Once air is warmed or cooled at the heat/cold source, it must be distributed to the various rooms of your home. This can be accomplished with the forced-air or radiant systems explained below.

A forced-air system distributes the heat produced by the furnace or the coolness produced by a central air conditioner through an electrically powered fan, called a blower, which forces the air through a system of metal ducts to the rooms in your home. As the warm air from the furnace flows into the rooms, colder air in the rooms flows down through another set of ducts, called the cold air return system, to the furnace to be warmed. This system is adjustable: You can increase or decrease the amount of air flowing through your home. Central air conditioning systems use the same forced-air system, including the blower, to distribute cool air to the rooms and to bring warmer air back to be cooled.

Problems with forced-air systems usually involve blower malfunctions. The blower may also be noisy, and it adds the cost of electrical power to the cost of furnace fuel. But because it employs a blower, a forced-air system is an effective way to channel airborne heat or cool air throughout a house.

Radiant Systems
Radiant systems function by warming the walls, floors, or ceilings of rooms or, more commonly, by warming radiators in the rooms. These objects then warm the air in the room. Some systems use electric heating panels to generate heat, which is radiated into rooms. Like gravity wall heaters, these panels are usually installed in warm climates or where electricity is relatively inexpensive. Radiant systems cannot be used to distribute cool air from an air conditioner.

Radiators and convectors, the most common means of radiant heat distribution in older homes, are used with hot water heating systems. These systems may depend on gravity or on a circulator pump to circulate heated water from the boiler to the radiators or convectors. A system that uses a pump, or circulator, is called a hydronic system.

Modern radiant heating systems are often built into houses constructed on a concrete slab foundation. A network of hot water pipes is laid under the surface of the concrete slab. When the concrete is warmed by the pipes, it warms the air that contacts the floor surface. The slab need not get very hot; it will eventually contact and heat the air throughout the house.

Controls for Heating and Cooling Systems
The thermostat, a heat-sensitive switch, is the basic control that regulates the temperature of your home.

It responds to changes in the temperature of the air where it is located and turns the furnace or air conditioner on or off as needed to maintain the temperature at a set level, called the set point. The key component of the thermostat is a bimetallic element that expands or contracts as the temperature increases or decreases in a house.

Older thermostats have two exposed contacts. As the temperature drops, a bimetallic strip bends, making first one electrical contact and then another. The system is fully activated when the second contact closes, turning on the heating system and the anticipator on the thermostat. The anticipator heats the bimetallic element, causing it to bend and break the second electrical contact. The first contact is not yet broken, however, and the heater keeps running until the temperature rises above the setting on the thermostat.

More modern thermostats have coiled bimetallic strip elements, and the contacts are sealed behind glass to protect them from dirt. As the temperature drops, the bimetallic elements start to uncoil. The force exerted by the uncoiling of the elements separates a stationary steel bar from a magnet at the end of the coil. The magnet comes down close to the glass-enclosed contact, pulls up on the contact arm inside the tube, and causes the contacts to close, completing the electrical circuit and turning on the heater and the anticipator. As the air in the room heats up, the coil starts to rewind and breaks the hold of the magnet on the contact arm. The arm drops, breaks the circuit, and turns off the system. As this point, the magnet moves back up to the stationary bar, keeping the contacts open and the heater turned off until the room cools down again.

The latest heat and air-conditioning controls use solid-state electronics for controlling the air temperature. They are typically more accurate and more responsive than older systems. However, repair to solid-state controls usually means replacement.

Understanding how the heating and cooling systems function in your home will help you head off problems before they become too serious.

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