Reviewed by George Baral
Updated August 31, 2022
When you’re feeling chilly, you turn up the thermostat. And miraculously your home gets heat. But how does it actually work? A lot of homes across Canada have forced air furnaces to combat the icy cold winters. In fact, it is by far the most common way to heat or cool your home.
A forced air furnace is usually located in the basement and is connected, by a system of ducts, to every room of the house. Basically, it moves cold air out of your living area through these ducts, into the furnace where it heats it up. It then returns nice warm air to the whole house, through a different set of duct work, and out through the heat registers.
A forced air furnace, as the name implies, uses hot air to carry heat through the house. Rooms have a cold air intake vent, which allows cool air to be sucked down into the furnace. Once there, it is heated using natural gas, propane, oil, or electricity. A fan then blows the heated air through another set of ductwork, and finally through heat registers, to every room in the house. The heat register has slats which can be adjusted to allow more or less heated air into the room. Forced air furnaces can be gas or electric powered.
All you need to do is set the thermostat to the desired temperature. This sends a signal to the furnace, and a valve opens, delivering fuel to the burners, and turning on the blower. The pilot light (or in some cases, electronic ignition) lights the burner. Heat is created in the heat exchanger, after which, the hot air is pushed out through the ductwork into the house, via the heat registers. Once the desired temperature is reached, flow of fuel stops and the fan will shut off a few minutes later. If the air cools below the set temperature, the thermostat sends a signal to the furnace. You’ll hear the burners turn on, then a few minutes later the fan will kick in, and the hot air will blow into the room.
Not much can really go wrong with a forced-air system. The airflow can become blocked, or sometimes the system will be noisier than usual.
Before you call a technician, there are a few things you can check yourself:
Is the furnace getting power? Take a look at the circuit breaker that controls the furnace to see if it has flipped. If so, turn it all the way off, then back to the On position. You can also look at the emergency shut off switch on the furnace. It should be in the On position.
Is the pilot light out? If you have a gas furnace, the pilot light may have gone out. It is a fairly simple matter to re-light the pilot with a long match. Here are some instructions from How Stuff Works and About.com.
Is the furnace getting fuel? If you have a propane furnace, check your tank’s gauge to make sure you have fuel left. Maybe you need a refill.
Is the thermostat dusty? If it’s an older, mechanical thermostat, try opening the thermostat cover to check the coils for dust. You could use a soft brush or compressed air to get rid of it. If the contacts are dusty, you can slide a piece of paper in and out between the contacts to remove the dust. More modern, digital thermostats rarely malfunction. Check the display. If it’s not visible, you may just need to replace the batteries. If the thermostat is not perfectly level, it could also affect how well it will control the temperature in the room. The Spruce tells you how to troubleshoot a thermostat.
Are the heat registers open? Check to make sure the registers are open, and that you don’t have heavy furniture sitting on top of them, blocking the heat from entering the room.
Are some rooms warmer than others? If so, you many need to “balance” your system. To do this, you can partially close the damper on the heat register in the warm rooms, and open the dampers fully in the colder rooms. If you want to really balance the heat, you can use thermometers to check the heat in each room. How Stuff Works provides detailed instructions, which involve getting several (depends on how many rooms you have.) thermometers to have close to the same temperature reading. This can be achieved by putting all the thermometers close together for about half an hour.Then you can tape the thermometers on the walls of each room about 3 feet up from the floor. Be sure they are not near a heat register or a cold air vent, and make sure they are not on exterior walls. After about an hour with the heat on, check the thermometer readings. You will then be able to tell if some rooms are getting more heat than others. You can then open or close the heat registers to get a balance. Follow this procedure for each room in your home, opening and closing dampers and registers until the same temperature is maintained in each room or the temperature balance you want is reached. The thermostat to the furnace should be kept at the same reading while you balance the system.
Is the system noisier than it should be? Air blowing through the ductwork can cause the ducts to shake and create noise. One solution is to add some extra duct hangers to give it more support. You can also check the joints to make sure air isn’t escaping. If it is, you may need to push the sections firmly together and wrap them with duct tape. If the noise is caused by anything else, you may need to call in a professional. It could be that the belts are worn, or the blower speed needs to be adjusted.
About.com lists some common problems that can occur with a gas furnace, and how to fix them.
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Some simple maintenance will keep your forced air furnace operating smoothly. Make sure you shut off the electricity to the furnace before you take off the access panel.
As you can imagine, clogged filters are going to slow down the flow of warm air through your house. Some experts recommend replacing the filters every month. If you have a permanent filter (made of plastic or aluminum), you can just wash and reuse it. Electrostatic air filters contain polyurethane and polypropylene fibers, and actually act like a magnet to attract dust, mold, and other airborne particles. These are generally quite a bit more expensive than a disposable filter, last a long time, and should be vacuumed or washed often. Check the manufacturer’s instructions before cleaning.
Check the ductwork to make sure warm air isn’t escaping through cracks or holes before it gets to your living areas. There are many different ways to do this. This Old House has a video which shows you a few ways, including how to use a smoke pencil to locate leaks. You can then make repairs in any accessible areas, using duct mastic in rough areas or places with sharp edges. The Green Building Advisor describes mastic as “a gooey, non-hardening material with a consistency between mayonnaise and smooth peanut butter.” At smooth joints, instead of mastic, people often wrap duct tape around the ductwork to prevent leaks. However, experts advise not to use standard cloth backed “duct” tape as it will come loose over time. Instead, use foil-faced duct tape.
If you have a direct-drive fan, you may just need to vacuum it. If there is a fan belt involved, get a service technician to check the tension.
Check around the cold air intakes and the heat registers, to make sure they’re not being obstructed by furniture or draperies.
The life expectancy of a good quality forced air furnace is about 15 years. After that time, the seals and the fan motor start to wear out. Of course, proper maintenance and the replacement of any worn parts have a bearing on the life expectancy of the furnace.
Like most heaters, some experts recommend that you have a certified service technician check your furnace each year to make sure it’s not losing warm air before it gets to the ductwork. They will also check the heat exchanger, flame sensor, pilot light, fan, and motor.
Your home insurance company may want to know the age and type of furnace in the house, as well as the type of fuel. Once a furnace has passed its life expectancy, not only may it operate less efficiently, causing your energy bill to go up, but it could break down, or the pilot light may go out, which could cause serious freezing issues if it happens in the middle of winter.
Keeping your furnace well maintained, and in good repair, will help you, and your insurance company, rest easy.
These two terms can be used interchangeably, as they are simply different ways of referring to the same heating process. There is a big difference if the term you’re using is central air, however, as this instead specifically alludes to air conditioning systems.
Most homeowners can expect to pay between $2,500 and $5,000 CAD to install a forced air furnace in their home. Prices can vary greatly depending on the specifics of the system you plan to install and the overall build of your house, however, so be sure to research diligently before committing to a purchase.
Forced air furnaces come in a variety of options, including not only gas and electric but also propane, oil, and other alternatives. You can inspect your furnace for gas lines or electrical wiring to discover for yourself what type is installed in your home, but be cautious not to tamper with the pilot light, wiring, or any other important or hazardous materials.
According to the U.S. Department of Energy’s Energy Savers website, properly installed and maintained radiant heating is more efficient than comparable forced air. The difference primarily comes from the immediate range of radiant heat, which can radiate directly into a room and therefore loses less heat than forced air does as it travels through your home’s ducts.
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About the expert: George Baral
George Baral has an MBA and a masters' degree in chemistry. He spent almost 35 years inspecting and evaluating heating and air conditioning systems before retiring. He obtained a California general contractor's license to start a company focusing on energy-efficient construction, became certified as a LEED AP and earned a NATE (North American Technical Excellence) certification, which provides advanced training for HVAC technicians.
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