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As a plumber this looks like a nightmare and the only solution in my eyes is remove more concrete and relocate that water line so you never have to deal with this again.
Why not just add a hatch to the box you considerd so you can open the box and access the valve? It shouldn’t be too hard to find a prebuilt valve box that looks decent in your bathroom. Most I know are for outdoor applications but no reason why they can’t be used indoors. And building it yourself also shouldn’t be too hard.
To make it look even better you could also consider putting a removable shelf on top of the valve box. That way it’s more hidden and you have a spot to put soaps or other bathroom essentials.
After making the floor/walls flush with the existing shower, I’d either:
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use pvc glue to construct a pvc cover/box out of (relatively thick) white pvc sheet and corner profiles. Bolt cover to wall/floor. Use a rubber seal(thick white PTFE tape?)on all edges of the box to make it water tight once you clamp it down. (this is assuming you don’t have a vacuum former, access to a 3d printer or are unable to find something to cannibalise)
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tiled/grouted corner box with a little door. Tile door, thick white rubber tape and bolts to ensure it’s a tight seal.
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leave the pipe exposed, but replace the ugly industrial valve with a nice chromed (and therefore largely rust resistant) one like this.
To be honest, I’d go with the last option, but pop some extra silicone around where the pipe comes out of the wall/floor. That’s another likely weakspot for rust.
The tiled concrete solution isn’t bad, but I don’t get whoever made it cheaped out on the valve.
e: oh and if there’s the option to install the valve in the basement or somewhere else, I’d 100% go that way and remove this valve entirely.
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This one’s certainly new.
They make specific valves for this purpose, which can be enclosed in concrete and where the cartridge can be replaced from the front without needing to break the concrete, with faucet cartridges like this: https://www.amazon.com/dp/B07HD6PX74/Interesting concept. I’m glad to be aware of that option. However, I have very low confidence of any fitting that attaches to PEX. So I would still want the joint of the fitting relatively unburied as well. One side of the valve is galvanized pipe. I suppose I could replace the PEX with steel as well and then join the PEX to the steel under the showerpan. I’ll have to keep that in mind.
@Damage I’m tempted to get a valve with a replaceable cartridge even though I hope to make the fitting accessible anyway— just because it seems like a smarter design.
What do you call that kind of valve?
I tried an image search for “water valve with replaceable cartridge” & most hits were taps & shower mixers. A local shop had one that looked kind of like this. The lower portion was simpler, but the handle is the same. My concern is that the handle is designed for a human hand and I would actually be running a steel rod to the handle so I can control it from an adjacent room. Most valves seem to have a removable handle so you can easily attach a tool to it.
I would like to find one that has a removable handle and a removable cartridge for full versatility.
If you are going to open up everything and shut off the water to replace the valve, the best solution is to put the valve somewhere else. Either up high on the wall so it won’t get wet or turn it to access it from the other side of the wall cavity.
If I move it higher, it will take on less water. But there would still be some water unless I move it all the way to the ceiling, which would really lengthen the path the water must go.
Regarding the other idea, it seems the house was extended at one point in time. So the wall was once an exterior wall and is very thick (~50cm?) and solid (concrete, brick, or combo of the two). Not sure if I’d be weakening it structurally to do that. It’s in a corner though, so there is perhaps an interior wall I could move it to the other side of. But I think it’s brick & thus also structural. Perhaps feasible though. It’s hard to be enthusiastic about demolishing part of a plaster & lath bedroom wall to reach it, but it could be the best option. I have to wonder if the original builders had a good reason for not doing that. Tricky though since I’ve seen plenty of dodgy work from the past builders of this house.
Is there a basement or crawl space underneath?
There is another shower underneath with easily removable PVC ceiling and access to the pipe. So indeed theoretically I could put the valve in the shower ceiling of the lower apartment. But that segment of pipe is galvanized steel, so I would have to cut it and add threads. I actually have the ratcheting tool to do that but I would still need to buy the threading die. It’d be a bit weird if the lower apartment had the control over the water above. Not a show stopper but I think there are better options.
If you have access then you can run a new line away from the bathroom, put a new shut off anywhere you want and then return it back to where you started. Think of a big loop with just a shut off on it.