Water heater shut-off valve: where to find it and how to use it

The three shut-offs on a water heater

A. O. Smith's installation diagrams show the cold water shut-off on the cold supply line above the heater and a gas supply shut-off on the gas line, and its manuals call for both. The code only requires a valve on the cold side.

What the code and manufacturers require

• The cold side valve is required. International Residential Code section P2903.9.2 says a readily accessible full-open valve must be installed in the cold water supply pipe to each water heater, at or near the heater.
• Use a full-flow valve. A. O. Smith's electric manual says to install the shut-off near the heater where it's readily accessible, use only valves compatible with drinking water, and use only full-flow ball or gate valves, because other types can restrict flow too much.
• Gas heaters need a gas shut-off. A. O. Smith's gas manual says to install a shut-off valve on the gas supply line, and to use a new CSA-approved gas supply line.

If your heater has no cold water valve, or one buried behind other equipment, have one added.

How to shut off a water heater in a leak

1. Close the cold water supply valve. A ball valve takes a quarter turn of the lever; a gate valve with a round handle turns clockwise several times until it stops.
2. Turn off the heat source. Gas: turn the gas control knob to OFF and close the manual gas valve on the supply line. Electric: switch off the breaker.
3. Relieve and drain if needed. If the tank itself is leaking, A. O. Smith's draining steps are to run a hot faucet until the water is cool, connect a hose to the drain valve, keep the cold supply off, then open the drain valve and a hot faucet.

Our step-by-step for gas and electric models is in how to turn off a water heater. If water is pooling at the base, see water heater leaking from the bottom; if it's coming from the fittings above, leaking from the top.

Do I need to turn off the water heater if the main water is off?

For a short shut-off where nobody drains the lines, the tank stays full and the heater can keep running. The danger is an empty tank. A. O. Smith says turning on power to an electric heater before the tank is completely full burns out the upper element (a dry fire), and its gas manual says the tank must be full before lighting. So if the plumbing is being drained, or the water will be off long enough that someone might run the hot taps dry, turn the heater off first. When the water comes back, open a hot faucet and let it run until the air is out before turning the heater back on; A. O. Smith suggests three minutes or longer.

Valve problems to watch for

• A valve left partly closed. A. O. Smith lists a partially closed supply valve as a main cause of low pressure on the hot side only (see low hot water pressure).
• A valve that won't turn or won't fully close. Don't force it. Close the home's main water valve instead and have the heater valve replaced.
• A gas valve that won't close. Leave it, don't operate the heater, and call your gas utility.

Know where yours is before you need it

Take a minute now: find the cold water valve on your heater, the gas valve or breaker, and the home's main water shut-off. Label them if they aren't obvious. A. O. Smith's manual is blunt that, because of the normal corrosive action of water, a tank will eventually leak, and a fast shut-off limits the damage. If you're replacing a heater, ask the installer to put in a quarter-turn ball valve on the cold supply if there isn't one. Replacing a heater may also need a permit (see do you need a permit to replace a water heater).