If your natural or propane gas furnace won't ignite, then a malfunctioning gas valve may be the cause. Below is how you can check to see if your gas valve is functioning properly:
Tools and materials need
Electrical multimeter with probe leads
Nut driver with bits
1. Remove the access panel - Before beginning, be sure to wear a pair of sturdy work gloves to protect your hands from possible burns and cuts caused by sharp sheet metal edges. In addition, turn off the furnace's main power supply at the nearby switch to protect yourself from possible electrical shock during the removal process.
The access panel will be held on by several screws, so locate them on the front and sides of the panel. Remove the screws while holding on to the panel to prevent it from falling, then set the panel aside in a safe location. Be careful not to pinch or cut any wiring while taking out the screws or handling the panel.
2. Verify the hot surface igniter is functioning - Once the panel is removed, be sure your hands and body are clear of the furnace electrical components, then restore power at the switch. Activate the furnace at the thermostat, then observe inside the burners through the sight glass. You should see a distinct, bright yellow glow from the hot surface igniter; this element heats the interior of the burners and allows for instantaneous gas ignition as soon as it reaches a preset temperature. If you do not see any "glow", then your igniter may have failed, and you will need to contact a technician for service or replacement.
3. Check the gas supply valves - If the hot surface igniter seems to be functioning properly, then the next step is to ensure your gas supply is intact. Check all of the following locations:
Main gas valve at the meter - Verify that your home's gas supply has not been disconnected from the provider's supply line.
Furnace supply valve - Locate the local furnace valve that supplies the unit; it will be in close proximity to the furnace itself and will be in the 'on' position if the valve handle is parallel to the gas line.
Gas valve shut-off - On the gas valve itself, verify the shut-off is turned to the 'on' position by ensuring the indicator is aligned with the 'on' marking on the valve body.
If any of the last two locations indicate the gas supply is disrupted or disconnected, then correct the problem by turning the appropriate valve back to its 'on' position. Never attempt to tamper with a gas meter valve; always contact your local utility company for assistance in getting your gas service restored.
4. Check the furnace gas valve for power - If you have gas supplying the furnace, then the next step is to check the gas valve for electrical power input. The gas valve will either contain two or three electrical wires leading into the unit; single-stage valves, which provide gas at one flow rate, have two wires connecting it to the furnace controller. Two-stage valves, which allow for decreased flow rates to improve efficiency, are attached to three wires. Testing these wires with a multimeter will enable you to determine if electrical power is connected.
To check for power, remove the two or three wires from the valve by pulling the connector from its plug on the valve. Next, set your multimeter to the the zero-to-two hundred volt range, or the closest comparable range. If the valve is a two-stage unit, insert one lead into a connector opening with the other lead in one of the other two connector openings. Set the thermostat to make the furnace start, then watch for a voltage spike on the meter. If you see 24 or more volts, then you can assume the valve is receiving power from the controller.
If you don't see any voltage, move one of the meter leads to the unused connector opening, then reset the furnace a second time. If you still do not read any volts, then you know the controller is not sending a powered signal. For one-stage valves, simply insert the leads into each connector opening, then repeat the same process of resetting the thermostat to check for a powered signal.
A valve that is receiving power, but not activating, is due to an internal component failure, and it will need replacement. Contact a technician for assistance with replacement of the faulty valve. If the valve is not receiving power, then the problem likely lies with the furnace controller and will need to be diagnosed and repaired by a technician.
For more information about furnace repair, contact a company like United Heating Cooling and Plumbing Inc.