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How many cfms is a high efficiency furnace?

Posted on November 13, 2013
by Appleby Systems

Modern central furnaces help maintain a comfortable healthy and energy-efficient indoor environment regardless of the weather conditions outdoors. Homeowners looking to replace an aging or obsolete furnace can make a better purchasing decision by understanding how sizing, options and features affect total performance.

Air Movement and Equipment Sizing

Forced air through ducts, air movement and equipment sizing

Forced air furnaces move conditioned air through the action of a mechanical blower. Air is drawn through the return side of the system and energy is transferred as the air travels through a specially designed heat exchanger.

Cubic feet per minute (CFM) is a measurement that calculates the volume of air the system can move over a finite period. Furnace capacity is established by running a complete set of ACCA Manual J load calculations. Manual J accounts for a variety of design characteristics that affect the total load including insulation levels, windows, local climate conditions, perimeter tightness and duct leakage.

Furnace Blower Options

Furnace efficiency and comfort is largely determined by the configuration of the burners and the blower assembly. Advanced features are designed to regulate the function of the burn process and motor operation to precisely track changing load demands.

High-efficiency furnace

Single-Stage Furnace: Basic furnaces include a single-stage gas valve and a fixed speed blower. In essence, the unit is either operating at full capacity when the thermostat calls for heating, or it is turned off when the demand is met. Single speed blowers are factory calibrated to operate between 1200-1800 CFM, which will generally meet the requirements of a 1000-2000 square foot home. Unfortunately, single-stage technology does not account for the fluctuating capacity requirements brought on by changing weather conditions.

Two-Stage Furnace: A two-stage furnace includes a low-fire and high-fire setting that is regulated by a two stage gas valve. The control module automatically determines capacity requirements and engages the appropriate flame setting to meet the current demand. On days that are relatively mild, the lower setting runs at 50 percent capacity, which saves about half the fuel used at the higher stage. Lower stage operation also extends run times, so the air in the home is mixed evenly and filtered more often.

In addition to improving efficiency, the lower stage provides superior comfort throughout the home. When the outdoor temperature falls, the thermostat recognizes the increased demand and turns on the higher stage. A two-stage furnace eliminates the need to constantly adjust the thermostat as the outdoor temperature changes.

Variable-Speed Motor: A variable-speed motor controls the quantity and velocity of air delivered through the air distribution system. Enhanced air flow at a lower speed can help improve indoor climate control while eliminating the effect of drafts and cold spots. A variable-speed fan is especially effective when combined with a two-stage furnace. Fan speed adjustments can help compensate for inadequate duct design, overloaded filters and other airflow restrictions. A variable-speed motor can be up to six times more efficient than a comparable fixed speed fan motor.

Modulating Furnace: A modulating furnace includes advanced gas valve technology and a variable speed fan for the highest level of comfort and efficiency among all HVAC furnace products. The gas valve modulates output automatically and incrementally adjusts the burn rate to match small fluctuations in the load. Since the thermostat tracks changes in demand so precisely, the furnace rarely operates at 100 percent capacity. Continuous fan operation provides exceptional humidity control and keeps the indoor temperature comfortable and healthy. Modulating furnaces offer the lowest noise levels among all gas furnace models.

Better Comfort and Efficiency

Today’s modern gas furnaces offer a variety of features and options designed to provide economical comfort through many years of reliable operation.


If you’re considering getting a new efficient furnace in Burlington, Oakville or the surrounding areas, contact us today.

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