Performance & Quality Factors

Learn more about types of furnaces..

Performance Factors

There are four basic factors that differentiate furnaces in terms of performance.


AFUE is an acronym for Annual Fuel Utilization Efficiency. It measures the amount of fuel converted to heat in the space in proportion to the amount of fuel which enters the furnace; the higher the AFUE the more efficient the furnace. A 95% AFE means that 95% of the heat from combustion is captured and 5% of the heat goes out the chimney

56480832Gas Burner

Gas burners range from single stage to fully modulated. The number of stages has a major impact on the efficiency of the unit and the comfort level in the home.

A single stage gas burner has only two speeds – off and high.  If the furnace is rated at 90,000 BTU’s, you get 90,000 BTU’s every time the furnace comes on; the furnace always runs at full speed.

A variable stage gas burners typically operate with 2 speed – Off, Medium and High. If the furnace is rated at 90,000 BTU’s, the furnace will produce about 40% of those BTU’s in medium stage. If that is not enough to satisfy the thermostat, the burners will go into high mode and produce the full 90,000 BTU’s. The increases efficiency by allowing the furnace to run at the lower stage in mild weather, saving energy and reducing costs.

A modulating gas burner has no defined stages; they provide just the amount of BTU’s needed to satisfy the thermostat in the existing conditions. As a result they are the most efficient and highest performing gas furnaces. These furnace dramatically increase comfort by running more often at low speeds to hold a specific temperature in a home.

Fan Motor

There are two aspects to fan motors that affect performance – motor type and stages. There are three types of motors used to run a furnace blower – PSC, Constant Torque, and ECM.

PSC (Permanent Split Capacitor) are traditional motors with copper winding’s and typically are set to a single speed. They operate at about 60% efficiency and consume a great deal of electricity, especially when first starting up. The start up phase also puts the most stress on the motor, so it is recommend that these motors are placed in a manual mode where they run constantly to increase the life cycle of the motor.

ECM (Electronically Commutated Motor) technology is based on a brush-less DC permanent magnet design that is inherently more efficient than the traditional PSC motor. They are 80% efficient less electricity and start up with minimal electrical usage.

ECM Constant Torque  – For comparison’s sake, constant torque motors are basically upgraded, next-generation PSC motors. What differentiates constant torque motors from PSC motors is their ability to deliver constant torque (i.e., rotational force or power output down a shaft).

ECM variable-speed motor technology can be compared to using a dimmer switch in lighting applications, meaning it is highly variable, making its precise performance ideal for a variety of advanced applications. ECM  variable-speed motors can run over a wide range of speeds. This is critical because blowers need to be flexible in order to deliver the airflow required by a multitude of system designs.


Advancing technology has allowed for the development of more sophisticated controls and thermostats.

Carriers Greenspeed intelligence, for example, is created by pairing adaptable-speed technology with their Infinity control’s intelligence. The unique, modulating gas valve of this unit, when managed by the Infinity control, allows it to literally adapt its output to the needs of the home. With tiny, 1% adjustments between 40 and 100% capacity, it gives the home only the amount of heating necessary.

This allows the furnace to operate longer at steadier, lower capacities, which ensures incredible energy efficiency and quiet operation with tighter temperature control than standard furnaces

Quality Factors

Heat Exchanger

A furnace’s heat exchanger is a set of tubes or coils which transfers heat from the combustion gas into the home. As the gas burns, the heat travels through the heat exchanger causing it to become hot. The furnace fan then circulates the air from the house over the outside of the heat exchanger. As the air moves over the heat exchanger, it absorbs the heat and distributes it through the house.

All heat exchangers fail eventually. This is because of metal fatigue. Metal when it is heated up expands, and when it is cooled off contracts. This expansion/contraction cycle is part of the normal furnace heating process. Over time this constant expansion and contraction has the same effect on a heat exchanger that bending a paperclip back and forth:   it breaks. And when that happens contamination occurs and it is no longer safe.


When you ask for a sound rating of a gas furnace today, you are liable to get just about anything. That’s because there is no industry-wide standard for rating the sound of an operating gas furnace.

A manufacturer might claim to have “the quietest furnace” when comparing high-fire sound levels on two-stage furnaces, despite the fact that the furnace spends less than 20% of its operating time at high-fire. The rest of the time the furnace is operating in low-fire, where it might not necessarily have an advantage.

Four factors that affect furnace noise are fuel efficiency ratings, burner type, fan motor, and duct work.