9
Se
qu
ence
of
O
pe
r
at
i
on
1
.
Thermostat
ca
ll
s
for
heat
.
2.
Combustion
b
l
ower
starts
.
3
.
B
l
ower
pre
-
purge
period
of
30
seconds
.
4.
Tria
l
for
I
gnition
.
(5
seconds,
3
tria
l
s)
.
a
. Fl
ame
estab
l
ishing
period
(3
seconds),
gas
va
l
ve
opens,
sparks
from
spark
rod
to
burner
surface
to
ignite
the
gas
.
b
.
Burner
on,
f
l
ame
proving
period
(
2
seconds)
.
Requires
a
minimum
of
1
.
0
microamp
through
f
l
ame
sense
rod
to
prove
f
l
ame
.
c
. I
f
the
b
l
ocked
vent
safety
switch
contacts
(norma
ll
y
c
l
osed)
opens,
then
the
ignition
sequence
wi
ll
not
start
and
an
error
code
67
(Vent
safety
switch
fai
l
ed
to
c
l
ose/open)
wi
ll
be
shown
on
the
disp
l
ay
.
5
.
Steady
State
Operation
:
Burner
continues
to
operate
unti
l:
a
.
Thermostat
circuit
opens,
gas
va
l
ve
c
l
oses,
b
l
ower
continues
to
operate
for
30
second
post
-
purge
period
.
b
. I
f
the
norma
ll
y
c
l
osed
b
l
ocked
vent
safety
switch
opens,
the
gas
va
l
ve
c
l
oses,
the
b
l
ower
continues
to
operate
indefinite
l
y
and
error
code
67
wi
ll
be
disp
l
ayed
after
a
coup
l
e
minutes
(up
to
5
minutes)
with
“Service
N
eeded
.
”
6
.
Thermostat
satisfied
.
7
.
Gas
va
l
ve
c
l
oses,
burner
e
x
tinguished
.
8
.
B
l
ower
post
purge
for
30
seconds
.
9
