TL;DR
A standard 40-50 gallon water heater holds 2-3 weeks of drinking water for a family of four. Draining it is straightforward: turn off the power/gas, attach a garden hose to the drain valve at the base, open a hot water tap somewhere in the house to break the vacuum, open the drain valve, and collect. The entire process takes 10 minutes. Know where your drain valve is before you need this.
Know Your Tank Before the Emergency
Walk to your water heater right now. Look for:
- The drain valve — a faucet-style or hose-bib valve at the base of the tank, typically 3/4-inch diameter. A standard garden hose fits it.
- The gas shutoff or circuit breaker — know how to turn off the heat source before draining.
- The capacity label — usually on the front of the unit, typically 30-80 gallons.
- The age — if it is more than 8-10 years old and has never been flushed, expect sediment.
This reconnaissance takes two minutes. After an emergency hits, you want to do this from memory, not from looking things up.
Step-by-Step Drain Procedure
Before draining:
- Turn off the heat source — gas shutoff valve (quarter-turn perpendicular to the pipe = off) or circuit breaker for electric heaters
- Allow the tank to cool if possible — at least 1-2 hours. Hot water is usable but requires careful handling.
Draining:
- Attach a garden hose to the drain valve at the base of the tank. Route the other end to your collection containers or outside.
- Open a hot water tap anywhere in the house — kitchen sink, bathroom. This breaks the vacuum in the system and allows water to flow out of the tank. Leave it open while draining.
- Open the drain valve slowly. Water will flow out of the hose immediately.
- Fill containers from the hose, close the drain valve when switching containers to avoid waste.
- The full tank (40-50 gallons) drains in 20-30 minutes by gravity.
If the drain valve won't open: Older drain valves can seize or fail. If the valve won't turn, try a pair of pliers for gentle additional torque. If the valve breaks or is stuck, call a plumber — but note that in a true emergency with no professional help available, you can access water through the pressure relief valve or by opening a plumbing union above the tank.
After draining: If you drain the tank dry, you must not turn the heat source back on until the tank is fully refilled — an empty electric heating element will burn out in minutes. When municipal water returns, the tank refills automatically through the cold water inlet; allow 30-60 minutes before restoring heat.
Water Quality Assessment
Clear water, no smell: Drink it or filter and treat as you would any stored tap water.
Slightly cloudy: Allow to settle in containers, pour off the clear upper portion, discard the cloudy/sediment bottom portion.
Rusty orange color: Heavy sediment or internal corrosion. Filter through a cloth multiple times, then treat with boiling or chlorine bleach before drinking.
Sulfur (rotten egg) smell: Indicates anaerobic bacteria in the tank, often associated with a magnesium anode rod. The water is not poisonous, but it is unpleasant. Boiling drives off hydrogen sulfide and makes it palatable.
How Much You Actually Have
| Tank Size | People Supplied at 1 gal/day | Days of Drinking Water | |-----------|------------------------------|------------------------| | 30 gallons | 1-2 people | 15-30 days | | 40 gallons | 2-3 people | 13-20 days | | 50 gallons | 3-4 people | 12-17 days | | 80 gallons | 4-6 people | 13-20 days |
This is drinking water only. Add sanitation, cooking, and minimal hygiene and you are consuming 2-3 gallons per person per day, which roughly halves these figures.
Preventing Sediment (Before an Emergency)
Flush your water heater annually. Open the drain valve with a hose attached, allow 2-3 gallons to flush through, and close. This removes sediment buildup that affects both heater efficiency and water quality.
An annual flush is a 10-minute maintenance task that significantly improves the quality of water available to you in an emergency.
Pro Tip
After using your water heater as an emergency supply, reopen the cold water inlet (typically always open, unless you manually closed it), restore water pressure from your main supply, allow the tank to refill completely, and then turn the heat source back on. Most people forget to close the drain valve before restoring the supply and flood their utility room. Close the drain valve. Then restore supply.
Sources
Frequently Asked Questions
Is water heater water safe to drink?
Yes, with some caveats. The water in your tank is the same water that comes from your municipal supply — it just has been sitting heated. If the heater has been maintained and the water is from a treated municipal source, it is safe to drink after cooling. If your heater is very old, has not been flushed in years, or if you have a whole-house sediment issue, filter the water through a cloth and boil or treat with bleach before drinking.
How much water is in a standard water heater?
Standard residential water heaters range from 30-80 gallons. The most common sizes are 40 gallons (single person or couple) and 50 gallons (family of four). A 50-gallon tank holds enough drinking water (1 gallon/person/day) for a family of four for 12 days — not counting any other stored water.
Can I drain the water heater while it is still hot?
You can drain it, but the water will be very hot — potentially 120-140°F. Allow it to cool for several hours before drinking. If you need water immediately, mix hot water with cooler water or let it cool in containers. Hot water also loses chlorine residual faster, so treat hot water with a small amount of bleach if you are storing it rather than using it immediately.
What if my water heater has an aluminum anode rod?
All water heaters have sacrificial anode rods (usually magnesium or aluminum) that corrode to protect the tank. After years of use, these can leach low levels of aluminum into the water. This is typically not a health concern at normal drinking levels, but if your heater is old and you notice a metallic taste, run the water through an activated carbon filter or boil and let it cool before drinking.