I'm looking at extending my existing gas line to the other end of my basement. The line comes into the house as 1" but doesn't go completely to the other end. I'm looking at around 40' of pipe to run.

I only need 3/4" on the other end, so I'm not sure what's the better option here in terms of pressure, supply, and cost. The current 1" line ends at a T with 3/4" going down to my furnace and 1/2" running forward about 10' and then up to my kitchen range. So I'm guessing in theory I could remove the range line (replace the T) and extend out from there with my new line. Then closer to the range T up to reconnect the 1/2".

So with that; should I extend on with 1" line and just connect up my 3/4" at the end and the 1/2" at the first third of the line, or extend with 3/4" all the way to the end and again reconnect the 1/2" where it goes? So generally, do I downsize the line to reach the end or extend what I've got? Will either help or hurt the pressure, supply, etc.

I do know that the difference in cost of the pipes themselves will be around $7. So I'd save $28 by running the 3/4". But is it better to run the 1" so that more could branch off of it later on if needed?

  • Do you plan to branch off the line in the future? – Tester101 Feb 20 '16 at 1:01
  • What kind of appliance are you planning to run? If it is a on demand water heater go full size. If it is a gas grill maybe 3/4. But the demand will help provide the right size pipe. – Ed Beal Feb 20 '16 at 1:38
  • @Tester101 I have no particular need to and currently have no plans to. The only things left that Could be converted to gas would be our dryer, which is at that side, and possibly an outdoor grill instead of propane to it - although again I have no plans for this yet. – TFK Feb 20 '16 at 2:32
  • @EdBeal Correct - a tankless, on demand water heater. – TFK Feb 20 '16 at 2:35

When designing gas distribution systems, one must consider the demand of every appliance in order to find the minimum pipe sizes.

But, to answer your first question, there is little disadvantage in using larger pipes (except for cost and space). It is very likely that the 1 in pipe will be sufficiently large.

The calculations needed require you to know the lengths of the pipes and the amount of gas used by each appliance. Also, you need to know the gas pressure fed into your system, and the required pressure at the appliance. For low-pressure gas systems (like are in most houses), pressure drops linearly with distance. Tables (generally in the fuel gas/housing code) state the pressure drop per distance with various pipe sizes and flow rates.

So, one consults these tables and calculates the pressure delivered to each appliance with your pipe configuration (assuming that all appliances are operating simultaneously). If the delivered pressure is above the minimum required, then all is well.

The relevant regulations (including tables) are copyrighted, so I'm posting them here. They are likely available in your local library.

Be sure to read up and follow all relevant regulations, including performing a pressure test of the newly installed pipes, in order to ensure that the installation is safe. (Also note that high pressure testing will damage regulators in your appliances, so close their shut-off valves before the test).

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  • @EdBeal Thanks everyone, they were all great answers. You all answered the question, but I did find the tables and was able to size it based on our line stats. To feed the 1/2" line to the range, we'd need to extend the 1" line up until at least the T to the range. Then we'd be able to run the rest with just 3/4", but we'll probably just finish it all off with 1". Thanks again. – TFK Feb 21 '16 at 3:13

If it were me, I'd run the full size all the way. I'd only reduce the size where branching to a consumer.

I have no technical data to back this up, it's simply my personal opinion. I'd rather pay a bit extra now, than redo it later. It sounds like you've at least considered what else to supply, which is a good enough reason in my mind to not reduce the line.

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With a tankless water heater you will need the 1" pipe. I have a older bosh Aqua star , gas furnace, gas cook top. The water heater draws more than the furnace and cook top combined. When I installed my unit I could not get over 120 degrees F. The gass company put in a new meter and regulator and set them for the max pressure (I think it was 3/4" water column). If I would had smaller pipe that unit would not have worked. My current max temp is 138. I love tankless you never run out of water. The only drawback I found was the flow rate. More than 1 shower at a time the water is a little cool. The biggest advantage I found, to change out the water in my hot tub I turn the temp down to 101-102 and fill it up. No more waiting for a day and a half for it to warm up. So you will need to go full size 1" and if you can't get the water temp call the gass company and ask for a pressure check.

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