We have a decent size house, 5 members with a pretty regular 7 total with grandparents visiting/staying for extended periods. 2nd floor has master bath with roman tub, 2 faucets, shower with regular head, hand held, and 3 body sprays. There is a washing machine, laundry sink, another two baths with one faucet shower/tub each. Main floor is kitchen sink, garage sink, 3 hose bibs. Basement has sink and bathroom with two faucets and shower.

Our street supply is on a 1 1/4" copper, ~56psi. The plumbers then step this down to 3/4" PEX which is the supply sent out to the house for hot/cold. I've seen a mix bag of information regarding the flow/pressure drop/velocity, it seems like one fairly safe number to use is velocity as it relates to noise/erosion, and that should be 8fps or less. For 3/4", that's around 9GPM. Which would mean the my flow should be limited to around 9GPM total hot and 9GPM total cold (granted it will be whatever it is but for sizing considerations...).

I've been testing flow, I have a way to check hot side flow from our tankless and to our master if I open up everything, including removing diffusers on the faucets, I see the most I can get there is 7.2-7.6GPM of hot only flow. Seems like in an unrestricted flow it should be more, the roman tubs are rated at 15-20GPM in the 40-80psi range alone (so maybe 7.5-10GPM hot only just at the tub).

I did another test where I put a pressure gauge on the hose bib outside, then opened up our garage sink which has quite a bit of flow. I saw the pressure at the hose bib drop from 56psi static down to 44psi when the sink is running. I did another test where I look at the street coming into the house (56psi), I opened up a ton of fixtures and the street dropped from 56 to 52 coming in.

In terms of real use operation, I absolutely see flow drops if kids are using our main tub/shower, I can notice at the kitchen sink I've lost flow rate there.

My question: is the problem the street pressure being too low, our did the plumbers undersize our supply trunks to the house using 3/4" instead of 1"? The 8fps (9GPM) velocity limit suggest to me that we're undersized for our fixture count/use cases at 3/4" but as I said, I've read a lot of post and most of the fights are over going to 3/4" from 1/2" vs anything about 1" PEX.

IMO:

The 3/4 PEX is undersized

1" copper would have been 'borderline'


also, the (internal) fittings used for pex will also be restrictive to flow


however


if the work meets code: unless you SPECIFIED otherwise in the contract and/or specifications you are SOL

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

4 bathrooms... 1 1/4 pex at a minimum.

Quote

the (internal) fittings used for pex will also be restrictive to flow

That is not the case with PEX A which is what he is using the fittings have the same ID as the tubing and they use a cold expansion method to expand the tubing over the fitting. PEX B the fittings are inserted into the tubing and then crimped or cinched so there are restrictions.

correct

BUT

the OP did not specify WHICH was used

==============================================

"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

PEX A was used, sorry it was buried in the title with a tiny 'a'. I'm working through the fixture calculations now, I still need the fully developed flow length and elevation but expect to be in the 46-60 psi range, and guessing in the 150 ft range for length. With 36 cold FU and 28 hot FU on the 2nd floor alone, hard to see how 3/4" PEX A supply for the whole house makes sense. 2nd floor alone would need 1 1/4" for cold and 1" for hot, again depending on where I land on elevation/developed length.

If you do not size properly against the 610.3/610.4 tables (which I believe are called out in the code book) is that a code violation, or is the only thing that matters is the minimum size called out in 610.3 per fixture?

there is something in my (MA) code that says ..i'm paraphrasing..
there must be sufficient water volume so that all fixtures operate as intended

with 3/4 pex you don't have that.

ditto


? what does MO code state ?

==============================================

"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

Local municipality is using 2015 International Plumbing Code it looks like. I did call to get my meter size (1" and they said there's no code violation for pipes being undersized as it is a convenience issue.

Ran/calculated some numbers. Supply pressure is 56psi, max fixture elevation from meter is ~20ft, puts me borderline on the 46-60 range of the 610.4 table. Best estimate on developed length is ~105ft, so using 150ft in the 610.4 table. We have 1 1/4" copper coming from meter

Total water fixture units is 65.5. Supply to master bath is 16.5 FU (hot) 19 FU (cold), total second floor is 28.125 (hot) 35.625 (cold), total hot for house is 36.375 (hot) and 51.875 (cold).

I could presumably install a boost pump to overcome some of the head loss due to elevation and reduced pipe size, but my concern is the velocity limits in the PEX before I start doing damage or risk blowing out couplings, so doesn't seem like a safe path. I can see quite a bit of the plumbing in the basement before it splits out. Wouldn't be easy but maybe I could manifold to salvage some as almost all my branches size to 3/4", but the master bath is a huge bottleneck, almost need to run separate lines up there and tie into the existing supply. I've got empty conduit running to the attic for future electrical use, not sure if it'd be code to use that conduit to run more PEX up and around. I may or may not be getting into that thing where you do more damage trying to fix a minor inconvenience...