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icomfort Wi-Fit 7−Day Programmable Communicating Thermostat
Page 35
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 setpoint with minimum system
cycling. The thermostat bases its decisions" for furnace operation control
using the following inputs:
S Room temperature,
S History of cycle times (how long does it typically take to bring the room
temperature to the desired setpoint),
S Target setpoint, and
S Differential temperature settings between 1st − 4th stage.
The thermostat uses this information to vary the heating capacity to efficient-
ly 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 op-
eration includes longer run times at lower heat stages.
Table 2 details the variable capacity furnace operation.
Staged operation
Some furnaces can be configured to provide up to four stages of gas heat
operation. When staged heating is chosen, the icomfort thermostat allows
you to choose between 1, 2, 3 and 4 stages of heat.
Single−stage heat: 1st stage provides 100% of full capacity.
Two−stage heat: 1st stage provides 70% of full capacity; 2nd stage
provides 100% of full capacity.
Three−stage heat: 1st stage provides 60% of full capacity; 2nd stage
provides 80% of full capacity; 3rd stage provides 100% of full capacity.
Four−stage heat: 1st stage provides 35 or 40% of full capacity; 2nd
stage provides 60% of full capacity; 3rd stage provides 80% of full ca-
pacity; 4th stage provides 100% of full capacity.
SLP98 variable capacity furnaces BTU’s 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 setpoint to determine the starting firing rate, and any required firing
rate increases or decreases during the thermostat demand.
During a heat demand, the icomfort thermostat monitors the differential be-
tween setpoint and room temp. As this differential increases and reaches
field adjustable levels (stages), the thermostat will increase firing rate ac-
cordingly to maintain room temperature and ultimately satisfy the heat de-
mand. On the next heat cycle, the icomfort stat will calculate a new starting
firing rate (in 1% increments) with the goal of maintaining room temperature
at, or near, thermostat setpoint with a minimum of system cycling. The prin-
ciples of operation are described in figure 36 and table 2.
Table 2. Variable Capacity Furnace Operation
T’stat Demands Operation
None Furnace is off
First level heat de-
mand.
1. Furnace BTU rate is calculated by the icomfort integrating al-
gorithm.
2. Furnace firing rate will continue at current heating rate until
first stage demand is satisfied or an additional thermostat de-
mand is received.
Second level heat
demand added
1. Thermostat will immediately increase variable capacity fur-
nace firing rate to the calculated BTU percentage represent-
ing the second stage thermostat demand.
2. Furnace firing rate will increase by 5% every 5 minutes (up to
100% if second stage thermostat is not satisfied.) If thermo-
stat second stage demand is satisfied the current firing rate
will continue until all thermostat demands are satisfied.
Third level heat de-
mand added.
1. Thermostat will immediately increase variable capacity fur-
nace firing rate to the calculated BTU percentage represent-
ing 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 ther-
mostat third stage demand is satisfied the current firing rate
will continue until all thermostat demands are satisfied.
Fourth level heat de-
mand added.
Icomfort thermostat will immediately set variable capacity
furnace firing rate to 100% until all thermostat demands are
satisfied.