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Real talk OP: From your post and comments it's pretty clear that you don't have much plumbing experience. There are a lot of things that can go wrong with replacing a shower valve.

If this is your main bath, this is not the job to learn on. Especially if you don't have any shutoff valves between your main and the shower. If you run into something unexpected during the repair, and you'd be stuck with all your water shut off while you wait for a plumber, and then you're also paying emergency rates.

As it is now, you have all the time in the world to consult plumbers and compare estimates. With your water shut off and your shower broken, your options will shrink really fast.

If getting into home improvement is something you're interested in, and this is a spare bathroom you can afford to be out of commission for an indefinite period, and it won't require your main valve to be shut off, then by all means give it a try. But expect there to be surprises and fuckups as part of the learning process.

So this is an old Mixet shower valve. You can try ordering the parts for a rebuild and changing the cartridge for it, but in my experience the valve body has issues with it. We usually end up replacing them with either a Delta or Moen single handle shower valve.

Source: Plumber for 13 years.

So there are definitely two things I don’t fuck with and I’ll call a professional for it.

Plumbing and electrical.

You can learn to do these things yourself but unless you’ve decided to fully commit yourself to these crafts you’re just better off hiring someone.

I had a contractor replace an older faucet a few years ago. He opened up the wall and found older, very brittle plastic pipes, I don't remember what exactly but they are not modern PVC. He was able to replace the faucet and fix the leak that precipitated the visit, but was emphatic that further leaks and issues were likely.

With an older fixture like that, after learning this, I wouldn't touch it.

My home was built in the late 90's.

This kind of shower controls are the best. I had this on my shower but right now my place is gutted for a remodel. I really want to get the same exact kind that you have. It's super convenient because you set the temperature once and then adjust the pressure separately. You don't have to constantly tune it to the right temperature with every shower.

I'm not very handy, but I learn when I need to.

For reference, I replaced the belt in my dryer last night. It took me 3 hours, I had to look up a few videos, I cut up my hand, and had a few "extra" screws at the end. But, the dryer works!

I've replaced the knobs and spout on my shower, and it wasn't hard at all. You just pull them off, and pop on the new ones. If you are trying to change anything about the pipes behind that, I'd call someone.

If you fiddle with anything electrical turn off the breaker/remove the fuse and wear protection, anything plumbing turn the shutoff valve and ensure the tap runs dry.

If you are:

• close to a well-stocked plumbing supply store you will make a few trips to *willing to potentially go a long weekend without a working shower and water
• ready to call a plumber anyway after trying if you fuck something up
• just wanting to replace with a compatible cartridge (which may not be easy to find/identify, and replacment options might be uglier), for anything at/behind the wall just call the plumber
• wanting to spend a few hours reading guides and watching 360p video tutorials

Then I say give it a shot.

Go on google or Amazon and search for shower valves, you'll find pictures of what's inside the wall behind the chrome plate you got there... seeing images will give you a better idea of what to expect than anything anyone can describe.

After that, go take the knob off your shower valve (probably the center cap pries out, you take out a screw, and it pulls off), then take off that chrome ring (Allen screw on bottom, or it might just get pried off carefully or unscrew the whole thing, they vary). Have a look... that'll tell you what your specific type of valve is and if you can do this without making a big ass hole in a wall...

Then figure out what's on the other side of the wall the valve is on... if it's a closet, maybe a big ass hole in the wall is ok? A big ass hole in the wall of the shower will involve tile. I'm thinking based on your description that you should 100% not fuck with tile. You'll get into all manner of special tools, special skills, etc. real quick with tile :) a hole in drywall in someone's closet might be a homeowner kinda deal though tbh.

Those three things are hopefully enough to answer your own questions, because the real "no bullshit" answer is that it depends... pex, copper, iron, delta, thermo, yadda yadda yadda... The shit inside the wall is as varied as the shit on the outside...

I could do this myself, but I wouldn't... sink, toilet, ice maker, leak in the crawlspace, move the hose bib outside to the complete other side of the house, replace the water heater, add an expandion tank, all that shit is an immediate "fuck no im not paying soneone to do that, ill do it", but a shower valve is a hard no :)

Have a look by all means, no harm in checking it out, but I wouldn't be too optimistic in your shoes.

I won't mess with plumbing again after trying to replace my kitchen faucet only to discover that the shutoff was old and busted the moment I turned the knob, leading to water spraying out everywhere. Called in a plumber to fix it and it took him several hours to resolve--no chance I had the knowledge, skills, and tools to fix that.

Do you know where your water main turnoff is?

Do you know a local plumbing supply you can buy from instead of Home Depot? I’ll never buy a faucet from HD.

To fix the leak and possibly replace the handle if you're interested check this video. It may not feature your exact model but it will give you an idea of what you're in for. I did this myself on both of our bathrooms at our old house with no issues.

https://youtu.be/MkPcCmYkG4Q?si=_VryQ6uZmwpj3lom

As far as reconfiguring the type of mixing etc.. others have explained it pretty well. to me it would be a definite "call a plumber moment."

Call a plumber. I am quick to do minor plumbing myself, but I’m glad I didn’t fuck with this when my faucet and valve needed replacement.

I replaced my shower valves with nicer looking ones. It is much more involved than changing a sink faucet, I did several of those before attempting this.

Easiest way is if you have access to the wall behind the shower controls. You cut out a square of drywall at the studs and tada, you can see the pipes. Hopefully you have copper pipes, you can sweat off the old connections and sweat on the new ones. You may have to shorten or lengthen (replace) some of the pipes because the new fitting connections are not in exactly the same spot. Leak test, fix any leaks you find, repeat until it runs dry on the outside.

If the wall is in a closet you can use a removable cover for easy future access. If it is a visible wall you'll have to reinstall the square of drywall, tape and mud the seams, sand, paint.