Thermidistat Mode

The wall thermostat ”calls for heat”, closing the R to W1 circuit for medium-heat or closing the R to W1-and-W2 circuits for high-heat. The furnace control performs a self-check, and 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.

The start up and shut down functions and delays described above apply to the 2-stage medium/high heating mode as well, except for switching from high- to medium-heat.

1.Switching from High- to Medium-Heat — If the thermostat R to W2 circuit opens, and the R to W1 circuit remains closed, the furnace control CPU will gradually decrease the inducer motor speed to the required medium-heat RPM. When the inducer motor IDM reduces pressure sufficiently, the highheat pressure switch HPS will open and the high-heat gas valve solenoid GV-HI will be de-energized. The gas valve solenoid GV-M will remain energized as long as the low-heat pressure switch LPS remains closed. When the inducer motor speed gets within 15% of the required medium-heat RPM the furnace control CPU will start a 5 second blower airflow change delay. After the 5 second blower airflow change delay is completed the blower airflow will transition to medium-heat airflow.

Cooling Mode

The thermostat “calls for cooling.”

2.Single-Speed Cooling

See Fig. 29 for thermostat connections.

The thermostat closes the R to G-and-Y circuits. The R to Y circuit starts the outdoor unit, and the R to G-and-Y/Y2 circuits start the furnace blower motor BLWM on cooling airflow. Cooling airflow is based on the A/C selection shown in Fig. 47.

The electronic air cleaner terminal EAC-1 is energized with 115 vac when the blower motor BLWM is operating. When the thermostat is satisfied, the R to G-and-Y circuits are opened. The outdoor unit will stop, and the furnace blower motor BLWM will continue operating at cooling airflow for an additional 90 seconds. Jumper Y/Y2 to DHUM to reduce the cooling off-delay to 5 seconds. (See Fig. 33.)

3.Single-Stage Thermostat and Two-Speed Cooling (Adaptive Mode)

See Fig. 59 for thermostat connections.

This furnace can operate a two-speed cooling unit with a single-stage thermostat because the furnace control CPU includes a programmed adaptive sequence of controlled operation, which selects low-cooling or high-cooling operation. This selection is based upon the stored history of the length of previous cooling period of the single-stage thermostat.

NOTE: The air conditioning relay disable jumper ACRDJ must be connected to enable the adaptive cooling mode in response to a call for cooling. (See Fig. 33.) When in place the furnace control CPU can turn on the air conditioning relay ACR to energize the Y/Y2 terminal and switch the outdoor unit to high-cooling.

The furnace control CPU can start up the cooling unit in either low- or high-cooling. If starting up in low-cooling, the furnace control CPU determines the low-cooling on-time (from 0 to 20 minutes) which is permitted before switching to high-cooling.

If the power is interrupted, the stored history is erased and the furnace control CPU will select low-cooling for up to 20 minutes and then energize the air conditioning relay ACR to energize the Y/Y2 terminal and switch the outdoor unit to high-cooling, as long as the thermostat continues to call for cooling. Subsequent selection is based on stored history of the thermostat cycle times.

The wall thermostat “calls for cooling”, closing the R to G and- Y circuits. The R to Y1 circuit starts the outdoor unit on low-cooling speed, and the R to G-and-Y1 circuits starts the furnace blower motor BLWM at low-cooling airflow which is the true on-board CF selection as shown in Fig. 47.

If the furnace control CPU switches from low-cooling to high cooling, the furnace control CPU will energize the air conditioning relay ACR. When the air conditioning relay ACR is energized the R to Y1-and-Y2 circuits switch the outdoor unit to high-cooling speed, and the R to G-and-Y1- and-Y/Y2 circuits transition the furnace blower motor BLWM to high-cooling airflow. High-cooling airflow is based on the A/C selection shown in Fig. 47.

NOTE: When transitioning from low-cooling to high-cooling the outdoor unit compressor will shut down for 1 minute while the furnace blower motor BLWM transitions to run at high-cooling airflow.

The electronic air cleaner terminal EAC-1 is energized with 115 vac whenever the blower motor BLWM is operating.

When the thermostat is satisfied, the R to G-and-Y circuit are opened. The outdoor unit stops, and the furnace blower BLWM and electronic air cleaner terminal EAC-1 will remain energized for an additional 90 seconds. Jumper Y1 to DHUM to reduce the cooling off-delay to 5 seconds. (See Fig. 33.)

4.Two-Stage Thermostat and Two-Speed Cooling See Fig. 58 for thermostat connections.

NOTE: The air conditioning relay disable jumper ACRDJ must be disconnected to allow thermostat control of the outdoor unit staging. (See Fig. 33.)

The thermostat closes the R to G-and-Y1 circuits for low cooling or closes the R to G-and-Y1-and-Y2 circuits for high cooling. The R to Y1 circuit starts the outdoor unit on low cooling speed, and the R to G-and-Y1 circuit starts the furnace blower motor BLWM at low-cooling airflow which is the true on-board CF selection as shown in Fig. 47. The R to Y1-and-Y2 circuits start the outdoor unit on high-cooling speed, and the R to G-and-Y/Y2 circuits start the furnace blower motor BLWM at high-cooling airflow. High-cooling airflow is based on the A/C selection shown in Fig. 47.

The electronic air cleaner terminal EAC-1 is energized with 115 vac whenever the blower motor BLWM is operating.

When the thermostat is satisfied, the R to G-and-Y1 or R to G-and-Y1-and-Y2 circuits are opened. The outdoor unit stops, and the furnace blower BLWM and electronic air cleaner terminal EAC-1 will remain energized for an additional 90 seconds. Jumper Y1 to DHUM to reduce the cooling off-delay to 5 seconds. (See Fig. 33.)

See Fig. 52-55 for thermostat connections.

The dehumidification output, DHUM on the Thermidistat should be connected to the furnace control thermostat terminal DHUM. When there is a dehumidify demand, the DHUM input is activated, which means 24 vac signal is removed from the DHUM input terminal. In other words, the DHUM input logic is reversed. The DHUM input is turned ON when no dehumidify demand exists. Once 24 vac is detected by the furnace control on the DHUM input, the furnace control operates in Thermidistat mode. If the DHUM input is low for more than 48 hours, the furnace control reverts back to non-Thermidistat mode.

The cooling operation described above also applies to operation with a Thermidistat. The exceptions are listed below:

1.Low cooling – When the R to G-and-Y1 circuit is closed and there is a demand for dehumidification, the furnace blower motor BLWM will drop the blower airflow to 86% of low cooling airflow which is the true on-board CF selection as shown in Fig. 47.

355CAV

355CAV specifications

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