Main water line sizing using UPC Appendix A

[JeffreyP]

Hi,

My two main questions are:
1. Which of the Appendix A 105.1 charts would I use for PEX pipe?
2. Do the below calculations seem correct?

I'm in Seattle where the city uses the UPC with some local changes (2021 Seattle Plumbing Code) and, for a large 2-unit residential remodel construction project, I'm trying to use Appendix A to calculate if my current 3/4" meter will be sufficient. Unfortunately, I can't easily upsize the meter without first installing a new water main for the street which would involve redoing a large section of the right of way at a high cost (which I'm hoping to avoid).

The stock sizing table 610.4 only includes up to 1-1/4" building supply lines for a 3/4" meter (I might need to do a bigger supply) and also is a rougher estimate with just 3 groups of pressure values. Given I'm likely going to be pushing the limits here, I'm trying to use the alternate approach from Appendix A to calculate more precisely.

Relevant info & calculations
Overall, I'm hoping to be able to provide supply for 68.5 water fixture supply units, from table A 103.1:
Clothes washer: 2 x 4 =8
Toilet: 10 x 2.5 = 25
Lav: 10 x 1 = 10
Shower: 8 x 2 = 16
Kitchen Sink: 2 x 1.5 = 3
Dishwasher: 2 x 1.5 = 3
Hose bib: 2.5 + 1 = 3.5

First I found the demand in GPM for the 68.5 units:

Then:

A 102.2 Water Meter
I'm not sure if I have a disk jet water meter or a multi-jet one (or some other kind) but using the disk meter chart (chart A 102.2) I noted 25 psi loss at 35GPM

A 104.1 Residual Pressure
Using 8 psi for flush tank

A 104.2 Elevation
Elevation difference is 33' so 33 * 0.43 = 14.19

A 104.2 Available Pressure
68psi (value provided by the water company for static pressure at meter) - 25psi (from meter) - 8psi (for highest fixture) - 14.19psi (from elevation) = 20.81psi

A 104.4 Developed Length
100' total so 20.81psi/100'*100 = 20.81 as the average permissible loss per 100'

For copper, per 610.12 the max allowed velocity is 8ft/sec so I believe I would need a 1-1/2" main supply:

Thanks!

 

[wwhitney]

Short version:

Pretty sure your residual pressure is too low, and your meter pressure drop is too high. Also the pressure info from the water supplier is likely insufficient.

Longer version:

1) Looks like there is no appropriate chart for PEX, which is SDR9 CTS (copper tube size). So the SDR11 chart is the right idea but will underestimate the pressure loss. Really all that matters is the C-factor (roughness), for which I understand 150 is appropriate for plastic pipe, and the actual inside diameter. So you could figure out the actual inside diameter (e.g. 1" PEX is 1+1/8" OD (that's what CTS means), and the ID is 9/11 of the OD (that's what SDR11 means, each wall is 1/11 of the OD, so 7/8" ID) and then check that against any of the other pipe types that are listed as C=150 to find one that is close but not over in ID (e.g. 3/4" Schedule 40 ID is actually 0.824").

Or you can skip the charts and use a calculator such as: http://irrigation.wsu.edu/Content/Calculators/General/Pipeline-Pressure-Loss.php Just remember you need the actual pipe ID, not the nominal size.

2) Appendix A seems a bit overly conservative to me. Several comments:

- No irrigation? Without irrigation 35 gpm seems a little high, even with 8 showers and 10 WCs. What's a realistic worst case simultaneous use? 1 clothes washer filling, maybe four WC tanks refilling, four showers running, and maybe one dishwasher filling? I don't think that would come out to 35 gpm. So you could investigate whether a lower peak demand of say 25 or 30 gpm would be defensible. Getting this number right is I think the biggest source of uncertainty.

- Find out the actual brand and model of water meter you have or the neighbors have. For example my water districut uses Badger Recordall meters, and their 3/4" Model 35 meter spec sheet says only 10 psi pressure loss at 35 gpm (which is above the recommended flow rate for the meter)

- Residual pressure is how much pressure is left to actually push water out the fixture, i.e. the input pressure to the fixture itself. Pretty sure for a shower you want at least 30 psi, below that would be anemic. Your shower is likely your highest fixture, and even if not, you'd just adjust each fixture's residual pressure requirement by its relative elevation, and then use the largest pressure requirement of those as your controlling fixture (and use the elevation of that fixture for your elevation correction).

- Similarly, static pressure at the meter is not what you want; static means a no flow situation. You want what the water company would call their residual pressure (since the water is leaving their system) at the meter during the design flow (35 gpm per the WSFU method). Since you mentioned that the water company might have to upsize the main if you want a larger meter, that suggests the property may be on a more distant or smaller sized portion of the distribution system, which suggests you might have significant pressure drop before the meter (compared to static) from that design flow. In fact your water company may have a policy like limiting the maximum demand on a 3/4" meter to say 25 gpm.

-This methodology of determining pipe size is a bit simplistic. If you say that your available presure is say 65 psi at the meter, less 10 psi drop over the meter, less 5 psi drop for elevation, and your residual pressure required is 30 psi, then you have 20 psi of available pressure. If your controlling fixture is 100' from the meter, and you size your pipe for less than 20 psi of drop over 100', then that would be accurate only if the full size pipe goes all the way from the meter to your shower while carrying all of the water demand over that entire length. Which is not realistic.

So this is a reasonable method to get in the ballpark of sizing for the pipe that will carry all of your demand, but a more accurate method for borderline or problematic cases is to draw out a graph of all the fixtures and all the (equivalent) pipe lengths connceting them, assign a flow rate to each pipe in the graph, pick a possible size for each pipe, compute the pressure drop based on the pipe size, length and flow for each pipe in the graph, and then compute the residual pressure at each fixture by adding up the pressure drops along the path from the meter to that fixture. If the residual pressures are all adequate for each fixture, great; if some residual pressure is too low, then figure out what you need to upsize and then recompute.

Cheers, Wayne

 

[Sponsor]

 

[wwhitney]

flow for each pipe in the graph

This choice would be necessarily some statistical--your total gpm is not determined by adding up a gpm for each fixture, as you don't expect every fixture to be in use simultaneously. So rather if you have a procedure to take a collection of fixtures and turn that into a gpm estimate (like the WSFU procedure), you could do that for each pipe using the set of downstream fixtures supplied by that pipe.

Cheers, Wayne

 

[John Gayewski]

If the stock table only includes a 1.25 supply with a 3/4 meter then that's the largest allowed by the code. If you need to upsize the pipe then you need to upsize the meter simple as that.

I didn't read if appendix a is allowed to be used unless there's a section saying you can use that appendix there's no reason to look at it.

As far as all of the other calculations in this thread wsfu is all that matters unless you hire an engineer to size the water supply and say it's OK. Calculations don't generally matter to an inspector if you not in compliance with the chapter 6 table. To go around that table you need an engineer to sign.

 

[Reach4]

If the stock table only includes a 1.25 supply with a 3/4 meter then that's the largest allowed by the code. If you need to upsize the pipe then you need to upsize the meter simple as that.

That seems odd to me. I wonder if that is ever enforced -- preventing, for example, 1.5 PEX with a "3/4" water meter.

I wonder if you could get around that by using 1.25 SIDR polyethylene pipe, which is substantially bigger inside than 1.25 PEX (cross-linked polyethylene) pipe.

 

[wwhitney]

If the stock table only includes a 1.25 supply with a 3/4 meter then that's the largest allowed by the code. If you need to upsize the pipe then you need to upsize the meter simple as that.

I didn't read if appendix a is allowed to be used unless there's a section saying you can use that appendix there's no reason to look at it.

As far as all of the other calculations in this thread wsfu is all that matters unless you hire an engineer to size the water supply and say it's OK. Calculations don't generally matter to an inspector if you not in compliance with the chapter 6 table. To go around that table you need an engineer to sign.

Good point about Chapter 6 of the UPC controlling, in particular Section 610 on "Sizing of Potable Water Piping." 610.1 says you shall use the methods described in 610. 610.4 says to use that section, which is based on a big table, or to use section 610.5. 610.5 says you can use Appendix A or Appendix C.

So the upshot is that you have 3 options, Table 610.4, Appendix A, or Appendix C. Any of those are fine. Except that WA state did not adopt Appendix C, so the OP only has the first two options.

Table 610.4 caps any installation with a 3/4" meter at 39 WSFU. So the OP would need to upsize the meter under that method. Appendix A lets you use the actual pressure drop across the meter at the flow rate determined by the WSFU count, as per the manufacturer data. I don't recall seeing any leeway in Appendix A on that gpm rate, so if I didn't miss anything, the OP is stuck with using 35 gpm.

The discussion at the end of my post about how to do sizing more accurately than Appendix A is along the lines of what an engineer could/would do to be more accurate.

Cheers, Wayne

 

[JeffreyP]

Thanks Wayne for all the thoughts! Here are some follow ups from me on them.

- No irrigation? Without irrigation 35 gpm seems a little high, even with 8 showers and 10 WCs. What's a realistic worst case simultaneous use? 1 clothes washer filling, maybe four WC tanks refilling, four showers running, and maybe one dishwasher filling? I don't think that would come out to 35 gpm. So you could investigate whether a lower peak demand of say 25 or 30 gpm would be defensible. Getting this number right is I think the biggest source of uncertainty.

Correct, no irrigation. I agree that 35 gpm seems high and I was simply basing that off of the Chart A 103.1(2) that provides a conversion from water fixture supply units to GPM. However, I have since started to read up on Appendix M Peak Water Demand Calculator, which provides a method for estimating demand load in for building water supply. This is done via the excel sheet from IAPMO which has a probability of use for each fixture. This provides a much more modest 8gpm (note, I'd still need to add hose bibs to this) demand.

1) Looks like there is no appropriate chart for PEX, which is SDR9 CTS (copper tube size). So the SDR11 chart is the right idea but will underestimate the pressure loss. Really all that matters is the C-factor (roughness), for which I understand 150 is appropriate for plastic pipe, and the actual inside diameter. So you could figure out the actual inside diameter (e.g. 1" PEX is 1+1/8" OD (that's what CTS means), and the ID is 9/11 of the OD (that's what SDR11 means, each wall is 1/11 of the OD, so 7/8" ID) and then check that against any of the other pipe types that are listed as C=150 to find one that is close but not over in ID (e.g. 3/4" Schedule 40 ID is actually 0.824").

Or you can skip the charts and use a calculator such as: http://irrigation.wsu.edu/Content/Calculators/General/Pipeline-Pressure-Loss.php Just remember you need the actual pipe ID, not the nominal size.

After I read appendix M, it actually points to two figures (one for 60 degrees and one for 120 degrees) from Appendix I that I believe can be used for PEX! From Appendix I:

- Find out the actual brand and model of water meter you have or the neighbors have. For example my water districut uses Badger Recordall meters, and their 3/4" Model 35 meter spec sheet says only 10 psi pressure loss at 35 gpm (which is above the recommended flow rate for the meter)

I've inquired with the utility provider to confirm, but I think it might be a MasterMeter and yes, I think my pressure loss number here was much too high. It is likely more like ~9psi at 30 gpm.

- Similarly, static pressure at the meter is not what you want; static means a no flow situation. You want what the water company would call their residual pressure (since the water is leaving their system) at the meter during the design flow (35 gpm per the WSFU method). Since you mentioned that the water company might have to upsize the main if you want a larger meter, that suggests the property may be on a more distant or smaller sized portion of the distribution system, which suggests you might have significant pressure drop before the meter (compared to static) from that design flow. In fact your water company may have a policy like limiting the maximum demand on a 3/4" meter to say 25 gpm.

I've inquired about this with the utility provider as well. I'm also thinking I might be able to test this out in practice by measuring the static pressure (at a laundry hook up, hose bib, etc) and then running a bunch of water all at once into buckets for a couple minutes. Then, I could measure the water in the buckets to find the GPM and watch the PSI gauge during the flow to see how much it dropped. If nothing else I think it might help me understand the relationship between the pressure and flow better!

- Residual pressure is how much pressure is left to actually push water out the fixture, i.e. the input pressure to the fixture itself. Pretty sure for a shower you want at least 30 psi, below that would be anemic. Your shower is likely your highest fixture, and even if not, you'd just adjust each fixture's residual pressure requirement by its relative elevation, and then use the largest pressure requirement of those as your controlling fixture (and use the elevation of that fixture for your elevation correction).

I agree I definitely want to design for more than the 8psi I had originally called out. I tried to call Delta to see if they had any information about how shower performance would be at lower PSI values but they weren't helpful at all. I believe if a shower fixture is rated for 2.5 gpm at 80 psi then (given 2.5/sqrt(80) = .28) I think it would put out 1.25 gpm at 20 psi (given sqrt(20)*.28 = 1.25). So basically, for that example shower fixture, I'd expect half as much water if the pressure was 20 psi instead of 80 psi, but as far as the impact on the shower experience, I'm not sure what that would be like.

-This methodology of determining pipe size is a bit simplistic. If you say that your available presure is say 65 psi at the meter, less 10 psi drop over the meter, less 5 psi drop for elevation, and your residual pressure required is 30 psi, then you have 20 psi of available pressure. If your controlling fixture is 100' from the meter, and you size your pipe for less than 20 psi of drop over 100', then that would be accurate only if the full size pipe goes all the way from the meter to your shower while carrying all of the water demand over that entire length. Which is not realistic.

So this is a reasonable method to get in the ballpark of sizing for the pipe that will carry all of your demand, but a more accurate method for borderline or problematic cases is to draw out a graph of all the fixtures and all the (equivalent) pipe lengths connceting them, assign a flow rate to each pipe in the graph, pick a possible size for each pipe, compute the pressure drop based on the pipe size, length and flow for each pipe in the graph, and then compute the residual pressure at each fixture by adding up the pressure drops along the path from the meter to that fixture. If the residual pressures are all adequate for each fixture, great; if some residual pressure is too low, then figure out what you need to upsize and then recompute.

I think this makes sense. Right now, I'm mainly trying to figure out if I could just know how to size the main supply from the meter to the building and, given the constraints here, know if that would provide enough flow.

 

[wwhitney]

However, I have since started to read up on Appendix M Peak Water Demand Calculator, which provides a method for estimating demand load in for building water supply. This is done via the excel sheet from IAPMO which has a probability of use for each fixture. This provides a much more modest 8gpm (note, I'd still need to add hose bibs to this) demand.

Appendix M looks interesting (I only glanced at it), but I don't see how there's a path through the code that let you use it. In so far as Chapter 6 only reference Table 610.4, Appendix A and Appendix C. A bit confused on this.

If nothing else I think it might help me understand the relationship between the pressure and flow better!

You can think of the piping system (in a given state, with certain outlets open) as having a characteristic pressure vs flow curve--the more pressure you provide at the inlet to the piping system, the more flow you will get out of the outlets. And you can think of your source (e.g. a well with a given pump) as having its own pressure vs flow curve--the more pressure the source has to provide, the less flow it can provide. When you connect the two, the flow through the system will be the one where the two curves cross--at that flow rate, the source provides just enough pressure to push that amount of flow through the system.

I believe if a shower fixture is rated for 2.5 gpm at 80 psi then (given 2.5/sqrt(80) = .28) I think it would put out 1.25 gpm at 20 psi (given sqrt(20)*.28 = 1.25).

It's not that simple. The shower head can be designed to have a pretty flat flow curve within some operating range of pressures.
See, e.g. the image below, which is from: https://www.phcppros.com/articles/11134-where-did-all-the-pressure-go

Cheers, Wayne

 

[John Gayewski]

That seems odd to me. I wonder if that is ever enforced -- preventing, for example, 1.5 PEX with a "3/4" water meter.

I wonder if you could get around that by using 1.25 SIDR polyethylene pipe, which is substantially bigger inside than 1.25 PEX (cross-linked polyethylene) pipe.

There is some theory about fungus in the pipe. If your meter isn't able to carry the flow on a large pipe, the idea is that the large pipe isn't staying clean enough to be sanitary.

 

[wwhitney]

Appendix M looks interesting (I only glanced at it), but I don't see how there's a path through the code that let you use it. In so far as Chapter 6 only reference Table 610.4, Appendix A and Appendix C. A bit confused on this.

Well, this fact sheet from IAPMO claims that 10 states including Washington have adopted Appendix M as an alternate sizing method.

https://iapmo.org/media/2xvl24k4/fact-sheet_wdc.pdf

Just confused why this adoption doesn't show up as an amendment to 610.5 that lists Appendix M in addition to A and C.

Cheers, Wayne

 

[John Gayewski]

Good point about Chapter 6 of the UPC controlling, in particular Section 610 on "Sizing of Potable Water Piping." 610.1 says you shall use the methods described in 610. 610.4 says to use that section, which is based on a big table, or to use section 610.5. 610.5 says you can use Appendix A or Appendix C.

I'm not up on legal-eze what does" except as provided" mean? In 610.5 except as provided in 601.4. Doesn't that mean there must be a condition that exists to negate 601.4?

It seems to me an inspector could say use 610.4 and there's not much you could do about it. As if someone would want to do something about it.

 

[wwhitney]

I'm not up on legal-eze what does" except as provided" mean? In 610.5 except as provided in 601.4. Doesn't that mean there must be a condition that exists to negate 601.4?

First sentence of 610.4: "Systems within the range of Table 610.4 may be sized from that table or by the method set forth in Section 610.5."

First sentence of 610.5: "Except as provided in Section 610.4, the size of each water piping system shall be determined in accordance with the procedure set forth in Appendix A."

So 610.5 says "use Appendix A unless 610.4 provides another option." And 610.4 says "if within the range of Table 610.4, you may use that table." Your choice, one or the other.

Per the last link I posted, both Appendix A and Table 610.4 are quite overly conservative for multi-family residential. So Appendix M is a good alternative. I'm just surprised that if Appendix M has been "adopted" there's no language in Section 610 saying that Appendix M is a 3rd option.

Cheers, Wayne

 

[Master Plumber Mark]

you probably would be wise to leave it all alone and maybe install a slightly larger meter if
the city will allow you to do this...... although I doubt it will help much
if the incoming pressure is too low its just a waste of time and money. The city
of Seattle is not gonna bend over backwards for you and dig up the streets to install
larger water lines.

Inside your home somewhere near by the meter I suggest you just install a booster pump
and forget about all this high technical bullshit math with pretty charts and graphs
that you are trying to figure out..

We have done this before in older parts of our city
where you could not run your outside lawn sprinkler and flush the toilet at the same time without
watching the sprinkler peter out to nothing.....LOL
You can also install a large well bladder type tank along with the booster pump to add a ton of
volume to your system...... we have seen that done a few times downtown......

Basically you are installing a 110 volt shallow well jet pump like is made for a well
in your basement....

Now, This brand seems to be the most long lasting I have found ....... It was installed back in 2018 and I have
not heard any complaints from the two sweet guys that lived there that
liked and wanted to take showers together all the time but did not have the
pressure needed to enjoy themselves......

We aim to please.....LOL

good luck

 

[John Gayewski]

First sentence of 610.4: "Systems within the range of Table 610.4 may be sized from that table or by the method set forth in Section 610.5."

First sentence of 610.5: "Except as provided in Section 610.4, the size of each water piping system shall be determined in accordance with the procedure set forth in Appendix A."

So 610.5 says "use Appendix A unless 610.4 provides another option." And 610.4 says "if within the range of Table 610.4, you may use that table." Your choice, one or the other.

Per the last link I posted, both Appendix A and Table 610.4 are quite overly conservative for multi-family residential. So Appendix M is a good alternative. I'm just surprised that if Appendix M has been "adopted" there's no language in Section 610 saying that Appendix M is a 3rd option.

Cheers, Wayne

I don't understand why either section is written that way if all three can be used, or I guess four now. The gas piping section is petty clear about thy different methods of sizing gas pipe. Not sure about this.