Maintaining the fan motor

OILING THE MOTOR. Not all furnaces need to be oiled; many have permanent bearings that are sealed at the factory. To find out whether your blower needs oiling, remove the access panel and look at the blower motor. Those that need oiling will have small holes at either end of the motor. Put a couple of drops of household oil (usually 10W non-detergent] through the holes.

TIGHTENING THE FAN BELT. If the fan runs off a belt (not all of them do), push down on the belt at a point midway between the motor and the fan. The belt should flex about 1 inch. If it doesn't, loosen the locknut on the bracket that holds the motor and turn the second nut to tighten or loosen the belt. Retighten the locknut once the belt is properly tensioned. If the adjustment bolt is frozen, apply penetrating oil until it turns easily. Replace cracked or stretched belts, and apply belt dressing for longer belt life.

ALIGNING THE MOTOR AND FAN PULLEYS. For the quietest and most efficient operation, the pulleys on the motor and on the fan should be in line with each other. To check the alignment, put a straightedge, such as a level or framing square, against the sides of both pulleys. A triangular space between one of the pulleys and the straightedge means they aren't aligned. To correct this, loosen the nuts or bolts that hold the motor bracket to the mounting bar. Once loosened, the motor should slide along the bar. Position the motor so that the pulleys are aligned and retighten the nuts or bolts.

Tracking fan or motor problems

If the blower fan pumps no air into the vents, you have at least two problems. The first is that you’re cold. The second is a bit harder to track down. In order to figure out where the problem is, trace the circuit from the thermostat through the other controls and finally to the motor. Do what the pros do—check the obvious first.

THE THERMOSTAT. The thermostat hanging on your wall is nothing more than a big switch. When the temperature falls below the point at which you’ve set the thermostat, its switch flips and the furnace comes on. When the temperature rises to the set point, the switch flips the other way and the furnace turns off. Verify that the thermostat is set to On and is turned up, and that the wires are properly connected. If the thermostat has batteries check them as well.

THE BLOWER RELAY. With the help of the blower relay, the 24-volt thermostat can start and stop the 110-volt blower.

THE FAN-AND-LIMIT CONTROL. The fan-and-limit control senses when the firebox is hot enough to send air through the ducts and into the house. Even if the thermostat and blower relay say it’s time for the blower to kick on, nothing happens until the fan-and-limit control says the furnace is hot enough. When the thermostat and blower relay are ready to turn off the blower, the fan-and-limit control lets that happen only when most of the residual heat has been pumped into the house. If the fan were to turn on at the same moment the thermostat started the flame in the box, the air it blew would still be reasonably cold. And if the fan shut off the minute the flame stopped, a lot of hot air would remain in the furnace, doing no one any good.

Diagnosing fan and motor problems

CHECK THE SAFETY SWITCH AT THE FURNACE. After you've checked the thermostat upstairs, head for the furnace. Make sure the access panels are in place and that they have depressed the pushbutton safety switch. Remove the panels and push the safety button down. If the furnace fires but the fan doesn't, test the thermostat. If the furnace doesn't fire, see the sections on oil furnaces and gas burners for more information.

TEST THE THERMOSTAT. Find the blower relay, one side of which is connected to the thermostat. Set a multi-meter to the 50 volts AC range. Put one of the leads on the terminal labeled G—this is one of the blower wires. Put the other on C, which is one of the transformer terminals. Set the thermostat as high as it will go. The furnace should light, the meter should read about 24 volts, and the blower should start up in a few seconds. If not, replace the thermostat.

TEST THE RELAY. Turn the power to the furnace off at the circuit breaker box. Set a multi-meter to the ohmmeter setting. Put the leads on the relay's G and C terminals again. The readout should be 0 when you touch them and 1 when you separate them. If not, the relay is faulty. Label the wires so that you'll know where each goes, then remove and replace the relay.

Testing The Fan-And-Limit Control

1 T0 TEST THE BLOWER, Look to see whether the fan-and-limit control has a button or switch on it labeled something like "summer/winter" or "auto/manual." Push or flip the switch to the summer or manual position. Restore power to the furnace. If the blower is working, it will turn on immediately, indicating that the fan-and-limit control needs to be replaced. If the motor starts, the problem is with the motor or its capacitor.

2 IF THE RELAY AND THERMOSTAT ARE FINE BUT THE MOTOR WON’T START, test the fan-and-limit control. The control is often enclosed in a small metal box. Turn the power off at the circuit breaker or fuse box and remove the fan-and-limit control cover.

3 IF THERE IS NO SWITCH, test the motor by changing the control's settings. As you look at the dial, you'll notice either two or three sliding tabs and a numbered scale. Hold the dial firmly and move the two tabs at the lowest setting as far clockwise as you can. Restore power to the furnace. If the motor is working, it will turn on immediately, indicating that the fan-and-limit control needs to be replaced. You’ll also need to replace the fan-and-limit switch.

Replacing The Fan-And-Limit Control

1 IF YOUR TESTS HAVE SHOWN THAT THE FAN-AND-LIMIT CONTROL IS BAD, YOU WILL NEED TO REPLACE IT. Label the wiring. Put a piece of tape next to each terminal, identifying it. Put a piece of tape on each wire identifying where it came from. Then remove the wires from their terminals.

2 WITH A PAIR OF PLIERS, GRAB ONE OF THE GROMMETS SURROUNDING THE WIRING THAT COMES INTO THE CONTROL. Squeeze the grommet and slide it out of the bottom of the control. If there is a second set of wires, remove it in the same way.

3 REMOVE THE SCREWS ATTACHING THE CONTROL TO THE FURNACE. A sensor tube extends into the firebox. Pull the control out until the tube clears. Take the control with you and get an identical replacement. You may have to reset the sliding tabs to match the settings on the old control. Don't move the dial. If there is a third tab on the dial, do not reset it.

Testing The Capacitor

1 IF YOUR TESTS UP TO NOW SHOW THAT THE CONTROLS ARE WORKING, even though the motor isn't, either the motor or the capacitor that helps start it is defective. The first step in checking them is turning off the power at the circuit breaker box and at the furnace itself. Remove the motor access panel and set it aside. Locate the capacitor, usually located above the motor itself.

2 DISCHARGE THE CAPACITOR. The blower motor has a capacitor to help it start. It charges up like a battery and discharges instantaneously to give the motor unit extra starting power. Until discharged, capacitors can give you a dangerous shock.

3 TEST THE CAPACITOR WITH THE OHMMETERONYOUR MULTIMETER. Resistance should drop as the battery in your meter charges the capacitor. On a meter with a needle, the needle should quickly rise as the capacitor discharges, and then fall. On a digital meter the initial reading should be one, then it should fall to zero and rise back to one. If no change occurs, replace the capacitor.

DISCHARGING CAPACITORS SAFELY - Capacitors store an electrical charge to boost the power to an electric motor, allowing it to achieve full torque immediately. The electrical charge remains in the capacitor until it's discharged, so even though the power is turned off the capacitor is still live. Obviously you could be shocked, so make sure the capacitor is discharged before you continue working on the furnace. Make a discharger from parts available at an electronic supply store: a 20,000-ohm, 5-watt resistor and two leads with alligator clips on both ends.


1 Turn off the main power at the breaker panel.

2 Clip the two leads to the resistor, as shown.

3 Carefully clip the other ends of the leads to the terminals on the capacitor and wait about 15 seconds while it discharges.

4 Test the capacitor with an ohmmeter by placing a probe on each terminal. Resistance should drop as the battery in the meter charges the capacitor.

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