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HVAC Forced Warm Air Heating Units

The Old House Web

Parts of this story:Introduction >> Fuel burning units, general >> Forced hot water (hydronic) heating systems >> Steam heating systems >> Electric resistance heating >>Air conditioning systems

Warm air heating systems are of two types, forced air or gravity.

heating register

Any cat can tell you that warm air registers are best positioned low on a wall.

Gravity systems are occasionally still found in older single-family houses, but most gravity systems either have been replaced or converted to forced air. Gravity systems are big, bulky, and easily recognizable. Lacking a mechanical means of moving air, such systems are inefficient and heat unevenly, can be dangerously hot, and are generally considered archaic. Plan to replace them unless there are overriding reasons for doing otherwise.

Most forced warm air systems use natural gas or fuel oil as a heat source, but some systems use electric resistance heaters or heat pumps. These heaters replace the heat exchanger and burner found in gas- and oil-fired furnaces or supplement the heat output of heat pumps.

Electric resistance heating systems have no moving parts and require no adjustment. The circulation blower and air distribution ductwork for electric resistance heating systems (and heat pumps) are identical to those of gas- and oil-fired warm air systems and should be checked as described below.

Assess the condition of forced warm air heating systems as follows:

Heat exchanger. The heat exchanger is located above the burner in gas- and oil-fired furnaces and separates the products of combustion from the air to be heated. (There is no heat exchanger in an electrically heated furnace.) It is critical that the heat exchanger be intact and contain no cracks or other openings that could allow combustion products into the warm air distribution system. Visual detection of cracks, even by heating experts, is a difficult and unreliable process.


Look for signs of soot at supply registers and smell for oil or gas fumes. Observe the burner flame as the furnace fan turns on; a disturbance or color change in the flame may indicate air leakage through the exchanger. Operate the furnace for several minutes and then feel the furnace frame for uneven hot spots. Similarly, another simple test requires turning on the fan only and placing a lighted match or candle in the heat exchanger enclosure. If there are leaks, the flame will flicker. A CO tester may also be used to detect combustion gases. For any of these tests, consult a heating contractor or HVAC service technician.

Look for rust on the exchanger -- a major cause of premature exchanger failure is water leakage from humidifiers or blocked air conditioner condensate lines. Check for other signs of water leakage. The durability of the heat exchanger determines the service life of the furnace.

Furnaces installed since the 1950s normally have a useful life of 25 years or less. Older furnaces with cast iron heat exchangers may last much longer.

Furnace controls. Gas- and oil-fired furnaces have two internal controls, a fan control and a high-temperature limit control. (Furnaces with electric resistance heating coils have high temperature limit controls and air flow switches.) The fan control prevents cold air from being circulated through the system. It is a temperature-sensitive switch, completely independent of the thermostat, and turns the furnace blower on and off at preset temperatures.

When the thermostat calls for heat, the furnace burner is turned on. After the heat exchanger warms to a preset temperature (usually 110 to 120 F [43 to 49 C]), the fan control activates the blower. The thermostat will shut off the burner when the building warms to the thermostat setting, and when the heat exchanger cools to about 85 F (29 C) the fan control will switch off the blower.


Observe the above sequence; if it is faulty, the fan control should be adjusted or replaced.

The high-temperature limit control is a safety device that shuts the burner off if the heat exchanger gets too hot (the control is usually set at about 175 F). Should the burner automatically turn off before the blower is activated, either the blower, the fan control, or the high-temperature limit control is faulty and should be adjusted or replaced.

Circulation blower. Remove the blower cover and inspect the blower motor and fan. Look for proper maintenance and oiling. Check for wear or misalignment of the fan belt, if any, and for dirt build up on the motor or fan.


When the system is operating, listen for unwarranted blower noise and determine its cause.

Distribution system and controls. The distribution system is made up of supply and return ducts, filters, dampers, and registers. Supply and return ducts may be made of sheet metal, glass fiber, or other materials. Glass fiber ducts are self-insulated, but sheet metal ducts are usually not insulated except where they pass through unheated (or uncooled) spaces. Sheet metal ducts are occasionally insulated on the inside; determine the presence of insulation by tapping on the duct and listening for a dull sound.

Check ducts for open joints and air leakage wherever the ducts are exposed. Examine them for dirt build up by removing several room registers and inspecting the duct. Ducts can be cleaned by a heating contractor. If there is a flexible connection between the furnace and the duct work, check it for tears and openings. There should be no openings in return ducts in the same room as a combustion furnace.

Air filters are usually located on the return side of the furnace next to the blower, but they may be found anywhere in the distribution system. Check for their presence and examine their condition.

Supply ducts are often provided with manual dampers to balance air flow in the distribution system. Locate them by looking for small damper handles extending below the ductwork. Check their operation. In zoned systems, automatically controlled dampers may be located in the ductwork, usually near the furnace.


The operation of all dampers should be checked by activating each thermostat, one at a time. If the dampers are working properly, air should begin to circulate in each zone immediately after its thermostat has been activated.

Check the location of supply and return registers in each room. Warm air registers are most effective when positioned low on the exterior wall; cold air registers when located high on the walls or in the ceiling.

Return registers should be on opposite sides of the room from supply registers. If return registers are located in a hallway or a different room, make sure intervening doors are undercut by about one inch.


When the furnace blower is on, check the air flow in all supply and return registers. Remove and inspect registers that appear blocked. Listen for sounds emanating from the ductwork and determine their source.

Humidifiers may be located in the supply ducts. They should not be located in return air ducts because the moist air will pass through the heat exchanger and evaporator coil, rendering the humidification ineffective and corroding the heat exchanger. Check humidifiers.

Editor's note: This story is adapted from the U.S. Department of Housing and Urban Development's Residential Rehabilitation Inspection Guide, 2000.Click here for other stories in this series.

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