Flushing a Water Heater: The Safe Order of Steps, Gas or Electric

Minerals in your water settle at the bottom of a tank water heater as sediment. The US Department of Energy’s Building America guide explains that the build-up can coat the heating element and reduce how well it heats. Draining some of it out is a beginner job. This guide covers tank heaters only. A tankless heater has its own descaling procedure in its manual.

Safety first. The water in the tank is hot enough to burn. Turn off the power or set the gas to pilot before you drain, keep the hose end away from people and pets, and wear gloves and eye protection. Never relight, move or disconnect a gas water heater beyond what its manual tells you to do. Gas work is for a licensed gas fitter.

At a glance

• Skill level: Beginner. The DOE guide rates it the easiest level.
• Time: Alberta Health Services says draining takes about 20 to 60 minutes, depending on the tank size. Allow extra time to flush until clear and refill, and expect to wait several hours for hot water afterwards.
• Cost: Nothing, if you already own a garden hose. The DOE guide lists the materials as free.
• Tools: a garden hose, a flat screwdriver (some drain valves need one), rags and a bucket.

Read your manual first

Both guides start the same way: follow the flushing steps in your heater’s manual when you have it. Gas valves in particular differ between makes. Find the model on the label and download the manual if you’ve lost it.

The order that matters

This is the sequence from the DOE and Alberta Health Services guides. Do it in this order.

1. Turn off the heat. For an electric heater, switch off the breaker (or unplug it). For gas, set the control to “pilot” or off, as your manual says. An electric heater that runs with an empty tank can burn out its elements.
2. Close the cold-water supply valve on the pipe into the tank. If there isn’t one, shut off the main. See how to shut off your water, power and gas.
3. Connect a garden hose to the drain valve at the bottom of the tank. Run it to a floor drain or outside. The outlet must be lower than the bottom of the tank, or it won’t drain.
4. Open a hot-water tap upstairs or above the tank, so air can get in as the water leaves.
5. Open the drain valve and let the tank empty. The water will be hot. Don’t force a stiff valve open or shut, or it can break.
6. Stir up the sediment. Open and close the cold-water valve a few times while it drains. The bursts of water loosen what’s settled on the bottom.
7. Flush until it runs clear, with the drain open and the cold supply on.
8. Close the drain valve, then let the tank refill with the cold supply open. Close the hot tap once it runs with a steady stream and no more sputtering air.
9. Restore power or gas only when the tank is full. For gas, follow the manual so the pilot stays lit. Never turn on an electric heater with an empty tank.
10. Check the drain valve for drips. Alberta Health Services says to open and close it a few times if it leaks. If it still leaks, shut off the cold supply and call a plumber.

The DOE guide also suggests holding off on laundry for a few hours, because the water may be cloudy and the tank needs time to reheat.

While you’re there: check the temperature

The CPSC urges households to set water heaters to 120°F (49°C). It says most adults will suffer third-degree burns from 150°F water in two seconds, and from 140°F water in six seconds. To check, hold a cooking thermometer under the hot tap first thing in the morning, or at least two hours after anyone has used hot water. If it reads too high, adjust the thermostat following the manufacturer’s instructions. For an electric heater, turn off the breaker before opening the thermostat covers.

When not to do it yourself

• The tank itself is leaking (not the valve or a fitting). A tank that leaks has failed and needs replacing.
• The drain valve is seized or breaks off. Don’t force it. Call a plumber.
• Anything to do with the gas line, the burner or the venting. In Ontario, the TSSA reported a $6,000 fine, plus a surcharge, for a plumbing company director who sent an uncertified employee to install a natural gas water heater, and it advises hiring only registered contractors for fuel-fired appliances. Replacing a water heater may also need a permit; check with your city. See when to call a licensed pro.
• You smell gas. Leave the house and call your gas utility from outside.

If the heater rumbles, pops or leaks, start at Water heater leaking or noisy.

Watch it done

Flush a water heater collects step-by-step videos. More basement jobs are in the Basement, and seasonal jobs are on our winter maintenance checklist.

Sources

1. Building America Solution Center, US Department of Energy (developed by Pacific Northwest National Laboratory). Flush water heater tank Source
2. Alberta Health Services, Safe Healthy Environments (MyHealth.Alberta.ca). Flushing your water heater Source
3. US Consumer Product Safety Commission. Avoiding tap water scalds Source
4. Technical Standards and Safety Authority (Ontario). Individual fined $6,000+ for employing unlicensed worker to install natural gas water heater Source