Sequence Of Operation (Tim E Delay Relay Equipped Furnaces) - Suburban DD-17DSI Service Manual

SEQUENCE OF OPERATION
For Furnaces Equipped W ith Time Delay
The therm ostat controls the operating circuit to the furnace
by reacting to room tem perature to open and close a set of
contact points which allows current to flow to the ON and
OFF switch then to the relay.
The relay receives the power and allows power to pass
through to the switch within the relay. This is done by a
heater coil within the relay which actuates a bi-m etal disc
closing the relay circuit.
The power then flows to the m otor and allows the blower to
operate. One end of the m otor shaft drives the room air
wheel. T he other end of the m otor shaft drives the
com bustion air wheel that delivers the required air to the
burner for com bustion.
The lim it switch is an in-line device which protects the
furnace from over heating conditions. The contacts in the
lim it switch open at a given tem perature setting, shutting off
power to the ignition system which controls the gas valve.
As the room air wheel com es up to speed, air flow closes
the sail switch com pleting the circuit. The sail switch is
placed into the system as a safety to prove there is
adequate air for com bustion.
The next operation is controlled by the Direct Spark
Ignition, (DSI) system as power is applied to the DSI board.
The system will do the following.
1. The board has a tim ing circuit which allows the blower to
purge the cham ber of any products of com bustion or gas.
2. The board will then apply power to the gas valve. At the
sam e tim e it produces a high voltage power supply to the
electrode producing spark at the burner.
3. The board will also confirm the presence of a flam e. If
the flam e is not sensed after 7 seconds, the m odule will try
two (2) m ore tim es and then go into lock-out. The flam e is
sensed through the spark wire and electrode.
W hen the therm ostat has reached the set point with the
room air tem perature, the contacts will open rem oving
power from the controls. The blower will rem ain on until the
relay opens and stops the m otor.
NO TE: On some m odels, sail sw itch is before lim it
sw itch.
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