Definition of Furnace Gas Valve

A furnace gas valve is a component of your furnace’s fuel system. The furnace gas valve opens and closes, which allows the flow of gas to the pilot light and burner(s). While the component itself if actually rather simple, a furnace gas valve is a crucial part of your HVAC system. Furnace gas valves are only seen on gas-fueled forced air furnaces or gas-powered boilers. There are also gas valves located within gas-powered hot water systems or fireplaces.

How A Furnace Gas Valve Works

There are essentially two valves that make up your furnace’s gas valve, and they are positioned in series, or one after the other. The furnace gas valve operates by electromagnets. The primary valve -- also known as the safety valve -- supplies gas to the pilot light. The second valve -- also known as the main valve -- allows gas to flow to the burner trays.

The thermocouple (or thermopile) generates the power holds the safety valve open. Additionally, the thermocouple remains immersed in the pilot light flame. Without adequate heating of the thermocouple or thermopile, the furnace gas valve closes. As a result, this cuts off the supply of gas to the pilot light. Ultimately, the thermocouple acts as a safety mechanism that prevents gas buildup within the home.

A 24 VAC transformer (or the thermopile) powers the main valve. This valve allows gas to flow to the burner trays via a much larger tube than the pilot light valve. Additionally, the valves are installed in series with all of the other furnace safety controls. As a result, if the system detects a problem, then the circuit interrupts itself. During this process, the system shuts off power to the main furnace gas valve, while still keeping the pilot light valve open.

As the thermostat calls for heat, the furnace gas valve opens and closes. This process maintains the desired temperature within the home. Plus, this design helps regulate the pressure of gas flowing into the furnace.

Types of Furnace Gas Valves

There are a few types of furnace gas valve systems: gas chain, consisting of a manual valve, solenoid valve, and pilot safety; and a combination gas valve. A gas chain valve system requires the homeowner to manually turn a valve handle in order to open or close the flow of gas to the furnace. A solenoid valve opens only when the furnace calls for heat. As a result, gas only flows if all the other valves in the system are open. Plus, the pilot safety valve only stays open if the thermocouple or thermopile heats to a certain temperature by the pilot flame. It can be manually overridden in order to relight the pilot light, but otherwise operates via electromagnets as described above.

The combination gas valve gained popularity in the 1960s and performs all the functions of the gas chain in one package. It contains a valve knob or handle, regulator, thermocouple, electric terminals, and a solenoid valve. As technology has progressed, combination gas valves have fallen out of regular use, with their job now being performed by electronic ignition controls or integrated furnace controls (IFCs).

Troubleshooting Furnace Gas Valves

If you think your furnace gas valve is not working, there are a couple of steps you can take to troubleshoot the problem and narrow down the root cause.

• Check to see if the pilot light is on. If not, follow the manufacturer’s instructions for relighting the pilot light. It may take a few seconds after relighting the pilot light for the main valve to have enough power to stay open.
• Check to see if the thermocouple or thermopile is generating enough voltage to keep the safety valve open. If not, you can replace it. Also check to ensure the other safety mechanisms within the circuit are receiving adequate voltage. If this doesn’t fix the problem, you will need to replace the entire furnace gas valve.

Cost of Replacing a Furnace Gas Valve

If you need to replace the gas valve in your furnace, costs will vary depending on the make and model, as well as your location as labor costs differ depending on region and even season.

Although many homeowners troubleshoot heating issues, the best option remains consultation with a qualified HVAC professional if your furnace isn’t operating properly. They are experts who can get your heat back up and running quickly and safely.