The Water Already in Your House: Heater, Pipes, and Tank
Figures last verified
A typical residential water heater holds 40 to 80 gallons of drinkable water. Household plumbing holds several more. Together that is often several days of supply for a family at the conservative planning figure, sitting in the basement, uncounted by almost every household water plan.
Getting it out correctly takes three steps in a specific order. Doing them out of order can destroy the heating element or leave you unable to draw anything at all.
Shut off the incoming supply first
The moment you know the municipal supply is compromised, close the main water shutoff valve to the house.
This matters because if the system loses pressure or becomes contaminated, an open main lets contaminated water or air into your plumbing and into the heater. Closing it isolates everything currently in the house as a clean reserve.
The main shutoff is usually where the supply line enters, near the meter, often in a basement, crawlspace, or utility closet. Finding it now rather than during an emergency is the entire preparation required, and it is worth turning it to confirm it actually moves, since valves that have not been operated in twenty years often seize.
Kill the power or gas to the heater
Before draining any water from the tank, turn off the heat source.
Electric heaters have heating elements that burn out within seconds if energized with no water around them. Turn off the dedicated breaker at the panel.
Gas heaters should be set to pilot or off at the control valve.
This is the step people skip, and it is the one that converts a water supply into a repair bill.
Draw the water
Open a hot water tap somewhere in the house, ideally on an upper floor. This breaks the vacuum. Without it the tank will not drain properly, which is the second most common failure and the reason people conclude their heater is empty when it is not.
Connect a hose to the drain valve at the base of the tank, or place a container under it. Open the valve.
Expect sediment. The bottom few inches of any water heater that has not been flushed regularly contain mineral scale and rust, and the first water out will be discolored. Let it run until it clears, or draw from it and filter and treat it. The water above the sediment layer is clean.
Draw what you need into containers rather than emptying the tank onto the floor. Do not restore power or gas until the tank is confirmed full again.
How much is actually there
Tank capacity is printed on the label, typically 40, 50, 60, or 80 gallons.
Usable capacity is somewhat less, since the drain valve sits above the very bottom and the sediment layer occupies some volume. Call it 90 percent of nameplate.
A 50-gallon heater therefore yields roughly 45 usable gallons, which at the conservative one-gallon-per-person-per-day figure is eleven days for a single person or nearly three days for a family of four. The scope question behind that figure is in where the one-gallon-per-person-per-day number comes from.
Tankless heaters hold essentially nothing. If you have one, this entire reserve does not exist for you, which is worth knowing before you count on it.
The pipes
Household plumbing holds a few gallons, more in a large house with long runs.
To recover it, open the lowest faucet in the house, usually a basement utility sink or an outdoor spigot, and open a tap at the highest point to let air in. Gravity does the rest.
This is a small supply and worth collecting only when the situation has gone on long enough to matter, but it is free and clean.
The toilet tank, and not the bowl
The tank behind the toilet holds two to five gallons of clean water that has never been in the bowl. It is the same water that comes out of the tap, and it is drinkable.
The exception is any tank containing a cleaning tablet, the blue or bleach-puck type. That water is not drinkable and the tablets are the reason to stop using them if you intend to count this supply.
The bowl is not a source. It can be used for flushing waste and nothing else.
Ice and other hidden volume
The freezer's ice supply is water, and a full ice maker bin holds several quarts. It is also doing useful work keeping the freezer cold during an outage, so it is a late-stage rather than early-stage source.
Canned vegetables and fruit are packed in liquid that is drinkable and adds up across a pantry.
A swimming pool or hot tub holds thousands of gallons that are not drinkable without treatment, and pool chemistry complicates that treatment considerably. Pool water is a sanitation and washing supply rather than a drinking supply, and treating it for drinking is a last resort requiring filtration beyond what chlorine addresses, covered in filter, purify, or distill.
Water beds, if anyone still has one, contain algaecide and are not a source.
Does it need treatment
Water drawn from the heater and pipes was municipal water and carries whatever chlorine residual it had when the supply was cut. It needs no treatment if drawn promptly into clean containers.
If it sits in open containers, or if there is reason to believe the supply was contaminated before pressure was lost, treat it. Dosing depends on the sodium hypochlorite percentage on the bottle, which has changed and which most published charts have not caught up with, covered in how much bleach per gallon.
Sediment from the heater is aesthetic rather than dangerous, and a cloth filter handles it.
Restoring the heater afterward
Close the drain valve. Open the main supply. Open a hot tap and let it run until water flows steadily with no air, which confirms the tank is full. Only then restore power or gas.
Energizing an electric element in a partially filled tank destroys it just as thoroughly as energizing it in an empty one.
The preparation this requires
Locate the main shutoff and confirm it turns. Locate the heater's breaker or gas valve. Own a short hose that reaches the drain valve, and a container.
That is all of it, and it converts an invisible 50 gallons into an accessible one. Counting it into the plan usually reduces what needs to be stored in containers by a third or more, which changes the container decision in what to store water in.