Annual Fuel Utilization Efficiency (AFUE)

The AFUE is the most widely used measure of a furnace's heating efficiency. It measures the amount of heat actually delivered to your house compared to the amount of fuel that you must supply to the furnace. Thus, a furnace that has an 80% AFUE rating converts 80% of the fuel that you supply to heat -- the other 20% is lost out of the chimney.
Note that the AFUE refers only to the unit's fuel efficiency, not its electricity usage.
The US Department of Energy (DOE) determined that all furnaces sold in the US must have a minimum AFUE of 78%, beginning January 1, 1992. Mobile home furnaces are required to have a minimum AFUE of 75%.

The DOE's technical definition of AFUE is as follows:

The measure of seasonal or annual efficiency of a furnace or boiler. It takes into account the cyclic on/off operation and associated energy losses of the heating unit as it responds to changes in the load, which in turn is affected by changes in weather and occupant controls.

Air Flow Configuration

The direction in which a furnace outputs heat. A furnace may have an upflow, downflow or horizontal configuration. (Horizontal configuration is sometimes called cross-flow configuration.)

Downflow Furnace

A downflow furnace takes cool air from the top and discharges warm air from the bottom and are generally placed in an attic.

Uplflow Furnace

An upflow furnace takes cool air in from the bottom and discharges warm air from the top and are generally placed in a basement.

Horizontal Furnace

A horizontal furnace lies on its side, pulling cool air from one side and pushing warm air out of the other and are generally used in locations with limited space.

BTU

British Thermal Unit. The quantity of heat required to raise 1 pound of water by 1 degree Fahrenheit.

BTUH

British Thermal Units per Hour

Combustion Efficiency

Combustion efficiency is a measure of the heating efficiency of a boiler. That is, it is a measure of the number of units of input energy required to produce a unit of output heat.
The combustion efficiency of a boiler is measured once the boiler is up and running (at steady state). This makes makes combustion efficiency a less accurate measurement of the overall efficiency of a boiler than the AFUE which takes into account the boiler (or furnace's) efficiency as it cycles through startup and shutdown.

Single versus Triple-Phase Furnaces

The vast majority of residential homes have single-phase electrical service. Many commercial and industrial buildings have triple- phase electrical service (usually in addition to single-phase service). If you are purchasing a furnace for your home, you will want to buy a single-phase furnace: Installing triple-phase electrical service in your house can be expensive, and since residential furnaces tend to be single-phase, there is little incentive to upgrade.

Indoor Blower Capacity

A furnace's output can be measured in several ways. One of those measures is the volume of air that passes out of the furnace and into the ductwork over a predefined span of time. This is a furnace's indoor blower capacity. (A second and related measure is the amount of heat created by the furnace, measured as British Thermal Units (BTUs) or BTUH).
Blower capacity is measured in cubic feet per minute (cfm).
The maximum indoor blower capacity represents the furnace's highest setting. The default capacity is the factory setting at the time that the furnace was manufactured.

Intermittent Ignition Device (IID)

IIDs are linked to a furnace or boiler's thermostat and light the pilot by means of a spark or other heat source when needed. IIDs are more fuel-efficient than the traditional approach of maintaining a continuously burning pilot flame.

Power Burner

Power burners control the mixture of gas and air that is injected into the boiler's combustion chamber. These burners increase the efficiency of the boiler by providing an optimal ratio of gas to air.

Steady State Efficiency (SSE)

Steady State Efficiency measures how efficiently a furnace converts fuel to heat, once the furnace has warmed up and is running steadily. However, furnaces cycle on and off as they maintain their desired temperature. Furnaces typically do not operate as efficienty as they start up and cool down. As a result, steady state efficiency is not as reliable an indicator of the overall efficiency of your furnace as AFUE.
SSE is calculated by allowing a furnace to warm up, (typically for at least 20 minutes), then measuring the input BTUs per hour (BTUH) from the fuel and output BTUH released to the house in the form of heat. You then divide the output BTUH by the input BTUH.
For example, a furnace that outputs 62,250 BTUs per hour once it has warmed up, and requires 75,000 BTUH of fuel has a SSE of 62,250 / 75,000 = 83%.

Watt (w)

The unit of electrical power equal to 1 ampere (amp) under a pressure of 1 volt. Equal to 1/746 horsepower.

Watt Hour (Wh)

An electrical energy unit of measure equal to 1 watt of power supplied to, or taken from, an electric circuit steadily for 1 hour.

Kilowatt Hour (kWh)

A unit of work or energy, measured as 1,000 watts (1 kilowatt) of power expended for 1 hour. Once generated, one kWh is equivalent to 3,412 BTU

The US Department of Energy (D.O.E.) recognizes several different classes of heating equipment, including:

• Central warm-air furnace
• Steam or hot-water system
• Heat pump
• Floor, wall, or pipeless furnace
• Built-in electric units
• Heating stove (which burns wood, coal or coke)
• Room heater (which burns gas, oil or kerosene)
• Fireplace
• Portable heater

Central Warm Air Furnace

A type of space-heating equipment in which a central combustor or resistance unit--generally using gas, fuel oil, or electricity--provides warm air that circulates through ducts leading to the various rooms. Heat pumps are not included in this category. A forced-air furnace is one in which a fan is used to force the air through the ducts. In a gravity furnace, air is circulated by gravity, relying on the natural flow of warm air up and cold air down; the warm air rises through ducts and the cold air falls through ducts that return it to the furnace to be reheated, thus completing the circulation cycle.

Steam or Hot-Water System

Either of two types of a central space-heating system that supplies steam or hot water to radiators, convectors, or pipes. The more common type supplies either steam or hot water to conventional radiators, baseboard radiators, convectors, heating pipes embedded in the walls or ceilings, or heating coils or equipment that are part of a combined heating/ventilating or heating/air-conditioning system. The other type supplies radiant heat through pipes that carry hot water and are inlaid in a concrete slab floor.

Heat Pump (Reverse-Cycle System)

A year-round heating and air-conditioning system in which refrigeration equipment supplies both heating and cooling through ducts leading to individual rooms. A heat pump generally consists of a compressor, both indoor and outdoor coils, and a thermostat.

Floor, Wall, or Pipeless Furnace

Space-heating equipment consisting of a ductless combustor or resistance unit, having an enclosed chamber where fuel is burned or where electrical-resistance heat is generated to warm the rooms of a building. A floor furnace is located below the floor and delivers heated air to the room immediately above or (if under a partition) to the room on each side. A wall furnace is installed in a partition or in an outside wall and delivers heated air to the rooms on one or both sides of the wall. A pipeless furnace is installed in a basement and delivers heated air through a large register in the floor of the room or hallway immediately above.

Home  •  About Us  •  Contact Us  •  Free HVAC Estimate  •  Products  •  HVAC Service Agreements  •  Company News  •  Client Testimonials  •  FAQs
Glossary of HVAC Terms  •  HVAC Vendor Profiles  •  Site Map  •  Specials  •  Link Partners  •  Privacy Policy