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YM9M - Sequence of Operation (Heating)

Article ID: 647
Last updated: 13 Nov, 2020

York Gas Furnace: YM9M / PM9 / YP96 / PC9

Re: Sequence of Operation (Heating)

There are two styles of York Modulating Gas Furnaces, a Variable Speed Blower model and a standard PSC motor.

Both styles have the same basic sequence of operation in the heating mode


Pressure Switch Proving

  • On a call for heat (24V applied to the W terminal), unless the pressure switch contacts are already in the "closed" position*, the control board will start the inducer fan motor and ramp its speed up until the pressure switch contacts "close".
    • If the pressure switch contacts are "closed" on a call for heat, the control board will keep the inducer motor de-energized and display a Fault Code 2.
  • Once the pressure switch contacts "close", the control board will slow the inducer fan to make sure that the pressure switch contacts "open".
  • After the switch contacts "open", the control board will increase the inducer motor speed to re "close" the switch.
    • The control board uses a pressure transducer to continuously monitor the actual negative pressure from the inducer and the proper make and break points of the pressure switch.
    • If the switch fails to "close" or "open" at the correct points, the board will enter either a Fault Code 2 or 3
  • ​​​​​​​After the 2nd "closure" of the pressure switch contacts, the control board will ramp the inducer motor to 1.0" w.c. negative pressure and start an ignition sequence.

​​​​​​​Ignition Sequence

  • The glow plug will energize for approximately 30 seconds.
  • The gas valve will open at 70% capacity (2 - 2.5" w.c. manifold gas pressure) and attempt to establish a flame (monitored by the flame sensor).
    • If flame is not established, the control board will make two more ignition attempts before locking out on a Fault Code 7.
  • One flame is established, the furnace will continue to fire at 70% for approximately 5 seconds and then drop to 35% capacity (1 - 1.3" w.c. manifold pressure).

Heating Cycle - Gas Valve

  • After a successful Ignition Sequence, the gas valve modulates down to 35% capacity.
  • The gas valve firing percentage will slowly increase to 100% based on a fixed, pre-programmed time algorithm until the thermostat is satisfied (no power on "W").

​​​​​​​Heating Cycle - Inducer Motor

  • The inducer motor speed will modulate to maintain a fixed, pre-programmed negative pressure which varies based on the furnace firing rate.
    • An increase in pressure (more negative pressure) will cause the motor to slow-down.
    • A decrease in pressure (less negative pressure) will cause the motor to speed-up.

H​​eating Cycle - Blower Speed

  • The blower speed is controlled by a temperature sensor in the top of the heat exchanger.
  • The blower speed will increase and decrease to maintain a constant heat exchanger (discharge air) temperature.
    • An increase in the heat exchanger temperature will cause the blower to speed-up.
    • A decrease in the heat exchanger temperature will cause the blower to slow-down.
  • ​​​​​​​The discharge air temperature can be raised or lowered by adjusting a dial or pin setting on the control board.

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Article ID: 647
Last updated: 13 Nov, 2020
Revision: 4
Access: Public
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