I just bought a house and it have a gas water heater. I need to know what is the water shut off valve to shut down water in the entire house. Also if you can help me label any of the other knob (which one is to adjust the temperature of the water inside the tank) .. that would also be very appreciated. Picture:

enter image description here

  • Are you looking for the hot water shutoff, or the cold water shutoff? Commented Aug 22, 2015 at 3:09
  • 1
    The temperature control is the big black knob at the bottom, mounted on the rectangular box.
    – Grant
    Commented Aug 22, 2015 at 3:14
  • The main shutoff may or may not be within this cabinet. It should be right where the main supply line enters the building. As for all the valves in the photo, it's difficult to say for sure what they're for, as most of the plumbing seems to be hidden behind the heater.
    – Tester101
    Commented Aug 22, 2015 at 3:17
  • Is this heater used to supply a hot water heating system, as well as domestic hot water? That would explain the extra valves.
    – Tester101
    Commented Aug 22, 2015 at 3:50
  • yes the water tank is used to supply a hot water heating system.
    – skyde
    Commented Aug 22, 2015 at 4:31

2 Answers 2


enter image description here

A - Expansion tank. This prevents the pressure increase due to heating water, from damaging the plumbing. The tank is filled with air, that is separated from the water by a bladder. When the water expands due to heating, water compresses the air absorbing the pressure.

B - Check valve (or back flow valve). This is designed to only allow water to flow in one direction.

C - Thermostatic mixing valve. This allows hot and cold water to mix, so the water is not too hot. You can adjust this to make the water leaving the tank hotter or cooler. Note, the water can only be as hot as the water in the tank, so you'll also need to set the control knob to the proper temperature setting.

D - Ball valve (or quarter turn valve). Used to isolate expansion tank and turn off cold water to water heater.

E - Globe valve. Outlet to heating system.

F - Temperature and pressure relief valve (T&P valve). This is designed to open if the water temperature, or pressure in the tank gets too high.

G - Hose bib. Not sure what it's used for here.

H - Globe valve. Inlet from heating system.

I - Ball valve. Used to turn the gas supply on/off.

J - Temperature control. Used to set the desired temperature for hot water. Adjusting this will change the temperature to which the heater heats the water to.

K - Drain. This allows you to drain the tank.

L - Ball valve. Not sure what it's used for here. Possibly shuts off cold water supply to heater, and/or other fixtures. If anything in this closet was a main shutoff, this would be it. Can't see where the plumbing goes, so can't tell if it only goes to the heater or splits to other fixtures.

M - Drip leg. This is used to catch any debris in the gas line, to prevent it from clogging the orifice.

N - Pilot view port. An opening where the pilot flame can be viewed from. Peek through here to see if the pilot is lit.

O - Burner cover. removing this would give you access to the main burner assembly.

It's not clear if any of these valves are the main shutoff.

You'll use a combination of the control knob and the thermostatic mixing valve, to set the temperature of the hot water delivered to the taps.

  • Made my letters a bit small, I'll make a better image next time I'm near a computer.
    – Tester101
    Commented Aug 22, 2015 at 3:56
  • D does look like the incoming cold-water cutoff for servicing this equipment. Might also cut off cold to the rest of the system, but I doubt it.
    – keshlam
    Commented Aug 22, 2015 at 4:31
  • Image is good and clear enough thanks a lot for labelling each valve.
    – skyde
    Commented Aug 22, 2015 at 4:42
  • Lettering was definitely a good approach; I'll have to remember that trick. Easier, and arguably more useful, than drawing anew diagram, as long as the picture is reasonably clear.
    – keshlam
    Commented Aug 22, 2015 at 16:43

The white pipe at the top left appears to have a thermostatic mixer valve. That's the output side of the water heater. To avoid running out of hot water too quickly, the tank is set to a somewhat higher temperature than is actually desired, and this valve mixes that with cold water to get the actual temperature and sends that to the rest of the house.

The box at the bottom is the burner control. The pipe coming into that is your gas supply, The knob there controls the tank temperature. There[s a safty interlock which shuts off the gas if the flame goes out; a service technician can show you how to relight it if necessary.

The PVC pipe is the output of the pressure-relief valve. That's a safety feature; while that valve should be tested/flushed during a service call, you can mostly ignore it.

The uninsulated copper tubing is mostly cold water in.

As others have said: you can shut off hot water from here, but the main water shut-off may be elsewhere.

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