iComfort Wi-Fi
®
7-Day Programmable Communicating Thermostat
Page 39
Gas Heat Control Mode
NO
NO
YES
YES YES
NONO
YES
YES
NO
NO
YES
YES
NO
YES
NO
Continue at
starting firing rate
T'stat
demand
satisfied
?
Starting firing rate
as determined by
variable capacity
algorithm (35 to
100%)
Differential
less than 2nd stage
differential
?
T'stat
demand
satisfied
?
T'stat
demand
satisfied
?
Firing rate
increased by
5% every 5
minutes up to
100% firing rate
Firing rate
increased by
5% every 5
minutes up to
100% firing rate
T'stat
demand sat-
isfied in less than
5 minutes
?
T'stat
demand sat-
isfied in less than
5 minutes
?
Increase firing rate
to calculated 2nd
stage firing rate
Increase firing rate
to calculated 3rd
stage firing rate
Increase firing
rate to 100%
until thermostat
demand is
satisfied
THERMOSTAT
DEMAND
SYSTEM
IDLE
YES
NO
Differential
less than 3rd stage
differential
?
YES
NO
Differential
less than 4th stage
differential
?
YES
NO
Differential
less than 2nd stage
differential
?
Differential
less than 2nd stage
differential
?
Differential
less than 3rd stage
differential
?
Differential
less than 4th stage
differential
?
Figure 43. Variable Capacity Operation Flow Chart
Table 9. Variable Capacity Furnace Operation
Thermostat
Demands
Operation
None Furnace is off
First level heat
demand.
1.Furnace BTU rate is calculated by the iComfort
®
in
tegrating algorithm.
2.Furnace firing rate will continue at current heating
rate until first stage demand is satisfied or an addi
tional thermostat demand is received.
Second level
heat demand
added
1.Thermostat will immediately increase variable capa
city furnace firing rate to the calculated BTU percent
age representing the second stage thermostat de
mand.
2.Furnace firing rate will increase by 5% every 5
minutes (up to 100% if second stage thermostat is
not satisfied.) If thermostat second stage demand is
satisfied the current firing rate will continue until all
thermostat demands are satisfied.
Third level heat
demand added.
1.Thermostat will immediately increase variable capa
city furnace firing rate to the calculated BTU percent
age representing the third stage thermostat demand.
2.Thermostat will increase the firing rate by 5% every
5 minutes (up to 100% if third stage thermostat is not
satisfied.) If thermostat third stage demand is satis
fied the current firing rate will continue until all ther
mostat demands are satisfied.
Fourth level
heat demand
added.
iComfort
®
thermostat will immediately set variable
capacity furnace firing rate to 100% until all thermo
stat demands are satisfied.
Variable Capacity Control of Gas Heat Mode
(G71MPP and SLP98V)
The thermostat includes a feature that provides variable
capacity control of the gas heat mode. The purpose of variable
capacity control is to keep the room temperature at, or near,
the desired set point with minimum system cycling.
The thermostat bases its “decisions” for furnace operation
control using the following inputs:
Room temperature,
History of cycle times (how long does it typically take to
bring the room temperature to the desired set point),
Target set point, and
Differential temperature settings between 1st - 4th stage.
The thermostat uses this information to vary the heating
capacity to efficiently satisfy the heating demand. The full
range of the furnace heating capacity (low end is a percent of
the 1st stage capacity; high range is 100% of full capacity) is
used. When in variable capacity gas heat mode, the furnace
operation includes longer run times at lower heat stages.
Table 9 details the variable capacity furnace operation.
SLP98 variable capacity furnaces BTUs firing rates can be
incremented by as little as 1% (35% to 100% range available)
as the thermostat may dictate. The thermostat monitors room
temperature, previous heat cycle times, and target set point to
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