Replacing Shower Valve Cartridge - how much water will actually be lost?

[Almighty1]

Okay, I have a question concerning replacing a shower valve catridge as I don't see a shut off valve for the water as it is probably behind all the tiles if there are any. So it seems the correct way to do this is to shut off the main water supply which till take care of the cold side and then shut off the cold water supply on the water heater but the water heater does not have a shutoff on the output side. As shutting off the main water supply basically turned off the water pressure so the water is no longer being pushed for both the hot and cold. My question is how much water will actually be lost since I would think it would only be a little because I can always just open other faucets like either the kitchen sink or bathroom sink to get rid of some of the water in the pipe and then basically the pipes will no longer have water where the shower valve is because the pipes will no longer be fully filled. Thanks in advance as never worked on something that did not have a shut off valve right before the thing I am working on.

 

[Reach4]

You could drain the WH down to low-enough.

You should remove the sediment from the WH periodically -- maybe every couple of years. This can be done by leaving the WH pressurized, and draining out several gallons. But draining water without pressure will also remove sediment, so a bonus.

 

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[Almighty1]

Thanks @Reach4 - I used to drain the WH but I heard someone who did that with a Bradford-White from from mid-2010's actually had a leak as a result. Mines is also a 2019 Bradford-White and I did do it every two years.