3/4" or 1" trunk extension to garage?

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Rossn

Member
TLDR:
In my home, I sized (Per IRC) the cold water trunk on the west side of the house at 1-1/4" copper and east side side of the house at 1" copper. I am now extending the 1" from the east side of the house to the garage and hoping to confirm sizing of that last section of the trunk as 3/4" or 1" type L copper.

The garage (and apartment, above) will include:
(1) 3/4 bathroom
1 kitchen
1 clothes washer
1 exterior hose bib (3/4" pex)
1 laundry tray

Any reason to not run the trunk extension (to the garage) as 1"? I'd mainly be concerned about velocities that are low at times, and any health concerns that could come from low water velocities.

I get 8.2 WSFUs, which translates to 12.8 GPM, but that is likely only to be seen if the space above the garage is actually rented, and when it is not, usage will be rather low.

From the former trunk, the extension it will run about 25' on a convoluted run to a pex manifold with (3) 90's & (4) 45's along the way. Pex runs from the manifold will be 20-35' to final destinations.

Pressure here is about 50 psi where it enters the home... about 110' by the pipe to the pex manifolds in the garage.

Copper.org handbook seems to indicate 3/4" pressure drop per foot starts to cap out at 10 GPM.

Rossn

Member
Based on pressure drop calcs from copper.org, I get the following pressure drops from entry point to the home:

3/4" Copper to garage
To Garage Manifolds: 6.8-9.9 psi
To Garage Hose Bib: 9.4-16 psi

1" Copper to garage
To Garage Manifolds: 3.9-5.6 psi
To Garage Hose Bib: 6.5-11.7 psi

Based on this, seems like my pressure drop is about 4 psi greater with the 3/4" over the 1". Given I'm already pressure/flow challenged (issue with main line to house) I'll go with the 1", unless someone tells me that is a mistake (for another reason).

That will also give me a little more wiggle room, should I decide to heat the garage apt water from a DHW heater in the garage (or POU).

Last edited:

I'd run 1".

Rossn

Member
I'd run 1".
Thanks - I did run 1" copper, and after running the pressure drop calcs for my low pressure situation, it is really good I went with the copper over pex, but that sure was time consuming.

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