Gas furnaces have numerous different components that all need to work correctly in order for the unit to ignite and heat effectively. One of these components is the gas valve, which opens and closes to control the flow of gas to the furnace burners. As with any other furnace component, the gas valve can sometimes wear out or break. Troubleshooting a faulty gas valve isn’t always easy as there are several other issues that can cause the same problems as a bad gas valve, but there are some signs that can help. With that in mind, here is an overview of how a furnace’s gas valve works and how to know if your gas valve is faulty.

How Does a Furnace Gas Valve Work?

To understand exactly what a furnace gas valve is and how it works, it is first necessary to know a bit about the sequence of operations that occurs whenever a gas furnace starts. When your thermostat detects that the temperature has dropped below what you have it set to, it sends an electrical signal to the furnace’s control board.

Once the control board receives the signal for the furnace to start, it then powers the unit’s draft inducer motor. The inducer creates a draft that draws any latent combustion fumes out of the furnace, which is important as these fumes are inflammable and could prevent the furnace from lighting if they remained in the combustion chamber. The draft created by the inducer produces suction that activates the furnace’s pressure switch.

Once the inducer motor is running, the control board sends a signal through the furnace’s safety circuit. The pressure switch is what closes the safety circuit so that the signal travels back to the control board. If the pressure switch doesn’t activate for any reason, then the safety circuit will remain open. This means that the signal won’t travel back to the control board and the furnace won’t continue trying to start.

If the safety circuit is closed and successfully sends the signal back to the control board, the control board then signals the furnace’s electronic igniter to start. Most modern furnaces use either a direct-spark or hot-surface igniter that works to light the unit’s gas burners. If your furnace is more than 12 years old, it may instead use a traditional pilot light to ignite the burners. Furnaces with a pilot light skip this step since the pilot has a constant flame that will light the burners as soon as the gas starts flowing.

As the control board signals the electronic igniter starts, it also signals the gas valve to open so that gas will start flowing to the burners. The furnace will then immediately light as soon as the gas starts flowing. The furnace’s flame sensor should then detect the heat from the burner flames almost immediately. If the furnace doesn’t light or the flame sensor is dirty or malfunctioning and doesn’t detect the flames, the control board will then automatically signal the gas valve to close. This is an important safety feature as it ensures that gas won’t continue to flow if the furnace doesn’t light.

If the furnace does light successfully, it will continue to run for a few seconds to ensure that the unit’s heat exchanger fully warms up. Once the heat exchanger is warm, the control board will then signal the blower motor to start so that air is drawn into the furnace and circulated throughout the ductwork. Now that you have a basic understanding of the role that the gas valve plays when your furnace starts, let’s look at the different signs that can indicate your furnace’s gas valve is faulty.

Furnace Fails to Start

A faulty gas valve will usually prevent the furnace from igniting. The furnace will still try to start, and you will hear the inducer motor run for a minute or so, but then nothing else will happen since the gas valve won’t open and no gas will flow.

In order for the gas valve to open, a current has to flow across it. If the current doesn’t flow across the gas valve because its wires are loose or frayed, then the valve won’t be able to open. The valve can also simply break or wear out and thus get stuck in the closed position.

If you hear your furnace’s inducer motor start and the unit doesn’t light within 60 to 90 seconds, you’ll need to have it inspected by a technician. The technician will first inspect the gas valve’s wires to make sure that they aren’t burned, frayed, or otherwise damaged.

If the wires aren’t in good condition, they will first need to be replaced before the valve itself can be checked. If the wires are fine, the technician will then use a multimeter to measure whether the gas valve is receiving a 24-volt current from the control board. If no current is flowing to the gas valve, it usually indicates that the control board is faulty and needs to be replaced. If the current is flowing and the gas valve isn’t activating, then it means that the valve itself is bad and needs to be replaced.

Gas Valve Doesn’t Make a Clicking Noise

You can also sometimes tell if your gas valve is faulty by listening to your furnace as it attempts to light. After the inducer motor has been running for around a minute, you should hear an audible clicking sound once the gas valve opens. If you don’t hear the gas valve click open, it indicates that either the valve or the control board is malfunctioning. If the valve does click open but the furnace still won’t light, the problem is most likely related to a faulty electronic igniter.

Furnace Was Flooded

If your home ever floods and your furnace ends up submerged in water, you will always need to have your gas valve replaced. This is because any exposure to water will instantly ruin the valve and its wiring. If your furnace is ever flooded, you will want to have it inspected before you ever try to use it as there is always the chance that the gas valve could get stuck open and create a major safety hazard.

Gas Leak

All modern furnaces have a number of different safety features that should prevent gas from flowing if the furnace isn’t lit. However, there is a chance that something could fail and cause the gas valve to not close when it should, and this can result in gas starting to leak out.

Although natural gas is odorless, gas companies add a special chemical to it that gives it a distinct rotten egg odor. If you do ever smell gas in your home, you will want to take immediate steps to protect your family due to the risk of a catastrophic explosion. If the smell is fairly faint and confined to the area around your furnace, you should immediately close the main gas shut-off valve and then quickly vacate the house until you can have your furnace and gas line inspected.

If the gas smell is strong or permeates throughout much of your home, you should immediately get yourself, your family, and any pets out and far away from the house. You may also want to quickly open a few windows on your way out and leave the door open as this will help the gas to disperse and reduce the risk of explosion. Once you are a safe distance away, you will then want to call the fire department or your gas company’s emergency line to report a gas leak.

Expert Heating and Cooling Services

If your furnace fails to light, the certified technicians from Crystal Blue Plumbing, Heating & Air can inspect it and quickly determine whether the issue is caused by a broken gas valve, faulty control board, or some other problem. We specialize in furnace and air conditioner repairs, as well as maintenance and installation, and we also offer a full range of plumbing services. Give us a call today to schedule a furnace inspection or if you need any other heating, cooling, or plumbing service in the Sacramento area.