Heating and Cooling System Basics

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Heating and Cooling System Basics

by Walter Curtis

How Heating and Cooling Systems Work

Most of us take heating and cooling for granted. We expect our heating systems to keep us warm during the winter, and we depend on air-conditioning to keep us cool during the summer.

When the house is cold in winter or hot in summer, the natural reaction is to call for professional service. Fortunately, there is an alternative. You can cut service costs drastically and keep your heating and cooling systems working efficiently by doing some maintenance and quick fixes yourself. But first, it's important to know how the basics of how heating and cooling systems function.

How Heating and Cooling Systems Work

All climate-control devices or systems have three basic components: a source of warmed or cooled air, a means of distributing the air to the rooms being heated or cooled, and a control used to regulate the system (e.g., thermostat). The sources of warm air, such as a furnace, and cool air, such as an air conditioner, in a house often use the same distribution and control systems. If your house has central air conditioning, cool air probably flows through the same ducts that heat does and is regulated by the same thermostat. When a heating or cooling system malfunctions, any of these three basic components may be causing the problem.

Both heating and air conditioning work on the principle that heat always moves from a warm object to a cooler one, just as water flows from a higher to a lower level. Furnaces and heaters put heat into the air to make your home warmer; air conditioners remove heat to make your home cooler.

All heating and cooling units burn fuel. Air conditioners use electricity. Most home heating systems use gas or fuel oil; other systems use electricity. The heat pump -- an electrically powered climate control unit -- both heats and cools air. In summer it extracts heat from the air inside your home. In winter it pulls heat from the air outside and uses this heat to warm the air inside.

When the furnace is turned on, it consumes the fuel that powers it, whether it be gas, oil, or electricity. As fuel is burned, heat is produced and channeled to the living areas of your home through ducts, pipes, or wires and then is blown out of registers, radiators, or heating panels. Older systems use the heat they produce to heat water, which in turn heats the air in your home. These systems use a boiler to store and heat the water supply, which is then circulated as hot water through pipes embedded in the wall, floor, or ceiling.

When an air conditioner is turned on, electrical power is used to cool a gas in a coil to its liquid state. Warm air in your home is cooled by contact with the cooling coil, and this cooled air is channeled to the rooms of your home through ducts and out registers or -- in the case of room air conditioners -- directly from the unit itself.

In the next section, we'll review the different distribution systems used for heating and cooling the home.
Heating and Cooling Distribution Systems

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, gravity, or radiant systems explained below.

Forced-Air Systems

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.

Forced-air heating systems use a fan to move warm air.

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.

Gravity Systems

Gravity systems are based on the principle that hot air rises and cold air sinks. Gravity systems, therefore, cannot be used to distribute cool air from an air conditioner. In a gravity system, the furnace is located near or below the floor. The warmed air rises and flows through ducts to registers in the floor throughout the house. If the furnace is located on the main floor of the house, the heat registers are usually positioned high on the walls because the registers must always be higher than the furnace. The warmed air rises toward the ceiling. As the air cools, it sinks, enters the return air ducts, and flows back to the furnace to be reheated.

Gravity systems rely on warm air rising naturally.

Another basic distribution system for heating is the radiant system. The heat source is usually hot water, which is heated by the furnace and circulated through pipes embedded in the wall, floor, or ceiling.

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.

Radiant heating systems function by warming walls, floors,
or ceilings, which then warm the surrounding air.

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.

Radiant systems -- especially when they depend on gravity -- are prone to several problems. The pipes used to distribute the heated water can become clogged with mineral deposits or become slanted at the wrong angle. The boiler in which water is heated at the heat source may also malfunction. Hot water systems are seldom installed in new homes.

In the next section, learn how the thermostat and other controls are used to maintain the indoor climate created by your heating and cooling systems.
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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