MGD Gas Furnace
Service Manual
If, however, the switch is found to be opening prematurely, then it should be replaced. When
replacing ANY limit switch, use ONLY a switch of EXACTLY the same temperature setting. Use of
a different temperature limit switch can create a dangerous situation. Be certain you have the
correct control part when replacing the limit switch.
AUXILLIARY LIMIT SWITCH - 140°F AUTO RESET
This limit switch is located inside the control box and automatically re-sets itself after the
temperature of the switch has dropped below the factory set point. This switch is normally closed
and operates much like the main limit switch. This switch can be tested using the same procedure
as described in the main limit switch section of this manual.
When replacing, use only exact replacement parts with the same temperature setting. Use of a
different temperature limit switch can create a dangerous situation.
PRESSURE SWITCHES
An air-proving pressure switch is used on all models to insure that a draft has been established
through the heat exchanger before allowing burner operation.
All Models
use a dual port switch which senses the pressure differential that is created by the
combustion blower.
All Models
use the same setting Normally Open pressure switch. The particular setting is
required to insure continued SAFE, RELIABLE, operation. NEVER SUBSTITUTE a pressure
switch with one that is similar in appearance. ONLY FACTORY PROVIDED or AUTHORIZED
SUBSTITUTES ARE ACCEPTABLE.
If you are servicing a unit whose pressure switch will not close, or remain closed during operation,
the operating pressure of that unit should be checked and compared to the switch setting.
Under normal operating conditions, sufficient negative pressure will be created to close the
pressure switch, and keep it closed to keep the furnace section operating. Under abnormal
conditions, however, such as a restricted vent (flue exhaust hood), or a leak in the heat
exchanger, sufficient negative pressure will not be created. This will result in the switch failing to
closed or failing to remain closed during furnace section operation.
COMBUSTION AIR BLOWER
All models use the same combustion air mounted on the inlet side of the heat exchanger. The
purpose of the combustion blower is to establish a draft (flow) through the heat exchanger, to
insure that all flue products are carried outside the unit via the flue exhaust hood.
There is a pressure tap on the induced draft blower housing, and one on the burner box, which
connect via a silicone rubber tube to the taps on the unit’s pressure switch. During normal
operation the exhaust blower develops a sufficient differential pressure to close the pressure
switch and keep it closed.
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