Hi - DIYer here. No one seems to know what this issue is.

We have added a detached addition to our house - mother-in-law suite style. Water is coming in from the main water line.

I've attached a diagram below to show how we ran the supply lines. They were run through the ceiling, which is vaulted and does not have attic space.

Our first mistake - I believe that I mistakenly ran hot water to vanity, toilet, and shower. And cold to vanity and shower. Not concerned about the hot water to the toilet, just need it to run properly.

Currently, the instant hot water heater is not installed. We have connected the cold line directly to the hot (feeding cold into hot line, creating a circuit to test water/pressure). And have also tried attaching a water hose to one supply line and turning main water line on for other line.

Here's the thing...
-Hot and cold are both making it to the vanity! Great pressure there.
-Water pressure to the shower head is abysmal when cartridge is in. When you take the cartridge out, it is awesome pressure! For both hot and cold.

Here's the weird part...Water pressure to the toilet is strange.
-Cold line does not go there (remember, mistakenly ran hot to toilet).
-When cartridge is in the shower valve, and hot water is turned on, no water comes to the toilet supply line.
-When you pull the cartridge our of the shower valve, and hot water is turned on, water pressure to toilet supply line seems good. And water flows powerfully out of the shower valve where cartridge was removed.

Could this be an issue with the shower cartridge? I can't understand why the pressure is so good to the valve of the shower, but terrible to the shower head. I also can't understand why the toilet only works when the shower cartridge is removed from the shower valve.

Is it somehow creating a vacuum?

Moen's customer support line is backed up. Our plumber friends literally have no idea without ripping out all of the drywall we just finished. Thoughts?

[i.postimg.cc]



Edited 1 times.

Both problems would occur IF there is no hot water pressure, even though you say there is.

Interesting and agreed. But why would pressure to the vanity (first in the series of fixtures) be ok? And pressure be fine to all three fixtures when cartridge is removed? No signs of leak.



Edited 1 times.

the cartridge has nothing to do with your pressure / volume problem.

Likely your piping is not sized correctly. My guess is that it's all 1/2" pipe when you likely need 3/4" main and 1/2" branches. Any idea what you have in terms of sizing?

Yeah - hard to see at first glance in the photo. But CPVC is 3/4in and reduces to 1/2in about half way down the supply line.

Written in on this photo [i.postimg.cc] next to where it says 'sink' is where it's reduced.



Edited 1 times.

why on earth did you reduce the pipe size ?

you need to 'open er up' and redo the piping and remove the hot/cold reversal

3/4" all the way with 1/2" branch or outlet to each fixture

THEN

check/clean/service the pressure balancing spool in the shower valve

BUT

perhaps, juuuust perhaps, the shower valve 'reverses' for a 'back to back' installation so the incoming hot and cold lines do not need to be crossed in a 'commercial' installation

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