Central Heating and Cooling Systems

A central heating system has a primary heating appliance such as a furnace or boiler located in an out-of-the-way spot such as a basement or garage. It delivers heat throughout the house, either by pumping warmed air through a system of air ducts or sending hot water or steam through pipes to room radiators or convectors. With both forced-air and gravity systems, one (or more) thermostats turn the heating (or cooling) plant off and on, operated either manually or automatically as room temperatures rise and fall.

Homes without central heating normally utilize electric baseboard heaters or, in some cases, in-the-wall or in-floor gas heaters or radiant heat.

Central ducted air systems
In contemporary homes, ducted air systems are the most common type of central heating and cooling. If your home has an air conditioner, heat pump, or furnace, it is a ducted air system. There are two main types: forced-air and gravity.

With a forced-air system, a furnace warms air, an air conditioner cools air, or a heat pump either warms or cools air, then a blower forces the air through the system.

With a gravity furnace, convection currents (caused by the natural tendency of heated air to rise) carry heated air through the system from a furnace that is located on or below the main floor level. Gravity systems do not have blowers, tend to have very large air ducts, and can only deliver warmed air. If your system includes an air conditioner or heat pump, it is a forced-air system.

Radiant heating systems
Radiant heat is a comfortable, even type of heat that is radiated into living spaces by hot water, steam, or electric elements.

With a hydronic radiant system, a central boiler heats water; the resulting hot water or steam then circulates through a system of pipes to room radiators or through circuitous routes of tubing that wind beneath a floor's surface to emit heat.

Electric radiant systems may have electric-resistance baseboard or wall registers or a system of electric cables or foils hidden beneath floors or above ceilings. When an electric current runs through an electric-resistance element, cable, or foil, it generates heat--and that heat is radiated into the room. Because radiant heating systems have very few moving parts, they rarely fail (most failures are caused by circuit overloads).

Cooling and heating/cooling systems
Air conditioners and heat pumps are also forced-air systems. With these, cooled (and sometimes humidified or electronically cleaned) air is usually delivered through the same ductwork and registers used by heated air.

An air conditioner runs on electricity and removes heat from air with basic refrigeration principles. A heat pump can provide both heating and cooling. In the winter, a heat pump extracts heat from outside air and delivers it indoors. On hot summer days, it works in reverse, extracting heat from room air and pumping it outdoors to cool the house.

Like air conditioners, nearly all heat pumps are powered by electricity. They have an outdoor compressor/ condenser unit that is connected with refrigerant-filled tubing to an indoor air handler. As the refrigerant moves through the system, it completes a basic refrigeration cycle, warming or cooling the coils inside the air handler. The blower pulls in room air and circulates it across the coils and pushes the air through ductwork back into rooms. When extra heat is needed on particularly cold days, supplemental electric-resistance elements kick on inside the air-handler to add warmth to the air that's passing through