When turning off incoming pressurized fresh water to a property for a few hours during non-freezing conditions, are there any special precautions that need to be taken?
Specifically, is there anything that needs to be done to protect a tank-based water heater?
Also, what about when the water is turned back on?
In addition to what FreeMan stated, if you have an electric water heater you want to turn off the breaker that feeds it. Depending on your plumbing, turning on some hot water faucets to finish some dishes or finish washing up could drain the water heater. If the thermostat for it kicks in and the water level is low, you'll burn up the element in seconds. When you turn your water back on, release any air in the water heater by opening a few faucets. Once your tank in full, turn your power to it back on.
If your tank is gas, it's not so important because your tank don't be damaged like your elements would be but it's still a good idea to shut it off.
I've turned the water off at the meter for our house more than once - I even bought my own "curb key" to do it. We've had a small variety of leaks and issues requiring this. The water company has turned it off for a variety of maintenance work too.
I've never taken any particular precautions prior to turning the water off, nor had any ill effects after turning the water back on.
When we first connected to city water, they started the meter at zero (the new meter had never been used!) and told us to run 100 gallons to flush the line from the meter to the house. They started billing us after the 100 gallon mark. Other than that - nothing.
You may get some air in the line if you're opening other valves or cutting into piping. The water will spurt and splutter as the air is forced through the pipes and out the taps, but this won't harm anything beyond possibly splashing you, and, depending on the length of pipe between where the air got in and the tap where you're letting it out should be over in well under a minute. It's possible that you'll get a small spurt from every faucet and tub, as the air bubbles might work their way all through the house, but it's not too likely. If you know your house's plumbing well, go to the farthest faucet from where the water main comes in and let it run for whatever amount of time it normally takes to get hot water there (assuming a single water heater located near the main inlet). This will ensure that you've flushed the vast majority of water out. Or, if you're up for some fun, don't warn the family and let them discover the air themselves and get a little splash as each faucet is used during the day. :)
You are lucky to live in a place where the utility company doesn't turn off your water on a schedule because there is a water shortage. Those of us who are used to it know that nothing particularily bad happens.
If you plan to work on hot water part, turn off your water heater no matter if it is powered by electricity, gas, coal or uranium rods (sun-powered are a separate case, refer to their manual). Drain any hot water from it beforehand. You don't want hot water leaking on your hands in some confined space where you cannot readily run away, do you? It probably has a valve that won't let the hot water into the cold side, but one could never know how this valve feels like, so it is better not to have hot water available to burn your hands it even if you don't plan working on the hot side.
Washing machines, dish-washing machines, water coolers, coffee machines, etc, ... anything connected to both water and electricity must be turned off and unplugged from the mains if applicable. Forgetting one or two of these is less important, they don't have the water heater potential for doing unpleasant things.
When turning off any rarely-used water valves, esp. old rusty ones, be prepared that (1) they may not close absolutely tight (you may get a small amount of water flowing from open pipes for the whole duration of the repair) and (2) they may start leaking from the shaft. The shaft leak usually, but not always, stops later by itself.
Ok, you turned off the water, did some repairs, now turn it back on.
Check for leaks.
Run water from all taps until it stops coughing air. Look for any signs of mud or rust in the water that comes out first. If there is any sign of mud or rust (it depends on the type of plumbing and the quality of your local utility water), you may need to run the washing machine on the shortest program without any clothes in. You don't want your clothes washed with rusty water, beleive me.
Check for leaks again, everywhere, even where you didn't touch. One can never be sure.
Turn back on anything you turned off.
Done (probably).
If your municipal water supply does not require a check valve (backflow preventer) where the water enters your house, then you probably do not have an expansion tank in your water system. In that case, if your water heater is not up to full temperature when you close your main valve and the water heater is running, the water in the tank will expand as it warms, and the pressure will increase until the temperature / pressure relief valve dumps the excess water on the floor. It's not the end of the world, but the T/P relief valve at that point might develop a permanent slow leak and require replacement. So just turn off the water heater before closing the main supply valve.
And if you have an expansion tank in your water system, no worries!
