York Gas Furnaces: PUDD

Re: Heating Sequence of Operation

These furnaces use either a Robertshaw SP735 or White-Rodgers 50E47 hot-surface ignition controller.

Sequence of Operation

1. Call for Heat

• On a call for heat, the thermostat sends 24V to the "W" terminal.

2. Inducer Motor

• 24V power passes through the N/C contacts of the 1LP (3-wire) pressure switch to energize that Heat Relay.
• The heat relay closes to send 120V power to the inducer motor. (A stuck-closed 1LP switch will keep the inducer from starting.)
• 24V power passes through the secondary pressure switch(es) (2LP, 3LP if present) and safeties to the ignition control.

3. Ignition

• Once 24VAC is received on "TH" and "TR (Common)", the control will start an ignition sequence by powering the on-board igniter relay (which breaks one leg of 120V to the glow plug).
• After 45 - 60 seconds of pre-heating the glow plug, the board powers the main gas valve "MV".
• The board will then have 6 seconds to receive a flame signal.
• The gas valve will stay energized until 24VAC is removed from "TH" or flame signal is lost.

4. Blower Motor

• The Fan Relay passes 120V through its N/C contacts to the Fan & Limit control. (On a call for "G", the Fan Relay N/O contacts close, sending power directly to the blower motor.)
• Once the heat chamber sufficiently heats, the N/O contacts of the fan switch close, sending 120V power to start the blower motor.

Diagnostic Light: Light On Continuous = Replace Controller, Flashing - Ignition Lockout