How to Flush a Water Heater
To flush a water heater, turn off the gas or power, run a hose from the drain valve to a drain, and let fresh cold water push through the tank until it comes out clear with no sediment. A.O. Smith says most tank heaters should be flushed at least once a year, and more often in hard water areas.
Flushing takes an hour or two of mostly waiting. It clears the mineral sediment that builds up at the bottom of the tank, and A.O. Smith links that sediment to leaks, bad smells and other common water heater problems.
Flush vs. drain
People use the words interchangeably, but they aren't the same job. Draining empties the tank. Flushing runs cold water through the tank with the drain open, so the water stirs the sediment off the bottom and carries it out. Most homeowners drain first, then flush. Our how to drain a water heater page covers the draining part in detail.
What you need
- Garden hose and a bucket, or a route to a floor drain or outside
- Flathead screwdriver for the drain valve
- Thick gloves and safety goggles
- Towels or rags
- A wet/dry vacuum for stubborn sediment, if you have one
Step-by-step flush (gas or electric)
Turn off the heat
On a gas unit, close the gas valve. On an electric unit, switch off the breaker or the disconnect. Let the water cool for a few hours if you can.
Set up the drain route
Attach the hose to the drain valve at the bottom of the tank and run it to a floor drain, outside, or a bucket.
Drain the tank
Close the cold supply, open a hot tap in the house, then open the drain valve. Draining takes 20 to 40 minutes on average.
Flush with cold water
With the drain valve still open, open the cold water supply. Let it run until the water at the end of the hose is clear and you see no sediment. Catch a cup of it now and then to check.
Close the drain valve
Remove the hose and empty the bucket.
Refill and purge the air
Leave a hot tap open while the tank fills. Run it for at least 3 minutes after the water comes out to clear trapped air.
Restore the heat
Turn the breaker back on, or open the gas and relight the pilot following the manufacturer's instructions. Our turn on a water heater page covers that.
One practical note: the first gallon or two off an old tank often looks like wet sand. If it stays gritty after several minutes of flushing, shut the cold supply, let the tank drain, then open the cold again in short bursts. The surge stirs up more of the bottom than a steady flow does.
Gas vs. electric differences
The flush itself is the same. Only the shutoff and restart change:
| Step | Gas heater | Electric heater |
|---|---|---|
| Shutoff | Close the gas valve or set the dial to OFF | Breaker off, or the disconnect switch |
| Restart | Open the gas and relight the pilot per the manual | Breaker on, and only after the tank is full |
Electric heaters have a particular reason to flush. The lower element sits in the zone where sediment piles up, and a buried element works harder.
How often should you flush a water heater?
At least once a year for most tank heaters. HomeGuide says the same, and adds that it improves performance and helps the tank last longer. Hard water means more often. Check your model's manual, since some manufacturers specify their own schedule.
If the tank has never been flushed and it's more than a few years old, go gently. The drain valve may not reseal after stirring up years of buildup. Have a hose cap ready for the drain valve's threads just in case.
Is sediment in a hot water tank dangerous?
Sediment is mostly a problem for the heater, not for you. A.O. Smith ties it to leaks, bad smells and other common water heater problems, and to the rumbling or popping noises from older tanks. It doesn't make the tank explode, but it does shorten the tank's life.
If the sediment is reddish or the water tastes or smells off, it's worth testing your water. Our hard water page explains what mineral content means for your plumbing and how to test it.
Sediment filters and traps
A sediment filter ahead of the water heater catches sand and grit before it reaches the tank. It's most useful on well water, where sand can come up with the pump. It does nothing for dissolved hardness minerals, which form scale inside the tank no matter what. A softener handles that.
Tankless heaters have a small inlet screen of their own that needs cleaning during the yearly descale. Some owners add a sediment pre-filter on the cold line too. RVs use inline filters on the fresh water hookup for the same reason.
Mistakes to avoid
- Skipping the heat shutoff. On electric units, a dry element burns out.
- Opening a stuck drain valve with pliers. A cracked plastic valve on a full tank floods the floor.
- Turning power on with the tank half full. Wait for steady water with no air at the tap.
- Forgetting the gas. Close the gas valve before draining a gas unit.
When to call a plumber
If the drain valve clogs with sediment, A.O. Smith says to call a plumber or service technician. Call one too if you hear rumbling that won't go away after a flush. A.O. Smith says noises can have other causes besides sediment. HomeGuide prices a professional flush at $75 to $200.
FAQ
How do you flush a gas water heater?
Close the gas valve, then flush it like any other tank: drain through a hose, run cold water through until it's clear, close the valve and refill. Relight the pilot according to the manufacturer's instructions afterward.
How do you flush an electric water heater?
Switch off the breaker or disconnect, then drain and flush through the drain valve until the water runs clear. Refill completely and purge the air at a hot tap before turning the power back on.
How often should you flush your water heater?
At least once a year, more often with hard water. Your owner's manual may give a specific interval.
How do you flush a tankless water heater?
Tankless units don't get flushed the same way. They get descaled with a pump and solution through the isolation valves. See how to flush a tankless water heater.
Sources
- A. O. SmithHow to Drain and Flush a Water Heater (opens in a new tab)
- HomeGuideWater Heater Repair Cost (opens in a new tab)
- A. O. SmithWater Heater Repair or Replace? (opens in a new tab)