Can leaving the shower faucet on, but the showerhead off, cause a leak?

Question

I have an apartment with a showerhead that can be shut off while the faucet is still on. Can I just leave the faucet on and only use the showerhead on/off buttons, or will that cause a leak?

Answer 1

You would definitely not want to leave the in wall shower valve on continuously and shut off the water at a shower arm valve. This would keep the main shower valve pressurized continually, something which they are not designed for. It could shorten the life of the expensive shower valve. It could also leak or seep inside the wall and cause damage before you would discover it.

The standard justification for these shower arm valves is to save water during showering. I personally like to save water, but I don't use one of these shower arm valves because we have a tankless water heater and I don't want to cycle it on and off unnecessarily. With a tank water heater this is not a problem, but any use of a shower arm shut-off valve will pressurize the piping and valves ahead of it.

The proper use would be if there was extreme need to conserve water due to local scarcity or if the water heater was not able to supply all the hot water needed during times of high use. Also I can see that in a minimal shower enclosure it would be a benefit to interrupt the flow of water while scrubbing with soap to conserve soap and to give time for the soap to work. In a tub/shower one can just step back from the shower to avoid having the soap immediately removed by the shower stream.

EDIT

@keshlam makes a very good point that leaving a shower (mixing) valve open with a shower arm valve closed establishes a connection between the hot and cold plumbing which would almost certainly pull hot water into the cold water lines or cold water into the hot water lines. This could happen even if the shower arm shutoff valve were just used momentarily, but would be a big problem if it were left that way all day.

Exactly this happened to me during a renovation. One bathroom was gutted and had a new shower valve in place with a plug on the outlet. The valves had been opened by the plumber to flush and to check for leaks and left open (or got turned open later). This is the equivalent of leaving the shower valve open with the shower arm valve closed.

With no handles in place on the valve I did not notice that both hot and cold were left on. The occupants of the house complained that they did not have hot water in the kitchen or the other bathroom. I immediately checked the water heater, but it was working.

Hummm . . .

Then it came to me about the bathroom under renovation. I went to the gutted shower, put the handles on and sure enough the hot and cold were open wide. I turned the valves off and hot water was restored in the kitchen and the other bathroom.

Answer 2

If everything was connected properly, the pipes shouldn't leak.

The showerhead or diverter may drip. Those are relatively cheap valves. Also, some manufacturers think it's important to drip in order to warn you so you don't leave it set too hot and scald yourself next time you turn it on.

You also may waste some hot water in this setup because, with the main mixing valves both open, water can flow between the hot and cold pipes. Depending on exactly how things are set up, turning on another tap or flushing the toilet may pull hot water back into the cold-water pipe or vice versa. There generally is not a check valve at the shower to prevent that, since under most normal use one isn't needed. This isn't a disaster if it happens, but it isn't desirable either.