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I have a brand new AO Smith 50gal gas water heater. Today there was a burst pipe from the cold spell we're having currently, so I had to turn off the water main to the house until the plumbers get here tomorrow.
With that, nobody had used any hot water all day so it should be full, but do I need to turn the water heater off until the water can be turned back on tomorrow to avoid any damage etc?
Water heaters have a dip tube on the cold water side that puts the cold water into the bottom of the tank. If water pressure is lost on the cold water side, the tank can siphon down through the cold water inlet till it reaches the bottom of the dip tube. Then since there's no water in the tank to absorb heat, the dip tube can melt and the tank can be damaged.
It would be best to at least set the heater so it is on pilot only.
Note: Manufacturer AO Smith states in their manuals that their gas water heaters must not be operated without being certain it's filled or with the cold water inlet shutoff valve closed.
Before the use of pressure relief valves, homeowners were always urged to shut off the gas or electricity to a water heater which has the water cut off.
The reasoning was simple: When the water heater came on to heat up its reservoir, it would cause the water to expand, and therefore pressurize, to the point where the tank or a pipe would rupture. As long as the inlet valve were open to the distribution system, heating water would expand slightly into the city system and not build up pressure.
That is still good advice because many overpressure/overtemperature valves discharge into an inconvenient area—which could cause water damage, cause electrical concerns, etc. If quick freezing conditions are present at the discharge, the valve's function might be impaired, resulting in 1960s over pressurization concerns.
Better to be safe than sorry. These cost very little, and may save some hassle:
This answer is my opinion as a technician to the simple question:
If my water heater is left on, and the water is shut off, will this damage the gas fired water heater if the hot water will not be used very much, or not at all?
Considering you wont be using the water, and it will only be off for a short time there is no risk at all. You can also shut off the cold feed valve to ensure water isnt siphoned out of the tank if cold water is used with the tank off. This would also not allow much hot water to be used out of the tank as you have now air locked the system.
The water heater will still turn on and off to maintain the water temperature inside of the tank. Think about if you have ever gone away for a prolonged period of time. Normally there will be no water use for days or weeks and this causes no problems to the tank. Even if water is used from the tank most (if not all) water heaters have low water cutoffs built into the unitroll (or aquastat). Now The tank has a dip tube for inlet. This dip tube normally stops 4 to 8 inches above the unitroll (which regulates temperature of the water). This ensures that for whatever reason, if the cold supply looses pressure, the water inst siphoned out of the tank completly. This avoids the tank being "tricked" to continuously run to try to maintain temperature if all of the hot water is used up. This also avoids the possibility of a melting dip tube or catastrophic failure of the tank.
Realistically there is no danger to having the tank on with the water off. Especially since the tank works off water pressure to supply hot water. The tank pressure will be equal to your water pressure. If there is no inlet water pressure, there will be no outlet water pressure.
If shutting off your water and leaving your water heater on could cause damage or catastrophic failure, then water heater manufacturers would be up to there necks in law suits. Even if it did cause issues, all the parts, and even the labor should be covered by the manufacturer.
When I go on vacation in the winter I always turn of the main water valve. I also turn off the water valve into the water heater, and drain the pipes in the house at the lowest faucet. I have done this for 25 years in three different houses, two with gas heaters and on an oil zone. I have had absolutely no problem with any of those heaters.
Here a tip for maintaining a healthy water heater: drain a bucket of water from its bottom valve every six months. The last hot water heater I had, AO Smith 75 gallon, was put in when I built my previous house in 1996, and is still working fine for the people who bought the house in 2011; that's 17 years so far. The tank in my current house is on an oil furnace zone, and does not have a lower drain faucet. That is because it is completely insulated, as it has not direct burner.
My understanding is that if you go away on vacation, turn off main water and turn off inlet water to the heater. Lower the temperature on the water heater to save energy. Don't bother turning off the water heater.
When I leave the house for any length of time, days, weeks etc. I always turn off the water at the incoming street source. I disconnect the electricity to the water heater since have electronic ignition. Now if a pipe in house fails, only the water in the pipes can drain out to house. Top floor, very little water will be in house, lowest level, more obviously. But at least that's it. Re the water heater, with whole system off, if it develops leak, only water in tank will drain out and being off, no chance of internal system failing and having water heater calling for more heat for water that isn't there and possibly creating overheating or damage. When get home, turn on the water and turn on the water heater, have a glass of wine and enjoy the memories while shower water heating.
