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ServiceF1rst · Posted: Sat Oct 06, 2007 5:07 pm Post subject: Pressure Regulating Gas Valves

Pressure Regulating Gas Valves

The techniques and skills used to service Smart Valves are the same or very similar to those already being used to service standard systems. This article will focus on the Pressure Regulating Valve (PRV) components that are part of the gas valve/ignition control system.

The gas pressure regulator's purpose, whether it is a functional component of the control valve, or a separate entity, must control the gas pressure (natural or L.P.) to the main gas burner. The gas utility provides a main (service-pounds to inches) regulator to the residence or building from which all of the building’s gas needs are to be met. Generally, the goal is to supply natural gas at 7" water column (W.C.) at the outlet side of the regulator. On occasion this may not happen. In extreme cases where the weather is severe and/or the customer is toward the end of the gas line, the available pressure, because of capacity issues, may actually be lower than desired.

Most gas burning appliances are designed to operate on 3.5" W.C. on natural gas ( 11.0" W.C. on propane). One thing is certain - THE GAS VALVE REGULATOR CANNOT RAISE THE INLET PRESSURE TO THE MAIN BURNER ABOVE THE INLET PRESSURE TO THE VALVE. When the technician checks gas pressures he/she should have all of the buildings appliances operating at full capacity.

It is important to note that over 95% of all combination gas valves DO NOT regulate the pressure of the gas to the pilot outlet. When you adjust the pilot adjusting screw, you are changing the flow rate - not the pressure - and you are basically changing only the pilot gas flame size. It is important to keep the pilot flame steady, (not like a torch) and in constant contact with the pilot burner hood for good pilot fame recognition in the sensing circuit. The pressure of the pilot gas will be the pressure of the gas at the valves inlet minus the pressure drop through the valve. On average, you could see 6.0 to 6.2" W.C. when you have 7.0" at the valves inlet (natural gas).

Troubleshooting Tips

# 1. - Each valves capacity is partially a function of the valves inlet and outlet dimensions, and the regulators range of pressures that it can safely operate. The valves complete model number will provide the information about its capacity, the regulators range of pressures it can provide and other special features the valve may include.

# 2 - Any time you find it necessary to replace a gas valve it is a good technique to, after replacing the valve, check the outlet pressure to the main burner, To insure that you do not overfire or underfire the burner (sooting, not enough capacity to meet the spaces requirements, tee..) always check the appliance’s rating plate and check to see the regulator is set correctly.

# 3 - You may change the regulator’s output by removing the cap screw and repositioning the underscrew. Turn the underscrew CLOCKWISE for RAISING the set point and turn the underscrew COUNTER CLOCKWISE to REDUCE the set point.

# 4 - All combination gas valves have outlet pressure taps for measuring outlet pressures and only Honeywell provides an inlet pressure tap for ease of getting inlet pressure readings .

#5 - A few valves (including Smart Valves) have outlet screens (filters) - in such event, these filters will reduce the output capacity by 5%. Most valves have additional filters imbedded in the body of the valve to protect the pilot gas passages and these are not replaceable in the field.

#6 - If the inlet gas pressure to the valve (including Smart Valve) is in the valves nominal range (consult the valves specification sheet) and you cannot provide the correct capacity / pressure to the main burner - you should replace the valve.
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Of all the instruments utilized in the HVAC/R trades - I trust analog gauges the least.