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What is a Variable Capacity Gas Furnace?

What is a Variable Capacity Gas Furnace?

We're glad you asked!

When you hear the term "variable," it refers to something that can assume any value, its one that can be ever changing, not one value and not another, but rather able to change and move as necessary with limits. This is much different that of two-stage or two-speed, because they have values of either one or another. Variable can anything from 0 to 100% or any set of values in between.

How does a variable capacity furnace work?

Furnaces are sized to be able to heat your home on the coldest day of the year. It’s when the heating capacity required to heat your home to a comfortable condition is equal to the heat loss of your home at the outdoor design conditions (-38C). However, not many days reach bone chilling temperatures of -38C. In fact this is one of the reasons for variable capacity furnaces, the vast majority of days that require 100% capacity are typically less than 2% of the heating season, as such the other 98% of the time your furnace is too big (oversized). The result is wasted energy, discomfort, and an increased concern for safety of occupants. Even if not directly related to the furnace carbon monoxide and/or combustion products can enter the home because older furnace designs are affected by tighter building envelopes that under negative air pressure conditions allowing combustion spillage into homes, a continuing cause for concern.

Let’s compare a gas furnace to a car - a standard gas furnace operates like a car in stop-and-go traffic-it’s either moving or sitting still, and burning lots of fuel in the process. The variable capacity gas furnace operates more like highway driving, with the cruise control on, making slight seamless adjustments to meet varying demand, all while improving overall efficiency. The furnaces capacity can vary between 40% and 100%.

Does the Blower (fan) work on a variable capacity furnace?

Older furnaces operate with single speed or two speed blower motors and when your furnace was installed, the ideal speed and airflow for your home was set (depending upon your specific situation such as the size of your home, etc.). However, there are numerous situations that can occur within the household to restrict this airflow such as ductwork design, unit location, zoning or dirty filters, just to name a few. Some newer single-stage and two-stage capacity furnace utilize variable speed high efficient smart motors. However, the new variable capacity furnaces apply an even greater use of the variable speed technology. Introducing AirFlex technology™, smart motors that now communicate two ways with the furnace. This allows for blower settings to be configured in a way that accommodates your lifestyle, home design, and geographic location. For example, the blower could be set to increase airflow during a fan-on operation to compensate for cathedral ceilings or long duct runs. If you live in a humid climate, you may require a slower fan-on setting to minimize off-cycle re-humidification (this is where water off the cooling coil is re-introduced to the home). Of course all this is accomplished with the latest in smart motor technology delivered by a variable-speed motor. In fact all the motors on this type of furnace are variable speed allowing for the maximum energy savings to be realized.

What are the benefits to having this technology?


Outstanding system efficiency can save you hundreds of dollars each year because they consume minute amounts of electricity compared to standard efficiency motors. By varying the gas and airflow capacities increased AFUE (Annual Fuel Utilization Efficiency) SEER (Seasonal Energy Efficiency Rating) are realized, resulting in year-round energy savings for your household. These furnaces are also Energy Star rated which means that they qualify for Government and Utility rebates.

Improved air quality


When you set your thermostat to “FAN,” the variable speed capacity furnace continues to slowly circulate air throughout your home resulting in two-thirds less energy cost than a traditional blower motor. That means you can run the fan constantly without significantly increasing operating costs. It also means better indoor air quality – low speed fan operation increases the effectiveness of your filters, allowing them to capture and remove more contaminants.


It's all about Control.

Homeowners know heating and cooling their home uses more energy than any other appliance. This inevitably shows up every month on the energy bill. Also, Energy Auditors involve and educate homeowners when conducting specialized testing on their homes. Today we all spend the vast majority of our time indoors and having a comfortable, clean and safe environment is essential to our well-being. The technology and equipment available today can and will meet these new demands. What most people don't know is that energy expenditures can now also be controlled!

Look for high efficiency products (95% AFUE or above). Not only will you see a difference in your monthly bill, you'll also feel the difference. Consult with a reputable contractor who can meet in your home and assess your needs and be sure to ask for referrals

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1567 Logan Avenue
Winnipeg, Manitoba
R3E 1S5
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T. 204-772-5799
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