How long does it take to flush a water heater?

Budget 60 to 90 minutes for a 40 or 50 gallon tank, most of which is waiting. Draining takes 20 to 40 minutes depending on how far you can run the hose and how much sediment is restricting the valve, the flush cycles add 10 to 15 minutes, and refilling and purging air takes another 10 to 20. Add an hour if the drain valve is a plastic one that clogs, which is common on tanks over five years old that have never been flushed.

Do I turn off the water heater before flushing it?

Yes, and the order matters. On a gas heater, turn the gas control dial to Pilot or Vacation — you do not need to extinguish the pilot. On an electric heater, switch the breaker off at the panel, not just the thermostat, because an element that fires with no water around it burns out in seconds. Then close the cold water inlet valve on top of the tank. Leaving the burner or elements on while the tank is empty is the single most expensive mistake in this job.

What if no water comes out of the drain valve?

The valve is blocked with sediment, which is extremely common on a tank that has never been flushed. Open a hot water faucet somewhere in the house to break the vacuum first — that alone fixes many cases. If it still will not flow, briefly open the cold inlet valve to push a surge of pressurized water through the drain and dislodge the blockage, or poke a stiff wire or a small screwdriver into the valve opening from outside. If nothing works, the valve needs replacing, and a brass ball valve is the upgrade worth making.

Should I flush a water heater that has never been flushed?

It is usually still worth doing, with one caveat: on a very old tank, sediment can be the only thing sitting over a corroded spot, and a small number of owners report a drip appearing after the first flush of a fifteen-year-old heater. That is not the flush causing the failure so much as revealing it. If your tank is under about ten years old, flush it. If it is well past its expected life and showing rust at the fittings, plan the replacement rather than gambling on maintenance.

How do you flush a tankless water heater?

Differently, and it is not the same job. A tankless unit has no tank to drain — it accumulates scale on the heat exchanger, which is removed by circulating a descaling solution or white vinegar through it for 45 to 60 minutes using a small submersible pump and two hoses connected to the isolation valves. If your tankless unit was installed without isolation valves, that has to be corrected first. Manufacturers generally specify this annually, and more often on hard water.

How to Flush a Water Heater: Step-by-Step

Written & researched by Priya Anand DIY & Tools Editor

Updated August 2026

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You can flush a water heater yourself in about an hour with a garden hose and no other tools, and it costs nothing. Shut the heat off, close the cold inlet, run a hose from the drain valve to a floor drain or outside, open a hot faucet in the house to break the vacuum, and drain the tank. Then flush it in bursts with the cold inlet until the water runs clear, refill, purge the air, and only then restore power or gas. No permit is involved and nothing here is licensed work — but the water leaving that hose is 120 to 140°F, and an electric heater that is powered up before the tank is full will destroy an element in seconds.

Doing this once a year is the cheapest thing you can do for a water heater. Sediment settles to the bottom of the tank and acts as insulation between the burner and the water, which makes the heater run longer for the same hot water, and it is what causes the rumbling and popping sounds people describe as the tank "boiling". Our page on how often to flush a water heater covers the interval; this page is the job itself.

Roger Wakefield, licensed master plumber, flushing a water heater step by step

Tools and materials

A garden hose long enough to reach a floor drain, a laundry sink, a driveway or a lawn. Check it for splits before you trust it with 140°F water.
A flathead screwdriver for the drain valve, if yours has a slot rather than a handle.
A bucket to catch the first cupful and see what comes out.
Gloves. The discharge is hot enough to scald — the CPSC notes that water at 140°F can produce a serious burn in as little as five seconds.
Optional but worth it: a brass 3/4-inch ball-valve drain replacement. Around $15, and it permanently solves the clogging plastic valve.
Optional: channel-lock pliers and a wet vacuum if you plan to replace the valve or expect a mess.

If you do not own basic plumbing tools, our plumbing tools worth owning guide covers the short list that handles most household jobs.

Low-angle view of a person's hands tightening a fitting under a kitchen sink with a wrench, in soft overcast light

Step 1: Shut it down correctly (5 minutes)

This is the step with the only real risk of damage, so do it in this order.

Gas heater: turn the gas control dial on the front of the tank from ON to Pilot or Vacation. You do not need to shut off the gas supply or relight the pilot afterward.
Electric heater: switch the water heater breaker off at the panel. Not the thermostat — the breaker. Electric elements are cooled by the water surrounding them, and one that energizes in an empty tank burns out almost immediately. This is by far the most common way people damage a heater during a flush.
Let it cool. Ideally give it one to two hours, or leave it overnight, so you are draining 100°F water rather than 140°F water. If you are in a hurry, run a hot tap somewhere in the house for several minutes to pull cooler water in.
Close the cold water inlet valve on the pipe entering the top of the tank.

Step 2: Drain the tank (20 to 40 minutes)

Screw the garden hose onto the drain valve at the bottom of the tank, hand tight plus a little. Run the other end to a floor drain, a laundry tub, or outside — remember it is downhill only, because this is gravity draining, not pumping. Nearly boiling water will kill lawn and garden plants, so aim it at a driveway or a gravel area rather than a flower bed.

Now open a hot water faucet at a fixture in the house and leave it open for the whole job. This is not optional. A closed tank draining from the bottom pulls a vacuum, and the flow either slows to a trickle or stops entirely. Opening a hot tap upstream lets air in and the tank empties properly.

Open the drain valve and let it run. Catch the first cupful in a bucket and look at it — that sediment is the reason you are doing this. On a tank that has been flushed regularly it will be nearly clear. On one that never has, expect grit, sand-like calcium carbonate, and cloudy gray water.

If nothing comes out — the valve is packed with sediment, which is normal on a neglected tank. Confirm the hot tap is open, then briefly crack the cold inlet valve to push a pressurized surge through. If that fails, poke a stiff wire into the valve opening from the outside. Replace a plastic valve with brass while it is empty.

Overhead view of a person's hands tightening a fitting under a kitchen sink with a wrench, in soft overcast light

Step 3: Flush in bursts until it runs clear (10 to 15 minutes)

An empty tank is not a clean tank. Sediment that has compacted on the bottom over years does not simply flow out with the water — it needs to be stirred up and carried out, and that is what this step does.

With the drain valve still open and the hose still running out, open the cold water inlet valve for 20 to 30 seconds, then close it. The incoming cold water enters through the dip tube near the bottom of the tank and blasts across the sediment bed, suspending it so it flushes out. Watch the discharge; you will see it go cloudy and gritty again.

Repeat this three to six times until the water leaving the hose runs clear. On a tank that has never been flushed, expect to do it more than you think, and expect the last two cycles to look much better than the first two. If after eight or ten bursts it is still heavily discolored, the sediment bed is thicker than a hose flush will move, and the honest options are a professional flush with a pump or accepting that the heater is near the end of its useful life.

Step 4: Refill and purge the air (10 to 20 minutes)

Close the drain valve firmly, then disconnect the hose — carefully, because there will be hot water left in it.

Leave the hot faucet in the house open and open the cold water inlet valve fully. The tank refills over several minutes. At the open faucet you will get spitting and sputtering as trapped air is pushed out, then a steady stream. When water runs smoothly with no air, the tank is full. Go around the house and run every hot tap for 30 seconds each to clear air from the rest of the system — air pockets in supply lines cause the knocking noise people mistake for water hammer.

Only now do you restore heat. Turn the gas dial back to ON, or switch the breaker back on. On an electric heater, be certain the tank is full first: this is the moment the damage happens if you have rushed. Expect 30 to 60 minutes for a gas tank to recover temperature and 60 to 120 for electric.

Before you walk away, check the drain valve and any fitting you touched for drips. A plastic drain valve that has been opened for the first time in ten years is the classic post-flush leak — the washer is old and the seat is scored by sediment. If it weeps, replace it with a brass ball valve rather than trying to tighten it.

Wide view of a person's hands tightening a fitting under a kitchen sink with a wrench, in warm late-afternoon light

Two things worth doing while you are there

Test the temperature and pressure relief valve. The T&P valve is the safety device that prevents a water heater becoming a pressure vessel, and it is designed to be exercised. Lift the lever briefly with a bucket under the discharge pipe, and let it snap back. Be aware of the trade-off: on a valve that has never been tested, mineral deposits sometimes stop it reseating and you get a persistent drip, meaning the valve needs replacing. That is a $15 part, and a T&P valve you have discovered is stuck is far better than one you have not.

Note the temperature setting. Most manufacturers ship tanks at 120°F, and that is the CPSC's recommendation for scald prevention. Some households set higher for Legionella control — the CDC's potable water systems guidance discusses the temperature ranges where the bacterium grows. The two goals genuinely conflict, and the usual resolution is storing hotter with thermostatic mixing valves at the fixtures. Our page on what temperature a water heater should be set to works through it.

Safety and when to call a pro

Flushing a water heater is homeowner maintenance and needs no permit. Stop and call a licensed plumber — or a gas fitter — if any of the following appear:

Water pooling under the tank, or rust at the bottom seam. That is tank failure, not a maintenance issue, and it will not improve. See how long a water heater lasts.
A T&P valve that discharges repeatedly without you touching it. That indicates excessive temperature or pressure and is a genuine safety issue.
Any smell of gas at any point. Leave, then call the utility from outside.
A drain valve that shears off while you are working on it. Shut the cold inlet immediately.

Replacing a water heater is a different matter from flushing one: most jurisdictions require a permit and an inspection for a replacement, and the requirements are set locally — check with your city or county building department. Our page on whether you need a permit to replace a water heater sets out what is typically involved, and the ENERGY STAR water heater guidance is a useful starting point if you are weighing a replacement instead.

Related reading: why hot water runs out so fast, what size water heater you need, how much it costs to run a water heater and best tankless water heater. Our full method is in how we rank, and the hub is DIY plumbing.

How to Flush a Water Heater