The gas piping from my gas meter comes into my house in a straight 10 foot run to my gas furnace

Two feet from the exterior wall the black pipe has a t connection with a run to a gas dryer. The gas dryer works fine but the 3 year old gas furnace has a supply problem.

The furnace initially ignites okay with a gas pressure reading of 7 but when the burners go to the second stage the gas pressure drops to a .5.

I have been advised that there is a possibility that there is an obstruction in the black pipe. Here are my questions

is it possible that the gas valve adjusting the flame might be bad?

How would the possible obstruction in the black pipe be cleared?

What type of professional should I hire to address this? An HVAC tech or a plumber?

Is black pipe still the only acceptable material used for gas distribution in the home?

Thanks for your help

  • if it's after the meter, i think you'll need a plumber...
    – dandavis
    Oct 7, 2017 at 10:38
  • Or a steamfitter.
    – Lee Sam
    Oct 7, 2017 at 16:26
  • 1
    Licensed HVAC technician, as you are not certain the problem lies within the pipes. Once the technician diagnoses and confirms proper operation of HVAC equipment (or fixes it), he will tell you if he is qualified to perform repairs on upstream gas supply piping if necessary. If not, then licensed plumber. Oct 7, 2017 at 22:19
  • HVAC repair tech, with a van that says they also do boilers; pretty good chance they have pipe fitting experience. Any legit 'hvac' company that only does new installs prob won't do electrical or any plumbing beyond a condensate line. - Hire the one who says the problem (was, now it's worse) is that there's no drop leg.
    – Mazura
    Sep 19, 2019 at 2:42

2 Answers 2


Why would it act this way?

There is a difference between pressure and flow. If you have two tanks connected by an incredibly small tube, the pressure in those two tanks will be the same. Drain the contents of one tank, wait long enough, and the pressure in the two tanks will again be the same (and lower).

The size of the tube controls the flow rate, which limits how quickly the pressure can equalize. But it will eventually equalize.

If your gas pipe is 99.99999 % clogged, eventually the pressure on either side of the clog will equalize. But if you then open a bigger hole to pull the gas out -- like burning it -- the rate of exhaust will be greater than the rate of equalization through the tiny hole in the clog, so the pressure on that side will drop lower and lower. Until your stop burning the gas, at which point the pressure will slowly go back up and equalize.

Can you DIY? Yes.

Since you're posting on DIY.stackexchange.com, I'll point out that this may well be something you can do yourself.

You have a gas furnace. Somewhere near the furnace you have a gas connection that can be disconnected. This is probably a union fitting, like this:

union fitting

There are other possibilities, but this is by far the most common. Union fittings are important because pipe is threaded together, and you cannot loosen one arbitrary location without also overtightening the threads at the other end of the pipe. Thus, a union is a fitting that allows you to create a disconnection in the middle of the pipe.

If you shut off the gas (at the meter, or at some shutoff valve that may exist upstream of the possibly blockage) and then drain the gas lines (by turning on the stove and burning off whatever is left in the pipes, for example), you can then open the union, and work your way backwards disconnecting pipe until you reach the known good spot.

You don't have to disconnect every single fitting. Most of the time you can skip a couple of fittings, if you pick your battles right and make sure there's enough room to twist a group of fittings around.

Once you're done, you can shine a light through the pipes looking for obstructions. Use whatever you have to hand - a skinnier piece of pipe, a swiffer handle, or even just a heavy weight on a string, to dislodge any obstruction you find.

It may be that the obstruction is caused by decay in the pipe -- perhaps there is one pipe where water has built up and rusted. You'd want to replace this piece. Just measure it and go to your local big box or plumbing supply store - they can cut to length and thread the pipe for you.

To reassemble, you'll want a wire brush to clean the pipe, and some gas-rated "pipe dope" which you can also get at the same store. These connections will also require at least one, but probably two pipe wrenches of substantial size - 10 to 16". And you'll probably have to put some serious weight on them at one point or another. So have a helper or some helpful tools, maybe including nail-up straps, wood blocks, a ladder, etc.

Other pipes than black iron?

There are other gas delivery technologies. One such is CSST, corrugated stainless-steel tubing. This is to natural gas what PEX is to plumbing -- much easier to route, faster to install, and it requires a whole different set of procedures and training. You don't want to just insert this in the middle of a run of black pipe, though. It's the kind of thing you get either installed as part of new-build construction, or when you redo your entire system.

Check your sediment trap

Other commenters and answers have mentioned this, but it bears repeating: if you have a blockage in your pipe, you may have the blocking crud also in your appliance. Before the gas enters your appliance -- right before -- there should be a sediment trap. This will appear as a 90-degree bend in the flow of gas, with an open leg pointing down:

gas sediment trap

The purpose of that "dead" leg is to collect whatever crud might be in the pipes before it can clog up your appliance. So... shut off the gas, empty the pipes, and then open that trap and see if anything is in there. You may have to dig it out with a pencil or a pipe cleaner or something. If there's a huge amount (like: the entire leg is full) then crud might also be inside your appliance -- maybe a "service call" is appropriate?

  • That's a great write-up. But why would you discourage use of CSST to connect between sections of black pipe? I see it used routinely for adding a new branch in an existing black pipe system; a re-route or repair in the middle of a black pipe system seems reasonable too.
    – Greg Hill
    Jan 11, 2021 at 17:31
  • @GregHill because of the tooling required, and the grounding. For something like replacing a 4-foot length of pipe, it's too much.
    – aghast
    Jan 12, 2021 at 0:29
  • For some CSST systems the only tooling required is a tubing cutter -- the same cutter one might already have for cutting copper tubing or other kinds of metal pipe.
    – Greg Hill
    Jan 12, 2021 at 16:18

To address the gas pipe blockage or a possible leakage the professional you should contact is your gas company. They should have service personnel just for this sort of work.

is it possible that the gas valve adjusting the flame might be bad?


How would the possible obstruction in the black pipe be cleared?

Replace the pipe.

What type of professional should I hire to address this? An HVAC tech or a plumber?

Neither one. Contact your gas company for a certified gas repair technician.

Is black pipe still the only acceptable material used for gas distribution in the home?

This might vary by location - In USA - state, city, county, federal codes govern this. Search for Gas certified pip and you will see a range of pipe available (not necessarily in compliance with building codes - but what is available to be used).

  • 1
    Ken, thanks for your reply but in MD the gas utility will only repair their meter and outside. The inside has to be contracted by the owner
    – provlima
    Oct 7, 2017 at 19:22
  • @provlima find an appliance repair company that services gas appliances - they should have a certified gas repair tech. Any blocked Gas pipe should be replaced . There is a reason it is blocked. If the pipe is blocked it is also possible the feed controls on your downstream appliances have a blockage as well , make sure they are thorough . Identify how it became blocked..from upstream or what ... ask and inform in your questions the concern for the origination of the problem not just the replacement of a blocked pipe. (Some repair people need a hint that it did not just block all by itself).
    – Ken
    Oct 8, 2017 at 19:41
  • Ken,thanks for your reply. What I can't understand is why does the furnace receive a 7+ inches of pressure during the 90 seconds of the first stage ignition, but then suddenly drop to .3 when the second stage kicks in?
    – provlima
    Oct 8, 2017 at 22:43
  • @provlima depending on how you are measuring the pressure - it is natural that your pressure drops when the second stage kicks in the volume of gas it requires to run. Think of a balloon or a tire - while the air is inside it maintains a pressure once you call for flow of air (releasing from the balloon or tire) the pressure drops.Since your supply line is not like a tire or balloon with a fixed amount of gas - in order to maintain the pressure - your flow rate must at least match the consumption rate.
    – Ken
    Oct 9, 2017 at 1:24
  • I’m not sure if this is common but in my area (Utah) many HVAC techs and plumbers have gas licenses or certifications so they could diagnose and repair a problem like this. Given that the furnace is the problem, I might start with a reputable HVAC contractor.
    – daneb
    Jan 11, 2021 at 5:14

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