Middleby Marshall PS555, PS570, PS360, PS200, PS220 Combination Gas Control Valve, Pilot pressure

SECTION 3 - SERVICING COMPONENTS

M. Combination Gas Control Valve

1. Part Number Reference - Combination Gas Valve

•PS360WB70 and PS360EWB ovens are equipped with a 3/4” (19.05mm) gas line. This requires the use of Combination Gas Valve P/N 32569. The valve has a tan plastic cover with a blue On/Off knob.

•All other oven models are equipped with a 1/2” (12.7mm) gas line.

-Through 10/01 - these ovens used Valve P/N 28092- 0017. The valve operated on a 60 Hz supply, had a tan plastic cover and a blue On/Off knob.

-11/01 and later - these ovens use Valve P/N 45688. The valve operates on a 50 or 60 Hz supply, has a grey plastic cover and a black On/Off knob. It is otherwise identical to the previous version, and may be used to replace it in all cases.

2. Valve Operation

When the ON-OFF knob on the valve is in the ON position, gas is allowed to enter the valve. Once 24VAC is supplied to the valve, gas is supplied to the pilot line.

When the pilot flame has been proved (see Pilot and Proof of Flame Rectification on Page 94), 24VAC is supplied to the combination valve by the ignition module, opening the main valve. Gas pressure from the inlet side of the valve is passed through the Pressure Regulator. The regulated pres- sure is supplied to a cavity below the main valve diaphragm. This pressure overcomes the spring force and moves the main valve to its open position. During operation, the main valve is opened and closed by the pressure regulator to keep a constant pressure at the outlet of the valve.

When power to the valve is cut, the cavity below the main valve diaphragm is depressurized and the valve spring closes the main valve rapidly to shut off the main burner.

When the 24VAC supply to the valve is cut, the valve stops gas flow to both the pilot and the main burner.

The combination valve includes taps to measure both the inlet pressure and the regulated pressure. To ensure accu- racy, the regulated pressure should be measured when the oven is on low flame.

3.Servicing the Combination Valve

• Pilot pressure

One of the more common service issues with the com- bination valve occurs on ovens with the on/off gas regu- lating system (with a solenoid valve), when the oven is on low flame. During low flame, the servo can “bounce” inside the valve as it searches for the proper level. The bouncing line pressure can also cause surges in the pilot pressure. A properly adjusted pilot will tend to smooth out the surges.

The best way to check for a properly adjusted pilot is to check the pilot line pressure. This requires a pilot pressure tap in the pilot line. Ovens built 3/94 or later should have the tap installed in the line. The tap is also available as Service Kit P/N 30185 which includes installation instructions.

Recommended pilot gas pressures are:

-Wayne burner, natural gas - 3½-4” W.C. (8.71-9.95 mbar)

-Wayne burner, propane - 8-10” W.C. (19.9-24.9 mbar)

-Midco burner (all gases) - 5-6” W.C. (12.5-14.9 mbar)

•Burrs in pilot adjustment portal

Some combination valves have been shown to have shipped with small burrs in the pilot adjustment portal. Although this will not interfere with proper operation of the valve in any way, it does make it difficult to adjust the pilot pressure until the burr is removed.

Check for a burr by attaching a manometer to the pilot pressure tap. If the manometer does not detect any change in pilot pressure when the screw is tightened all the way down, there is probably a burr in the portal. To remove a burr, perform the following procedure:

-Turn the adjustment screw all the way down.

-Apply more pressure to break through the burr.

-Back the screw out until the top of the screw is level with the top of the access hole.

-Turn the adjustment screw back down to adjust the pressure.

-Repeat these steps until the burr is cleared.

Figure 72 - Combination Gas Control Valve

Pressure
regulator	Regulated
adjustment
	pressure
(under cap
	tap
screw)
Inlet	Pilot line
pressure	connection
tap
	Pilot pressure
On/Off knob (should	adjustment (under
remain in ON position)	cap screw)

98