This article is an extension of HVAC School’s “How to Measure Gas Pressure” video. Additional input was provided by Jim Bergmann. Bubble test procedures and standards were provided by Honeywell. Links to source materials are available at the end of this article.

If there’s one complaint that HVAC techs should never take lightly, it’s a report that a customer smells gas in their home. Even here in Florida, we receive a fair volume of calls from customers who smell natural gas. The customers may not know what is leaking, but they’re sure they have a leak on their hands. For many of us, our minds will immediately jump to the gas valve.

However, many gas valves are replaced unnecessarily. Needless valve replacement can waste your time and the customer’s money, so everybody loses. We’re here to help reduce some of that heartache.

A note about gas leaks

All gas valves leak. You’re not going to get a 100% leak-proof system. The question really isn’t about if there is or isn’t a gas valve leak.

The real question about gas valves is if they leak significantly or not. Some residual gas will always remain in the manifold and slowly dissipate, even when the unit isn’t running. It has nowhere else to go but out. Still, the presence of residual gas alone does not indicate a significant leak.

Even though we recommend using an electronic leak detector in our video, Jim Bergmann advises against it for the manifold (outlet of the valve). Depending on the electronic leak detector’s sensitivity, it may pick up residual gas or minor leaks that do not affect the customer’s air quality or safety. Instead, Bergmann recommends using a bubble test to determine the extent of leakage (not its presence).

A bubble test measures the gas bubbles that escape from a valve over 10 seconds. Manufacturers have standards for allowable leakage, and a bubble test reveals if the leakage is within normal limits or excessive. Gas valves only need to be replaced if the leakage is above the maximum standard value.

The bubble test has two parts: setting up testing conditions and restoring the system to normal. We will walk you through the steps of each part.

Bubble test: part 1

The first part of the bubble test sets up the testing conditions to produce measurable bubbles. We need to divert the gas flow to a water source. When we redirect the gas, we can physically see the leak if we watch the gas move through a fluid in a different state of matter.

For a visual aid, you may want to consult the diagram below to see the positioning of the valves you will be opening and closing throughout the test:

1. De-energize the control system.

In plainer terms, “shut off the control system.” When you de-energize, you will want to make sure that no power runs from the gas supply source. Ensure that the safety shutoff valve (SSOV) does not have power running to it. You will perform the test just beyond the SSOV.

2. Close the upstream manual gas cock.

The upstream manual gas cock lies between the gas source and the pressure regulating valve (PRV).

3. Make sure the manual test petcock is closed in the leak test tap assembly.

Depending on your gas piping system, this could also be a permanent petcock. Regardless, you will want to ensure that the petcock is closed.

4. Remove the leak test tap plug and connect the testing apparatus to the leak test tap.

The leak test tap plug is on the petcock. Remove it and fasten the testing apparatus. The testing apparatus you’re going to use is ¼” wide flexible tubing attached to ¼” wide aluminum or copper pilot tubing.

5. Close the downstream manual gas cock.

The downstream manual gas cock is located between the petcock and the burner itself. Make sure it is closed.

6. Open the upstream manual gas cock.

Again, the upstream manual gas cock is between the gas source and the PRV. We closed it during Step 2. You are going to want to open it this time.

7. Run the safety shutoff valve (SSOV) to its fully open position. Then, immediately de-energize the system to close the valve.

You will want to make sure the SSOV is fully open, but you don’t want to let it stay open for too long. You have to be quick to shut it down right after you completely open the valve.

8. Immerse your testing apparatus (tube system) ½” deep into a glass of water.

Dipping the testing apparatus in water will allow you to see the gas against a liquid. The tube will act like a straw, and a gas leak will act like a person blowing bubbles out of the straw and into a drink. Make sure the end of the tube that enters the water is cut to a 45-degree angle.

9. Slowly open the test petcock.

Take your time with this one, and don’t be too surprised if you see a short, quick burst of bubbles, even if you open it pretty slowly. The bubbles will stabilize. Remember, you don’t want to test the immediate release of gas. You want to test the constant flow of leaking gas.

10. When the rate of bubbles coming through the water stabilizes, count the number of bubbles that appear during a 10-second interval.

As I said during Step 9, the bubbles may flow irregularly at first, but they will stabilize. When they even out a bit, count the number you see within 10 seconds. The acceptable number of bubbles will vary depending on the gas piping’s size and the manufacturer’s recommendations. The chart below shows the appropriate amount of bubbles for each pipe size with a Honeywell V4730C, V8730C, or V4734C gas valve. It also presents rough conversions (and formulas) to measure leakage in cubic centimeters per hour.

Bubble test: part 2

The second part of the bubble test deals with restoring the system to normal and making sure that there are no leaks at the test tap (not the valve!).

1. Close the upstream manual gas cock.

Once more, this is the valve between the source and the PRV. We closed this valve during Step 2 of the first part of the test and reopened it during Step 6. We are going to close it again.

2. Close the test petcock and remove the testing apparatus. Replace the leak test tap plug.

Remember, the leak test tap is on the petcock. You will want to make sure the petcock is closed and that you remove the apparatus before you try to plug the tap again.

3. Open the upstream manual gas cock and energize the safety shutoff valve (SSOV).

Didn’t we just close the upstream manual gas cock? Yes, we did, but we need to reopen it for this step. Now is also the time to re-energize the SSOV.

4. Test with soap bubbles to ensure that there is no leak at the test tap.

Liquid soap is a suitable medium for testing gas leakage in a single small area because it’s thick, and the bubbles tend to stick for a while. You’ll want to apply soap bubbles to the test tap to make sure the area you just tested is secure.

5. De-energize the safety shutoff valve (SSOV).

You should have powered the SSOV in Step 3. You will now remove power from the SSOV.

6. Open the downstream manual gas cock.

We have finally reached the downstream manual gas cock again. Remember, this is located between the burner and the test tap location. You will want to open it.

7. Restore the system to normal operation.

This is the end of the test procedure. You are going to restore the system to its usual operating mode. Be sure to double-check the SSOV closure’s tightness, especially if the unit has more than one SSOV.

After repeatedly opening and closing many valves, you have completed the bubble test. There is no way you’re going to spot a gas leak if the gas dissipates into the air, so it’s a good idea to analyze gas movement against a liquid medium. This test method also allows you to apply and sharpen your visual inspection skills.

Even though it may seem simple (or silly), counting bubbles is a more reliable way to detect significant valve leaks than an electronic leak detector. Don’t get us wrong, we still love our electronic leak detectors for finding gas leaks elsewhere, but they can be a bit too sensitive for valve testing.

Is this process practical for normal field testing? I will leave that up to you and your own company operating procedures to decide.

For more information on trace leakage and Bubble-O-Meter tests, check out this Honeywell article on the topic (provided by Gray Cooling Man).

For more information on V48 and V88 gas valves, check out Honeywell's PDF on their product data.