How to Flush a Water Heater — Step by Step

Flushing a water heater is genuinely a homeowner job, it takes about an hour, and it is the single most effective thing you can do to keep a tank out of the emergency category. Here is the order that matters, including the step most guides skip.

What you need

• A garden hose long enough to reach a drain or outside
• A flathead screwdriver, for the drain valve on some units
• A bucket, to catch the first run and see what comes out
• Somewhere for 50 gallons of hot, rusty water to go that is not your lawn

Step by step

1. Turn off the heat and let it cool. Gas: control dial to OFF. Electric: breaker off. Then leave it for a couple of hours — draining 140°F water through a garden hose is how people get burned.
2. Shut the cold inlet valve on top of the tank.
3. Connect the hose to the drain valve at the bottom and run it somewhere that can take hot, discoloured water. Not a flower bed.
4. Open a hot tap somewhere in the house and leave it open. This breaks the vacuum — without it the tank drains at a trickle or not at all.
5. Open the drain valve and let the tank empty. Catch the first gallon in the bucket so you can see how much sediment is coming out.
6. Once empty, briefly reopen the cold inlet to stir the bottom, and let it run out again. Repeat until it runs clear — usually two or three cycles.
7. Close the drain valve, remove the hose, reopen the cold inlet. Leave the hot tap open until it runs without spitting air, which means the tank is full.
8. Only then turn the heat back on. Firing an electric element in a partly empty tank destroys it in seconds. This is the most expensive mistake on this list.

Things that go wrong

• The drain valve will not close again. Old plastic valves are brittle and sediment gets under the seat. Have a hose cap to hand, and expect to replace the valve with a brass one.
• Nothing drains. Almost always a vacuum — open a hot tap. If it still will not, sediment has blocked the valve.
• It leaks afterwards. On a very old tank, a thick sediment bed can be effectively plugging a weak spot. Disturbing it reveals what was already there.
• Elements burn out. Turning the power back on before the tank is full. It takes seconds and it is not recoverable.

If yours has never been flushed

On a tank over about eight years old that has never been serviced, think carefully. The flush is still the right thing to do, but a heavy sediment bed on an ageing tank is exactly the situation where a flush uncovers a problem. If the unit is already near the end of its expected life, the money is better spent planning the replacement.

If you would rather someone else did it — including the anode rod check, the T&P test and the combustion check — that is our annual maintenance service.