Heating Dictionary: Furnace, Boiler and Heat Pump Terms Explained

Furnaces, boilers and heat pumps have their own vocabulary, and the technician who looks at yours will use it on the estimate. This page translates the words you are most likely to meet: on a written quote, in an inspection report, or on the phone when you describe what is going wrong. It is a dictionary, not a repair manual. When a term below describes your house, the next step is still a call.

The terms, A to Z

AFUE

Annual fuel utilization efficiency, the share of fuel a furnace or boiler turns into heat over a season. New equipment runs from about 80 to the high 90s.

Air handler

The indoor unit that moves air over the coil or heat exchanger and through the ducts.

Aquastat

The control on a boiler that watches water temperature and tells the burner when to fire.

Baseboard heat

Heat delivered along the base of the wall, either hot-water fin-tube fed by a boiler or electric resistance elements.

Blower motor

The motor that pushes air through the ducts in a furnace or air handler. A failing one shows up as weak airflow or a whine.

Boiler

A unit that heats water or makes steam for radiators, baseboards or radiant floors. Common across older New England homes.

BTU

British thermal unit, the standard measure of heating output. Furnaces and boilers are sized in BTU per hour.

Carbon monoxide detector

A required alarm in any home with fuel-burning heat. A cracked heat exchanger or a blocked flue makes it the most important device in the house.

Circulator pump

The pump that moves hot water through a boiler system. When it seizes, the boiler runs but the radiators stay cold.

Combustion air

The air a furnace or boiler needs to burn fuel. A sealed-up mechanical room can starve it.

Condensate line

The drain that carries away the water a high-efficiency furnace or a heat pump produces. A clog trips a safety switch and shuts the system down.

Condensing furnace

A high-efficiency furnace that pulls extra heat from its exhaust until water condenses out of it. It vents through plastic pipe.

Direct vent

An appliance that draws combustion air from outside and vents outside through a sealed pipe.

Ductless mini-split

A heat pump with an outdoor unit and one or more wall-mounted indoor heads, with no ducts.

Ductwork

The sheet-metal or flexible channels that carry heated air to the rooms and pull return air back.

Emergency heat

On a heat pump thermostat, the setting that runs the backup electric strips alone. Costly to run for long; meant for a failure.

Expansion tank (boiler)

The tank that absorbs the pressure change as boiler water heats. A waterlogged one makes the relief valve drip.

Flame sensor

A rod in the burner that proves the flame is lit. A dirty one shuts the furnace off seconds after it starts.

Flue

The pipe or chimney passage that carries exhaust gases out of the house.

Forced air

Heat delivered by blowing warmed air through ducts, the most common system outside the Northeast.

Furnace filter

The filter in the return that protects the blower and coil. A clogged one is the most common reason a furnace overheats and shuts off.

Heat exchanger

The metal chamber inside a furnace where combustion heats the air passing over it. A cracked one can leak carbon monoxide and usually means a new furnace.

Heat pump

A system that moves heat instead of making it, cooling in summer and heating in winter. Its output drops as the outdoor temperature falls.

HSPF

Heating seasonal performance factor, the winter efficiency rating of a heat pump.

Hydronic heating

Any system that heats with water, from radiators to radiant floors.

Ignitor

The part that lights the burner in a modern furnace, usually a hot-surface element. A common failure with a small fix.

Limit switch

A safety switch that shuts the burner off if the furnace gets too hot. Repeated trips usually point to airflow.

MERV

The rating scale for how fine a filter is. Higher catches more, but too high for the system restricts airflow.

Oil burner

The burner assembly on an oil-fired furnace or boiler. It needs an annual cleaning and a nozzle change.

Pilot light

The small standing flame that lights the burner on older gas equipment. Newer units use electronic ignition.

Pressure relief valve (boiler)

The safety valve that opens if boiler pressure climbs too high.

Radiant heat

Heat from warm surfaces, usually tubing in the floor, rather than blown air.

Radiator

A cast iron or steel unit that gives off heat from hot water or steam. Cold radiators on one floor usually mean air in the line or a valve problem.

Return air

Air pulled back from the rooms to be reheated. Blocked returns starve the system.

Short cycling

A furnace or boiler that starts and stops in short bursts. It wears parts fast and points to a control, airflow or sizing problem.

Steam heat

A boiler system that sends steam to radiators through one or two pipes. Common in pre-war homes and unforgiving of low water.

Thermocouple

The safety sensor that shuts off the gas if the pilot light goes out.

Thermostat

The control that calls for heat. A dead thermostat looks exactly like a dead furnace.

Zone valve

A motorized valve that opens to send hot water to one part of the house. A stuck one leaves a zone cold or overheated.

Zoning

Dividing a house into separately controlled heating areas.

When a word on the estimate worries you

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