CSST sizing question

[Giantsean]

Hi All,

Our home has a 2PSI gas system piped with Gastite Flashshield (the anti-lightning stuff). Right now it runs a Navien CH-240, cooktop, dryer, as well as an outdoor quick connect hookup for an NG grill. The original trunk is 3/4 from meter to tees where 1/2 lines for the cooktop/dryer/grill split off (in a couple of places due to the way they were added). Right now everything works as it should.

I really want to add an 80K BTU NG garage heater and trying to budget for it. If I want to stay w/ Gastite the cost difference of the 1/2 vs. 3/4 is considerable, so much so that it may be a dealbreaker (esp. at the local going rate).

I have tried using some "longest length" gas capacity formulas along w/ Gastite's own standard 2PSI max capacity tables, using very conservative lengths (longer than needed, using actual line lengths w/ some fluff) and it APPEARS that I can tee off close to the furnace and run 1/2" to the garage, but my original gut feel was I'd need 3/4. The only way I got the tables to not work was with Gastite's "custom" table creator where you enter PSI and min/max length, but it said I needed 1" to the Navien by itself so I'm guessing I did it wrong. Still the 2PSI service looks like it buys me a lot of flexibility.

Just to make clear... I have no plans to install gas lines myself - I'm comfortable w/ appliance hookups but that's about as far as I'll go. Also nobody in the area will sell Gastite to homeowners unless they are certified. Again, just trying to plan out as best I can. If I can save a buck (or hundreds lol) I'm looking to do so. Attached a crappy map and some of my calculations.

Sincere thanks for any advice, and stay safe!

 

[wwhitney]

You seem to have the necessary information in your diagram and in the first half of your spreadsheet (I guess you should include the tee fittings and an equivalent length allowance for them, but unless things are really close I don't think that's going to make a difference).

But I don't understand the 2nd half of your table, A + B would be 280,000 for example.

What you can do with the Gastite table you have is this: label each segment with the length, size, and segment CFH if all the appliances are running at max. Look up the allowable CFH for the segment's length and size. Divide that into the segment CFH; the ratio is the pressure drop in psi (since the table is for 1 psi drop). Do that for every segment. Now for each outlet, add up the psi drops from the meter to that segment. If each outlet sees a total drop of under 1 psi, the design works. If not, upsize an appropriate segment and repeat.

Cheers, Wayne

 

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[Giantsean]

Thank you! I will try that. The A+B+C stuff I believe is to allow you to size the particular trunk runs between splits. The tool (really a guide) that I was using had you add the CF/h of all the outlets on the run that the segment covers, I guess with the idea that the final run feeds only a couple of outlets, but the first run from the meter to the first set of outlets has to have the capacity for the whole system downstream (ie it starts big and gets smaller). I was conservative there too and at least by my numbers, 3/4 appears to be more than enough and may make no difference... 2PSI really seems to be a good setup.

(you'll laugh but I started out thinking that I'd have to tee off at the meter and run the entire length, thinking that the entire length of the furnace line "belonged" to the furnace lol)

 

[wwhitney]

The A+B+C stuff I believe is to allow you to size the particular trunk runs between splits.

Sure, but the numbers in that part of your table are wrong. Segment 1 supplies A and B, the garage heater and the "furnace", and the CFH is 280, not 145.

Cheers, Wayne

 

[Jeff H Young]

Figuring 1000 BTU per cu ft your chart tells me the main needs to be 3/4 to handle 363k BTU 100 feet column and the new heater requires 80k BTU at 100 foot requires a 3/8 line. I've never worked w 2 psi but its your chart. Is this how your reading your needs as well?

 

[Giantsean]

Sure, but the numbers in that part of your table are wrong. Segment 1 supplies A and B, the garage heater and the "furnace", and the CFH is 280, not 145.

Cheers, Wayne

DOH!!! I mean... yeah.. just testing you No obviously my bad... I had my formula misplaced but I still should have caught it, and I misinterpreted your question when you pointed it out. This is why I'm not a plumber . Good news is that the existing sizing still works for the big line, and the small one actually drops (I don't even know if Gastite has 3/8 available, so 1/2" it stays ).

To be honest I also don't know whether the math on Section 2 is right as it all tees off in the same place. It's probably wrong somehow but perhaps it doesn't even matter as long as the existing 3/4 covers all the outlets.

Thanks again!

 

[Giantsean]

figureing 1000 btu per cu ft your chart tells me the main needs to be 3/4 to handle 363k btu 100 feet colum and the new heater requires 80k btu at 100 foot requires a 3/8 line. Ive never worked w 2 psi but its your chart. Is this how your reading your needs as well?

More or less, and I may be overthinking it w/ all the calculators. What I was really trying to determine was whether a) I'd need to replace the existing 3/4 with 1" line (looks like no, thankfully) and b) should I run 1/2" or 3/4" to the garage. Looks like I can get away with 1/2", though someone told me elsewhere that gastite 3/4" pipe and fittings are not that much more expensive than 1/2" Maybe it is worth it to go bigger unless there is some performance concern. A regulator will be needed no matter what as it's 2# service, but who knows maybe it will be more futureproof if I need to tee off in the future.

 

[Jeff H Young]

I only looked at the main and end didn't look at anything else . all the ABC and 123 was confusing I didn't
just run 1/2 inch its good You want to save money run black pipe

 

[wwhitney]

What you can do with the Gastite table you have is this:

I'm sorry, this was bad advice. That procedure is for the opposite kind of table, one where the entries are maximum allowable length, and you look up based on CFH and size. Because pressure drop is linear in length, but not in CFH or size.

You could do something similar with the table using lots of interpolation. E.g.:

Section (3) carries 363 CFH. Per the table, at 125', a 3/4" CSST can carry 364 CFH. Call segment one 35' to account for fittings. Then the pressure drop on section (1) is 35/125 = 28% of total allowable.

Section (1) carries 280 CFH. Interpolating, the table says at 225', a 3/4" CSST can carry 280 CFH. Call the segment 20' to account for fittings; then the pressure drop on section (2) is 20/225 = 9% of total allowable.

The new section carries 80 CFH; call it 55' with fittings. Per the table, a 1/2" CSST at 750' can carry 80 CFH. So the pressure drop on the new section is only 55/750 = 7% of allowable.

That makes the total pressure drop 44% of allowable, no problem. [That means each segment could be twice as long as you have, and you could still handle the flows for your appliances. It doesn't mean you could double the flow for each appliance with the current lengths and sizes, as the pressure drop is superlinear in flow rate.]

Cheers, Wayne

 

[Jeff H Young]

My estimates don't work 3/4 main , 3/8 to new location ? Minimum technically bare minimum , don't think I would ever run 3/8 inch. shorter run to a 40 ,ooo BTU w/h or clothes dryer maybe. I will say though I've never had a problem with a minimum sized gas system the problems I've seen have always been grossly undersized

 

[wwhitney]

my estimates dont work 3/4 main , 3/8 to new location ?

Not sure what you mean by "my estimates don't work".

The simple, conservative way to use the table is like this: the farthest fixture away from the meter is the garage heater, call it 100'. Then just look at the 100' column. It says 3/8" = 80 CFH ; 1/2" = 218 CFH ; 3/4" = 403 CFH. So for the whole system, any pipe that carries 80 CFH or less can be 3/8" ; any pipe carrying 218 CFH or less can be 1/2" ; and any pipe carrying 403 CFH or less can be 3/4".

[This method tells you that if every appliance were 100' from the meter, and every pipe were carrying its full allowable CFH, then the pressure drop at each appliance would be exactly the allowable maximum, when everything is running at its rating. Since some appliances are closer, and some pipes are carrying less than their full allowable CFH, the actual pressure drop at the appliances will be less than the maximum allowable. The previous method is a way to account for that, and it will OK some installations that do not pass the simple, conservative test.]

Cheers, Wayne

 

[Jeff H Young]

OK got it the method was correct . but to use the 100 foot column on all runs is not required and sometimes you can have a heavy 200K BTU demand very close to meter and have a really long run to the end and be tricked into going 2 or 3 pipe sizes larger. of course on a 2 PSI system piping is so small not much of issue.

 

[wwhitney]

sometimes you can have a heavy 200 k btu demand very close to meter and have a really long run to the end and be tricked into going 2 or 3 pipe sizes larger.

An improvement on the simple method avoids most of this problem:

Label each appliance with its total distance from the meter and with its CFH demand. For each pipe segment, look at all the appliances that are served downstream of it; add up their CFH demands, and take the greatest distance from the meter just for those appliances. Then use the column corresponding to that distance to find the pipe size for that segment.

So in your example, the pipe segment that is common to the far away appliance and to the close heavy demand appliance would get somewhat oversized, as you have to use both the long length column and the high demand figure. But for a pipe dedicated to the heavy demand (or to the far away appliance), you only have to use the column for the shorter length (or use the lower demand, respectively).

Cheers, Wayne

 

[Jeff H Young]

Pretty much Yes. I follow the example in my code book its quite old I don't think changed but should get newer with data for CSST tables

 

[Giantsean]

Where I got a little messed up is with a design like mine, where you are teeing off almost sequentially in several places. But I think you guys have given me plenty of examples, so thank you for going above and beyond.

My only last question... I talked to my plumber (really my HVAC guy lol) and he told me that I was wrong... 3/4" line is only like 20 bucks more than 1/2" for same length. Is there any danger or disadvantage to just going w/ 3/4 for the garage run? Who knows like I said, if I decide to tee off from it in the future, I will be covered (don't see a need yet but like always, too big is better than too small )

 

[wwhitney]

No functional downside to bigger pipe.

Cheers, Wayne

 

[Plumber69]

1" will usually do everything

 

[Jeff H Young]

are 2 psi systems standard in Conneticut? pretty much unheard of in my world Ive never seen that in a house. Ive heard of it but never seen 2 psi on a house here

 

[Giantsean]

Not sure... probably not older houses that have had gas service a while but gas is relatively new in my neighborhood. The last homeowner (my mom lol) switched our house over to gas in 2012, so maybe they are doing it standard for new service installs. Either way, I'm sure pleased they went that way!

 

[Jeff H Young]

you get 2 psi we get 1/4 pound 8 times more pressure. Never understood that but thats how it is