Allied A97UH2E - Page 26

508433-01Issue 2345Page 26 of 70
Combustion Air Inducer (B6) and Cold End Header Box
All A97UH2E units use a two-speed combustion air inducer
to move air through the burners and heat exchanger during
heating operation. The blower uses a 120VAC motor.
The motor operates during all heating operation and is
controlled by integrated control control A92. The inducer
also operates for 15 seconds before burner ignition (pre-
purge) and for 5 seconds after the gas valve closes (post-
purge). The inducer operates on low speed during rst-
stage heat, then switches to high speed for second stage
heat.
The combustion air inducer is installed on the cold end
header box (CEHB). The cold end header box is a single
piece made of hard plastic. The box has an internal
channel where the combustion air inducer creates negative
pressure at unit start up. The channel contains an orice
used to regulate ow created by the combustion air inducer.
The box has pressure taps for the combustion air inducer
pressure switch hoses. The pressure switch measures the
pressure di󰀨erential across the cold end header box orice
or di󰀨erence in the channel and the box. If replacement
is necessary the gaskets used to seal the box to the
vestibule panel and the combustion air inducer to the
box, must also be replaced.
A pressure switch measures the pressure di󰀨erential across
the CEHB orice to prove inducer operation. The CEHB
orice will be di󰀨erent for each model. When the proving
switch opens, the furnace control (A92) immediately closes
the gas valve to prevent burner operation.
Combustion Air Inducer Pressure Switch (S18) (Figure
14)
A97UH2E series units are equipped with a dual combustion
air pressure switch (rst and second stage) located on the
CEHB. The switch is connected to the cold end header box
by means of exible silicone hoses. It monitors negative
di󰀨erential pressure across the cold end header box orice.
The switches are a single-pole single-throw pressure
switches electrically connected to the integrated control.
The purpose of the switches is to prevent burner operation
if the combustion air inducer is not operating, the inlet or
exhaust pipes or heat exchanger are restricted.
On heat demand (rst or second stage) the switch senses
that the combustion air inducer is operating. It closes a
circuit to the integrated control when pressure across the
CEHB orice becomes greater then the switch set point.
Set points vary depending on unit size. See Table 9. Both
pressures sensed by the switches are negative relative to
atmospheric pressure. If the inlet or exhaust pipes, or the
heat exchanger become obstructed, the switch senses the
loss of di󰀨erential pressure and opens the circuit to the
furnace integrated control and gas valve. A bleed port on
the switch allows relatively dry air in the vestibule to purge
switch tubing, to prevent condensate build up.
Figure 14. Combustion Air Inducer Pressure Switch
3/16 Terminals
Low Fire Switch
High Fire Switch
1/4
" Terminals
Tap (negative - )
Tap (positive +)
NOTE: The switch is factory set and is not eld adjustable.
It is a safety shut-down control in the furnace and must
not be by-passed for any reason. If switch is closed or
bypassed, the control will not initiate ignition at start up.
Unit Set Point Low Heat Set Point High Heat
030 0.40 0.70
045 0.50 0.80
070 0.55 0.95
090 0.50 0.90
110 0.55 1.00
Table 9. 0 - 4,500 ft.