Instruction manual

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The furnace will start up in either intermediate-, or maximum-heat.

The furnace will operate in minimum-heat after starting and

operating for 1 minute at intermediate-heat before transitioning to

minimum-heat.

The wall thermostat ”calls for heat”, closing the R to W1 circuit for

minimum-heat or closing the R to W1-and-W2 circuits for

maximum-heat. The furn ace con trol perfo rm s a self-check , and

veri fies th e low, medium and high pressure switch contacts LPS,

MPS and HPS are open, then de-energizes the PSR relay to close

th e NC contact.

The start up and shut down functions and delays described above

apply to the 2-stage minimum/maximum heating mode as well,

except for switching from maximum- to minimum-heat.

1. Switching from Maximum- to Minimum-Heat —Ifthe

thermostat R to W2 circuit opens, and the R to W1 circuit

remains closed, the furnace control CPU will gradually de-

crease the inducer motor speed to the required intermedi-

ate-heat RPM. When the inducer motor IDM reduces pres-

sure sufficiently, the high pressure switch HPS will open

and the gas rate will be changed to intermediate-heat. The

gas valve solenoid GV (pin 5) will remain energized as long

as the low pressure switch LPS remains closed. When the

inducer motor speed gets within 15% of the required inter-

mediate-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 air-

flow will transition to minimum-heat airflow. At this point

the furnace control CPU will energize the PSR relay to open

the NC contact and slowly decrease the inducer motor

speed to th e required min im u m -h eat R P M . Wh en the

PSR relay is energized and the NC contact opens the fur-

nace control CPU will reduce the gas rate to minimum-heat

RPM.

Two-Stage Thermostat and Two-Stage Intermediate/

Maximum Heating

See Fig. 34 and 35 for thermostat connections

NOTE: In this mode the intermediate-heat only switch SW4-2

must be ON to select the intermediate-heat only operation mode in

response to closing the thermostat R to W1 circuit. Closing the

thermostat R to W1-and-W2 circuits always causes

maximum-heat operation, regardless of the setting of the

intermediate-heat only switch.

The wall thermostat ”calls for heat”, closing the R to W1 circuit for

interm ed iate-h eat o r clo sin g the R to W1 -and -W 2 circuits fo r

maximum-heat. The furnace control performs a self-check, and

verifies the low, medium and high pressure switch contacts LPS,

MPS and HPS are open , then de-energizes the PSR relay to close

the NC contact.

The start up and shut down functions and delays described above

apply to the 2-stage intermediate/maximum heating mode as well,

except for switching from maximum- to intermediate-heat.

1. Sw itch ing from Ma x im u m - to Interm ed ia te-H ea t — If

th e thermostat R to W2 circuit opens, and the R to W1 cir-

cuit remains closed, the furnace control CPU will gradually

decrease the in du cer mo tor speed to th e req uired interme-

diate-heat RPM. When the inducer motor IDM reduces

pressure sufficiently, the high pressure switch HPS will

open and the gas rate will be changed to intermediate-heat.

When the inducer motor speed gets within 15% of the re-

quired intermediate-heat RPM the furnace control CPU will

start a 5 second blower airflow change delay. Af ter t he 5

second blower airflow change delay is completed the

blower airflow will transition to intermediate-heat airflow.

Cooling Mode

The thermostat “calls for cooling.”

1. Single-Speed Cooling

See Fig. 34 and 35 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

isbasedontheA/CselectionshowninFig.54.

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. 34.)

2. Single-Stage Thermostat and Two-Speed Cooling (Ad-

aptive Mode)

See Fig. 34 and 35 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 o p e ration, wh ich

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. 34.) 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

CFselectionasshowninFig.54.

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.

54.

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. 34.)

987MA