How to Replace a Lennox Defrost Board

Turn off the furnace's circuit breaker. The furnace's circuit breaker, located in the building's electrical service panel, has a "Furnace" or "Air handler" identification label. The furnace transformer produces the 24-volt power source that operates the thermostat. When activated, the thermostat sends the 24-volt signal to the defrost board's 24-volt terminal strip.

Turn off the outside heat-pump air-conditioning unit's disconnect box, the electrical service box that the unit's flexible conduit connects to. If the disconnect box uses a pull-out buss bar, pull the buss bar out of the box and set the buss bar on top of the box. If the disconnect box uses a circuit breaker-style switch, flip the switch to the "Off" position.

Remove the hex-head screws holding the Lennox heat-pump unit's electrical compartment cover in place, using a 5/16-inch nut driver to turn the screws counterclockwise. Pull the cover down until its top edge clears the unit's fan shroud lip, then set the cover aside. The electrical cover, the curved cover above the refrigeration pipes, shields the unit's electric control components.

Locate the defrost board, which mounts on the electrical compartment's back wall. The square, black-colored terminal with four pins, located near the center of the defrost board, is the timer setting terminal. Notice which pin -- labeled "T1," "T2" or "T3" -- the jumper wire connects to. Technicians use the pin labeled "Test" to diagnose the system.

Wrap masking tape around each wire that connects to the defrost board. Write each wire's terminal identification label, the letters printed on the defrost board next to the wire terminals, onto its respective masking tape tag.

Loosen the screws on the defrost board's 24-volt terminal strip, using a Phillips screwdriver to turn the screws one full turn counterclockwise. The 24-volt terminal strip mounts on the bottom, right-hand corner of the defrost board. Slip each wire out of its respective wire terminal.

Pull the push-on wire connectors from their respective defrost board wire terminals, using needle-nose pliers for leverage.

Remove the four hex-head screws holding the defrost board against the electrical compartment's back wall, using the 5/16-inch nut driver to turn the screws counterclockwise. Save the screws for the new defrost board. Pull the old defrost board out of the Lennox heat pump unit.

Hold the new defrost board against the electrical compartment's back wall, keeping the end of the defrost board with the terminals labeled "Fan" facing up. Align the defrost board's screw holes with the electrical compartment's screw holes and mount the defrost board with the saved screws, using the nut driver to twist the screws in a clockwise direction.

Slip the ends of the wires with the "C," "R," "W1," "O" and "Y1" masking tape labels into their respective slots in the 24-volt terminal strip, using the letters printed on the defrost board as a guide. Tighten the terminal screws with the Phillips screwdriver.

Slip the push-on wire connectors onto their respective defrost board wire terminals, using the masking tape labels and the letters printed on the defrost board as a guide. Use the needle-nose pliers for leverage, if needed. Usually, the wire connectors slide onto the terminals with little resistance.

Compare the jumper wire's location on the new defrost board's timer setting terminal with the jumper wire's location on the old defrost timer setting terminal. Pull the jumper wire off the incorrect terminal pin and slide it onto the correct terminal pin, if needed.

Slide the top of the Lennox heat-pump unit's electrical compartment cover under the fan shroud's lip. Slip the cover's hex-head screws into the cover's screw holes and tighten the screws with the nut driver.

Turn on the Lennox heat-pump unit's disconnect box. If the disconnect box uses a circuit breaker-style switch, flip the switch to the "On" position. If the disconnect box uses a pull-out buss bar, align the buss bar's tab with the slot on the disconnect box and push the buss bar in.

Turn on the furnace's circuit breaker.