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APPLICATION
The dual fuel thermostat is designed for the following applications:
1-stage cool, 2-stage heat: 1-speed HP with 1-stage furnace
1-stage cool, 3-stage heat: 1-speed HP with 2-stage furnace
2-stage cool, 3-stage heat: 2-speed HP with 1-stage furnace
2-stage cool, 4-stage heat: 2-speed HP with 2-stage furnace*
*This combination must use furnace algorithm to control furnace
staging.
Balance Point Temperature
The "balance point" temperature is a setting which affects the
operation of the heating mode. This is a field-selected input
temperature (range 5-55°F) where the dual fuel thermostat will
monitor outdoor air temperature and decide whether to enable or
disable the heat pump.
If the outdoor temperature is above the "balance point," the heat
pump will energize first to try to satisfy the indoor temperature
demand. If the heat pump does not make a sufficient improvement
within a reasonable time period (i.e. 15 minutes), then the gas
furnace will come on to satisfy the indoor temperature demand. If
the outdoor temperature is below the "balance point," the heat
pump will not be allowed to operate (i.e. locked out), and the gas
furnace will be used to satisfy the indoor temperature.
There are three separate concepts which are related to selecting the
final "balance point" temperature. Read each of the following
carefully to determine the best "balance point" in a dual fuel
installation:
Capacity Balance Temperature:
This is the point where the heat pump cannot provide sufficient
capacity to keep up with the indoor temperature demand because
of declining outdoor temperature. At or below this point, the
furnace is needed to maintain proper indoor temperature.
Economic Balance Temperature:
Above this point, the heat pump is the most cost efficient to
operate, and below this point, the furnace is the most cost efficient
to operate. This can be somewhat complicated to determine and it
involves knowing the cost of gas and electricity, as well as the
efficiency of the furnace and heat pump. For the most economical
operation, the heat pump should operate above this temperature
(assuming it has sufficient capacity), and the furnace should
operate below this temperature.
Comfort Balance Temperature:
When the heat pump is operating below this point, the indoor
supply air feels uncomfortable (i.e. too cool). This is purely
subjective and will depend on the homeowner’s idea of comfort.
Below this temperature, the gas furnace should operate in order to
satisfy the desire for indoor comfort.
Auxiliary Heat Lockout Setting
The "auxiliary heat lockout" setting is an optional lockout feature.
This is in addition to the "balance point." The purpose of this
feature is to lock out the gas furnace whenever the outdoor
temperature is above the selected setting. The temperature range is
5-55°F. When the outdoor temperature is above the "auxiliary heat
lockout" setting, the heat pump will become the primary source,
and the gas furnace cannot come on (except in defrost or
emergency heat mode).
The "auxiliary heat lockout" setting can never be set below the
"balance point" setting, and likewise, the "balance point" can never
be set above the "auxiliary heat lockout" setting. The range of each
of these is from 5-55°F (or equivalent values in Celsius), or they
can be turned off. When off, they do not limit the equipment
operation. If the "auxiliary heat lockout" setting is moved down, it
will "push" the "balance point" so the two settings do not overlap.
Likewise, moving up the "balance point" will push the "auxiliary
heat lockout" setting.
When the two settings are the same, the system views this as a
single outdoor "balance point" temperature, and the heat pump is
used for above (warmer outdoor temperatures) and the furnace for
below (colder outdoor temperatures.)
DESCRIPTION OF OPERATION
Cooling Operation—Single Speed
HP cooling:
O/W2 energizes reversing valve to select cooling operation.
G energizes furnace blower.
Y/Y2 energizes compressor and selects high blower speed at
furnace.
When cooling demand is satisfied, G and Y/Y2 are de-energized.
O/W2 will remain on to minimize cycling of reversing valve. It
will turn off only when a call for heat occurs.
Cooling Operation—Two Speed
Low-speed HP cooling:
O/W2 energizes reversing valve to select cooling operation.
G energizes furnace blower.
Y1/W2 energizes compressor at low speed and may determine
furnace blower speed.
High-speed HP cooling:
Y/Y2 is added to low-speed cooling call to operate compressor at
high speed and increase blower speed.
Heating Operation—Single-Speed HP
HP heating:
O/W2 remains off to select heating operation.
G turns on furnace blower.
Y/Y2 energizes compressor and increases furnace blower speed.
Heating Operation—Two-Speed HP
Low-Speed HP heating:
O/W2 remains off to select heating operation.
G turns on furnace blower.
Y1/W2 turns on compressor at low speed and may also adjust
blower speed for low-speed HP operation.
High-Speed HP heating:
Y/Y2 is added to low-speed HP heating call to operate compressor
at high speed and adjust blower for high-speed HP operation.
Heating Operation—Single-Stage Furnace
W/W1 causes furnace to operate. Furnace controls its own blower
at heating speed.
Heating Operation—Two-Stage Furnace—Thermostat Control
Low heat:
W/W1 causes furnace to operate at low heat. Furnace controls its
own blower at low heating speed.
High heat:
Y1/W2 is added to low-heat heating call to operate furnace in
high-heat mode. Furnace controls its own blower at high heating
speed.
Heating Operation—Two-Stage Furnace—Algorithm Control
W/W1 causes furnace to operate. Based on amount of time it is
required to be on, it determines when to operate at low heat and
when to operate at high heat. It controls its own blower at proper
speed.
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