Getting to Know Your HVAC System
How’s the climate in your home? Whether you feel too hot, too cold, or just right depends on the temperature and humidity of the air around you. Your home’s air circulation system—whether it be forced-air, piped, or radiant— adds or removes heat. It may also control the humidity level by adding or removing moisture. This chapter shows how heating and cooling systems work, how you can maintain and improve their efficiency, and what your choices are in new equipment. It also offers tips about what you can do to lower your expenses for gas, oil, or electricity.
As a homeowner, you should have a basic understanding of your system. Leafing through this chapter will help; however, because system technology varies, there is no substitute for consulting a pro. These pages identify and explain the three most common HVAC (heating, venting, and air-conditioning) systems. You may also have a heat pump or central air-conditioning. Your heating plant may be a furnace (which heats air) or a boiler (which produces hot water or steam). Either is usually fired by gas or oil, but wood is also still used. Or you may have smaller, room units.
Though heating and cooling components vary enormously, all take advantage of heat’s inherent tendency to move from a warmer object or space to a cooler one. This means you can couple a heat source, such as a burner, electric resistance element, or heat pump, with a heating plant, usually a furnace or boiler, to send heated air, water, or steam via a distribution network to a home’s various rooms. After the medium gives off its heat, it then recirculates to the heating plant. A control unit, almost always a thermostat, maintains a preset temperature by switching the heating plant on and off.
Ducted systems circulate air from a furnace to registers in each room via a network of ducts. Modern forced-air versions have a blower that pushes treated air through supply ducts and pulls it back through return ducts. An older home may have a less-efficient gravity system, which works the same way but without a blower. Warm air rises from the furnace to the rooms, and cool air falls through the returns. Supply and return registers may be on the floors or in the walls. Blower-driven forced-air heating systems have two advantages over steam and radiant systems: They can be easily adapted to centrally cool a home, and the system can be easily adapted to humidify the home. Troubleshooting and maintenance tips for air furnace systems. To add to or modify a forced-air system.
How the air moves. In a typical forced-air system, a furnace, usually fired by gas or oil, heats the air inside. A blower, powered by an electric motor, sends air out of the furnace and into the main duct, called the plenum. Smaller supply ducts tap into the plenum and carry warm air to individual rooms via supply registers, which usually can be controlled by simple levers. A separate system of return ducts carries cool air back to the furnace.
Hot-Water or Steam Heat
In a piped system, water or steam distributes the heat. The heating plant, called a boiler, sends hot water or steam through supply lines to radiators in each room of the house. In a hot-water system, warm water circulates up to the radiators, often (but not always) helped by a circulating pump. Cooler water flows back to the boiler via return lines. Heat rises and falls slowly, so the radiators maintain fairly even temperatures. Most steam systems have only one pipe running to each radiator. Because steam rises, no circulation pump is needed. Steam radiators are usually either very hot or completely cold.
How the water circulates. A hot-water system is “closed,” meaning that the same water keeps circulating through the pipes, often without being changed for years. A steam boiler must have water added regularly, because steam dissipates into the rooms during heating.
Radiant heating warms the floor or ceiling in a room, silently providing even heat. Some systems use electrical cables; others circulate hot water through small-diameter tubing. In most areas, electric radiant heat is expensive, and usually used only for added-on powder rooms and other small spaces. Older hot-water radiant heat often consists of copper pipes run through concrete floors. This type tends to corrode and often needs to be completely replaced after a number of decades. Newer tubing is made of extremely durable polystyrene and can be attached under a wood floor or run under ceramic tiles. If you have radiant heat, do not bore holes or break into the floor unless you are certain where the lines run.
Maintaining radiant heat. In a radiant system, pipes or tubes usually run through or under the floor, so the heating takes up no wall or floor space in a room. The heat is wonderfully even and gentle; many people love the feel of a warm floor in the winter. Radiant heat is easiest to install in the ceiling, but because this is less efficient, it should be done only in areas with mild winters. Because the tubing or cables are buried, fixable problems are either at the electrical circuit panel or at the connections to the boiler.
Maintaining a Forced-Air System
A forced-air heating/cooling system completely recirculates the air in your home as many as three times every hour—a big job. Fortunately most components of a forced-air system are easy to take care of—once you know what they are and where they’re located. Begin by noting which registers supply treated hot air and which ones return cold air to the furnace. Most rooms have at least one hot-air supply register. Supply registers usually have adjustable dampers that let you modulate the airflow or shut it off entirely.
Cold-air return registers typically are larger and less numerous. Many homes have only one per floor, located in a hallway or other central spot. Returns never have dampers, because shutting one off would partially suffocate the system. Make sure registers aren’t blocked by furniture, draperies, or carpeting. Supply registers must have unobstructed space above. You may choose to equip some registers with plastic diverters that channel air horizontally for more even heating distribution. Return registers must be open to air currents from all directions.
To trace the entire duct network, take a trip to your crawlspace or basement. The drawing below shows how ducts eventually connect to the furnace and how air moves within it. While the blower is operating, feel the joints of all accessible ducts. Any leaks will decrease your system’s efficiency. Seal joints using professional-quality duct tape, which remains durable even when exposed to heat. If a joint is widely parted, drill pilot holes and drive sheetmetal screws before taping. In an older home, the ducts may be clogged with debris, or may contain mold or mildew that disperses into the air when the blower operates. Have a heating pro check for these possibilities.