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Figure 50 - Air pressure safety switch models
1.
Original-style round air switch
Replace with #2
2.
Dungs square-case air switch with dial adjustment
Replace with same
3.
Dungs square-case replacement air switch with screw adjustment
Replace with #2
4.
Yamatake-Honeywell air switch, non-adjustable
Replace with same
Adjustment
screw
Adjustment
screw
Adjustment dial
(in. W.C. or
mbar)
Adjustment
screw
(fixed at proper
setting - cannot
be adjusted)
Negative
pressure
Positive
pressure
Negative
pressure
Positive
pressure
Negative
pressure
Positive
pressure
Positive
pressure
because of the presence of a centrifugal air switch inside
the motor housing (see Section VII, Burner Blower Motor
Centrifugal Safety Switch, on the previous page).
The switch is an assembly that includes a normally-open
microswitch and a diaphragm. When the blower is at speed,
pressure is applied to the diaphragm which in turn closes
the microswitch. This completes the heat circuit, allowing
the burner/heating elements to activate.
If a blower is not operating, the microswitch receives no
signal from the diaphragm and cannot close, preventing
burner/heating element activation.
A. Part Number Reference - Air Switches
Middleby has used four different types of air switches for
these applications. See Figure 50.
When replacing one of these switches, note that the cur-
rent Yamatake-Honeywell air switch (#4 below) cannot be
used as an upgrade for an older switch. If you are replacing
Switches #1, #2 or #3 as shown in Figure 50, you should
use the Service Kits described for Switch #2.
VIII. AIR PRESSURE SAFETY SWITCH (PS200-SERIES,
PS555/570-SERIES, PS360EWB, PS360WB70)
Middleby ovens have a safety feature that prevents the
burner/heating elements from activating if the blower(s) are
not in operation. On PS200-series ovens and PS555/570-
series ovens, this feature is controlled by a separate air
pressure switch that reads air flow from the blower. One
switch is used for each blower (1 for the PS200-series, 2
for the PS555/570 series).
These ovens cannot use a centrifugal switch on the motor
because they use an indirect, belted drive system. If a belt
fails, it is possible for the motor to operate normally while
the blower itself is stopped.
In addition, gas ovens with a line-voltage burner blower mo-
tor use an identical air switch to check that the burner blower
motor is operating before burner activation. These ovens
include the PS360EWB, PS360WB70, PS555, PS555G,
and PS570G, as well as PS200-series ovens equipped with
the modulating gas system. This additional air switch is
not necessary on ovens with a 120V burner blower motor
SECTION 3 - SERVICING COMPONENTS