iComfort Wi-Fi
®
7-Day Programmable Communicating Thermostat
Page 23
HIGH BALANCE POINT
If the outside temperature is above the programmed high balance point (set by default at
50°F), auxiliary electric heat operation or furnace operation (in dual fuel system) is not
allowed. This ensures that the lower cost heat pump operation will satisfy the heating
demand, rather that the more expensive auxiliary electric heat. The high and low balance
points will not lock out both compressor heat and auxiliary heat/furnace at the same time.
DUAL-FUEL APPLICATIONS
The following information is applicable to communicating systems only.
1.
Staged Operations
Dual fuel applications, which include both a Heat Pump (XP17 or XP21 only) and a
gas furnace, will provide multiple stages of heating. For example, a two-stage heat
pump would deliver two stages of heat. The gas furnace can add two to four more
stages of heat. Figure 37 flow chart illustrates Dual Fuel operation with Balance
Points.
2.
Variable Capacity (Modulating) Operations
Dual-fuel applications, which include both a variable capacity heat pump (XP25)
and a variable capacity gas furnace (SLP98), will provide variable capacity heating.
3.
Tempering — Staged Furnace
Dual-fuel tempering staged furnace employs no sensor implementation. Furnace
cycles on and off at first stage rate.
First cycle on-time is longer than subsequent times to bring temperature up.
Subsequent times are shorter to maintain temperature.
4.
Tempering — Variable Capacity (Modulating) Furnace
(SLP98)
Dual-fuel tempering staged furnace employs no sensor implementation. Furnace
starts heating at minimum heat rate while defrost is active.
Testing has shown the entering air is within acceptable limits.
During defrost the airflow will run at second-stage CFM.
Abbreviations:
FURN = auxiliary heat provided by gas furnace
HP = compressor heat
LBP = Low Balance Point
HBP = High Balance Point
T'stat = thermostat
Shut down FURN
Stg1...Stg 2
Operate HP
Stg 1...Stg 2
at or
below
LBP
FURN
heat
lockout
HP stage(s) operate first;
then FURN stage(s) (if
needed; see NOTE 1)
T'stat
heat
demand
?
NO
Shut down HP
Stg 1...Stg 2
YES
if
outdoor
temperature
is...
T'stat
heat
demand
?
Operate HP
Stg 1...Stg 2
NO
YES
Operate FURN
Stg 1...Stg 2
T'stat
heat
demand
?
YES
NO
Operate FURN
Stg 1...Stg 2
Heat pump
lockout
T'stat
heat
demand
?
T'stat
heat
demand
?
YES
YES
NO
between
HBP and LBP
at
or above
HBP
COMPRESSOR ONLY
EITHER COMPRESSOR OR
FURN HEAT
FURN ONLY
50ºF
O
U
T
D
O
O
R
T
E
M
P
E
R
A
T
U
R
E
(d
e
fa
u
lt
s
s
h
o
w
n
)
HBP - FURNACE LOCKOUT
TEMPERATURE
NOTE 1
- Each Heat Pump and Furnace Heat stage will operate
until it meets the demand or until its stage timer lapses (20
minutes by default). If the timer lapses before the demand is
met, the system brings on the next stage in order of:
HP-stg-1, HP-stg-2, FURN-stg-1, FURN-stg-2.
NOTE 2
- The chart below illustrates how the balance points
control whether HP or FURN will operate to provide heat.
(See NOTE 2)
YES
NOTE 3
- If emergency heat is selected, Furnace is used.
25ºF
LBP - COMPRESSOR LOCK
OUT TEMPERATURE
Figure 37. Dual Fuel Operation with Balance Points
