Middleby Marshall PS220, PS570, PS360, PS200, PS555, PS224 PS310 Air pressure safety switch models

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

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.

Figure 50 - Air pressure safety switch models

1.		2.
Original-style round air switch		Dungs square-case air switch with dial adjustment
Replace with #2		Replace with same
	Positive
	pressure	Adjustment dial
		(in. W.C. or
		mbar)
Adjustment
screw
		Positive
		pressure
		Negative
		pressure
3.		4.
Dungs square-case “replacement” air switch with screw adjustment		Yamatake-Honeywell air switch, non-adjustable
Replace with #2		Replace with same
Adjustment
screw
		Adjustment
		screw
		(fixed at proper
Positive		setting - cannot
		be adjusted)
pressure
Negative
pressure
		Positive	Negative
		pressure	pressure

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