The water problem is a difficult one, and the real solution for this family will involve something some people shun -- confrontation.
 
It would be lovely if there were a device that would automatically shut off the flow after a set interval and not restore flow for a set interval. It would be as appealing as the Vchip in households where parents can't or won't monitor children's televiewing. However, I have not heard of such a device. Also, it is a mistake to assume that the device would eliminate confrontation; on the first occasion, the daughter will emerge from the bathroom expressing her ire, and demand that the gadget be removed. Then, until it is removed, she will devote much energy to some kind of protest, and to figuring a way around the thing. The way around would be to tie up the bathroom for en even longer period, and that could be more inconvenient, for instance, making the son late to school.
 
I have seen and used flow regulators that reduce the volume emitted by showerheads and faucets. Some people will take these devices out and leave the landlord (or parent or working spouse) holding the water bill. Newer versions of these devices, and better fixtures that reduce flow without added regulators, actually aerate the water more for a better result with less water.
 
Another gadget I've heard of provides hot water on demand. Some of these can be installed in the plumbing during construction, and others are built to be retrofitted with a minimum of damaged walls. One shown in a magazine actually hangs below the showerhead. This would solve the problem of the son having cold showers, but it would do nothing to control costs or encourage the daughter to adopt greener behavior. However if such devices are used in all the locations where hot water is needed, the main heater could be set to a much lower temperature, and for some households this would reduce the cost of hot water.
 
Another alternative that some families choose is to install a bigger water heater. Along the same line, this family could check the temperature setting on their existing heater, and see if it has been set low back when the children were younger to reduce the risk of accidental scalds. If the temperature is to be reset higher, it would be prudent to add insulation to the heater and to as many of the hot water pipes as possible.
 
The ideas that seem better to me will require confrontation. One would be to simply require that the girl let her brother go first. Then, he could have a nice hot shower, and she could make do with whatever hot water he left for her. There is a possibility that he'd discover the fair play of turn about.
 
Another method is to set a timer, and when it rings, the water is to be shut off promptly. If that seems too difficult to enforce, with a wet unhappy daughter complaining from her bath, a control valve could be installed between the heater and the bathroom, which could be turned off when the timer buzzes, without going near the bathroom.
 
At 16, it is a bit late to begin controlling behavior with rules that parents may not like to enforce, but parent do have the right to limit use of things that cost money when they are paying the bills. This suggests another approach, a refinement of the timer idea. If the daughter wishes to use more than her proper allocation of hot water, she could have more if she pays for it out of her own earnings. This could and should be computed with reference to the water and electric bills and a reasonable estimate of the hourly electrical usage for running the hot water heater. Some teens are wise enough to get the lesson in the process of calculating the cost; they have simply not considered that hot water doesn't grow on trees.
 
An approach that has worked with my young people has been to keep a yardstick in the bathroom. The rule is that the drain is to be closed before a shower, and when the water gets to a depth of three inches, the shower is over. The reasoning is that since a bath requires only 2-3 inches of water, a shower should not use more. (In cold weather, we let the water stand until it has given up its heat to the air, saving a bit on the furnace. The tub needs cleaned a bit more often, but that takes less time than to earn the wages for more electricity.)
 
If the bathroom is so cold that it is uncomfortable to suds with the water off, a small portable heater might solve that. I chose a heater that is also a fan, so that it can ventilate the bath after a shower.
 
We have found that shutting off the water between rinses not only saves water, it saves time and produces more clean. It is more efficient to scrub and to shampoo when the water is not running. Also, we have discovered that one lathering is sufficient, saving on shampoo.
 
Rose B, mother of three (all teens now), in NC
 
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