Forced Air Furnaces

Home Inspection Gas Burners

In order to heat our homes and be comfortable in cold weather, there are two things our heat equipment needs to do for us. First, we need exploit some form of energy source like burning natural gas or using electricity to create heat. Secondly, we need a method of transferring that heat around our home. Forced air furnaces are one of the most common house heating systems as they can safely turn energy into heat and distribute that heat to us through the air in our home.

How Do Forced Air Furnaces Work?

Furnaces heat and circulate air but the thermostat is what tells the furnace when to operate. Thermostats are typically mounted on a wall in the centre of our homes and they measures the current temperature and have an adjustable desired temperature. When the actual temperature drops below the occupants desired temperature, the thermostat sends a signal to the furnace to begin operating.

When the call for more heat goes to the furnace, the furnace goes into a startup sequence. The sequence varies with furnace features and technology but essentially it will ignite the energy source, confirm ignition and combustion air flow, and warm the heat exchanger. Once the heat exchanger has some heat, the main house blower fan starts and pulls household air through the furnace from the air returns, past the heat exchanger, and out the heat registers in the home carrying warm air from the furnace. Once the heat level at the thermostat reaches the occupants desired temperature, the furnace heat source and blower fan turn off.

Electric Furnaces - Electric furnaces operate on the same principle as gas furnaces except instead of igniting a fuel to create heat, they warm a series of heating elements similar to an electric stove. 

What is a Heat Exchanger?

When a furnace burns oil or gas to create heat, it also creates toxic gasses that we do not want mixed in our household air. A heat exchanger separates household air from combustion air but is designed to transfer heat from the combustion process to the household air. Then the cooled combustion air can be exhausted safely from the home out the chimney. Heat exchangers are subjected to a lot of heat expansion stresses and cracks in heat exchangers are very serious issues as this can allow toxic gasses into the household air.

Electric Furnaces - With no gas fumes, a heat exchanger is not necessary. The electric coils can sit directly in the household air. There is no chimney on an electric furnace.

Furnace Efficiency

As furnaces consume energy to create heat, there has been an increased interest at both governmental and consumer levels to make furnaces more efficient. In the last 20 years we have seen consistent improvements in efficiency in gas burning furnaces.

  • Electronic Ignition - In order to light the gas burners when heat was called for, furnaces used to have a ‘standing pilot’ gas flame. A standing pilot is like an always burning match ready to ignite the main burners. Keeping a small flame always on wastes energy so newer furnaces use an electronic spark or electric hot surface to light the main burners only during furnace startup.
  • Chimney's - Chimneys (also called 'vents') exhaust toxic gasses from the combustion process. Hot air naturally wants to rise which is great when the furnace is operating but when it is not operating, warmed household air will rise out of the chimney wasting energy. Newer furnaces control the chimney exhaust with a closing 'dampener' or by using a forced draft fan effectively stoping warm air loss up the chimney.
  • Heat Exchange - Heat exchangers transfer heat from the hot combustion air to the household air. Newer furnace designs have more efficient heat exchanger designs, multiple hear exchangers, and improved materials to facilitate heat transfer improving efficiencies.
  • Latent Heat of Vaporization - You may remember from high school physics that when matter changes from a gas to a liquid, latent energy in the form of heat is also released. When our furnaces burn natural gas, one byproduct is water which quickly turns to steam in the heat of the combustion process. Heat exchangers in newer furnaces will cool the exhaust enough for the steam to turn back into water releasing the latent heat energy which previously had been exhausted. These ‘condensing’ furnaces have exhaust so cool that it won’t rise out of the chimney so exhaust fans push combustion air outside then sidewall of the home in rust resistant plastic piping.

Electric Furnaces - Electric fireplaces are nearly 100% efficient as there is no loss of energy in a combustion process.

Energy Efficiency Classes

The innovations of furnaces typically have fallen into three classes, regular, mid, and high efficiency. The efficiency level of these classes is expressed as a percentage of energy input versus output. Another way to consider the measurement is for each $1 of heating fuel applied, what % of it is used to heat the home.

  • Regular Efficiency (60-77% Efficient) - Traditional furnace designs. These can no longer be sold in most markets due to low efficiency levels. Most often if these are found in a home, they are past their expected service life and owners should budget for replacement.
  • Mid Efficiency (78-82% Efficient) - These furnaces use electronic ignition, improved heat exchangers, and control chimney air flow to increase efficiency.
  • High Efficiency (90-98% Efficient) - These furnaces build on the mid-efficiency improvements but are also condensing furnaces capturing more latent heat. In many markets, these are now the only efficiency class of furnaces available

Safety Concerns

While furnaces today are very reliable, there are major safety concerns that we need to be aware of with gas and oil furnaces:

  • Gas or Oil Leaks - Natural Gas is explosive when mixed in the right levels with household air and oil spills are flammable. Fuel lines both inside the furnace and in the home need to be free of leaks and protected from accidental damage. (see more information on gas line safety)
  • Exhaust Air Leaks - When fuels burn they produce toxic gasses like carbon monoxide which are deadly. Exhaust gasses can enter the household air through holes in the heat exchanger, failures in the chimney system, or incorrect drafting of gasses. All homes with gas burning appliances should have carbon monoxide alarms to alert occupants to exhaust leaks. (see more information on carbon monoxide alarms)
  • Combustion Air Supply - Furnaces need a supply of combustion air to supply oxygen for the burning process in the furnace. This combustion air can come from a dedicate outside line (the most efficient) or be drawn from household air. If household air is used, a make up air source is needed. This is often accomplished by an air vent to the utility area. In homes with no visible make up air, the furnace needs to be in a well ventilated space as it relies on household air leaks to replace combusted air. A furnace that lacks combustion air may allow exhaust gasses to spill back into the home.


During the heating season, your furnace will be pulling thousands of cubic feet of air past the heat exchangers and through the household air ducts. To keep heat exchangers clean and efficient and to clean household air, furnaces are designed to use air filters to pick up airborne contaminants. These filters need to be changed about every 4-6 weeks through the heating season. Common filter problems we see on home inspections are missing filters, filters installed backwards, missing filter covers, and incorrect filter sizes. See our article on furnace filters here.

Because of the safety issues with burning fuels inside our homes, gas burning furnaces should be serviced annually by a heating technician with particular attention to exhaust gasses. Technicians through regular maintenace and minor repairs can help prevent unwanted emergency repairs in the heating season.

Heat Distribution

Forced air furnaces, like the name implies, use a blower fan to force air from the furnace to the heat registers in the home. In order for heat to cycle evenly through the home, air needs to circulate from the heat registers in each room to the cold-air return creating a constant loop. Ideally, every room in the home would have both a heat register and a return air return however most homes only have one air return per level.

Forced air heating systems are very difficult to balance perfectly. Some heat registers allow occupants to restrict some air flow into rooms however if rooms have limited air supply at the registers, this problem is difficult to fix. Air leaks in the piping system and reducing friction in the air ducts are concerns for heating system installers in building a good air distribution system.

Final Thoughts

Forced air heating systems have become very reliable but it is important not to overlook safety and maintenance. During a home inspection, your inspector will help you identify the installed heating equipment and efficiency class, check for correct operation, look over gas lines and safety equipment, and check the quality of furnace installation. 

With all the options available in heating your home and available equipment, always call in the professionals when you are buying a new home so you know your home is safe and solid.

By James Bell - Owner/Operator of Solid State Inspections Inc.