The thermostat controls the operating circuit to the
furnace by reacting to room temperature to open and
close a set of contact points which allows current to
flow to the relay.
The relay receives the current and allows current to pass
through to the circuit breaker by closing a switch within
the relay. This is done by a heater coil within the relay
which actuates a bi-metal disc closing the relay circuit.
The circuit breaker is placed in line to monitor the Amp
draw of the motor. It is an overload and safety protector
for the motor.
The current then flows to the motor and allows the
blower to operate. One end of the motor shaft drives the
circulating air wheel and the other end of the motor shaft
drives the combustion air wheel that delivers the
required air to the burner for combustion.
As the circulating air wheel comes up to speed, it blows
against the sail switch completing the circuit. The sail
switch is placed into the system as a safety to prove
there is adequate air for combustion.
The limit switch is an in line safety device which protects
the furnace from any over heating conditions. The
contacts in the limit switch open at a given temperature
setting, shutting off power to the valve.
The next section of operation is controlled by the valve
and pilot. Once the power is applied to the valve, the
following steps are:
1. Set gas valve knob to the pilot setting to light the
a. light pilot.
2. Set gas valve knob to the ON position for burner
3. While ON stand by, if the pilot goes out and the
thermostat closes, the blower will come on, but the
valve will remain closed. At this time, the pilot must
be relit for burner operation.
Note: The blower will remain running until the
thermostat contact opens.
As the thermostat senses the room air temperature, the
contacts will open removing power from the valve which
will shut off the gas. The blower will remain on until the
heater coil within the relay cools and the relay opens
and stops the current flow to the motor.
Sequence of Operation