Keeping Air Moving
Sizes and shapes may differ, but all forced-air furnaces work essentially like the one illustrated at right. Return air passes through an air filter into a blower compartment. The blower then pushes air into a second compartment, where it’s warmed by a heat source. This source might be electrical heating elements , a gas- or oil-fired heat exchanger, or a heat pump. If you have central air-conditioning, a cooling coil in the supply duct above the heat source extracts heat from the moving air during hot months.
The heated or cooled air then moves into a plenum from which the supply ducts radiate to carry air to the various rooms. Air enters this particular furnace at the bottom and exits at the top. Known as an upflow type, it’s best suited to basement installations. Other types send the heated air downward or to the side.
Replacing a Filter
Blowing air from a furnace stirs up dust and perhaps pollen. A high-quality filter catches nearly all of these particles. A neglected filter chokes airflow and makes the unit work harder, thus wasting energy. A severely clogged filter can cause a furnace to overheat and shut down. Some filters can be cleaned, while others must be replaced. Inexpensive filters trap only larger particles. To keep your air even cleaner, purchase the highest-quality filter available for your furnace. Some types trap some mold and bacteria as well. To effectively remove pollen, mold, and bacteria as well as nearly all dust, have a contractor install an electrostatic filter, which is typically attached near the plenum.
1. Open the door. Most blower doors lift or swing open. On counterflow models, the door will be at the top of the furnace. Purchase an exact replacement filter made for your furnace make and model.
2. Replace the filter. Many filters slide out. Look for dirt on or around the blower, too. Vacuum, if necessary. A hammock-type filter wraps around the base of the blower; it is also easy to replace.
Oiling and Adjusting Blowers
Make blower maintenance part of your seasonal tune-up schedule—and check out the unit whenever air seems to be moving faster or slower than usual. Some blowers have direct-drive motors; others operate with an adjustable V-belt-and-pulley setup (shown at right). You needn’t adjust the direct-drive type, but the V-belt type may need oil; check the manufacturer’s instructions. With either model, look for lubrication ports on the motor and on the blower pulley on belt-drive units, as shown at right. Lubricate at the beginning of each heating and cooling season. An adjustable motor pulley lets you change the speed of a belt-drive model by loosening a setscrew and adjusting the distance between pulley faces.
Oil the ports. If a motor has oil ports, they’ll be at each end of the shaft. Also, check for lubrication points on the blower fan. With an oil can, squirt a few drops of SAE-10 nondetergent oil into these ports.
Adjust the belt. Adjust the belt so it’s loose without slipping; it should depress about 1 inch. Keep mounting bolts tight. Check the belt for fraying, cracks, and signs of wear. Keep a spare belt on hand.
Balancing an Air System
If some rooms in your home are consistently too hot or cold, a simple balancing project may solve the problem. A forced-air system can be balanced by reducing the airflow to a room that is too warm. Warm air can then reach colder areas, typically those farthest from the furnace. Partially closing registers in a hot room will cool the room off, but won’t redirect the air. Instead, adjust dampers in the ductwork. Dampers are controlled by a handle or a locknut arrangement. You may find one at the point where each duct takes off from the furnace plenum. If your system lacks dampers, consider installing them. Wait for a cold day to begin the procedure. If only one or two rooms have airflow problems, you might be tempted to adjust only one or two dampers. However you’ll get better results by tuning the entire system.
Summer and winter settings. If you have central cooling, you may have to rebalance the ducts every summer and winter. Mark seasonal settings on the ducts.
1. Open the dampers. To open a damper, loosen the locknut and turn the handle parallel to the duct. To identify which ducts serve each room, close them one at a time to determine which room isn’t getting air. Label the dampers. Open the registers in all the rooms, and open all the dampers in the ducts.
2. Tape thermometers in rooms. Synchronize several thermometers by laying them together for 30 minutes and then noting any differences. Tape a thermometer on a wall in each room, 3 feet above the floor but not above a supply register.
3. Adjust dampers. Adjust dampers to the rooms, starting with the one where your home’s thermostat is located.
4. Recheck the temperatures. Note any increase in air delivery to other rooms. Recheck the temperatures and continue adjusting dampers until you achieve the balance you desire.