How to Flush a Water Heater
Remove sediment buildup from the bottom of your water heater tank to restore efficiency, extend the unit's lifespan, and improve hot water output. This task takes about an hour and should be done once a year.
What You'll Need
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Tools
Garden hose
Standard 5/8-inch garden hose long enough to reach a floor drain, utility sink, or exterior drain point from the water heater.
Flathead screwdriver(optional)
Used to open the drain valve if it has a slotted screw head rather than a handle.
Adjustable pliers or channel-lock pliers(optional)
Helpful for loosening a stiff drain valve or removing a hose cap.
Work gloves
Protect hands from hot surfaces and sediment-laden water, which can reach 120-140°F.
Safety glasses
Protect eyes from splashing hot water or dislodged sediment during draining.
Bucket (5-gallon)(optional)
Useful for catching the initial flush water and checking sediment levels before connecting the hose.
Materials
Replacement drain valve (brass, 3/4-inch NPT)(optional)
A spare ball-style drain valve in case the original plastic valve cracks or fails to close fully after flushing. Match the 3/4-inch NPT thread size standard on most residential tanks.
$8-$20
Teflon (PTFE) thread tape(optional)
Wrap around drain valve threads if you replace the valve to ensure a watertight seal.
$2-$5
Step-by-Step Instructions
Turn off the power or gas supply to the water heater
2-5 minutes
For an electric water heater, go to your main electrical panel and switch off the dedicated 240-volt breaker labeled 'Water Heater.' Verify power is off with a non-contact voltage tester held at the cable entering the top of the unit (most conventional electric water heaters have no indicator light to check). For a gas water heater, turn the thermostat dial on the gas valve to the 'PILOT' position - do not turn it to 'OFF,' which would extinguish the pilot light entirely and require relighting. Never attempt to flush a water heater while the heating elements or burner are active, as exposing electric elements to air can burn them out instantly.
Warning
Failing to cut power or gas before draining can permanently burn out electric heating elements or cause the gas burner to overheat the empty tank. Confirm the breaker is off with a non-contact voltage tester if you are unsure.
Connect a garden hose to the drain valve
3-5 minutes
Locate the drain valve near the bottom of the tank - it resembles a hose bib (spigot) with 3/4-inch garden hose threads and is typically made of plastic or brass. Thread your garden hose clockwise onto the valve until hand-tight, then snug it one quarter-turn with pliers if needed. Run the other end of the hose to a floor drain, utility sink, driveway, or yard area that can handle hot, sediment-laden water. Make sure the hose end is lower than the valve so gravity assists drainage.
Pro Tip
Check that your drain destination can handle hot water - PVC drain pipes in some utility sinks can warp if water is above 140°F. Let the water heater sit unused for 30-60 minutes before draining if you want cooler water, or run a hot-water tap until it cools slightly.
Open a hot-water tap to allow air into the tank
1-2 minutes
Go to the nearest sink or bathtub and open the hot-water faucet fully. This breaks the vacuum inside the tank and allows water to drain freely - without it, the tank will drain very slowly or not at all, similar to keeping your thumb over a straw. Leave this tap open for the entire draining process. Water will keep flowing until you shut off the cold supply in the next step; once draining begins, air will gurgle through this faucet - that is normal.
Pro Tip
A bathtub tap or utility sink tap works best because they have larger openings and allow more airflow.
Turn off the cold-water supply to the tank
1-3 minutes
Find the cold-water supply line entering the top of the water heater and close its dedicated shut-off valve by turning it clockwise until it stops. This valve is usually a ball valve (lever handle that turns 90 degrees to perpendicular) or a gate valve (round wheel handle requiring multiple turns). If there is no dedicated valve, shut off the main house water supply at the meter or main shut-off. This step prevents new water from entering the tank while you drain it.
Warning
If you skip this step, the tank will continuously refill as you drain it and you will never fully clear the sediment.
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Track This Project FreeOpen the drain valve and begin draining
20-45 minutes
Put on your work gloves and safety glasses. Open the drain valve by turning it counterclockwise (or lifting/turning its lever if it is a ball-style valve) until it is fully open. Water will immediately begin flowing through the hose. The water will likely appear rusty, brown, or full of white/gray flakes - this is the sediment you are removing and is completely normal. Allow the tank to drain completely, which typically takes 20-45 minutes depending on tank size (40-gallon vs. 80-gallon) and sediment level.
Pro Tip
If water flow is very slow or barely a trickle despite the air vent being open, there may be heavy sediment blocking the valve. Try gently wiggling the valve or use a wet/dry vacuum at the hose end to pull water through. Seriously blocked tanks may require a pro.
Warning
The draining water can be extremely hot (up to 140°F). Keep children and pets away from the hose discharge area. Do not touch the hose or valve with bare hands.
Flush the tank with fresh water to clear remaining sediment
10-15 minutes
Once the tank has fully drained and flow slows to a drip, briefly open the cold-water supply valve (turn it counterclockwise) for 2-3 minutes to stir up and flush out any remaining sediment sitting on the tank floor. Watch the water coming out of the hose - it will initially run murky again as sediment is disturbed. Close the cold-water supply valve again and let the fresh water drain out completely. Repeat this flush cycle one or two more times until the water running out of the hose appears clear or only slightly discolored.
Pro Tip
Three flush cycles is usually enough for annual maintenance. If you haven't flushed in several years, you may need 4-5 cycles before the water runs clear.
Close the drain valve and remove the hose
3-5 minutes
Once the final flush water runs clear, close the drain valve by turning it clockwise until it is snug - do not overtighten, especially if the valve body is plastic, as it can crack. Disconnect the garden hose by turning it counterclockwise off the valve threads. If the valve does not seal fully and continues to drip after closing, wrap the valve stem with a rag temporarily and note it for replacement; a continuously dripping drain valve must be repaired before you restore power to the unit.
Warning
A cracked or leaking drain valve that won't fully close is a common failure point on older tanks. Do not restore power or gas until any drip has stopped - a slow leak can damage flooring and go unnoticed for months.
Refill the tank
15-25 minutes
Fully open the cold-water supply valve (turn counterclockwise) to begin refilling the tank. Keep the hot-water tap you opened in Step 3 open - air will sputter and gurgle out of it as the tank fills. When water flows steadily from that tap with no sputtering or air bursts, the tank is full. Close the hot-water tap. Check the area around the drain valve and supply connections for any drips before proceeding.
Pro Tip
Refilling a 40-gallon tank typically takes 15-20 minutes. A 50- or 80-gallon tank will take longer.
Restore power or gas and confirm normal operation
3-5 minutes (plus 30-60 minutes reheat wait)
For an electric water heater, return to the electrical panel and flip the 240-volt breaker back to ON. For a gas water heater, turn the thermostat dial from 'PILOT' back to your desired temperature setting (typically 120°F, marked as 'Hot' or a specific degree marking). Allow 30-60 minutes for the water to reheat before testing. Run a hot-water tap and confirm that hot water is being produced within the expected time. Check around the drain valve and base of the unit one final time for any new leaks.
Pro Tip
Setting your water heater to 120°F (not higher) balances hot water performance with energy savings and reduces the risk of scalding.
Warning
Never turn the breaker back on while the tank is empty or only partially filled - exposed electric heating elements will burn out within seconds. Confirm the hot-water tap ran steady and full before restoring power.
Common Mistakes to Avoid
- Skipping the cold-water supply shutoff, which causes the tank to continuously refill and prevents sediment from being fully removed.
- Forgetting to open a hot-water tap before draining, creating a vacuum that slows or stops water flow entirely.
- Restoring power to an electric water heater before the tank is completely full, which instantly burns out the lower heating element.
- Overtightening a plastic drain valve when closing it, causing the valve body to crack and creating a slow leak at the base of the tank.
- Only draining without flushing - doing a single drain without the cold-water flush cycles leaves a significant layer of sediment on the tank floor.
Safety Warnings
- Water heater water can reach 120-140°F and cause serious burns - wear work gloves and safety glasses during draining and keep children and pets away from the hose discharge area.
- Never restore power to an electric water heater until the tank is completely full - dry-firing the heating elements destroys them immediately and can create a fire hazard.
- For gas water heaters, do not set the gas valve to the full 'OFF' position unless instructed to - relighting a gas pilot requires following the manufacturer's specific procedure and, if the pilot won't relight, you should call your gas utility or a licensed plumber.
- If you smell gas at any point during this procedure, stop immediately, leave the area, and call your gas utility's emergency line from outside the home - do not attempt to continue the flush.
When to Call a Professional
Call a licensed plumber if: the drain valve is completely seized and cannot be opened (forcing it risks cracking the tank); the valve cannot be fully closed after flushing and continues to drip despite being hand-tightened; you notice water pooling under the tank or corrosion on the tank body (signs of internal failure or active leak); or your water heater is more than 12-15 years old and showing reduced hot-water output - flushing a tank in late-stage failure can dislodge sediment that was previously sealing a pinhole leak. For gas water heaters, any work beyond the thermostat dial adjustment - including repairing the gas valve, relighting a pilot, or addressing a yellow/orange flame - should be handled by a licensed plumber or gas technician.
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Frequently Asked Questions
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What if no water comes out when I open the drain valve?
My drain valve is leaking after I closed it. What do I do?
Will flushing a really old water heater cause it to leak?
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Project Overview
- Difficulty
- Beginner
- Time
- 45-75 minutes
- Est. Cost
- $5–$30
- Tools
- 6 items
- Materials
- 2 items
- Steps
- 9
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