Gas piping size question...

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Brian Ferri

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Havre de Grace, MD
I don't recall the gas pipe size entering the house but there are spare manifolds were reduced with 1/2" shut off's. I had a tankless installed that requires 3/4" gas piping. The inspector hasn't inspected it yet and if it's wrong it'll get corrected, but I'm just curious. If you attach 1/2" to 3/4" gas fitting to the 1/2" shutoff, then the 3/4" gas piping to the it really 3/4"? I'm sorry I'm not articulating this correctly.

I'm just curious if the tankless is getting the adequate gas psi as it should. The 3/4" gas piping to the tankless is about 15 to 20 feet if that matters.


In the Trades
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Berkeley, CA
A pretty good approximation is that 1 foot of 1/2" black iron gas pipe is equivalent to 4 feet of 3/4" black iron gas pipe.

So if you have, say, 2 feet of 1/2" black iron gas pipe feeding 20 feet of 3/4" black iron gas pipe, there's comparable to 28 feet of 3/4" black iron gas pipe. Now you can check the guidance in the tankless manual to see if that length is below the maximum allowed for that size of supply pipe.

This assumes you're talking about black iron pipe. If you're talking about CSST, that's a different story.

Cheers, Wayne
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