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I'm planning to purchase and install a new condensing tankless water heater (Takagi T-H3J-DV-N) that will supply both domestic hot water and space heating (radiant via heat exchanger, separate closed loop).

The gas connection on the water heater is 3/4", but my current line running to my existing water heater is 1/2", off of a gas manifold with a 1" service from the meter (manifold is on the other size of the wall from the meter, ~8" away).

The distance from the manifold to the new tankless heater is less than 8'. According to the natural gas sizing chart in the Takagi manual pg 26, a 1/2" line will carry over 172,000 BTU/hr over the 8' distance. The heater maxes out at 160,000 BTU.

I have two questions:

  1. Since the 1/2" pipe will carry the required amount of gas to operate the appliance, would increasing the line from 1/2" to 3/4" at the appliance cause any functional issues?
  2. In general, do plumbing codes prohibit increasing from 1/2" to 3/4", as long as the line is sized properly (for 1/2")?
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    There is not a restriction on the pipe size You can increase the size (I would) in this case a little bigger is better if there are any 90's they do reduce the flow and you are approaching the max flow. I had a 150000 heater in my shop at my last home it has 1/2" and I had to upsize the pipe to 3/4 because the flex line used to whistle quite loud, after upsizing to 3/4 there was no noise at all. – Ed Beal Oct 7 '16 at 19:04
  • Will you have any issue running a stove/furnace at the same time? – topshot Oct 7 '16 at 19:12
  • @topshot The line is dedicated to the new water heater, no other appliances on the same line. The stove has a separate 1/2" line running from the (1"-fed) manifold. – Jason Hoetger Oct 7 '16 at 19:53
  • I meant what else was at the manifold. Should be OK with just 2 1/2" lines. – topshot Oct 7 '16 at 20:05
  • @topshot The manifold currently has the water heater on Line 1, range on Line 2. A gas dryer is also piped but currently I'm using an electric. All lines are 1/2", the longest (range) is 15 ft away from the manifold, and the service is 1", so even if I someday got a gas dryer, I think the capacity is there for everything to be 1/2". It's just a question of whether or not increasing to 3/4 immediately before the appliance will cause a problem. – Jason Hoetger Oct 7 '16 at 22:09

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