Try this at home:
Turn off the water supply to only your water heater.
Check the cold water flow at your tub/showers that have only the rotating control handle.
Not sure how older valves will work but newer ones will not have hot water as expected but also very little to no cold water flow too.
The sink type valves still produce cold water but not the showers.
That’s how a pressure balanced shower valve should work. Loss of pressure on one side translates to a loss of pressure on the other side.
Auto-Temperature control type shower valves do that.
Something else we could check, except it requires “operating a valve”.
You would be crazy to break the solid lime and calcium seal on those valves around these parts.
Kinda as foolish as me washing the van.If not for the rust my bumpers would fall off
that 's the safety feature that keeps you from getting scalded when Mrs. wonderful flushes the toilet while You are in the shower…
I don’t turn any valves that haven’t been in steady use. One day, a client started turning valves in the basement wall after I told him the laundry taps were disconnected. We went into the kitchen and “What the …” no water at the sink. I went back to the laundry room and turned the kitchen supply back on which had been turned off. When I got back the realtor was fussing with the shutoffs under the sink. Now, they got water and there’s a drip UNder the sink that wasn’t there before. :roll: Oh well, edit the plumbing page. 
Bob. Your van’s got Rusty Bumpers? They’re metal?
John Kogel
www.allsafehome.ca
How timely.
My most recent install of a delta T/S faucet with mixing valve left my customer very angry at me.
“HEY ,whats up with the new faucet and no water??!!”
Well, sir the faucet is doing exactly what it is supposed to do considering the fact you have turned your hot water off at the waterheater because the hot side on every faucet in your rental property is leaking(more like streaming) and the stem on the shut off to the heater needs to be repacked.Would you like me to repair these items so that someone can use your new faucet?
Once he figured out I wasn’t BS’ing he was a bit red faced but all turned out well.
This is a good thing to point out Mr.King as new products come to use and we all need some re-education and updates.
This can be particularly interesting if you have a remodeled home that has galvanized piping and the hot water supply is lost volume.
You now lose the cold water volume as well.
A good thing to know.
Your client will be impressed with how you know about how things work.