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A.  Maintaining a Forced-air System
B.  Replacing the Air Filter
C.  Maintaining the Blower Motor
D.  About A Gas Pilot

A.  General Information
        The most common method of home heating is the use of air warmed by a furnace and forced through ducts that carry it to the rooms in the house. This method is known as forced air heat. Forced air is, by far, the most common form of home heat, because it uses natural gas or liquid propane, which are generally more affordable than other fuels. Gas forced-air systems are widely used in cool climates worldwide.
Heater Diagram          A gas forced-air furnace - running on natural gas or liquid propane (LP) - draws in surrounding air, channels it across a set of heated plates, known as a heat exchanger, and then uses a blower to circulate the air throughout the house.

A chamber on top of the furnace, known as a plenum, leads the warmed air from the furnace to a network of ducts that carry the warm air to heat registers or vents mounted on walls or ceilings. To keep the cycle going, return ducts carry cooled air from each room back to the furnace so it can be reheated and recirculated. Older systems use gravity to carry warm air throughout the house and cool air back to the furnace. Advances in home design have required some changes in today's forced-air systems. Conventional forced-air heat operates by recycling indoor air. In drafty older homes, this worked well since fresh air trickled in from outdoors. Problems arise in newer, superinsulated homes, where air contaminants can be constantly recirculated, causing respiratory ailments and other health problems. Many Building Codes now require a fresh air intake in new construction to reduce such hazards. Some homes use a heat recovery ventilator, which improves air quality without significant heat loss by drawing prewarmed outdoor air into the system. Builders have also begun installing high-velocity (HV) forced-air systems. These systems increase living space by using small-diameter tubes that require far less space in ceilings and walls than sheet-metal ducts.

A. Maintaining a Forced-air System
You can handle most routine furnace maintenance yourself. Generally, the newer the furnace, the simpler the maintenance, since a number of heavy-maintenance components have been eliminated on newer models. Most furnaces installed since the 1980s do not have a thermocouple-controlled pilot light. In fact, the standing pilot light found on older furnaces has been eliminated completely. In most cases, it's been replaced with either an intermittent pilot light that's lit only when there's a call for heat from the thermostat, or a glowing element, known as a hot-surface igniter. An intermittent pilot light must be repaired by a professional technician, should it fail. You can replace a hot-surface igniter yourself. Before doing any maintenance, always turn off the furnace's main gas supply and the pilot gas supply, if your furnace has a separate one. Then, switch off the furnace's main power switch and the power to the furnace at the main service panel. Check your owner's manual for any warnings or special instructions concerning your furnace. Then, clear the area, so you have a safe workspace. Start with the most important and simplest furnace maintenance procedure - inspecting the air filter. There are many types of filters. Read the section below to find out how to clean yours and how often it must be changed.

B. Replacing the Air Filter
The air filter on your forced-air furnace is designed to capture dust, pollen, and other airborne particles that would otherwise recirculate whenever the furnace blower is on. The filter must be cleaned regularly, according to the manufacturer's specifications, and should be inspected once a month. Remove cover Replace air-filter Locate the filter compartment and remove the access cover. The location of the compartment depends on the furnace type and the style of filter. Many filters fit in a slot between the return air duct and blower. A few styles are located inside the main furnace compartment. An electrostatic filter is installed in a separate unit attached to the furnace. Slide the filter out of its compartment taking care not to catch it on the blower housing. Hold the filter up to a light. If the filter blocks much of the light, replace it. Electrostatic filters can be reused after cleaning. Always read the manufacturer's instructions for your filter.

C. Maintaining the Blower Motor
Inspect the blower motor before the start of the heating season. Inspect it again before the start of the cooling season if your central air conditioning uses the same blower. Turn off the power to the furnace. Remove the access panel to the blower housing and inspect the motor. Some motors have oil ports and an adjustable, replaceable drive belt. Others are self-lubricating and have a direct-drive mechanism. Wipe the motor clean with a damp cloth and check for oil fill ports. The access panel may include a diagram indicating their location. Remove the covers to the ports (if equipped) and add a few drops of light machine oil. Place the covers on the ports. With the power still off, inspect the drive belt. If it is cracked, worn, glazed, or brittle, replace it. Check the belt tension by pushing down gently midway between the motor pulley and blower pulley. The belt should flex about an inch. Remove Access Panel Light Machine Oil Check Belt Tension
To tighten or loosen the belt, locate the pulley tension adjustment nut on the blower motor. Loosen the locknut, and turn the adjustment nut slightly. Check the belt tension, and readjust as required until the tension is correct. If the belt is out of alignment or the bearings are worn, adjusting the belt tension will not solve the problem. With the power off, hold a straightedge so it's flush with the edge of both pulleys. To align the belt, locate the motor mounting bolts on the motor's sliding bracket. Loosen the bolts, and move the motor carefully until the pulleys are aligned. Tighten the bolts and check the belt tension and alignment again. Repeat until the pulley is aligned and the tension is adjusted. Replace the furnace access panels that have been removed. Restore power at the main service panel and switch on the furnace.
Locate Mounting Bolts Pulleys Align Replace Access Panels

D. About A Gas Pilot
Every part of the heating system depends on a correct pilot light setting. The pilot light (it's actually a flame used to ignite gas flowing through the burners) plays a large role in the efficiency of the entire system, and a clean-burning pilot saves money, improves indoor air quality, and extends furnace life. If your furnace has a standing pilot light, always check the flame before the start of the heating season to ensure that it's burning cleanly and with the proper mix of air and fuel. Start by removing the main furnace access panel. If you can't see the pilot flame clearly, turn off the gas supply and the pilot gas shutoff switch (if equipped). Wait 10 minutes for the pilot to cool, and remove the pilot cover. Relight the pilot, following the instructions on the control housing or access cover. If the pilot won't stay lit, shut off the gas supply once again and inspect the thermocouple

Pilot Light Once the pilot is burning, inspect the flame. If the flame is too weak (left flame), it will be blue and may barely touch the thermocouple. If the flame is too strong (center flame), it will also be blue, but may be noisy and lift off the pilot. A well-adjusted flame (right flame) will be blue with a yellow tip, and cover ½" at the end of the thermocouple.

Flames Turn the pilot adjustment screw on the control housing or gas valve to reduce the gas pressure. If it's weak, turn the screw in the other direction to increase the gas pressure. If the flame appears weak and yellow even after adjustment, remove the pilot jet and clean the orifice. The thermocouple creates an electrical charge from the heat of the pilot flame. If the pilot in your furnace or boiler goes out quickly, and you have made sure the gas supply is sufficient, you may need to replace the thermocouple. Turn off the gas supply. Using an open-end wrench, loosen the thermocouple tube fitting from the control housing or gas valve. Unscrew the thermocouple from the pilot housing and install a new one. Tighten it with a wrench just until it's snug.