Bryant 355CAV Sequence of Operation, Purge Gas Lines, Adaptive Mode, Fire And Explosion Hazard

Purge Gas Lines

If not previously done, purge the lines after all connections have been made and check for leaks.

!CAUTION

FIRE AND EXPLOSION HAZARD

Failure to follow this caution could result in a fire, explosion, personal injury, or death.

Never purge a gas line into a combustion chamber. Never test for gas leaks with an open flame. Use a commercially available soap solution made specifically for the detection of leaks to check all connections.

Step 3 — Sequence of Operation

!CAUTION

UNIT OPERATION HAZARD

Failure to follow this caution may result in intermittent unit operation.

Furnace control must be grounded for proper operation, or control will lock out. Control is grounded through green/yellow wire routed to gas valve and burner box screw.

Using the schematic diagram (See Fig. 48), follow the sequence of operation through the different modes. Read and follow the wiring diagram very carefully. ! !

NOTE: If a power interruption occurs during a call for heat (W/W1 or W/W1-and-W2), the control will start a 90-second blower-only ON period two seconds after power is restored, if the thermostat is still calling for gas heating. The amber LED light will flash code 12 during the 90-second period, after which the LED will be ON continuous, as long as no faults are detected. After the 90-second period, the furnace will respond to the thermostat normally.

The blower door must be installed for power to be conducted through the blower door interlock switch ILK to the furnace control CPU, transformer TRAN, inducer motor IDM, blower motor BLWM, hotsurface igniter HSI, throttling valve TV, and gas valve GV.

Single-Stage Thermostat and Step-Modulating Heating

(Adaptive Mode)

See Fig. 29 or Fig. 59 for thermostat connections.

NOTE: Low-heat only switch SW1-2 selects the low-heat only operation mode when ON. Medium-heat only switch SW4-2 selects medium-heat only operation mode when ON. If both switches are ON the furnace control will default to medium-heat. If either or both switches are ON the furnace control will operate at two-stages only as referenced in Section 2 below. If both switches are OFF the furnace control will operate in the adaptive heating mode in response to a call for heat. (See Fig. 33.) When the W2 thermostat terminal is energized it will always cause high-heat operation as long as the R to W circuit is closed, regardless of the setting of the low-heat or medium-heat only switches.

This furnace can operate as a step-modulating furnace with a single-stage thermostat because the furnace control CPU includes a programmed adaptive sequence of controlled operation, which selects low-heat, medium-heat, or high-heat operation. This selection is based upon the stored history of the length of previous gas-heating periods of the single-stage thermostat.

The furnace will start up in either medium-, or high-heat. The furnace will operate in low-heat after starting and operating for 1 minute at medium-heat before transitioning to low-heat. The furnace control CPU determines the combined low-heat and medium-heat on-time (from 0 to 16 minutes) which is permitted before switching to high heat.

If the power is interrupted, the stored history is erased and the furnace control CPU will select medium-heat for 1 minute, low-heat for 15 minutes and then switch to high-heat, as long as the thermostat continues to call for heat. Subsequent selection is based on stored history of the thermostat cycle times.

The wall thermostat ”calls for heat”, closing the R to W circuit. The furnace control CPU performs a self-check, verifies the low-heat and medium-heat pressure switch contacts LPS and MPS are open, then de-energizes the HPSR relay to close the NC contact.

1.Inducer Prepurge Period - The furnace control CPU turns on inducer motor IDM and slowly increases the inducer motor speed. After the low-heat pressure switch LPS closes the furnace control CPU continues to increase the inducer motor speed until the medium-heat pressure switch MPS closes. When the medium-heat pressure switch MPS closes, throttling valve TV is energized, inducer motor RPM is noted by the furnace control CPU, and a 25-second prepurge period begins. The RPM is used to evaluate vent system resistance. This evaluation is then used to determine the required RPM necessary to operate the inducer motor during medium-heat prepurge, the first minute of medium-heat mode, and low-heat mode.

NOTE: The heat cycle can start in either high- or medium-heat. If a high-heat cycle is initiated, the furnace control CPU will continue to increase the inducer motor speed after the medium-heat pressure switch MPS closes. When the medium-heat pressure switch closes, throttling valve TV is energized, inducer motor RPM is noted by the furnace control CPU, and a 25-second prepurge period begins. The RPM is used to evaluate vent system resistance. This evaluation is then used to determine the required RPM necessary to operate the inducer motor in high-heat pre-purge, and high-heat mode.

2.Igniter Warm-Up — At the end of the prepurge period, the Hot-Surface Igniter HSI is energized for a 17-second igniter warm-up period.

3.Trial-For-Ignition Sequence — When the igniter warm-up period is completed the main gas valve relay contact GVR closes to energize the gas valve solenoid GV-M. The gas valve solenoid GV-M permits gas flow to the burners where it is ignited. Five seconds after the GVR closes, a 2- second Flame-Proving period begins. The HSI igniter will remain energized until flame is sensed or until the 2-second flame proving period begins.

If the furnace control CPU selects high-heat operation, the high-heat gas valve solenoid GV-HI is energized when the high-heat pressure switch HPS closes.

4.Flame-Proving — When the burner flame is proved at the flame-proving sensor electrode FSE, the furnace control CPU begins the blower-ON delay period and continues to hold the gas valve GV-M open. If the burner flame is not proved within two seconds, the furnace control CPU will close the gas valve GV-M, and the furnace control CPU will repeat the ignition sequence for up to three more Trials-For-Ignition before going to Ignition-Lockout. Lockout will be reset automatically after three hours, by momentarily interrupting 115 vac power to the furnace, or by interrupting 24 vac power at SEC1 or SEC2 to the furnace control CPU (not at W/W1, G, R, etc.).

If flame is proved when flame should not be present, the furnace control CPU will lock out of Gas-Heating mode

355CAV

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