GAS FURNACE
Page 30
505023M 08/11
VSM Furnace System Operation (G61MPV & G60UHV)
This section describes the operation of the zone control in
a system that uses a gas furnace.
Zone Thermostats
Only electronic thermostats with a C" terminal may be
used with the zone control system. The zone control sys-
tem distinguishes between heat pump and heat/cool ap-
plications via the SYSTEM jumper placement on the zone
control panel.
1. Cool / Heat / Auto−Changeover ModesZone thermo-
stats send a heating or cooling signal to the zone con-
trol panel. Thermostat servicing zone 1 is the central
control thermostat. Zones 2, 3 and 4 each have their
own thermostat. Thermostats may be standard or au-
tochangeover type.
Heat and cool demands present at the same time from
different zones (opposing demands) are satisfied on a
first come first served basis. If a heating demand and a
cooling demand reach the zone control panel at the
same time, the heating demand is satisfied first. If op-
posing demands persist, the system will work to satis-
fy the current demand for a maximum of 20 minutes,
then switch over and try to satisfy the opposing de-
mand for a maximum of 20 minutes. When either de-
mand is satisfied, the system works to satisfy the other
demand.
NOTE − Allowing opposing demands to persist may
consume excess energy. If this condition continues,
check the installation (i.e. zone arrangement, supply
registers, return registers, zone loads etc.) and make
adjustments as necessary. Table 9 shows the time de-
lays that may be expected when opposing demands
are received from the zone thermostats.
The zone control system acknowledges a new or op-
posing demand as soon as it is received by illuminating
that zone’s thermostat input lights. If the first demand
is not satisfied by the time the delays elapse, the sys-
tem switches over and begins satisfying the opposing
demand. During the switch-over, a delay may be im-
posed before the system begins satisfying the new de-
mand.
2. Fan On / Auto ModeZone thermostats can send a
continuous fan signal to the zone control system. The
zone control system will signal the blower to supply air
to zones calling for continuous fan while no other con-
ditioning calls exist. When the zone control system re-
ceives a conditioning call while satisfying a demand for
continuous fan, it signals the damper controlling the
continuous-fan zone to close. After the conditioning
demand is satisfied, the continuous-fan zone damper
is signaled to reopen.
Balance Point Setting
(Dual Fuel Systems) Balance Point Sensor (kit 56A87)
communicates with the zone control panel whether or not
to force the Gas Furnace to satisfy heating demands,
based on a comparison of the Balance Point setting with
the outdoor temperature. Terminals 2−3 close on tempera-
ture fall to lock out heat pump.
Zone Mode
The zone control mode (Vacation switch OFF) utilizes the
zone control system’s full potential. While in this mode, the
zone control system will respond to demands from any
zone, controlling dampers and regulating blower CFM to
maintain comfort. When the system is in zone mode, the
zone control system responds to demands from any zone
thermostat. (Switch settings are shown in figures 17
through 26.)
The only OPEN supply-air dampers are those zones from
which a demand was received; all other dampers are
CLOSED. The blower operates at a speed based on the
position of the Zone PIAB selection jumpers (and the heat-
ing air reduction jumper, if a call for heat is present).
NOTE − To ensure that the zone control performs optimally,
avoid mixing air between the zones.
Central (Vacation) Mode
When in central mode (Vacation switch ON), the system
responds only to heating or cooling demands from the cen-
tral control (Zone 1) thermostat; all zones will receive con-
ditioned air. All dampers remain open and the blower oper-
ates at full speed (minus the amount selected by the
heating air reduction jumper). (Switch settings are shown
in figures 17 through 26.)
In Fan−Auto mode, the blower will cycle on and off with
each demand. During gas or electric-strip heating, the
blower may continue after the end of a demand until the
heater is cooled sufficiently.
Cooling Operation
When the Harmony IIIt zone control system receives a
thermostat cooling call, the following events occur:
S The zone control checks to make sure it has been at
least 5 minutes since the last cooling call ended to pre-
vent starting against high head pressures.
S When timed−off delay is satisfied, the zone control
starts the outdoor unit with 1st−stage compressor and
slowly increases the indoor blower speed to achieve
proper CFM. Four minutes must elapse at this state to
allow the system to reach steady−state operation be-
fore staging again.
S The zone control checks the discharge air sensor for
proper temperature. If measured temperature is 7ºF or
more above the cooling staging jumper setting, then
Y2 energizes (if available). If both stages of cooling are
energized and 4 minutes has elapsed since the last
staging event, and the discharge air sensor (DAS) de-
tects a temperature less than the cooling staging jump-
er, then Y2 is staged off.
S If, at any time, the discharge air sensor measures a
temperature of 45ºF or below, the zone control de−en-
ergizes the Y1 and/or Y2 output, stopping the com-
pressor and preventing the indoor coil from freezing
up. The compressor will not be energized again until
the temperature at the DAS rises by 10ºF and the
timed−off delay expires. During this time, continuous
fan is supplied to the zones calling for cooling.
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