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Combustion motor assembly replacement
Using the information from above you have determined the
combustion motor assembly needs to be replaced, follow the
outlined procedures.
1.
Disconnect Molex plug
2.
Remove screws from the top and break silicone loose
from orifice ring on top side of ventor assembly. Figure
Picture #26 inlet pipe may also break loose, this is
normal.
3.
Remove screws from bottom of assembly and remove
the whole assembly.
4.
Mount new assembly, reattach screws to bottom remove
from step 3.
5.
Re-silicone orifice ring and combustion air pipe and
reinstall screws to top removed in step 2.
6.
Reconnect Molex plug from step 1 and test.
Pressure Switch (Picture 19) – All M1M furnaces use a
differential type pressure switch. The purpose of this switch
is to insure that a good supply of combustion air is supplied
to the burner. The combustion blower creates a differential
in negative pressure (less than atmospheric pressure) across
the orifice ring. The switch is normally open and closes on a
drop in pressure, read in negative inches of water column.
Under normal operating conditions, once the ventor motor
builds up to speed sufficient differential (negative) pressure
(approximately -0.30” wc) will be created to close the differen-
tial pressure switch and keep it closed the whole heating
cycle. Under abnormal conditions, such as ventor motor
failure, or restricted air inlet or vent, sufficient differential
pressure will not be created.
Under most circumstances, when the pressure switch is not
going closed, insufficient differential (negative) pressure is not
being created. See Table 9 for opening and closing settings.
Pressure switch check out procedure:
1. Remove orange wires from pressure switch. Place tees
in the bases connecting pressure switch to combustion
blower assembly.
2.
Connect a differential pressure gauge (magnehelic or
equivalent) to the tees. The magnehelic connection
marked low connects to the lower tee, and the one
marked high connects to the top tee.
3.
Start motor.
4.
Negative pressure created by the forced draft motor must
be greater than -0.30” w.c. for the switch to close.
5.
Use the ohm meter to check for continuity across the
switch.
6.
If continuity is established, switch has closed. If ohm
meter shows infinite reading, and a pressure of -.30” w.c.
or higher is present, switch is open, and must be replaced.
If the pressure differential reading will not pull down to -.30”
w.c., possible causes could be:
1.
Restriction in air inlet pipe, this causes a greater negative
at the inlet side connection. This will act to open switch. To
prove this, disconnect the hose going to the top connection.
If switch closes, there is too much negative pressure being
created on the inlet side. Look for air inlet restriction.
Table 9. Lower (lesser) Differential Negative Pressure Than Closing Pressure
Insufficient negative pressure measured at the
combustion blower may be caused by:
1.
Restriction on outlet side of combustion blower (blocked
flue or debris building up in flue).
2.
Leak (lack of restriction) on inlet side. Inducer inlet
leaking, inducer blower props loose, or wrong restrictor
orifice.
3.
To test for restriction in outlet pipe (exhaust) to verify
problem is outside of furnace, disconnect exhaust for
test period only and start furnace. If furnace starts, look
for problem in vent pipe. Reconnect after testing.
Higher than normal negative pressure at top
connection (acts to open switch) may be caused by:
1.
Restricted combustion air inlet pipe.
2.
To verify if problem is in inlet pipe, disconnect
pressure switch hose at top connection and start
furnace. If furnace starts, look for problems men-
tioned above in inlet pipe. Note: top connection acts
to open contacts on differential switch.
NOTE:
Inlet Pressure - Outlet Pressure = Differential Pressure
Lower (Lesser) Differential Negative Pressure Than Closing Pressure
Picture 19. Pressure Switch