68
Rev. 0
SERVICING
S-17 CHECKING PRESSURE CONTROL
A pressure control device is used to measure negative pres-
sure at the induced draft blower motor inlet to detect a par-
tial or blocked flue and also a pressure control device is
used to measure negative pressure at the recuperator coil
cover to detect a partial or blocked recuperator coil or drain.
WARNING
Disconnect Electrical Power Supply:
1. Remove burner compartment door to gain access to
pressure switches.
2. Remove wires from the two electrical terminals on each
switch.
3. Using a VOM check from common terminal to NO (Nor-
mally Open) - should read open.
If switches read as above proceed to Step 4, otherwise
replace control.
4. Remove the pressure control hose from the control and
interconnect with an inclined manometer as shown in
the following illustration:
Coil Cover
Pressure Switch
Open to
Atmosphere
Inclined Manometer
Hose to Induced
Draft Blower Tap
1/4" Tee
Induced Draft Blower
Pressure Switch
Pressure Switch
Hose
Induced
Draft
Blower
*
*
*
*
*
*
*
M
1
2
3
P
C
ON
O
F
F
ID Blower Pressure Switch
Negative Pressure Measurement
(Upflow Shown - Counterflow Similar)
Inclined Manometer
1/4" Tee
Hose to Coil
Cover Tap
Induced Draft Blower
Pressure Switch
Induced
Draft
Blower
Coil Cover
Pressure Switch
Pressure Switch
Hose
Open to
Atmosphere
*
*
*
*
*
*
*
M
1
2
3
P
C
ON
O
F
F
Coil Cover Pressure Switch
Negative Pressure Measurement
(Upflow Shown - Counterflow Similar )
5. Reconnect wires YL-20 to C (Common) and OR-22 to
NO (Normally Open) terminals on ID Blower Pressure
Switch and GY-34 to C (Common) and GY-12 to NO
(Normally Open) terminals on Coil Cover Pressure
Switch. Refer to wiring diagram for proper wire connec-
tions.
With Power ON:
WARNING
LINE VOLTAGE NOW PRESENT.
6. Energize furnace for heating cycle. The induced draft
blower motor will begin to run. The inclined manometer
for the induced draft blower pressure switch should read
approximately negative
0.60" to 1.00" w.c. and the in-
clined manometer for the coil cover pressure switch
should read approximately negative 0.65" to 1.40" w.c.
with no combustion. Refer to the chart on page 67 for
pressure switch usage and trip points.
7. Remove and check the electrical wires using the VOM,
check from Common to NO (Normally Open) - should
read closed.
If not as above, replace control.
8. Reconnect all wires to each control and place in a heat-
ing cycle.
9. Begin to restrict the flue outlet until the pressure control
trips - cycling OFF the burner. Also a blocked drain test
will need to be performed until the coil cover pressure
control trips - cycling OFF the burner. The trip points
should be as shown on the page 67.
10. If not as listed, replace control.
11. After check and/or replacement of pressure switch(es),
reinstall burner compartment door and verify proper unit
operation.
The "ID Blower" pressure control is a safety device to pre-
vent the combustion cycle from occurring with inadequate
venting caused by a restricted or blocked vent pipe, or too
many effective feet of vent pipe.
The "Coil Cover" pressure control is a safety device to pre-
vent the combustion cycle from occurring due to a restricted
or blocked recuperator coil or drain.
The pressure readings listed in the Pressure Switch Usage
and Trip Point table on page 67 must be adhered to for
proper operation.
S-18 HIGH ALTITUDE APPLICATION (USA)
For those altitudes starting at 7000 feet and above, it may
be necessary to replace the induced draft blower pressure
switch and orifices. The coil cover pressure switch does
not need to be changed.
These changes are required to compensate for the reduc-
tion in atmospheric pressure (less available air for com-
bustion) as the altitude increases.
