# Seeking advise on some residential low water pressure topics related to plumbing design

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#### Rossn

##### Member
Have a few pressure challenges, and looking for some insight.

TLDR: have some long distances to run cold water in my house, and would like to understand:
Questions:
- For an above-garage apartment (1 bedroom, small kitchen, 3/4 bath, washer dryer), is it appropriate oversize the plumbing from the planned 3/4" PEX trunk (31 psi) to 1" Pex (33 psi) or 3/4" copper (34 psi)?
- For a hose bibb off the garage, is it appropriate to oversize the plumbing from the planned 3/4" PEX line (36 psi) to 1" pex line (39.5 psi)?
- Are there shower heads that perform acceptably in the 30-35 psi range? My understanding is that most require 40 psi.
- If I cannot get sufficient pressure at my furthest fixture, what are my options?

Background:
My house is on city water, and sees about 51 PSI at the entry point to the house, which has a calculated demand of 40 WSFUs/26 GPM (4-1/2 baths, 1-1/2 kitchens). As part of a deep remodel, the (new) main trunk (1-1/4", then 1" Copper) runs about 155 (equivalent) feet to the garage. From the garage to the furthest fixture (shower) in a garage apartment is another 65 (equivalent) feet. Additionally, there is a hose bib that will run from the garage another 50 (equivalent) feet, and will need to see 7.5 GPM.

Calculations, per IRC, give the pressures I've listed.

Main distribution line:
1" Meter is at the street and supply line runs about 280' to the house. Currently 1" copper, it is insufficient to meet the needs of the 40 WSFUs/26 GPM calculated demand of the house (does not consider irrigation needs). That will need to be upgraded to 1-1/2", but will be more complicated/expensive due to a county irrigation ditch it must traverse. So, that will be handled after the remodel is completed. Certainly the apartment shower isn't going to work well before that happens.

Thank you!

#### wwhitney

##### In the Trades

The IRC WSFUs calculations and conversion to GPM strike me as very conservative (high), at least as far as domestic use. I mean, to actually get 26 GPM, you'd need to be running eleven 2.5 GPM showers simultaneously.

On the other hand, some uses do have fairly high GPM demands, like fire sprinklers, irrigation, or possibly hose bibs.

So I'd suggest starting with a calculation of the pressure available to each fixture if it is only fixture in use. Then maybe look at the worst case of 3 or 4 fixtures in use simultaneously. And if you have irrigation, look at what fixture pressures you get when the irrigation is running. Don't forget the pressure loss due to elevation.

For fixtures such as WCs and washing machines, if you're concerned their water usage would increase the dynamic pressure loss for other concurrent uses, you can install a flow limiter on the supply to those fixtures to reduce that effect. The WC will take longer to refill, and the washing machine cycle may take an extra minute or two, but you'd probably notice those effects less than you notice the pressure change in the shower when those fixtures cycle on and off.

Cheers, Wayne

#### Rossn

##### Member

The IRC WSFUs calculations and conversion to GPM strike me as very conservative (high), at least as far as domestic use. I mean, to actually get 26 GPM, you'd need to be running eleven 2.5 GPM showers simultaneously.

On the other hand, some uses do have fairly high GPM demands, like fire sprinklers, irrigation, or possibly hose bibs.

So I'd suggest starting with a calculation of the pressure available to each fixture if it is only fixture in use. Then maybe look at the worst case of 3 or 4 fixtures in use simultaneously. And if you have irrigation, look at what fixture pressures you get when the irrigation is running. Don't forget the pressure loss due to elevation.

For fixtures such as WCs and washing machines, if you're concerned their water usage would increase the dynamic pressure loss for other concurrent uses, you can install a flow limiter on the supply to those fixtures to reduce that effect. The WC will take longer to refill, and the washing machine cycle may take an extra minute or two, but you'd probably notice those effects less than you notice the pressure change in the shower when those fixtures cycle on and off.

Cheers, Wayne

Hey Wayne!

As always, you have some really good thoughts and ideas, even if not directly answering my questions. I appreciate it.

Good call-out on the flow limiters for WC's and washing machines. Not something I had given thought, and will be good to keep in my back pocket (especially until main line is replaced, where pressure drops to 40 at the house when 10GPM is in-use).

In my case, irrigation pulls off the main distribution line before it enters the house. So, that is good for the house's internal plumbing, but still an issue in the house when one zone (~10 GPM) runs.

My observations align with your thoughts on WSFU's being conservative. It's a good call-out about running some practical numbers, as well. I think the IRC water system sizing is good for physical sizing, but if you're right that it will be conservative and won't predict performance well.

As to real world numbers... hose bibs are a real use in our household, as we have a few large gardens and potted fruit trees. It's not uncommon for me to be out doing some watering of those not on drip in the morning (the water breakers we use are around 8GPM), when my wife hops in the shower. And I can see my kids using a toilet and washing their hands as the toilet fills. This totals about 15 GPM concurrent. Add 4.5 GPM for the apartment (washing hands while toilet fills).

I re-ran the calcs with real-world numbers, and they look better, but are still in the 30's. For that furthest shower head, now looking at:

3/4" ProPex trunk for apartment: 37.2 PSI (35.9 PSI if +1GPM)
3/4" Copper trunk for apartment: 38.2 PSI (37.6 PSI if +1GPM)
1" ProPex trunk for apartment: 37.8 PSI (36.8 PSI if +1GPM)

So, it seems I should either go to 3/4" copper (less sensitive to pressure drop as flow increases) or 1" ProPex for the trunk.... at these pressures, every PSI seems to count.

At the end of the day, I think I just need to minimize pressure loss, then check real-world performance and then start using those tricks like flow limiters.

I guess the thing I'd like to understand if there are any practical issues upsizing the main branch/trunk for the apartment (3/4" Copper or 1" Pex). The only ones that come to mind are stagnation/biofilm buildup (I think less of an issue with Cu) - and I'm not sure how practical of an issue that is. If you or any plumbers out there have any information to share, that would be awesome.

One thing I don't understand is why Uponor takes temperature into account for sizing (presumably due to dynamic viscosity changes in the water), whereas copper.org's copper handbook does not take temperature into account (and does not list their temperature assumption). Any idea on that?

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