I've got a water heater and a hot water radiator furnace sitting next to each other, both on gas. The strange thing is that there are two parallel gas lines feeding each one of these which come from across the basement but branch off from the same gas line. Why was there a need for two lines? Cant they "tee" at the last moment right before the appliances?

I've seen a lot of strange, stupid and crazy things in my house so it wouldn't surprise me if this is one of them. But before I combine the lines I'd like to get some feedback from people. What does the code say about this?

Edit: I should have added this detail in my original questions but all my pipes are 3/4" including the shared pipe before the "T". Basically there's one 3/4 pipe that goes out of the meter (I have the gas meter in the basement) and there are 4 branches (each is 3/4 as well) out of that. But two of those run in parallel across the whole basement. I think it's insane.

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    Sounds like the pipe installer was being paid by the foot.
    – BMitch
    Commented Apr 3, 2011 at 12:48

4 Answers 4


The concern that I know of is about the size of the pipe and all appliances that can be running at the other end. So if they ran out of a larger dimension pipe (or just had a lot of the smaller dimension) this would almost make sense. But I'd think any normal installer would try to minimize the parts cost and split it closer to the appliances.

However, if the second appliance was added later, the existing line may have been too small, and then it would be cheaper to run a separate small line rather than replace the existing one with a new large line.

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    Agree, but that's not the case. All pipes are 3/4". Go figure.
    – Peter Q
    Commented Apr 3, 2011 at 3:41

It's got to do with the volume of gas delivered to each appliance. The gas coming into your home is at some standard pressure, through a large-diameter pipe (2 inch or so diameter, perhaps). That means that some particular volume of gas can come into your home per second.

Each appliance has a certain volume of gas per second that it needs to operate, but it usually works on a smaller pipe than the one that enters your home (most of the stuff in my house is on 3/4" pipe).

If you simply tee off an existing 3/4" pipe from an existing appliance, then if BOTH appliances run at once, there may not be a sufficient volume of gas in the 3/4" pipe for them to run correctly.

If you're installing everything new, you'd just run one bigger line over to where the two appliances are, then tee off to the pair of them. In an existing house, however, where you've got the smaller lines going to the appliances, then proper installation requires running new lines from where the larger pipe is.

I had to have this done when I installed a gas stove - the nearby line for the dryer was right on the other side of the wall, but I still had to run 3/4" line a fair distance in order to make sure I'd have sufficient gas volume.

  • I understand this but please see my edit. Every pipe at home is 3/4" including the shared ones.
    – Peter Q
    Commented Apr 3, 2011 at 3:40
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    I'd bet that both the water heater and the furnace have 3/4 inch inlets, meaning that they each need the volume provided by a 3/4 inch line, and meaning that you can't share the line between them. If you wanted just one line, you'd need a larger one than 3/4 from the meter back to them, then T it off and neck down to whatever the appliances need. I don't know how large that line would need to be. Commented Apr 4, 2011 at 20:03

Before anyone can answer this you would need to know the length from the meter to the appl. and the BTU's of each appl. also is there a reg at the tee as is your system 7" wc or 2 psi del. pressure. It sounds like it was a good install - may have run 3/4 so you would have a easlier time finishing the ceiling.

  • Nice answer, Brian. Welcome to DIY!
    – lqlarry
    Commented Mar 5, 2012 at 6:22

I would think the extra pipe might give a bit better earthquake safety.

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    I don't get this. Can you elaborate? BTW, we don't have many earthquakes on the East Coast.
    – Peter Q
    Commented Apr 3, 2011 at 3:39
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    @Peter Q: A short pipe between the appliances would have much less ability to flex in an earthquake than ones going across the room before being joined. Commented Apr 3, 2011 at 22:32
  • 2
    Aha, thanks, that makes some sense although can't flexible gas pipes be used if that's a concern?
    – Peter Q
    Commented Apr 4, 2011 at 3:00

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