The water heater.
40–50 gallons hiding in plain sight.

A standard residential tank water heater holds 30 to 80 gallons of water. Most households have a 40- or 50-gallon unit. That water is already treated municipal water — the same water that came out of your tap. It has been sitting in the tank at 120°F, which is hot enough to inhibit most bacterial growth. In a water emergency, it is your first and most available supply.

A family of four needs about 1 gallon per person per day for drinking and basic sanitation — 4 gallons per day, 28 gallons per week. A 40-gallon water heater covers that household for a week and a half without any other source. A 50-gallon tank covers nearly two weeks.

How to access the water.

The process takes about five minutes and requires no special tools.

Step 1 — Shut off the cold water inlet valve. This is the valve on the cold water supply pipe going into the top of the heater. Turn it clockwise until it stops. This prevents any contaminated water from entering the tank if the mains become compromised, and keeps the tank sealed as a clean reservoir.

Step 2 — Turn off the heat source. For a gas heater, turn the dial to PILOT. For an electric heater, switch off the circuit breaker. This prevents the heating element from running dry and burning out once you begin draining.

Step 3 — Relieve the pressure. Open a hot water faucet somewhere in the house — any sink will do. This breaks the vacuum and allows the tank to drain by gravity. Leave it open while you draw water.

Step 4 — Connect a hose to the drain spigot. The drain valve is at the very bottom of the tank — it looks like a hose bib. Attach a standard garden hose, run it to a bucket or container, and open the valve. Water will flow by gravity.

Step 5 — Collect and store. Fill clean containers. The water is already potable — you do not need to filter or treat it under normal circumstances.

When to boil first.

If the municipal water system has been compromised — a boil-water notice, flooding, or a break in the main that allowed contaminants in — treat the tank water before drinking. Boil for one minute at a rolling boil, or run it through a gravity filter rated for biological contaminants. The tank held treated water, but if contamination entered the system before you shut the inlet valve, the precaution is worth it.

If the outage is simply a loss of pressure (pump failure, storm damage to lines) with no known contamination event, the tank water is fine as-is.

The sediment caveat.

Water heater tanks accumulate sediment at the bottom over time — minerals from the water supply that settle and build up. In tanks that have never been flushed, the first water out of the drain valve may be discolored or gritty. Let it run for 30 seconds and the water will clear. If sediment is a concern, run the water through a cloth or coffee filter before drinking. It is not harmful, just unpleasant.

Flushing your water heater annually (opening the drain valve briefly to let sediment out) both extends the life of the heater and keeps the emergency supply cleaner. It takes ten minutes once a year.

What this doesn't cover.

The water heater is a one-time supply. Once it's drained, it's drained. The right use is as a bridge — covering the first week or two while you access other sources: stored water, a gravity filter drawing from a rain barrel or stream, or restoration of municipal service. It is not a long-term strategy on its own.

Pair it with at least one other source. A gravity filter and a rain catchment barrel give you the redundancy to extend well beyond the tank's capacity.