I am renovating my fireplace that was original to the home built in 2007. It was not to our liking, so I am doing a resizing to make it fit better in the room. In doing this I needed to change the length of LP gas line going to the firebox. It is set up for gas, perhaps a log lighter or gas log, but I am using it as a woodburning unit as the prior owners have in years past.

I have already changed the length of the pipe so what extends into the firebox is the same as original. I have tested it for leaks, there are none. To test it, I turned the emergency valve on to pressurize the new length of pipe, and sprayed a soapy water solution on the 3 joints that are new. I did this 3 times over a 3 hour period and will do it again before I put the last sheet of drywall in place to cover it. I will close the valve after I find everything satisfactory.

The question is, since this was done years ago and the firebox did not blow up, will my charging the line the way I have trap gas between the valve and the cap that is in the firebox? I presume that some possibly may have entered the pipe beyond the valve since it had to pressurize the little that it did.

Not knowing the procedure the original installer did, makes me wonder if they did the same as I, but knew that it would do nothing as proved over time, that the amount of expansion from the pipe getting hot would not allow the pipe to bust (blow up). If there are any plumbers or fireplace installers out there that have done the same set up as I have here, and the firebox lived to tell about it, please chime in.enter image description hereenter image description hereenter image description here

  • Gas is only explosive when under dense conditions. The gas leftover from a pipe will do nothing but burn for a few seconds if it does that.
    – DMoore
    Mar 7, 2017 at 6:49
  • Yes, I am aware that it would be a small amount of gas in the pipe, if any, and if the pipe were open on one end, it would do no more than go poof when lit. It is when it is in pipe sealed on both ends, one end with a cap, the other with a valve, and set in a fire is what concerns me. We have burned the fireplace for 2 seasons, so I know it could be safe, but the pipe in the fire most likely was "airlocked" It may still be, since the line is "teed" off below the floor. FWIW, I did not let the gas flow while the pipe was open, just to test for leaks.
    – Jack
    Mar 7, 2017 at 15:54
  • 1
    What does turned the emergency valve on mean? Does that mean the valve is open or closed?
    – wallyk
    Mar 10, 2017 at 19:26
  • When I bought the house the valve was closed all the time until I extended the gas line to the firebox after I moved the supporting framing back. That is when i turned it on the only time I know of. It is back off again.
    – Jack
    Mar 11, 2017 at 7:32

2 Answers 2


Since you don't say exactly where this is, I used your vague profile location ("NW Washington state") and referred to the code for Seattle.

The proper leak test is to pressurize the pipe to at least 15 psi with a pressure gauge monitoring and verify that there is no visible decrease in pressure reading after 15 minutes. See section 406.4 of this.

Note that most appliances cannot withstand that kind of pressure, so the testing procedure specifies that individual appliance shut off valves should be set to block flow or be disconnected and the appliance supply line capped.

Typical natural gas residential pressure is 7 inches of water column, which is about 0.25 psi. So the standard test is a factor of 60 times over the working pressure.

... since this was done years ago and the firebox did not blow up, will my charging the line the way I have trap gas between the valve and the cap that is in the firebox?

I think the answer is yes, but I don't think it is the relevant question. If I understand correctly, you leak tested with gas line pressure only and that is insufficient to reliably detect a leak.

If it were me, I'd be doing a whole gas pipe test by:

  • Cutting off all the appliances (range, furnace, hot tub, dryer, water heater, etc.) with their individual input line valves.
  • Operate the cutoff valve at the output side of the gas meter to isolate the system.
  • Remove a conveniently accessible appliance flex line and attach a pressure gauge assembly.
  • Pressurize the system (a bicycle tire pump is useful) and observe the steadiness of pressure.
  • When I have done this, the test gauge itself was a significant source of leakage. Read about it here.

As for concerns about heating causing a leak, it should be okay if the cap and the pipe are made of the same material so they both expand at the same rate.

It looks like black pipe in your photos. The melting point of "malleable iron pipe" is 2,570 °F. The hottest temperature a wood burning fireplace experiences is from the glowing coals after the wood has been consumed at about 1,200 °F. The pressure rating of the threads at that temperature looks (by extrapolation) to be 10+ psi according to this table.

  • It may not change the procedure you described but I am in Whatcom County, about 1/2 hour Northeast of Bellingham, about 3 hours north of Seattle. I also understand that what you describe is the proper process for testing gas lines in my case, liquid propane. But what the question really asks is what concerns should I have about the trapped gas from the not so proper way I tested the gas for leaks? I will pursue testing the gas the way you describe if I can find the means to do so with my limited resources.
    – Jack
    Mar 10, 2017 at 16:49
  • Just to clarify the process that I was not sure of, whether or not they tested the line all the way to the cap in the fireplace, and when they tested it was gas present in that capped pipe in the fireplace beyond the emergency cut off valve. In reality I know there is no way to determine that, but if somebody has done this type of install before and knows that there are no problems with the testing done with the cap and gas present in the line that's the type of reference I need.
    – Jack
    Mar 10, 2017 at 16:58
  • @Jack: Aha! I interpreted LP gas to mean low pressure natural gas. Acronyms and abbreviations are a bitch. Liquid Propane isn't something I have worked with (except a barbecue as an appliance user).
    – wallyk
    Mar 10, 2017 at 19:25
  • 1
    The testing procedure is the same for LP as natural gas in Oregon 15psi the main difference is the orifice size. There should be a regulator on the tank to limit the pressure coming into the house.
    – Ed Beal
    Sep 25, 2017 at 15:40

Like the above reply low pressure gas pipe installations are required to be tested and hold 15 psi for fifteen minutes. Appliance connector caps due to being removed for installation of appliance. are typically only tested when the system or extension is installed and often the gas test gauge is used ad the de facto cap. In your case you have several fittings before the fireplace shut off valve that would be needed to be tested.You also have some corrugated gas pipe that looks like it was installed as part of your project. That would also need to be tested. I would put the test gauge on the fitting were the corrugated piping begins or cap that and put the gauge on the fireplace stub out if it is being inspected. There is always one untested fitting on a gas system. I see you used rectorseal which is a great product. An old plumber told me he uses teflon tape and rectorseal on every joint and never had a leak.You could test the whole house but it is a pain disconnecting every appliance and not necessary although I have done it before. Inspectors will usually allow you to test only the segment you are adding.As far as gas being in the short segment it is that way for every home that has a fireplace.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.