How do you know what rough in pipe size (for your new valve)your shower is without opening the wall. My house is 7 years old. There is a bathroom next to it as well.

It should be 1/2" as branch circuits are normally that size. Even if it was plumbed with a larger size up to the valve it would have to be reduced to 1/2" because that's the size used on shower valves-at least on all the ones I've seen. If your shower valve has a cover plate on the wall- as most do-remove the shower valve knob and the screws holding the plate and pull the plate from the wall. There should be enough of a hole in the wall to allow you to see the manifold valve and the pipe. You might not need to remove the knob to gain access via the plate. Are you replacing the whole valve or just the cartridge/ball/disk portion? If the latter, pipe size is not relevant as you're replacing the guts of the valve. If your replacing the whole valve assembly, you will most likely have to break out part of the wall to gain access to the piping.

Thank you for your response. The plumbing store quoted me for a 3/4" thermostatic valve by grohe. also 3/4" volume control valve for shower head and hand held shower. I am having the wall open but wanted to order the roughs so my men could begin work immediately after opening the wall. My house was not built with the highest of quality but it is decent. I didn't understand why the plumbing guy assumed 3/4". I will take your recommendation and remove the lever handle and trim plate if I can. Thanks

Good morning Candy,
There is a reason that the big box plumbing salesmen aren't out in the field as plumbers. Because they aren't! Listen to Shader when he tells you the correct size for shower valves are 1/2". Mainly because in all the years that I've installed tub/shower valves I've never piped a shower supply at 3/4"
You won't have to open up the valve to see the size of the supplies. Will you take the word of a plumber with over 50 years out in the field that your valve has 1/2" supplies? Regards, tom

As with anything there are always exceptions, especially if your using a special valve-i'll admit I'm not sure what a thermostatic valve is:confused: maybe a valve with an anitscald feature? To be sure the best way is to actually look and measure the pipe if possible...

Just saw speedballs response. Would have to agree that the pipes would be 1/2"-no matter how well your house was not constructed :(

Thank you all, I did remove the trim and saw it is indeed 1/2". the bad news is the valve is tight to a 2x4 and the new valve is wider. just for my education why would one ever use a 3/4" valve if they were starting from the bottom up , does the 3/4" pipe give you better volume? Just wondering. Thank you all

Candy, You ask," just for my education why would one ever use a 3/4" valve if they were starting from the bottom up , does the 3/4" pipe give you better volume? just wondering"

Yes a 3/4" pipe would give you more volume,(as would a 1" pipe) if you could locate a 3/4" tub/shower valve. But I've never run into one that wasn;t 1/2".
Can you come off the 1/2" inlets in the new valve with 1/2" street ells to save installation space? If not then you'll have to cut into the stud. Good luck, Tom

I'M BACK with new questions pertaining to this same valve replacement. We are now going with a rohl (perrin & rowe) shower system. They only have a 3/4" valve. So we are going to change the pipes from the main line to the new valve. However i noticed in the specs that the shower arm is for 1/2" and the outlet for handshower is also 1/2". I feel my plumber (being a side job) needs a little looking over. Please advise me why these dimensions are different, and why have a valve accepting 3/4" water pressure and reduce to 1/2" on both outlets (shower head and hand held shower). Thanks so much.

Hi Candy, Welcome back.

"So we are going to change the pipes from the main line to the new valve."
Unless you really want the hassle or resizing you will take 3/4 thread X 1/2 slip bushings and bush the valves down to 1/2". This way all the pipes will be 1/2". I'm surprised your plumber didn't tell you this. Regards, Tom

I'm concerned about his capabilities because my plumbing supplier told me the same thing. He is working on a side job and completed two gorgeous functioning bathrooms for friends. We do have to rerun the pipes anyway due to the fact this is a much larger valve and there is a vent in the way. Bare with me I have terrible use of terminology and articulation. We are going to run hot and cold water lines into the valve 3/4" b/c we have to reroute those anyway. The question is
at the top of the valve where the pipes runs to the shower head, do i start with the 3/4" up to 84" (where my husband wants it) and before i get there
switch to 1/2" via your method. The 2nd question do I then come off that
Vertical 3/4" line to shower head for my water source for the handheld shower. DO i just T off that with a 1/2" line. Thanks for your help.

I looked up your valve to find out why any shower valve would need 3/4" supplies and why any one would pay extra for it. According to the manufactures specs. you're building a custom shower with multiple shower heads. Hey! They're fun!!


The high flow 3/4" Grohtherm thermostat valve is perfectly suited to meet the demands of large multiple-head custom shower systems. With adequate water pressure, one 3/4" Grohtherm thermostat valve can supply enough water to simultaneously power a shower head, a personal hand shower and up to four body sprays, and maintain a specific, pre-selected water temperature as well. If more outlets are needed than can be accommodated by one valve, two or more valves can be installed, but each must have access to a main supply line.
Grohthemp
The 3/4" Grohtemp thermostat valve requires a separate volume control and is ideally suited to large multiple-head custom shower systems. Given adequate pressure, the 3/4" Grohtemp valve can supply enough water to simultaneously power a shower head, a personal hand shower, and up to four body sprays while automatically maintaining a specific water temperature at each outlet."

A personal hand shower, and up to four body sprays? Sounds swinging but it needs some black lites and a strobe light plus a 100 amp sound system to make it a real "party shower".

You ask,"The question is---
at the top of the valve where the pipes runs to the shower head, do i start with the 3/4" up to 84" (where my husband wants it) and before i get there
switch to 1/2" via your method. The 2nd question do I then come off that
vertical 3/4" line to shower head for my water source for the handheld shower. DO i just T off that with a 1/2" line?"
Come off the top of the valve with 3/4" to a 3/4X1/2X1/2" tee and pick up the shower head and hand held with that.
And just how tall is hubby any way? As a rule we set our shower arm connections at 76" off the shower floor. Have fun and invite me to the next party. Tom