Carrier Condensing gas furnace Operations Instructions download pdf (Page 45)

NOTE: Low-heat-only switch, SW1-2, selects either the low-

heat-only operation mode when ON, (see item 2. below) or

adaptive heating mode when OFF, in response to a call for heat.

(See Fig. 37.) When the W2 thermostat terminal is energized, it

will always cause high-heat operation when the R to W circuit is

closed, regardless of the setting of the low-heat only switch.

This furnace can operate as a two-stage furnace with a single-stage

thermostat because furnace control CPU includes a programmed

adaptive sequence of controlled operation, which selects low-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 low- or high-heat. If the furnace

starts up in low-heat, the furnace control CPU determines the

low-heat on time (from 0 to 16 minutes) which is permitted before

switching to high-heat.

If power is interrupted, the stored history is erased. When this

happens, the control CPU will initially select low-heat for up to 16

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 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 high-heat pressure switch contacts LPS and HPS are

open.

1. Inducer Prepurge Period-The furnace control CPU turns on

inducer motor IDM and slowly increases the inducer motor

speed. When the low-heat pressure switch LPS closes, inducer

motor RPM is noted by the furnace control CPU, and a 25-sec.

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 the low-heat prepurge period and low-heat mode.

NOTE: The heat cycle can start in either high-or low-heat. If a

high-heat cycle is initiated, the furnace control CPU will de-

energize the high-heat pressure switch relay HPSR to close the NC

contact and continues to increase the inducer motor speed after the

low-heat pressure switch LPS closes. When the high-heat pressure

switch closes, inducer motor RPM is noted by the furnace control

CPU before the 25-sec 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 mode.

2. Igniter Warm-Up-At end of the prepurge period, the Hot

Surface Igniter HSI is energized for a 17-sec igniter warm-up

period.

3. Trial-For-Ignition Sequence-When the igniter warm-up pe-

riod is completed, the main gas valve relay contacts GVR

close to energize the gas valve GV, the gas valve opens. The

gas valve GV permits gas flow to the burners where it is

ignited by the Hot Surface Igniter HSI. Five seconds after the

GVR closes, a 2-second flame period begins. The HSI igniter

will remain energized until the 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 also energized.

Fig. 50—Filling Condensate Trap

A99119

Fig. 49—Inducer Housing Drain Tube

A99118

Table 14—Blower Off Delay Setup Switch

Position

DESIRED HEATING

MODE BLOWER

OFF DELAY (SEC)

SETUP SWITCH (SW1-7 AND SW1-8) POSITION

SW1-7 SW1-8

90 OFF OFF

120 ON OFF

150 OFF ON

180 ON ON

1 2 ... 40 41 42 43 44 45 46 47 48 49 50 ... 59 60

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Carrier Condensing gas furnace Operations Instructions Page 45

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