Forced air heating systems that use a natural gas-powered furnace are very popular particularly in northern North American climates. Over the years, furnaces have achieved very high efficiency ratings of up to 98% meaning that only 2% of fuel used for the heating is lost.
New high efficiency furnaces draw fresh air from outside the home through a PVC pipe with another pipe used for exhaust gas exiting the home. Older furnaces rely on air supplied from within the home which decreases efficiency and may draw contaminated air into the system.
Does a furnace draw air from outside or inside the home?
Your furnace may draw air from either inside or outside the home depending on the age and design of the furnace.
Newer high efficiency furnaces of 90%+ AFUE rating have two PVC pipes leading outside the home: One draws fresh air from outside the home into the furnace which gets heated and the other is an exhaust pipe for air exiting the system and home.
Older furnaces have grates that allows air into the system that gets heated and then distributed through the air ducts of the home as warm air. One grate is usually on the furnace itself and there may also be air intakes throughout your home typically located on a wall panel perhaps in your living room or a hallway. Your system may also draw air from a crawl space or other area in your home.
Drawing air from inside the home is less efficient since it inevitably pulls cold air through cracks in windows and doors which offsets the heat being produced by your furnace. With modern homes being fitted with more efficient windows and doors that are more air tight in the past, drawing air strictly from outside the home for the furnace is preferable.
But there is another health-related reason why drawing air from inside the home isn’t the ideal method to supply your furnace.
Why do high efficiency furnaces only pull air from outside the house?
Furnace manufacturers are increasingly required to adhere to more stringent government regulations regarding energy efficiency and air loss with regards to their products. Many regulations stipulate that new furnace models use fresh air intakes because they ensure better ventilation in the home and also cleaner air inside the home.
In the past, older furnaces would draw “fresh” air from ducts within the home, directly from the area around the furnace and sometimes from the crawl space or other areas in your basement. These and other sources of air can be musty or contaminated and contribute to poor air quality by circulating dirty air throughout the house.
Basements tend to be the dampest and potentially moldiest part of your home due to the fact that they are built underground and often aren’t heated as well as upper floors. Drawing “fresh” air from your crawl space to supply the furnace
is not the best idea.
According to the American Lung Association, we breathe around 2,000 gallons of air every single day. Your furnace should ensure that it’s as clean as possible.
AFUE ratings and why higher is better
Over time as furnaces and HVAC in general have evolved, manufacturers have become more aware of how to produce energy efficient products and wring every bit of efficiency out of their units so that they can market them as such. A 90% AFUE-rated furnace won’t sell for as much as a 98%-rated model. The 98% AFUE-rated furnace is more appealing even with the higher price tag to energy-knowledgeable consumers who will spend a bit more up front want to save money on natural gas costs for the life of the furnace.
It should be noted that the AFUE rating is for the furnace itself and not your air duct system since that is specific to each home. A 90% AFUE rating means that 90% of the gas fuel used actually results in heat whereas the remaining 10% is lost through venting.
In your home, your air duct system likely has leaks and cracks where heat that is produced is then lost. Your home could for example have the same AFUE-rated furnace as your neighbor but ultimately your heating system is more or less efficient depending on specifics inside each of your homes.
Air intake or exhaust. How can you tell?
If your home has multiple pipes or vents on the side of your home, how can you easily tell which is which: Where does a furnace draw air from and where does a furnace vent?
The two easiest ways to tell which pipe or vent is which are as follows:
Take a look in the vent or pipe itself. If you see a vent with a wire screen, this means it’s an air intake. The screen is there to prevent rodents or debris from being sucked into the air intake. If there is a flapper door, this means it’s an exhaust. The flapper opens to let exhaust out when required.
Put your hand up to the vent or pipe when the furnace is running. If you feel air being sucked in, it’s an an air intake. If you feel air being expelled onto your hand it’s an exhaust.
Do you see a boxed vent with a shield over it? It’s probably your dryer vent especially if it’s outside the house near where your dryer is located. Similarly, air intake and exhaust pipes for your furnace will be located outside the house near the furnace.
Why are furnaces so popular and widely used?
Natural gas-powered furnaces are very common in the US and Canada but they are also found in other far flung countries like the UK, China, India and Australia due to the wide range of benefits. In Europe, while gas is still commonly used you tend to see wall-mounted heaters but underfloor heating is also more frequently being used in homes.
Operating using an adjustable or programmable thermostat control, gas furnaces provide consistent on demand heating and the convenience of automatically delivered natural gas.
They typically also have a disposable furnace air filter which collects dust and debris from the cold air before it is sent through the furnace and blower to be heated. In the photo above, the air filter is located vertically right where the bottom of the long inverse J-shaped silver duct work meets the brown Goodman-labeled furnace unit. It slides in and out for easy and quick replacement usually every 3 months.
The purpose of the furnace air filter was traditionally to protect the furnace motor and blower from damage from debris in the air. Increasingly, disposable furnace filters are being sold with various benefits for allergy sufferers and people who want more pollutants filtered out of the air they breathe such as smoke, candle wax, exhaust fumes, mold spores and more.
What benefits do gas furnaces offer to homeowners?
Most homes that require heating in the US use gas-powered furnaces. In Canada about 2/3 of homes that have heating use a furnace about 2/3 of those use natural gas. Natural gas is a popular option due to its widespread availability, its relatively low cost compared to other sources like oil and the fact that it can be automatically delivered to each home by pipe.
Natural gas furnaces are still widely used for a number of reasons:
- Natural gas furnaces are very efficient certainly compared to other forms of heating like fireplaces to earlier furnace models from decades past.
- There are many furnace models and manufacturers to choose from and it’s easy to find repair services. So it’s a bit of a self-fulfilling prophecy: Furnaces have been widely available and preferred for decades so it’s easy to find one that suits you.
- Natural gas is widely available, clean burning and doesn’t need to be delivered in truck to your home like oil.
- Gas furnaces are powerful more so than electric-operated furnaces.
- If you already have a gas connection in your home, you may consider adding a gas fireplace for space heating as it would be available during a power failure which a gas or electric furnace or pellet stove would not.
- Gas furnaces are relatively affordable and long lasting.
Summary
High efficiency furnaces rated at 90% AFUE and greater are built with a PVC pipe vent from outside the house down into your basement into the sealed combustion chamber of your furnace. Fresh cold air is sucked through the pipe into the furnace and is heated and then distributed throughout your home through the air duct system.
This system ensures that fresh air from outside the home is used to supply your furnace rather than air inside the basement of your home that might not be clean and may have contaminants given that the basement tends to be dampness part of your home.
Older homes will often have furnaces supplied by air from within the home. This is how they were previously designed before the advent of high efficiency furnaces and before more stringent government regulations regarding airflow in homes came into effect.
To learn more about forced air heating and cooling, check out my article called Is Duct Cleaning Necessary?
Main photo credit: oasisamuel – stock.adobe.com