Honeywell AQ25A Series Technical Reference Booklet page 73

Temperature Sensor Wiring
Connect the lead wires of each sensor to the corresponding
terminals on top of the AQ151 Control Module. See Fig. 5 on
page 6.
The Boiler Supply, Return and Secondary loop sensors can be
installed either as strap-on sensors or inserted into an
immersion well that is packed with thermally-conductive paste.
BOILER SUPPLY AND RETURN SENSORS.
Both the Supply and Return Sensors should be installed on the
supply and return piping of the boiler for proper operation of
the AQ251 Control Panel. Even if the AQ251 is connected to a
modulating condensing boiler with its own supply and/or return
sensors, the AQ251's sensors should still be installed for the
control to operate.
The Boiler Supply water sensor should be installed on the
supply piping close to the exit port of the boiler, using one of
the AQ12C11 strap-on sensors supplied with the AQ251. See
Fig. 6.
The Boiler Return sensor should be installed on the return
piping as close to the entrance port to the boiler as practical,
using the other AQ12C11 strap-on sensor supplied with the
AQ251.
The correct location is one that will measure the temperature of
all combined sources of water returning back to the boiler.
Insulate strap-on sensors with pipe wrap to ensure accurate
boiler temperature sensing.
The Boiler Supply and Return water sensors come with 10 ft.
(3m) of wire to minimize the need for splicing.
Fig. 6. Strap-on temperature sensor installation.
OUTDOOR SENSOR
The outdoor sensor should be located:
• in a shady location out of direct sunlight
• at least three feet from dryer, bathroom, or other vents
• above the expected snow line where ice and debris cannot
cover it
• on the north side of the building.
See Fig. 7 for typical placement. Outdoor sensor comes with
10 ft. (3m) of wire to facilitate splicing the sensor on the interior
of the building. Alternatively, if the building is equipped with a
continuous fresh air supply using, for example, an air-to-air
heat exchanger, the outdoor sensor may be installed in the
insulated portion of the ventilation intake duct, taking care not
to expose the sensor to direct or indirect sunlight.
IMPORTANT
System Demands Wiring
M13763 Additional information about settings for the various System
Demands is discussed in Table 6 on page 30.
AUX-IN
f the optional Aux In. contacts (terminals 9 and 10) will be
used, wire them now. These inputs are powered with 24 Vdc
and must connect only to a dry closure contact (unpowered
switch).
The Aux-In contact closure sets the system into a specific state,
as determined by the installer setup using the EQUIPMENT
SETUP > AUXILIARY I/O sub-menu (see Fig. 25 on page 39).
HEAT DEMAND
If the optional Heat Demand (terminals 11 and 12) will be used,
wire them to a system setpoint demand (dry contact closure),
such as a pool or spa Aquastat®.
The HEAT contact closure drives the system to control either at
the Reset temperature or the Setpoint temperature, as
determined by the installer setup using the EQUIPMENT
SETUP > ZONING > HEAT DMND selection (see Fig. 25 on
page 39). Heat Demand priority allows only heat for the first 30
minutes of a call for heat and then allows the space heating
needs to be added in for the next 30 minutes. This cycle
continues until the call for heat is satisfied.
7
AQ251 SERIES BOILER RESET CONTROL PANELS
Fig. 7. Outdoor temperature sensor installation.
Do not run sensor wires parallel, or close, to
telephone, Ethernet, or power cables. Cross all
power, Ethernet, and telephone wiring at right angles.
If sensor wires are located in an area with strong
sources of electromagnetic interference, or EMI, (e.g.,
if sensor wires are run in the same electrical chase as
line voltage wiring) use twisted pair, shielded cable, or
run wires in a grounded metal conduit. This is
important, since the calculated temperature - based
on the sensor's resistance reading - can be distorted
by high EMI, potentially causing the AQ251 to not
operate properly. If using shielded cable or conduit,
connect the shield wire to earth ground only at the
AQ251 panel. Do not ground the shield or conduit at
any other location or electromagnetic shielding will be
ineffective. If shielded cable is used, Honeywell
recommends the use of shielded cable with a
continuous ground plane, such as foil, with an integral
drain wire for bonding to earth ground.
M13775
69-1974—04