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SECTION 17: SEQUENCE OF OPERATION
DANGER
Electrical Shock Hazard
Disconnect electric before service.
More than one disconnect switch may be
required to disconnect electric from
equipment.
Equipment must be properly grounded.
Failure to follow these instructions can result
in death or electrical shock.
WARNING
Carbon Monoxide Hazard
Do not recirculate air from the heated space
over burner.
Air supply to burner must be from outside.
Failure to follow these instructions can result
in death or injury.
17.1 Air Handler Configuration
Based on the air handler application, the air handler
may be configured in any of the four styles described
in the upcoming sections. These configurations are
available on all air handlers, except the XT-112 which
is only available in the MUA style. For a comparison
of these configurations, See Page 48, Table 11 and
Page 48, Figure 26.
17.1.1 Make-Up Air (MUA) Style
The MUA style air handler has a constant speed
blower that is designed to deliver a constant volume
of air to the heated space. This style air handler
supplies 100% outside air to the heated space and is
not capable of supplying return air.
17.1.2 Fixed Recirculation (FR) Style
The FR style air handler has a constant speed
blower that is designed to deliver a constant volume
of air to the heated space. In all cases, however, the
air being delivered directly over the burner for
combustion must always be 100% outside air. The air
handler delivers 20% outside air and 80% of return
air. The outside air is heated then mixed with the
return air prior to being delivered to the heated
space.
17.1.3 Air Management (AM) Style
The AM style air handler has a constant speed
blower that is designed to deliver a constant volume
of air to the heated space. A building pressure sensor
is used to detect the differential pressure between
the outside and the heated space. The total volume
of air delivered by the air handler may at times
consist of 0% to 80% return air. An automatic control
is provided for the outdoor air and return air dampers.
A null pressure switch is used to sense a positive
space pressure (room or duct). This null pressure
switch is mounted in the control enclosure. More or
less outdoor air is brought into the space to satisfy
the pressure setpoint. Most installations require a
setpoint of 0.02 in wc to 0.03 in wc to achieve
adequate pressure control in the building space. 5/
16" diameter tubing connected to the positive
pressure port of the switch must run to the inside of
the building. 5/16" diameter tubing connected to the
negative pressure port of the switch must run to the
outside of the building.
17.1.4 Variable Air Volume (VAV) Style
The VAV style air handler uses a building pressure
sensor to detect differential pressure between the
outside and the heated space. The air handler will
deliver 100% outside air and will not deliver return air.
The air handler will ensure proper air velocity over
the burner by using a photohelic air sensor to
measure the pressure drop across the burner and by
using a damper to modify the airflow when a
pressure change is required. A second photohelic air
pressure sensor is used to measure the differential
pressure between the outside and the heated space.
Most installations require a setpoint of 0.02 in wc to
0.03 in wc to achieve adequate pressure control in
the building. The second pressure sensor is used to
provide a control signal to the variable frequency
drive that, in turn, controls the blower speed. 5/16"
diameter tubing connected to the positive pressure
port of the pressure sensor must run to the inside of
the building. 5/16" diameter tubing connected to the
negative pressure port of the pressure sensor must
run to the outside of the building.
SECTION 17: S
O
EQUENCE OF
PERATION
47
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