Branch Design Calculation

Old 06-25-21, 03:27 PM
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Branch Design Calculation

Hi There ... I'm going to be removing a rat's nest of gas piping left behind by previous homeowners in the crawlspace and replacing it with Pro-Flex Black CSST and I'm having a difficult time determining a trunk line calculation ( using the branch method for design ). Hoping someone here can shed some light on this.

As you can see from the attached image, my tankless rises straight out of a 3/4" meter tee ( street tee from meter ). It would be easy to simply remove all of the piping mess from under the house back to that tee, then give it a spin so that it is oriented toward the crawlspace, attach a 24" x 3/4 pipe in order to move into crawlspace ( strapped to joist here and used to bond CSST to ground system ) then transition from the 3/4" NPT to 3/4" CSST all the way to the cooktop branch CSST tee, leaving out the tee for future potential fireplace on the way ( 3/4 x 3/4 x 1/2 & capping off the 3/4 extra outlet for potential future BBQ here ), then continuing on with 1/2" CSST all the way to the dryer oxbox.

Where I run into a problem with this fairly simple approach is that according to my load of 209,000 BTUs, charts state that I am required to have a 1" trunk line until the Tankless drops out of the equation. And here is where I need clarification from an expert.

Since the tankless rises straight out of my meter, I believe the only "trunk" line is the meter side of the tee. The tankless side of the tee is then Branch 1 and only requires 3/4" .Then, with the tankless btus out of the equation the last side of the tee going into the crawlspace would also only be required to be 3/4". Would I really be required to have the meter side of the tee be 1"?

If it is required to be 1 inch, I am not sure how I would go about making that happen in regards to fittings. I've put together a drawing with my best guess of how to achieve it and it would of course make for a much bigger job as I'd have to resize things in the rise to the tankless, which is why I'd probably go ahead with the other extra tee work that is in my drawing but that I left out in the more simple version above.

Ugh ... heh ... anyway, any clarification or advice would be greatly appreciated ;-)

Old 06-27-21, 07:35 PM
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Okay, well, I guess I'm going to be conversing with myself here, so, I've decided to just play it safe and do the following:
1. Remove all gas piping back to the meter union ( of course being careful not to lose the important union bits ).
2. Disengage the pipe section to the tankless via the union and put aside.
3. Insert the shortest 3/4 nipple allowed into meter union and tighten ( I believe close nipples are disallowed due to inspectors not being able tell how well they are threaded ) so that I have a thread to work from.
4. Install a 3/4 x 3/4 x 1 bullhead tee oriented toward crawlspace opening.
5. Reinstall the tankless pipe section to the top of bullhead and tighten.
6. With the bullhead properly oriented insert entire pre-assembled section back in place and tighten meter and tankless unions.
7. Install a 1" x 16" nipple to the bullhead branch and strap to top of crawlspace opening.
8. Attach 1" Female CSST adapter to 1" MIP Nipple and commence CSST design in crawlspace with a clear conscience that I've got a 1" trunk line and I'm good to go on future meter switch if needed ... including having to move the meter so it isn't directly below the tankless and has its proper top clearance.

Still eager for any input should anyone have any regarding any flaws in any of these plans~
Old 06-27-21, 07:37 PM
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Welcome to the forums.

You need a plumbing expert to respond. There isn't always one online.
Old 06-30-21, 10:27 AM
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I think you're good. I do think upsizing to 1" CSST is well worthwhile based on the number of appliances (current and future) you have on that line.

I would also consider using black steel pipe for your range and possibly dryer valves, with a sediment trap (drip leg) on each.
Old 10-31-21, 09:25 AM
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