If main water supply line is shut off and thermostat is turned up on my gas water heater could relief valve open ?my girlfriend was taking a bath and told me the water pressure was weakening and water wasnt real hot so i turned up the water heater not knowing that the main water supply was turned off. the relief valve opened why did this happen?

Add a domestic water expansion tank to the cold water line going into the water heater or at the water main coming into the building.

sounds like defective thermostat/gas valve

the tank needs flushed out ! heres how. Water heater manufacturers recommend flushing sediment from your storage type water heater periodically.
If your water heater is gas, set the gas valve to "Pilot" to prevent the burners from coming on while you are flushing it. If your heater is electric be sure to turn off the circuit breakers. With an electric water heater, if the water level drops below the heating elements and the thermostat turns the elements on, the heating elements will probably burn out quite rapidly. Connect a garden hose to the drain valve at the bottom of the tank, Close the shut off valve on the cold inlet to the water heater. open the temperature/pressure relief valve at the top of the tank by lifting the lever. Leave the valve open. Open the drain valve at the bottom of the heater allowing the water to flow out through the garden hose. when the water is clear your finished. close pressure relief valve on top of tank and turn water back on.
Open a hot water faucet in your house, and let it run until no air bubbles come out.

Turn the heater back on, and with gas units re-light the pilot light if necessary. good-day !

you can go to get another relief valve because if its dripping its probably bad. if you have a clark supply center nearby, you can get one there! or another plumbing place or water heater supply. you may have to drain out some water to bring the level below the valve. Shut off the power to water heater. If electric, turn gas to pilot setting, if a gas water heater. Cut off water supply to water heater as well. It is not necessary to drain the entire tank. Open a hot water faucet, drain water from tank, drain for approximately three to five minutes. Prepare the new relief valve with Teflon tape. Unscrew the relief valve counter-clockwise with a wrench to remove.
Turn the water supply back on to water heater. Make sure water is running from the open faucet before turning the power back on. good-day!

Shut off the water supply valve over the heater. Shut off the electric or gas to the heater.
If you have any house or basement water valves below the level of the heater, open them and open a valve that is higher to vent it.
If necessary, open the drain at the bottom of the heater and using hose or buckets, lower the level below the relief/thermostatic valve. Be careful, the water is hot and softens even heavy hoses.
Once the water level is down, unthread the downtube (if there is one) from the discharge of the relief valve, unthread the relief valve from the heater and either dope/tape the threads or replace the relief valve.

Write back with any questions.

If you can shut the cold water off at the water heater you should not have to turn the water off outside. Drain hot water from taps at lowest point in house, hook a hose up to boiler drain at base of WH and drain some water from here. Do as you stated and shut gas off to be safe. Change supply line.
RJ

The thermostat probe on most residential water heaters is part of the gas valve. The thermostat is a non replaceable part of the gas vale and if your t-stat is bad you will need to replace the gas valve also.
This is generally not a very hard job just start off by shutting gas supply valve to off position.Disconnect main gas line to burner,line to pilot light, and thermocouple from gas valve. Remove pipe fitting from gas valve and use large pipe wrench to loosen gas valve.
Before you loosen gas valve you will need to shut water supply off and drain water from tank.MOST WATER HEATER GAS VALVES ARE PRETTY STANDARD ITEMS AVAILABLE AT MOST HARDWARE STORES. Just make sure you purchase valve for the type of fuel you have,such as l.p. or natural gas.Remove gas valve by turning counterclockwise. Use teflon tape or pipe joint compound on all threaded connections for reassembly. Hook up gas line,burner supply line,pilot and thermocouple lines. Refill tank and check and check for leaks. Light pilot and turn gas control to on. Set to desired heat setting . Also check all gas fitting for leaks. Good luck. Thanks

The water heater is over heating the water, possibly boiling it and causing steam pressure build up. The thermostat is not sensing the temp to shut off the flame. It would be wise to either have a repairman come out or replace the heater. If pressure builds too high and the relief valve cand keep up ,theer could be a rupure of a water lineSoooooooooooo, turn the gas off and git er done.

It could be back flowing into the hot water heater tank which is not full because of the leak. Even though the valve is closed on the cold water entry side, there is probably no valve on the exit side. That is how it is returning to the tank.

Is the relief valve in the open position, try closing the relief valve.
To shut the water off you have to shut the main water supply off were it comes out of the floor in your basement.