and how...

Let's start by asking

what

the basic job of the oven is. You could say, well, it's obviously to provide heat to a living space, and you'd be more or less right! But more specifically, its job is to safely release as much thermal energy contained in a combustible fuel as possible into the living space and distribute it with the help of a fan and ducts. Almost everything in the oven is designed around safety and its sequence of operations is specifically performed to demonstrate the integrity and functionality of each of its components before it is allowed to operate.
forced air furnaces the what why and how
If it doesn't pass its own tests, it locks up and you'll have to call your local HVAC company. Better that than being dead! We'll go over that sequence of operations, as well as the components themselves, shortly, but first let's discuss why we use these things. This seems to cause a kind of bewilderment among those who do not live in North America. If you're used to a boiler, it makes sense, but all too often I've come across people who seem to think our boilers are somehow inefficient machines. Not at all, they are incredibly efficient. This particular furnace is capable of capturing 96% of the thermal energy of a fuel.
forced air furnaces the what why and how
That's great, but some models offer even more than that! Hello, it's me from the future with an embarrassing correction. So we're about to talk about AFUE or "A-few," which stands for Annual Fuel Use Efficiency. And for this oven it's 95. Not 96. I've had countless opportunities to look at this label before, but unfortunately I didn't take advantage of any of them. And my memory failed me. So for the rest of this video, if I say "96" I probably meant 95. Anyway, back to AFUE. It is measured as a roughly annual average to help account for differences in efficiency at start-up and close-out.
During a state of constant heating, it is actually slightly higher. Being able to bring 96% of the thermal energy available in a fuel into the living space is tremendous, but even the worst natural gas boilers on the market have at least an AFUE of 80. A long time ago I made a video specifically about the There is a reason behind burning fuel for space heating, but that's essentially it. Furnaces excel at burning fuel and capturing almost everything we can of it. That said, they are probably (and hopefully) on their way out as we move towards electrification. Now, we don't typically use resistive electric heating to heat entire houses because, while electric heating itself is 100% efficient, electrical generation is not.
And even if it were, the electrical requirements of resistive heat are not practical on a large scale. But heat pumps are capable of turning this math around. Since heat pumps, which are basically air conditioners that run in reverse, can move much more energy than they consume, they are undoubtedly the future of domestic space heating. Even now, with our largely fossil grid, they generate fewer carbon emissions than direct fuel burning because their efficiency can approach 500%, more than what generation losses represent. But we'll save that topic (and its current challenges) for a video on I Prometo in the not-too-distant future.
For now, let's get back to this! One of the things we have learned over the years as we embarked on this human endeavor is that burning things is unpleasant. Burning virtually any fuel, including so-called "clean-burning" natural gas, produces particles that are not good to breathe and, of course, there is carbon monoxide, a much more immediately deadly byproduct of combustion that, due to poor evolutionary luck, it is much more attractive to red blood cells than oxygen. Which is bad. Generally, we shouldn't burn things indoors. And yet, here's a device that burns things... indoors. Ah! But this particular furnace is a condensing furnace, and thanks to these two tubes, the combustion of the fuel happens outside, but we'll talk about that later.
You may also know of another device that burns fuel indoors without ventilation: the common stove and oven, and research is starting to suggest that this may not be so good either, but there is the small saving grace that the amounts of fuel that burning for cooking is generally small fractions of what an oven uses, and there are also ways to mitigate this through ventilation, but I digress. Anyway, a furnace like this needs to burn its heating fuel and then has a bit of a backbreaking job ahead of it. Exhausting. We want the byproducts of the combustion process to reach the outside, where they will only be problematic in about... a decade or two.
But of course we want to be able to extract the heat generated by the combustion process and release it into the living space. We can do it with a heat exchanger. In a typical furnace, this takes the form of a series of tubes. But actually! The heat exchanger in a standard 80+ furnace is simply a series of typically U-shaped steel tubes. You could call them U-tubes. Fuel burns inside these tubes, which of course causes them to get quite hot. Those tubes are then placed in the path of air that is

forced

through the furnace with the help of a blower motor, and that air prevents the heat exchanger from melting by cooling the tubes, which incidentally also heats the air.
That's a nice advantage. The practical result of this is that the heat from the fuel is released into the air that flows through the furnace and eventually out through the vents that heat the house, but the byproducts of combustion remain separated within those tubes. And, of course, we need those byproducts to disappear somehow. And also, wait, you need oxygen to burn fuels and burning fuel uses up that oxygen, so you'll need a way to supply fresh air to those tubes in addition to getting rid of the byproducts. AHA! Well, now we get into the more practical and safety parts of the oven.
If you've ever lived with a boiler like this, you've probably noticed that there are two blower motors. There's the big one that makes air come out of your registers. But before that one appears, you hear another one. What is that for? Well, that's arguably the most important part of the oven. This blower is called a draft inducer and is actually more of a dummy. It draws air (and later other fun gases) through the heat exchanger. In other words, it induces a draft. That air is then pushed up through some sort of chimney where it will eventually exit the house, or as is the case here, it exits through this PVC pipe.
The draft inducer lives on the outlet side of the heat exchanger, so it draws air through the tubes, and what lives on the other side of those tubes are the flamethrowers. Now we come to the issue of burns. If we are burning a fuel, it has to come from somewhere, and where it comes from is deep underground, having been trapped there for millennia before we extracted it. But when it comes to your oven, it comes from the burners. Basically, these are specialized nozzles that release a certain amount of gas into the heat exchanger tubes. Thanks to the constant supply of fresh air provided by the draft inducer, it will burn quite well.
The draft inducer ensures not only that there is fresh air to burn, but also that what comes out the other side of the heat exchanger tubes does not reach the living space, but is safely expelled outside. Because this device deals with the, frankly, dangerous combination of combustible fuels and enclosed spaces, it is designed with a number of safety devices and a sequence of operations to ensure that things don't go too wrong. But before we get into that, I know this is the second "but before we get into that," apologies, let's discuss the condensation part of this oven.
When a fuel like natural gas is burned, one of the byproducts is water vapor. A standard 80+ furnace doesn't really know what to do with it, so it sends it out along with the carbon monoxide and other nasty stuff. But that water vapor is hot and, more importantly, contains energy in the form of latent heat. When it condenses into a liquid, as it inevitably will (it's all the steam you see coming off the roofs of houses in the middle of winter), it releases that energy. But that's no use if it just happens outside. If we could get it to condense inside, we could get more energy out of heating oil.
And so, a condensing furnace has what is essentially a second heat exchanger after the main fire tubes with a larger surface area that can further cool the exhaust gases, helping to remove more heat from the fuel on its own. , but the most important thing is that you get as much heat as possible. of water vapor to condense and release its latent heat inside the furnace instead of to the outside air where it would otherwise be wasted. This is how this furnace is capable of bringing 96% of the energy available in fuel to the home. And that's also how it can come out through a PVC pipe.
I mean, think about this, it's burning flammable gas, there's literally fire inside it, and it produces 70,000 BTUS or about 20 kilowatts of heat. And yet it loses so little heat to the exhaust that it can safely exit through a barely-warm plastic tube. Compare that to an 80+ furnace with a steel exhaust chimney that gets so hot you can't even touch it, and you'll understand why condensing

furnaces

are so important and a great idea. A condensing oven is, of course, a little more complicated than a conventional one. Most of that has to do with the fact that dealing with condensate is not the easiest task for one, well now you have to deal with it when you didn't before.
This oven has a drain tube on the side for that very reason. But water is also not just plain water, it is quite acidic thanks to other combustion byproducts, so the secondary heat exchanger must be made of materials, such as stainless steel, that resist corrosion. This adds to the cost of the boiler to some extent, but getting an extra 10-15% of the energy from heating oil makes it obviously worth it in my opinion and we should definitely find ways to help subsidize the extra cost for those who need financial assistance because it will always save money and resources in the long run.
Frankly, at this point it's a no-brainer. Oh, and that other pipe over there? Well, here's another way to increase efficiency. This furnace receives its oxygen supply from outside. This tube simply supplies outside air to the combustion section, in fact you can see that it simply opens into this space (and the other end is outside). Why do that? Well, a conventional furnace gets its combustion air from the room it is in. That means it creates negative pressure every time the draft inducer is running, and that brings in a certain amount of cold outside air to replace what comes out of the exhaust.
Instead, this supply pipe, along with the seals on this panel, makes the combustion section of the furnace essentially completely open air, without creating any negative pressure. Of course, particularly now in the human experience we are finding that we really should have a little more air exchange than we do. Most of our efficiency-first construction practices arose thanks to the energy crises of the 1970s, and we have been living in mostly sealed boxes, whichWhich turns out not to be very good, and not just because of respiratory disease pandemics. The simple buildup of carbon dioxide from our exhalation can be dangerous, so some amount of negative pressure is probably a good thing and this topic deserves more attention and study.
But anyway, let's finally go back to the beginning with one of the questions I asked; Why does the oven seem to do things in different steps? Ah well, that's all about security. There really aren't that many components in a basic home furnace: just the draft inducer, heat exchanger, main fan, an igniter, and a gas valve. But it is absolutely necessary to make sure everything is working before letting flammable gas out of a pipe. And to do this, we rely on a few sensors and a sequence of operations. Since this oven is fairly modern, we have a circuit board with a microprocessor that takes care of everything, but the same basic things have been happening in forced air ovens for decades.
When the thermostat calls for heat, the sequence of operations begins by turning on the draft inducer. Now, it is absolutely necessary for this component to function properly for the safe operation of the oven. If you opened the gas valve without air flow through the heat exchanger, it could be a disaster. So to prove that the inductor actually works, the circuit board looks for the output of this pressure switch to change. If so, that means there was a change in air pressure where the switch is located, which would only occur if the draft inducer is operating. Checking for a change in the switch output also allows the oven logic board to detect a stuck switch and refuse to operate.
Once the draft inducer has been tested for operation, there will be a pre-programmed delay period to ensure that no unburned gases remain in the heat exchanger. Assuming the oven is turned off correctly, there shouldn't be any, but better safe than sorry. While that delay occurs, this oven sends power to the hot surface igniter. This is a lighter that ignites fuel by being a hot surface. An alternative ignition method is a sparking thing, but it seems to have gone out of fashion for some reason. Anyway, because it takes a while for the hot surface of the igniter to get hot enough that its surface can ignite things, the delay period for cleaning the heat exchanger is a perfect time to warm it up.
Next comes the opening of the gas valve. This is a very urgent step because natural gas and propane, which is actually what powers this furnace. I'm really in the middle of nowhere...they can be explosive! We don't want that, so we need a way to know if the fuel has actually ignited and isn't being dumped unburned into the heat exchanger, where it could explode. And this is actually very easy to do: we simply use a thermocouple as a flame sensor to detect a rapid increase in heat caused by flames. And this is done quite cleverly. This oven, like most, has several burners arranged linearly.
The hot surface igniter is located here, next to the rightmost burner. The burners are designed so that the flame spreads quickly from one to the other and, assuming nothing happens to them and the fuel supply is adequate, this should happen almost instantly. And so, to determine that ignition occurred correctly, the flame sensor is placed on the burner furthest from the igniter. Therefore, it will only register a flame when all burners have ignited correctly. When the oven opens the gas valve, it looks for a rapid rise in temperature at that sensor and you should see it immediately.
If it doesn't, usually in just two seconds, it closes the gas valve and aborts its mission. This flame check step is very quick because if any of the burners do not ignite immediately, they will pump unburned fuel into the heat exchanger. If this is allowed to happen for more than a couple of seconds, there may be a dangerous amount of fuel in the heat exchanger which, if ignited, would likely damage it. And that is very bad and also dangerous. So if you don't see flames right away, the gas valve closes again and the draft inducer remains running to remove unburned fuel from the heat exchanger.
After a pre-programmed period of time, the oven will make another ignition attempt. And after a certain number of failed attempts (there are five in the case of this boiler), the system locks up and does not provide you with heat. 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, including temporary loss of gas pressure, so it automatically restarts to prevent the pipes from freezing, if you can avoid it. However, whenever the flame sensor detects flames, the logic board gives the go-ahead for the next step.
Which is waiting a little. Instead of turning on the blower motor right away and giving it a nice blast of cold air, the furnace will simply sit pretty and allow the heat exchanger to warm up a bit before turning on the blower. But once it does that, you now have heat coming from the vents. When the thermostat is satisfied and stops calling for heat, the gas valve closes, extinguishing the flames, and the inducer fan remains running for about thirty seconds to ensure that all remaining exhaust is replaced by fresh air. At the same time, the fan motor continues to run to cool the heat exchanger and, of course, remove the rest of the heat it contains from it.
But after about a minute, it turns off completely and waits patiently for the next heat call. Now there will also be at least one other safety device on the oven, and that would be a limit switch. This is a switch that activates at a certain temperature and is designed to protect against overheating. If for some reason the fan motor stopped working or the airflow was otherwise limited, the heat exchanger will overheat in a short time because it does not have enough airflow to cool it. So, that limit switch is there to detect such a scenario and turn off the oven if it occurs.
So, there's clearly a lot of safety built into a typical oven, and some clever ingenuity as well. But there's one thing it generally won't protect you from: carbon monoxide poisoning. If everything is working correctly, that shouldn't be possible, but... well, whenever you burn fuel there is a risk of carbon monoxide production. Furnaces can be particularly tricky because the heat exchanger wears out over time. Going from room temperature to having a fire inside and back again a dozen times a day for 15 years is difficult, and over time, blemishes can form due to this thermal stress. The most dangerous type is a crack in the heat exchanger tubes, which can allow exhaust gases and therefore carbon monoxide to exit the heat exchanger and into the living space.
That is why it is vitally important that you have carbon monoxide alarms in your living spaces, and especially in your bedrooms. Carbon monoxide poisoning is rare, but an alarm is the only thing that can protect you against it. Annual furnace inspections by a trained technician can detect problems before they become dangerous, but accessing the heat exchanger for a thorough inspection can be difficult and may not be performed on your typical "tune-up special." preseason". So please, if your house is heated by burning fuel, even if it is not a forced air boiler but something like a boiler or even if fuel is burned anywhere in your house for cooking or heating water or anything else, invest in carbon monoxide. alarms and test them periodically.
They may simply save your life. This is how the typical North American gas-fired forced air boiler works. It's really quite simple, but thanks to modern technology, it's also quite efficient. In that sense, condensation heat exchangers are by no means limited to forced air

furnaces

. Boilers can take advantage of this technology, even water heaters! As long as natural gas or another fuel remains a prominent way to heat spaces and water, we really should do it as efficiently as possible. However, as we move into this decade, we will no doubt begin to see natural gas being phased out in more and more places.
In areas with mild heating needs, heat pumps are already viable with a slightly larger investment than an air conditioner. And for colder climates like mine, geothermal heat pump systems are available and are dropping in price quickly. But we'll address that weed maybe in a month or two. Maybe three. It will be this heating season. I now have a thermal camera and there is a reversible mini split in the garage, so no excuses. OK Bye. ♫ smooth, efficient jazz ♫ Eugh, this is a clumsy line. Almost all the small and medium sized buildings built here geh gu, ugh... No, that's not the line I wrote! ...immediately deadly combination - combustion.
Damn! And, of course, there's carbon monoxide, a byproduct of combustion far more immediately deadly than --- ohhh, carbon was misspoken! "ca ma ma ma fehhhhr" ... path of air that is forced through the oven with the help of a *blower* motor Ehh... *blower* motor. That's it... with a *blower* motor. The fan *motor* is stressing the wrong thing. ...with a larger surface area that can further cool the exhaust gases, helping to get more heat out of the aid, eff frark! Yes, our ovens do not smell at all and have no connection with metallurgy. That is the terrible scope of regional slang.
At least they have something to do with the heat! I mean, we could call them "dog kennels," so, hey.

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