You Should Know How Your House Works. Here’s a Guide.

Not long ago, my wife, Lindsay, mentioned that she was afraid of our house. Two years after we bought it, it still felt like a mysterious machine to her, obscure in its workings and liable to break if she made a wrong move. It was the perfect prompt for us to do something we should have done from the jump: test our gas and water valves, map out our electrical panel, and label all of it clearly.

Whether you own or rent, knowing how to shut off your power and your pipes — and then turn them back on — is a matter of safety and practicality, especially in an emergency.

The wires and plumbing are the nerves and veins of your home, carrying vital signals and substances to where they’re needed. As you figure out how to safely turn them on and off, a mental picture will start to develop. You won’t just know that a given valve or breaker shuts off this faucet or that outlet. You’ll get an idea of where the connecting pipes and wires are hidden inside the walls. You’ll begin to know how your home works.

If you’re tired of your old light-switch plates, you’ll be ready to safely replace them. If you wake up to water dripping from a bathroom ceiling, you’ll be able to prevent the leak from becoming a flood. You may also discover that you don’t have a good way to shut down part of your home in an emergency. The best time to get that rectified is long before you need to.

A breaker panel is essentially a collection of switches for your switches. They’re called circuit breakers. In every panel, one or several of them control the main electrical feed line into your home. They will be marked with a high amperage number — often 100 — and we’ll ignore them for today.

The breakers you’ll be working on will typically be marked 15, 20 or 30 amps. Each one controls a single electrical circuit that supplies power to certain outlets and light switches. If you need to do electrical work on any of those outlets or switches, you shut off the breaker, just like flipping a light switch. If you ever overload one of the circuits (by plugging in too many devices, for example), the breaker automatically “trips” — shuts itself off to prevent a dangerous situation.

Ideally, all your circuit breakers are already labeled clearly with the outlets and lights that they control. But often, many are not, especially in older homes. I already knew that the labeling on our breaker panel was too broad to be useful (“2nd floor general lts-power” doesn’t tell you much), and sometimes dead wrong (“kitchen outlet” actually controls the staircase light and an upstairs bathroom outlet).

So Lindsay and I decided to map the panel anew. To begin, we labeled each breaker with a number. Then one of us would shut off a single breaker while the other tested the outlets we suspected it might control, using the noncontact voltage tester. For ceiling fixtures, we treated the lightbulbs as proxies for the voltage tester — it’s easier than dragging a ladder around, removing a bulb and testing the socket. When a breaker shuts a light off, you can assume the two are in circuit. (Of course, confirm that all your bulbs are working before you do this. A dead bulb will just confuse things.)

If you have any switched outlets, which only carry current when a switch on the wall is flipped, make sure that they are flipped on when testing. If the switch is in the off position, the voltage tester will give a false negative.

Mapping a breaker panel is a process of elimination. For the map to be accurate, only a single breaker can be shut off at a time. Electrical circuits can follow counterintuitive paths, resulting in a breaker that controls outlets and fixtures in multiple rooms or on different floors. You will likely wind up retracing your steps several times before your map is finished.

As you work, write down exactly which outlets and ceiling fixtures go with each breaker. Or, even more foolproof: Label the outlets and fixtures themselves with sticky notes as you go.

When you think you’ve gone through the entire breaker panel, compare your notes with reality. Is every outlet and fixture now mapped? If not, go back and retest the unknowns until everything is defined.

Finally, put new stickers on your breaker panel and label them with the outlets and fixtures they control. Or, type up your notes and mount them on the inside of the panel door. That’s my plan. Some of our breakers control so many things that no sticker could hold all the information.

Water Valves

Every toilet and sink will have its own shut-off valve or valves (two on most sinks, for hot and cold water). They’re usually easily accessible behind the fixture or in the sink cabinet. If you’re repairing a leaky toilet, for example, or if you’re upgrading an old faucet, you’ll need to shut off the valve(s) first. Bathtubs and showers generally don’t have separate shut-off valves; rather, the on/off handles themselves are the valves.

If you live in a free-standing home, you will also have a master valve that controls the water supply for the entire house. Most of the time this valve stays open, but if you suffer a burst or leaking pipe, shutting it off is often the only way to prevent a flood.

If you have a basement, your master valve is probably down there. If you don’t, it will likely be in a valve box in the ground near the house. In either case, look for clues to its identity: a single large pipe running into the valve from the direction of the street, and often an odometer-like water meter.

All valves do the same thing, but they do it in different ways. Ball valves are the most common these days. They open and close quickly with a quarter-turn of their handles, and are exceptionally robust and reliable. Gate valves screw open and closed, which requires multiple turns of the handle, and are susceptible to jamming due to mineral buildup.

If you have gate valves under your sinks and toilets, don’t try to force them closed if they’re acting stubborn when you go to make a repair. You could snap the handle stem, unleashing a geyser of pressurized water from the pipes. Instead, shut off the master valve, drain your pipes by opening the highest plumbing fixture in the house (typically a top-floor sink or shower), make your repair, and turn the main valve back on.

Gas Valves

Our house has a gas furnace, water heater, stove and clothes dryer. Each has its own shut-off valve. They’re the reliable, quarter-turn type. The problem is that the valves for the stove and dryer are tucked out of reach behind them. I had to move the 400-pound stove to get the video below, and I would have to wriggle behind the dryer to reach its valve.

That would be a headache if I ever needed to shut them off for servicing or repairs. In a true emergency — like a broken gas line in your home — you can shut off your main gas valve. It will be outside on the gas meter, and you’ll need a wrench. Many utilities and fire departments offer online video demonstrations or written instructions.