sms wrote:
I have a problem with certain family members and exceedingly long
showers. Pleading and screaming has no effect. The water bills and the
gas bills are outrageous.

I've seen some commercial products that control long showers but they
are poorly reviewed and expensive.

I was thinking of a solenoid controlled valve on the hot water line of
the shower. I'd plumb in a bypass to keep a tiny amount of hot water
going in, but cut off the bulk of the hot water with a valve like
this: http://www.amazon.com/dp/B00ESZPUR6/ (rated to 80°C).

To trigger the micro-controller that controls the valve I'd use a flow
sensor such as http://www.amazon.com/dp/B00HG7F9SS (rated to
100°C). A dishwasher valve would also work.

So basically it would be a flow sensor and a solenoid controlled valve
in series with the hot water line, and a bypass with a manual valve.

Just give them the bill to pay.