A.O. Smith VB/VW- 750 Preventative maintenance, Troubleshooting Gas Valve, Main Burner

Troubleshooting Gas Valve

Problem: Non-linear CO2 curve on adjusted boiler (deviation more than 3-6%)

1.Possible blockage at the air inlet that prevents uniform, constant air flow.

2.The gas inlet pressure is too low at full load.

Problem: Unstable CO2 level at minimum load

The minimum load (low fire) is too low. The pressure difference on the pressure regulator at minimum load must be at least

0.2in wc (50 Pa). If the pressure difference does not meet this requirement, it may be necessary to adjust low fire setting, see Setting Test Mode.

The pressure difference on the regulator can be measured between the venturi and the feedback tube. It is necessary to unscrew the pressure tap on the venturi for this measurement, and to close it afterwards.

With 17% of the reference load and a CO2 level of 8.7%, it should be possible to reach this minimum pressure difference. If the CO2 level is higher, the minimum power also has to be proportionally higher than 17%.

Problem: No gas, no burner start up

Check whether both coils of the two shutoff valves are energized and whether both shutoff valves are opening (you should hear a double “click” in the valve). Also check the gas inlet pressure.

Problem: Gas OK, no burner start up (both shutoff valves are open, fan turns)

There is not enough gas to accommodate a start up. Turn throttle screw ½ turn counterclockwise and try to start again with more gas.

Problem: Boiler makes noise at high CO2 level

Premix-boilers may have a loud whistle noise problem at high CO2 levels. This can sometimes be alleviated by decreasing the CO2 level.

Problem: Strong oscillations at burner start

It is normal for a small oscillation to occur in the first second of the burner start. If the oscillation is very strong or lasts very long, decrease the resistance of the chimney by using a shorter tube with a larger diameter. An additional solution is to change the start up power of the burner.

Problem: Nominal power of the mixing unit is unattainable Possible causes for this issue are:

1.the boiler and burner head pressure drop is too large;

2.fan not turning at correct speed.

3.sensing tube is clogged or broken.

Problem: After some time, the pressure regulation of the valve fails

Dirt in the valve finer than 0.013” (0.34mm) can prevent the correct function of the servo pressure regulator. Replace the valve and add a gas filter in front of the new valve.

preventative maintenance

This is a pre-mix burner system. The flame is not supposed to be directly on the burner. The flame should be just above the burner deck approximately 1/8” and blue in color, see Figure 21.

FIGURE 21. BURNER FLAMES

Visually check flame characteristics through the view port located on the top head of the boiler. Figure 21 shows the normal flame condition.

These boilers are designed to give many years of efficient and satisfactory service when properly operated and maintained. To assure continued good performance, the following recommendations are made.

The area around the unit should be kept clean and free from lint and debris. Sweeping the floor around the boiler should be done carefully. This will reduce the dust and dirt which may enter the burner and heat exchanger, causing improper combustion and sooting.

Main Burner

Check main burner every three months for proper flame characteristics. The main burner should display the following characteristics:

•Provide complete combustion of gas.

•Cause rapid ignition and carry over of flame across entire burner.

•Give reasonably quiet operation during initial ignition, operation and extinction.

•Cause no excessive lifting of flame from burner ports, see Figure 21.

If the preceding burner characteristics are not evident, check for accumulation of lint or other foreign material that restricts or blocks the air openings to the burner or boiler. To check burners:

1.Shut off all gas and electricity to unit. Allow unit to cool.

2.Remove main burners from unit.

3.Check that burner ports are free of foreign matter.

4.Clean burner with vacuum cleaner. DO NOT distort burner ports.

5.Reinstall burners in unit. Ensure that all the screws on the burner flange are tightened securely so that the gasket will provide a good seal.

6.Also check for good flow of combustion and ventilating air to the unit.

After placing the boiler in operation, check the ignition system safety shut-off devices for proper operation. To accomplish this with the main burner operating, close the valve on the manifold. Within four seconds the main burners should extinguish. If this does not occur immediately, discontinue gas supply by closing main manual shut-off and call a qualified service agent to correct the situation. If the burners extinguish, then light boiler in accordance with lighting and operating instructions.

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