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How Your Furnace Works: All About a Natural Gas, Forced-Air Furnace

You Know What a Furnace Is, Even If You Can’t Explain It Exactly

The majority of homes in Alberta have a furnace for heating. A furnace is a mechanical thermal conversion system, which is a complicated way of saying that it is a machine that turns fuel into heat. That was maybe a bit more technical than general, but you know that stuff gets hot when it’s on fire. So you pretty much already know that the fuel your furnace uses holds potential energy that–during the chemical process of burning–is released as heat. This heat is transferred to the air and that hot air heats your home.

The most common furnace configuration in Alberta is a natural gas supplied, forced-air system. This system uses natural gas as a fuel and has a blower fan that forces cool air through the unit to heat it, and then out through the ducts where, now hot, it keeps your home warm.

Parts of a Forced-Air Furnace

Any forced air furnace has these basic parts. Each is a separate system and all work together to heat your home: distribution, fuel conversion and heat exchange, gas supply, combustion supply and exhaust, and a thermostat.

The Distribution System

A distribution system is metal ducts that run throughout your home to send warm air to every room. The blower fan that does the work of pulling in cool air, pushing it through the furnace where it gets warmed, and pushing out hot air is also part of the distribution system.

The Fuel Conversion and Heat Exchange System

This system is the part that actually burns the gas and transfers that heat to the air. These are kept in two separate compartments to keep dangerous exhaust out of your home. The heat exchanger is essentially a chamber that holds burning gas. A series of metal pipes carrying fresh air run through the chamber. As the fresh, cool air passes around these pipes, the metal transfers the heat from the chamber to the fresh air. The hot air is then pushed out through the heat registers in your home.

The Gas Supply System

This is the system that brings natural gas into your home to the furnace. Your furnace is connected to a network of natural gas piping that supplies gas to homes throughout your community. A gas valve controller in the furnace allows gas only when your furnace kicks in for heating.

The Combustion Supply and Exhaust System

If you have ever sat around a campfire, you know smoke is not good to breathe. The exhaust from the natural gas combustion is not exactly the same as woodsmoke, but also not good to breathe. Your furnace actually controls two separate air flows, one of fresh air to warm your home that is safe and breathable, and one to allow the combustion of natural gas. An air intake pulls air from outside the furnace system–either inside or outside the home–and uses that air to ignite and burn the natural gas. The exhaust–containing carbon monoxide and other dangerous gases–is then vented outside the home.

The Thermostat or Controller

Probably the most familiar part to any homeowner is the wall-mounted device that allows you to control the temperature in your home. The thermostat is hard-wired to the furnace and, as you adjust the temperature on your wall unit, controls what systems in the furnace operate, and, in newer furnace models, at what speed.

Same Job as the Old Furnace, Only Better

Whether an old model or a newer high-efficiency model, all forced-air, natural-gas furnaces have these components. They might look different between various brands and models, but all perform the same essential functions. Older furnaces usually have only on or off functions, while newer, high-efficiency furnaces can operate the various systems at multiple speeds or levels to regulate the temperature more accurately. A newer furnace will not have hot and cold spikes above and below your desired temperature.

Condensing Heat Exchanger

One system that is unique to a high-efficiency furnace is a second heat exchanger that pulls the heat from the combustion exhaust. This exhaust, like smoke from a campfire, is still very hot. This second heat exchanger efficiently pulls the heat from the exhaust, using it to heat your home. The exhaust then expelled from your home is so cool that it can be vented through plastic pipes. As the exhaust cools, it forms condensation which is drained into your sewer system. So a high-efficiency forced-air furnace will also need to run a small hose to a drain.

The Natural Gas Part of Your Furnace

Though it is possible to use other fuel sources, such as propane, hydrogen, or even use electric coils for heat, the most common fuel type in Alberta is natural gas. It is a naturally occurring fossil fuel, extracted from the earth through drilling or hydraulic fracturing (or fracking). The extracted natural gas is processed to remove certain compounds and impurities, leaving it almost pure methane. The natural gas that is piped to your home is safe and clean.

More about fuels in another article → link!

The Benefits of The Furnace You Already Have

A natural gas-supplied, forced-air furnace has quite a few benefits, most of which you might already be familiar with.

It is a central system that can serve an entire home, so there is no need for additional units to install or maintain. Some very large homes or houses with suites may have multiple furnaces.

The system can also provide ventilation and fresh air. The ventilation function can run independently of the heating system so two functions are provided by only one unit.

This type of furnace has a very high heat output capacity. It can heat your space very quickly. Anyone who has seen a child huddled in a blanket on the floor register appreciates how much heat blows out.

And there are no limitations on the heating. As long as gas is connected to burn, and electricity is connected to run the fans and controller, the furnace will heat.

The Downsides That You Might Not Have Thought About

There are considerations against a natural gas-supplied, forced-air furnace, too.

Methane is a carbon fuel. If released into the atmosphere, through spills or leaks, it is 70 times more impactful than carbon dioxide for global warming potential. And the exhaust from completely burned methane still contains carbon dioxide. This type of furnace will always have a significant negative carbon impact. There is no fix for this. (The solution is an entirely different type of system. Read more about options here → link to that article.)

The natural gas distribution infrastructure is subject to wear and leakage. The pipes may be damaged, which could cause leaks, but the design of the pipes and the quality of construction is also subject to a small amount of leakage. Any gas leaking inside a home is a serious health concern and is very difficult to detect at small concentrations.

This system also requires multiple access points through the house's exterior wall for air intake and exhaust. In practice, these places are not really well sealed by builders. It has a significant impact on how well a house is insulated.

There is also significant cost variability for natural gas. All homeowners have seen this firsthand and then felt it keenly through the months after the COVID-19 pandemic. The market price for natural gas, a globally significant commodity, is influenced by global factors that are beyond the control of any provincial or national government. Price stability is not guaranteed.

And, beyond natural gas supply, the system requires also an electrical supply to function. The fan, thermostat, and combustion system are electrically powered systems. If the electricity is out, so is your heating.

Good Decisions Come From Good Information

As one of the most common heating systems, you are better prepared to maintain your home knowing how a forced-air, natural gas furnace works. Although very common and becoming quite efficient, with new high-efficiency models over 98% efficient, there are other options for heating your home. [Contact a Peace Energy Advisor today] to discuss all the options and to get advice on the best choice for your home.


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