How to Test a Furnace Gas Valve (5-Step Guide)

Have your furnace been shutting off unexpectedly? Or maybe it’s running, but there’s no warm air!

If either of these sound familiar, you’re in the right place!

In this proHVACinfo guide, you’ll learn:

  • What You Need To Know About Furnace Gas Valves
  • Tools And Supplies You’ll Need For Testing Your Furnace Gas Valves (ensure you have these before starting the job)
  • How To Test a Furnace Gas Valve (5- step guide)
  • And much more!
proHVACinfo - How to Test a Furnace Gas Valve

What You Need To Know About Furnace Gas Valves

The gas valve is a critical component within the furnace’s inner workings.

As with any valve, your furnace gas valve opens and closes to provide a flow of gas to the pilot light and to the burners.

You will only find furnace gas valves on gas powered forced air systems – so if you’re running a different kind of furnace, this article isn’t for you!

How A Furnace Gas Valve Works

Your furnace actually requires two separate gas valves in order to operate.

The two are located next to each other, but take on different tasks.

The first, or primary, valve we’ll talk about is the safety valve.

This particular valve is responsible for the gas supply to the pilot light.

The main, or secondary, valve, is responsible for the supply of gas to the burner heads.

This is the valve that the majority of this guide will be referencing.

Modern furnace gas valves are actuated with electromagnets for opening and closing. To operate the electromagnets, a thermocouple is mounted within the flame of the pilot light.

When the pilot light goes out, the thermocouple cools and the primary valve closes, preventing the flow of gas into your home – hence the name “safety valve”.

The main valve’s thermocouple is powered by a 24-volt AC transformer.

This active power supply is necessary owing to the fact that the secondary valve is connected to a much wider bore pipe than the pilot light’s.

As an extra precaution, the gas valves are wired in series with all of the other failsafe devices within the system.

This ensures that if a problem is detected at any point in the system, the main gas valve will close to prevent dangerous gas leaks.

When the thermostat indicates that the home is below the desired temperature, the main valve opens and closes in order to meet the demand and maintain the set temperature.

Tools And Supplies You’ll Need For Testing Furnace Gas Valves

In order to tackle this job, there are a few tools you absolutely must have.

The good thing is that some of these items are useful for other jobs around your home, too, so they are actually a nice investment in your DIY arsenal!

  1. A  socket set – This will be required to gain access to the inner workings of your furnace, where the gas valves are located. Different manufacturers have been known to use different size bolts, so having access to a whole set is particularly useful. This is one of the tools that is generally useful to have around the house, too.
  1. A good quality multimeter and clamp meter combo kit. Not only is this great for your gas valve, it’s useful for many other electrical checks within your HVAC system, and elsewhere in the home.
  1. A set of jumper wires – Different systems will have the gas valves at varying distances from the transformer, so to ensure you’re able to stretch the gap when checking the valve operation, be sure to have a wide selection available.

How To Test A Gas Furnace Valve (5 Steps)

Before we get on with the business of testing the gas valves, eliminate the possibility of a gas supply problem.

The simplest way to do this is to test any other gas powered appliances in the home; if your stove runs, you’ve got gas to the home.

If it doesn’t that’s a whole other story, and you’re probably going to need to call in professional help.

You should also note that each furnace model can potentially have different sequence of ignition, which can affect the manner in which the gas valve operates.

We are providing an overview of how to test the gas valves of the most commonly found furnace types.

Pro tip! Before you get started, take detailed pictures of the wiring arrangement and the gas valve before you start.

This will serve as a handy reference when it comes time to returning wires to their correct positions.

Step 1 – Attempt To Fire Up The Furnace

In order to carry out the following steps, we need to activate the furnace and get a call for heat signal to the control board.

The simplest way to do this is to set the thermostat to a higher temperature.

This will energize the induced draft motor and turns on the hot surface igniter, and all being well, this should energize the gas valve.

Step 2 – Check That The Gas Valve Is Drawing Current

There will be two wires leading to the valves depending on your system – choose any of them to begin as it doesn’t really matter at this stage.

Take your clamp meter and clamp one of the wires. Observe the display on the meter as you start the furnace. If it reads 0.0, you should try clamping the other wire and read again.

If you have the correct wire this time, you should see a small amp draw of up to 0.2 amps for a few seconds.

This will confirm whether the valve is drawing power or not.

Note that this doesn’t necessarily mean that the gas valve is good, we have simply ruled out the possibility of electrical failure.

Step 3 – Check The Voltage

It’s a good idea to now double check the power situation by checking the voltage, too.

Take your multimeter and set it to Volts AC (VAC) and place the probes on the connection between the wires and the gas valve (black to white, red to red).

Within about three seconds of the call for heat, you should see the meter reading around 24 volts.

If you see from the meter that the gas meter is indeed drawing the correct amount of power, but we still cannot hear gas flowing, we can eliminate power as a cause of the problem.

Step 4 – Deenergize The System And Continue The Checks

Now we have concluded that power isn’t the issue, for safety reasons, we should now deenergize the furnace system to prevent the risk of shock or accidental startups.

We always recommend turning the system off at the breaker to prevent anybody who unknowingly or accidentally touches the thermostat from reenergizing the system.

Next, disconnect the two low voltage wires from the gas valve, and in their place attach two jumper wires.

Before carrying on with this test, please note that you shouldn’t allow this check to run for more than 2 to 3 seconds.

At the transformer, you will see two pairs of wires, line voltage in (black and white) and 24v (red and brown).

Connect the jumper wires from the gas valve accordingly to the connectors on the transformer.

Turn the power back on, then close the door switch for 2 to 3 seconds while you listen for the flow of gas. If you don’t hear gas flowing, we now know for certain that the gas valve itself is bad.

Step 5 – Reset And Close Up The Control Box

At this stage, you will have either deduced that your furnace issues are related to power, the gas valve itself, or some as yet unknown issue.

If you haven’t been able to narrow down the problem at this point, please be sure to shut the system down and call an HVAC professional immediately to prevent potentially dangerous situations from occurring.

In any case, return all wires to their original positions, and close the panel back up.

If you have established that your gas valve is faulty and you are competent and confident to replace it yourself, this is absolutely possible.

We will be covering how to replace a faulty gas valve in an upcoming proHVACinfo guide, so make sure to check back regularly for that tutorial.

If you’ve been suspecting that your gas valve is faulty, we do hope this has provided you with some insight (and hopefully a solution) to your problem!

Stephen Marks

Stephen Marks

Stephen is an HVAC and home-repair enthusiast. He's here to answer any of your questions about HVAC!

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Stephen Marks

Stephen Marks

Stephen is an HVAC and home-repair enthusiast. He's here to answer any of your questions about HVAC!

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