Put this under the title “Conflicting Information:â€
In planning for the rehab of our old home, we are installing new gas lines. I spoke to a few plumber friends and inquired online here as well. All the plumbers looked at my total BTUs, length of runs and determined that for my main “trunk line†that 1.25 inch pipe was sufficient (with smaller pipe being used as branches off the main trunk). Most people here concurred that this configuration was probably enough….In fact, when I submitted my plans I used this configuration, and the Building Department checked and approved my plans.
BUT, I also have an engineering friend with the local utility company. He determines pipe size for cities. He took my plans and ran an analysis—taking into account length, elbows, horizontal and vertical flows, pressure, etc. It was a rather comprehensive report (with numbers, neither my plumber friends nor I completely understand)—but the one number we all understood was the recommendation that my main “trunk line†be 2 inch diameter. My engineering friend warned me that all the plumbers would say this is crazy and would say this is waaaay too big. As an example, he indicated that a 1.25 inch pipe to a planned commercial range (120K BTU) would LOOK like it is burning fine—with a pretty blue flame—but all the money that we spend on such a range would be wasted because the range would not be running as it was intended….
Well my plumber friend who is running the pipe has years of experience as a plumber, and sure enough, he thinks I’m crazy for wanting to run 2 inch pipe. He thinks (and code suggests) that this is waaaaay too big. He’ll do it if I want (at more labor and material cost), but thinks it is a waste.
So….to my question: What do you think? Should I go with the engineer’s “analysis†or should I go with what every plumber in town seems to think and code will allow. My personal feeling is: I want to do this once and never do it again (i.e. I’m happy to spend the money, but don’t necessarily want to throw money away).
Replies
Did you post a gas question a few weeks ago?
When I had my gas service redone, the gas company upgraded me to a 2lb system. So my guess pressure is higher than normal and I have regulators placed at strategic points to get down to regular gas pressure (appr. 1/4 lb). My whole house BTU load is a little over 600k. My main trunk line is 3/4" trac pipe and the gas guys doing the work had a book with charts where they looked up the btu rating for length/diameter of the trac pipe. They tranisitioned to black pipe at some places and they likewise sized that based on charts.
I'd think 2" is totally excessive.
MERC.
DJMerc—
Yes that was me…I also raised your suggestion to both the local utility and plumbers regarding a 2lb. system….both thought that it wasn’t worth it…The utility company would charge for the new meter (about $2800 was their estimate) and then I would incur the extra costs for all the regulators…I guess that is my point—lots of conflicting opinions on this one…. Code/plumbers seem to think 1.25 is fine because that is what “their books” say is correct. Engineer seems think 2 inch is right because that is what his computer/charts/analysis says for this particular project. Both are “experts” unlike me. I guess, part of my question is: “Which expert would you go with and why?”
FWIW, My total BTU for the house would be about 470K and an additional 400K for a future pool/spa. I do appreciate your input…I like to make informed decisions, and you provided information that I hadn’t thought of….Thanks.
There are only 2 things a plumber needs to know. shlitz flows down hill and payday is friday.
Go with the engineer
That is odd that your local utility is trying to charge you for the meter. My local utility just tossed me a new one (everybody pays some monthly equipment fee - about $10), but I guess these things are very local.
As far as I know the BTU rating of a pipe is a well known quantity. Can you ask your engineer buddy why the plumbers can't come up with the same number? Is it all over the elbows?
Honestly, I would go with the plumber because a residential gas system can't be nearly as stringent as city or commercial. Really do you care if your WH or gas cooktop runs 5 or 10% short? I don't think you will ever notice and the plumber has probably installed many houses with your load and used his charts. If you don't have some jackleg on the line, his reputation is on the line and he doesn't want you calling back say appliance X isn't working right.
So I'd inquire with the engineer, but defer to the guys that have done it and will continue to do it.
Do you know the cost difference? My guess is 2" pipe is at least 3x cost of 1.25, not to mention the expense of the labor and bigger hangers and maybe even more hangers, etc.
But now that I type all this, I think the engineer could be right. I have 600kBTU, and I was pretty close to needing a 1" trac pipe. So think a 3/4" trac pipe, which is more resistive than black pipe, produces 600k @ 2lb. That is 8x normal gas pressure. It's probably not linear with pressure. So he could be right?
Tough call I guess. Gas company have any input? How is the gas company going to get 470k BTU through a meter with 1/4lb pressure. The orifice is going to have to be pretty big. If the orifice isn't 2", why even run 2" pipe? Something is definitely wacky here. I think you need to keep digging. I've never fealt so blessed to have a good gas company here.
MERC.
Soak and rinse the beans well before you cook them. Buy a bottle of Beano and add a little to your meals.
I'm thankful for the loyal opposition! It's hard to learn much from those who simply agree with you.
In this post you have not provided enough information to completely dispute your "Engineer" friend.
However, having been a mechanical enigineer for the last 20 years, and having been directly involved in the design of hundreds of commercial, instituional, industrial, and residential, plumbing, heating and air conditioning systems, including many natural gas piping systems, I do have some experience in this field. My initial impression is that your friend's determination of the required pipe size is either grossly conservative (i.e. unnecessarily wasteful and expensive) or indicative of incomeptence.
Assuming that you have low pressure gas (0.5 psi), a 2" pipe sized for a realistic conservative pressure drop of 0.5" WC and a total pipe length of 200 equivalent length, you could feed the entire load of 800 MBH at once.
In reality, you can take diversity into account. You will never run all of your gas appliances at once. Anything over 1-1/4", sch 40 black iron pipe is a waste of your money. I would suggest you not take technical advice of any sort from this "engineer".
Timbo/DJMerc (and even Hasbeen and Bob W :-)),
Thanks for the feedback. When I can get the engineer's report I'll post the numbers that he used to come up with the suggestion of the 2 inch pipe....Maybe that will shed some light on his thinking. The engineer does work for the local utility company and also deals with Residential service as well so its not like he does only the big "street" projects.
Based on what people have said, I've got a feeling he is basing his suggestion on the notion that everything could possibly be on at once (almost an impossibility)....and I know he took into account resistance of elbows, and the fact that I have a vertical rise of 20 plus feet from the crawl space to my attic (in addition to the furthest unit being nearly 75 feet away fromt he meter). It just seems like overkill, based on what everyone else has said....But I'm not a rocket scientist, engineer, or even a plumber. I'll get the numbers when I can and post.
Thanks for your input!
I suspect he may also be assuming that pressure drop must be maintained at "negligible". But since there are pressure regulators at the appliances, a good deal of pressure drop is absorbed with no consequence.
>>He determines pipe size for cities.
Next time you build a city, give him a call.
Cities don't scale down to residential.
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Sojourners: Christians for Justice and Peace
Okay, here are the numbers from the engineer in which he determined that 2 inch pipe was recommended…Much of this information, I honestly don’t know what it means…The bold number indicates pipe diameter recommendations. I tried to recreate the table as best I could. This was the table the engineer came up with after taking my plans, determining total BTUs of appliances (470K), total length of runs, elbows, horizontal and vertical flows, etc. Does this make sense?
He also provided tables for all the laterals as well as the Main Trunk/Negative Results for what that’s worth. Thanks to all for your input….
Main Trunk w/Restrictions: Low Pressure/Solving for flow
UNKNOWN: KNOWNS
Load U/S press D/S Press Int. Diam. Length U/S vel D/S velocity
(Mcf/hr) (in. w.c.) (in w.c.) (in.) (ft.) (ft/sec) (ft/sec)
1.31 7.50 7.0 2.067 80 10 11
I think DanH might have it. Looks like your engineer has given you a minimum design pressure of 7.0WC and an inlet pressure of 7.5WC, so he is only giving you a 0.5WC pressure drop through the system. Take a look at a gas regulator for an stovetop or something. It usually says 4 or 5WC.
You might ask him what happens if you drop that pressure to whatever your appliances really need. That might drop the pipe down.
MERC.
Hmmmm...starting to make some sense....
Thanks to you and Dan...
How much more money does the plumber want to install 2" over 1.25"?
Offer to pay the plumber for the estimate of 2".
How much did you pay the engineer?
SamT
Edited 1/7/2005 1:40 pm ET by SamT
SamT
The engineer is a friend who really went out of his way to help--for free. Very nice guy, who has some experience in this area. He works for the local utility company and did this on his own time/expense....
The plumber is also a friend, who is working at a very reduced hourly wage plus materials. Very experienced guy who is basically doing me a big favor (and not working on more lucrative projects while working on mine). He is willing to do what ever I want. Material cost between the pipe size is not that much (in the grand scheme of things), but it will take longer to install...Its just that he and other plumber friends that I have talked to thought the pipe was overkill (just as the engineer predicted they would say...)
Hope this answers your qs's
Edited 1/7/2005 2:02 pm ET by PNUTIII
I looks like the 2" line would provide the gas for ~ 1.3 milliion btus. (1.31 x 1000 cubic feet) x (1000 btus per cubic foot of gas). Btu content can vary between 950- 1100 btus per cubic foot here in Dalllas, this varies by local gas companies. This seems to a little high for me, by comparison a residential furnace is usually between 50,000 and 125,000 btus you would need 10 of the big ones to get close to maxing it out. I am usually one for overkill, but if it was to cost more than $100.00 I can't see doing it myself.