506566−01 09/10
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Gas Heat Control Mode
Modulating Control of Gas Heat
The thermostat includes a feature that provides modulating
control of the gas heat mode. The purpose of modulating con-
trol is to keep the room temperature at, or near, the desired
set point with a minimum of 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), and
Target set point.
The thermostat uses this information to modulate 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 modulating gas heat mode, the furnace opera-
tion includes longer run times at lower heat stages.
Table 1 details the furnace operation in the modulating mode.
Staged operation
Some furnaces provide up to four stages of gas heat opera-
tion. 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 capac-
ity; 2nd stage provides 60% of full capacity; 3rd stage pro-
vides 80% of full capacity; 4th stage provides 100% of full ca-
pacity.
Table 1. Stage table for Modulating Furnace Control
Current Gas Heating Stage as calculated above
Off 1st Stage 2nd Stage 3rd Stage 4th Stage
Thermostat demand
Off No heating demand No heating demand No heating demand No heating demand No heating demand
1st Stage
Use starting rate
calculated by Inte-
grating Algorithm
Continue current
heating rate
Continue current
heating rate
Continue current
heating rate
Continue current
heating rate
2nd Stage
Immediately in-
crease rate to per-
centage represent-
ing 2nd stage
Jump to 2
nd
stage rate
Increase rate by
5% every 5 minutes
(up to 100%)
Increase rate by
5% every 5 minutes
(up to 100%)
Continue current
heating rate
3rd Stage
Immediately in-
crease rate to per-
centage represent-
ing 3rd stage
Jump to 3
rd
stage rate
Jump to 3
rd
stage
rate
Increase rate by
5% every 5 minutes
(up to 100%)
Continue current
heating rate
4th Stage
Immediately in-
crease rate to
100%
Immediately
increase rate to
100%
Immediately
increase rate to
100%
Immediately
increase rate to
100%
Continue 100%
heating rate
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