|Gas Furnace Components: The Gas Valve
The gas valve component of your gas furnace connects the gas line to the manifold and controls the gas feed to the manifold and burners. The gas valve component of the gas furnace is considered a gas control. The components inside the gas valve are slightly different from the natural gas gas valve to the propane gas gas valve. Many new gas furnaces come equipped with a natural gas gas valve but have a propane conversion kit with the new natural gas gas valve so it can be quickly converted in the field depending on the type of fuel delivered to the gas furnace. After a new installation of a gas furnace the furnace should be inspected by the local inspections department. The Fuel-Gas Code requires all new installations of gas piping receive a pressure test in accordance with the guidelines in the Fuel-Gas Code book. The gas line near the furnace should be disconnected before this gas line pressure test is performed. If not the gas valve can be damaged as result of the excessive pressures placed on it during this gas line pressure test. This is also in accordance with many furnace manufacturers and gas valve manufacturer’s installation instructions which usually take precedence with many code officials and inspectors over the code book. Too much pressure on the gas valve and the diaphragm could rupture or other components inside the gas valve will be damaged because they are not rated for the higher pressures involved with testing the gas line. It is also in the Fuel-Gas Code that the gas line be disconnected from the meter or a live gas connection before this pressure test is performed. It is necessary after the gas furnace is put into operation that the gas valve have the proper pressure drop across the valve. This is dependent on the gas pressure supplied to the gas valve and a small adjustment on the gas valve which requires a special instrument for properly setting the right pressure drop across the gas valve. The HVAC installer or HVAC start up technician should have this instrument and the ability to make this adjustment. The gas valve gets its signal to open from a gas furnace control board or gas furnace control module. There is a sequence of operation that must occur before the gas valve will open allowing gas to get to the manifold and the main burner. This sequence of operation includes proving all safety switches in the gas furnace safety circuit are closed and ready for main burner firing. The gas valve usually requires no maintenance but if it is ever submerged under water or becomes excessively wet from moisture it is recommended it be replaced and precautions taken to prevent the new gas valve from getting wet.
Last edited on Mon Oct 18th, 2010 12:19 am by Bobice
Retired US Army Refrigeration Specialist / NYC Board of Education HVAC/R (Retired)
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