Why won't the main water supply knob turn to allow water to the water heater

turn off gas to heater and drain unit of water so it don't freeze

Thermostat is likley problem , or limit switch control they regulate temp.

You need to replace the thermocouple.
When you light the pilot it is supose to heat the thermocouple.
the thermocouple produces a small electric current which allows the gas valve to open and keep the pilot going. and allowing the main burner to come on.

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 first thing I would do is turn up temp control on gas valve.Control is located on front of gas valve at the bottom front of heater.They usually have a indicator dial on front with an arrow pointing direction.Turn dial to higher setting when burner is not on and this should turn burner on and provide hotter water. The control is normally turned clockwise to increase temperature.If control is already turned all the way up then water heater may need to be replaced.Depending on water quality in your area heater may have a large amount of mineral build up and it may not be capable of producing enough or hot enough water. If this the case all I can suggest is to try to drain tank and flush some of the deposits out. If you have installation manual it will give you instructions on how to do this. If not see if you have working shut off valve on cold water line ,if it works shut it off and connect drain hose to drain valve on bottom right side of gas valve. Put hose to floor drain or sump basin if you have one. Turn control knob on top of gas valve from on to pilot,this will leave pilot light on but prevent burner from working while draining. Open drain valve ,flow rate may be slow or not at all. Open hot water side of some faucet to provide air to help draining. You may try opening water supply valve to heater a few times to aid in draining. If you can get it to drain,shut drain valve off when done,turn cold water supply back on and shut off hot water outlet. Then when tank is full open hot water vavle to bleed air out of heater and piping. Turn control knob on top of gas valve back to on position. This should bring burner back on .It will take about 1/2 hour to heat. If you can't get it to drain or this doesn't help you might consider getting a new 40 gallon heater. Hope this helps. Thanks

No. However you may be able reset the thermostat to its factory setting.
Turn the thermostat off.
Shut of the gas and water supply to the unit.
Unplug or disconnect the unit from its electrical power source. Disconnect the thermostat from the heater.
Wait approximately 5 to minutes, and then reattach the thermostat wires to the heater.
Turn the water supply on and open the tub faucet to its hottest setting for about a minute.
Turn the faucet off and make sure no water water is flowing anywhere in the house.
Turn the gas supply back on.
Reconnect the thermostat wires to the heater.
Connect or turn the power supply back on.
Wait at least 2 minutes before turning the thermostat power button back on.
After the thermostat power button has be pressed, wait an additional 2 minutes before running water.
Test the heater at this time. Turn the water on at the farthest faucet to its hottest setting. measure the temperature with a cooking thermometer.
If you need to make adjustments to the water temperature, either up down, shut all the faucets off first.
The thermostat should read 100 degrees Fahrenheit when it first starts.
If you are receiving an error massage, consult with the manufacturer.
If the thermostat is reading the correct water temperature but the unit is not operating, there are other issue need to deal with. Let us know if this works or you need further assistance.

There are several reason your Whirlpool water heater pilot may not light.

a. The gas supply is not turned on, or the main gas supply is off or blocked.
b. The gas supply is turned on, but you have air in the line. Holding the pilot control knob down and sparking the igniter every 60 seconds will eventually purge the air.
c. The water heater control valve is bad.
d. The oxygen safety sensor has tripped, and the control valve is locked out.
e. The pilot tub is clogged or the pilot orifac is blocked.
f. The thermocouple is bad and needs to be replaced.

Your water heater may still be under warranty. Call the 1-800 number on the tank and ask for help and free parts.

pressure would go down any time the water is cut off. Temp went down no make up water relief went off cause you turned it up. put it back and put the main back on.

An on-demand, or tankless, water heater is a system for heating water as it passes through the pipes, usually very near the point of use. Traditional tank water heaters bring the water to approximately 140 degrees Fahrenheit (60 C), in order to ensure the water supply lasts long after the heater turns off. Since this is obviously far too hot to be pumped out of a shower of faucet, cold water is added to the mix in the pipes. Coupled with the loss of thermal energy over time, the average tank heater has an efficiency of approximately 80%. On-demand water heaters have a number of advantages over their tank-using counterparts. While tanks often degrade over time and begin leaking, the apparatus used in an on-demand water heater is much less susceptible to the burdens of age, and will often outlive the house they're installed in. The amount of energy used is a fraction of that used in a tank water heater, due to increased efficiency and a much smaller window during which a heating source is in use. While tank hot water heaters can, and often do, run out of hot water after a number of showers, on-demand heaters provide a never-ending supply of hot water, ensuring that warm water is always available when needed. A tankless water heater can cost between $500 and $1500, and they most commonly use either electric or natural gas energy to heat the water. Most mid-sized households report an energy savings of approximately 25-45% for a gas or propane heater, and up to 50% with an electric heater, over using traditional tank heaters. Water is usually set to a maximum temperature of 120 degrees Fahrenheit (49 C), though many heaters allow for custom setting of the maximum temperature via remote control. Outflow rates range from 8-14 gallons (30-53 litres) per minute. Installation for most tankless water heaters is incredibly easy, involving simple plumbing. In the case of electric heaters, all that is required for a fuel source is to plug the heater into an outlet and begin heating your water. Some modern on-demand water heaters can incorporate a solar preheating system. This system uses a traditional solar heating system and tank to bring the water nearer to the desired temperature, saving substantial energy use in the final on-demand heating. A solar preheating system costs between $500 and $800.