Rapid 7500 136 Installation, operation & service manual - page 65
SECTION 14: E
LECTRICAL
53 of 143
Table 16: Safety Systems
14.5.1 Fan / Limit Switch
The fan / limit s
w
itch acts as a fan control and a high
temperat
u
re control. It com
b
ines the f
u
nction of a
high temperat
u
re limit control
w
ith that of a fan
controller. It has t
w
o control relays: one controls the
air t
u
rno
v
er
u
nit's fan(s) and the other acts as a high
temperat
u
re controlling relay. The fan relay has t
w
o
settings or set points - one for t
u
rning on the air
t
u
rno
v
er
u
nit's fan(s) and one for t
u
rning it off. The
limit's relay only has one set point.
The fan portion of the s
w
itch has its set point set at
120° F (48.8° C). After the air s
u
rro
u
nding the heat
exchanger reaches set point, the fan relay(s) close(s)
and po
w
er is s
u
pplied to the air t
u
rno
v
er
u
nit's fan
motor starter coil(s),
w
hich t
u
rn(s) the fan motor(s)
on if it is not already r
u
nning. If the sensor for the fan
relay(s) cools do
w
n
b
elo
w
its set point (
b
ased on the
second set point), it
w
ill open sh
u
tting do
w
n the air
t
u
rno
v
er
u
nit's fan(s) if req
u
ired. The second set point
for the fan portion of the s
w
itch is la
b
eled "HYS"
w
hich stands for Hysteresis. This is the degrees
b
elo
w
the fan set point at
w
hich the relay
w
ill open. A
setting of 15° F (8.3° C) is recommended for this
eq
u
ipment.
No
t
e
: This feature of the fan/limit switch could cause the air turn-
over unit's fan(s) to cycle on and off a number of times after a
heating cycle ends to cool down the heat exchanger. This could
continue even with other controls turned off. Only turning off the
main power disconnect will disable the feature.
The limit portion of the s
w
itch has its set point set at
200° F (93.3° C). After the air s
u
rro
u
nding the heat
exchanger reaches set point, the limit relay(s)
w
ill
open the circ
u
it to the
bu
rner system and discontin
u
e
all
bu
rner f
u
nctions. Restarting of the
bu
rner can only
b
e accomplished after the sensor for the limit has
cooled do
w
n
b
elo
w
its set point.
14.5.2 Auxiliary Manual Reset Hi
g
h Temperature
Limit Switch
If for any reason, the temperat
u
re of the air
s
u
rro
u
nding the heat exchanger reaches the limit set
point of 250° F (121.1° C), the high temperat
u
re limit
s
w
itch
w
ill open the circ
u
it to the air t
u
rno
v
er
u
nit's
control system and sh
u
t it do
w
n. Restarting of the
bu
rner can only
b
e accomplished after the limit has
cooled do
w
n and the reset
bu
tton on the s
w
itch is
depressed.
14.5.3 Pressure Switches
14.5.4.1 Cabinet Airflow Switch
The ca
b
inet airflo
w
s
w
itch monitors the airflo
w
thro
u
gh the fan section of the air t
u
rno
v
er
u
nit; its
f
u
nction is to protect the air t
u
rno
v
er
u
nit and
do
w
nstream components from improper lo
w
airflo
w
conditions. The
bu
rner is allo
w
ed to f
u
nction
v
ia the
w
arm-
u
p
b
ypass timer contacts e
v
en tho
u
gh the
fan(s) may not
b
e r
u
nning. Once the air t
u
rno
v
er
u
nit's fan(s) are t
u
rned on, the air t
u
rno
v
er
u
nit's
airflo
w
s
w
itch closes and the
w
arm-
u
p
b
ypass timer
completes its time o
u
t cycle, remo
v
ing the
b
ypass
circ
u
it. This only allo
w
s the
bu
rner to f
u
nction as long
as airflo
w
is present thro
u
gho
u
t the air t
u
rno
v
er
u
nit
(except d
u
ring
w
arm-
u
p).
This airflo
w
s
w
itch is adj
u
sted
b
y t
u
rning the
adj
u
stment scre
w
clock
w
ise, till the scre
w
is t
w
o
t
u
rns from fl
u
sh of its ho
u
sing.
14.5.5.2 Gas Pressure Switches
Gas press
u
re s
w
itches are standard on certain
models (UL & FM compliant gas trains a
b
o
v
e 2,500
MBH and XL compliant gas trains a
b
o
v
e 400 MBH)
and are also a
v
aila
b
le as an option on the others.
The
v
ersion
u
sed com
b
ines the f
u
nction of
b
oth into
one component.
The f
u
nction of the gas press
u
re s
w
itches is to
protect against ins
u
fficient, lack of gas press
u
re and
excessi
v
e press
u
re in the system.
On the lo
w
gas press
u
re s
w
itch side, this s
w
itch
opens its internal s
w
itch
w
hich sh
u
ts the
bu
rner do
w
n
and pre
v
ents its operation d
u
e to ins
u
fficient gas
press
u
re.
On the high gas press
u
re s
w
itch side, its internal
s
w
itch
w
ill open, sh
u
tting do
w
n the
bu
rner d
u
e to
excessi
v
e gas press
u
re passing thro
u
gh the gas
train.
The settings of the gas press
u
re s
w
itches are field
adj
u
sta
b
le. The one monitoring the incoming gas
Safety Controls
Location
Volta
g
e
Fan / Limit S
w
itch
Air T
u
rno
v
er Unit
Control Enclos
u
re
120
A
u
xiliary Man
u
al Reset High
Temperat
u
re Limit S
w
itch
Air T
u
rno
v
er Unit
Control Enclos
u
re
120
Ca
b
inet Airflo
w
S
w
itch
Air T
u
rno
v
er Unit
Control Enclos
u
re
120
Gas Press
u
re S
w
itches
Air T
u
rno
v
er Unit/
Gas Train
120
Flame Control
Air T
u
rno
v
er Unit
Control Enclos
u
re
120