User Guide

506920−01 09/12
Page 36
Load−tracking Variable Capacity FAQs
(SLP98V only)
What is Load−tracking Variable Capacity? When an SLP98 Furnace is
connected to an icomfort Wi−Fi thermostat, the thermostat takes complete
charge of the variable capacity staging of the furnace. Load−tracking Vari-
able Capacity is only available with icomfort Wi−Fi thermostats.
How is Load−tracking Variable Capacity different from Variable Ca-
pacity? Load−tracking Variable Capacity will smoothly track the load (sensi-
ble temperature changes) up and down and adjust the furnace heating rate
both ways. Variable capacity only tracks the load upward (rising tempera-
ture). Variable capacity uses the thermostats stage differentials but not
stage timers. LVC disregards both stage differentials and stage timers.
What makes Load−tracking Variable Capacity work? With icomfort
Load−tracking Variable Capacity, the thermostat uses a Proportional Integral
Algorithm (PIA) to control firing rates of the furnace. The algorithm in the
Load−tracking Variable Capacity provides much more precise control over
the furnace than the Variable Capacity algorithm.
Interesting points about the Proportional Integral Algorithm
(PIA):
 The further temperature is from the current set point, the higher PIA
sets the heating rate.
 The longer away from a programmed set point, the higher the heating
rate.
 If needed, PIA will turn heating on and off in short cycles when there
is a heat demand that is lower than the lowest the furnace can provide.
 If the heat demand is higher than the minimum the furnace can provide,
the furnace will run without shutting off.
 As the room temperature rises nearer the set point, the heating rate will
decrease.
 As the room temperature falls further away from the set point, the heat-
ing rate will increase.
 The stage differentials have no effect when running the PIA.
 The second stage timers have no effect when running the PIA.