How to Flush a Water Heater: Gas, Electric and Tankless Step-by-Step

Last updated: October 2026

To flush a tank water heater, turn off the gas or power, shut the cold water supply, connect a garden hose to the drain valve, open a hot faucet, drain the tank, then briefly turn the cold water back on to stir up and rinse out the remaining sediment until the water runs clear. Tankless water heaters aren’t drained this way; they’re descaled by pumping white vinegar or a descaling solution through the unit for up to about 45 minutes using a flush kit. Most manufacturers, including Rheem, recommend flushing once a year, or every six months if you have hard water.

This guide covers exactly how to flush a gas water heater, an electric water heater and a tankless unit, which tools you need, how often to do it, what a professional flush costs, and when you shouldn’t flush an old tank at all.

Quick reference: water heater flush at a glance

Why flush a water heater?

Every gallon of water that enters your heater carries dissolved minerals, mainly calcium and magnesium. Heating water makes these minerals come out of solution, and in a tank they settle on the bottom as sediment. In a tankless unit they form hard scale inside the narrow heat exchanger passages. Over time this causes problems:

• Rumbling, popping or crackling noises as water trapped under sediment boils on gas units.
• Lower efficiency and higher bills, because the burner or element has to heat through a layer of mineral insulation.
• Less hot water and slower recovery, as sediment takes up space and lower electric elements get buried.
• Overheating of the tank bottom, which can accelerate corrosion and shorten the heater’s life.
• Burned-out elements on electric tanks, and error codes or reduced flow on tankless units.
• Clogged drain valves that make future flushing harder.

Rheem lists rumbling or popping noises, rusty or cloudy hot water, running out of hot water faster, longer recovery times and rising energy bills as signs it’s time to flush.

Tools and supplies

For a tank water heater

• Garden hose long enough to reach a floor drain or outdoors (the drain point must be lower than the drain valve)
• Flathead screwdriver (many drain valves have a slotted stem)
• Bucket for checking the water and catching drips
• Towels and work gloves
• Optional: a brass drain valve to replace a plastic factory valve, Teflon tape, and a short-handled wet/dry vac
• Optional: a flashlight for relighting the pilot on older gas units

For a tankless water heater

• A tankless water heater flush kit: a small submersible pump, two washing-machine-style hoses and a 3–5 gallon bucket. Kits typically cost around $100–$120.
• About 3–4 gallons of undiluted white vinegar (Rinnai’s procedure calls for 4 gallons), or a manufacturer-approved descaling solution
• An extra hose or towel for the pressure relief valve area
• The owner’s manual for your model

Your tankless heater must have service (isolation) valves with purge ports on the hot and cold lines for this method to work. Most professionally installed units do. If yours doesn’t, a plumber can add a valve kit.

How to flush a gas water heater

1. Turn the gas control to “Pilot” or “Vacation.” On most gas water heaters, turning the knob on the gas control valve to Pilot or Vacation stops the main burner from firing while the tank is empty. Follow your manual; some recommend turning the control fully to Off. Never let the burner fire with the tank drained, as this can damage the heater.
2. Shut off the cold water supply at the valve on the cold inlet pipe above the tank. If there’s no valve, close the main water shutoff for the house.
3. Open a hot water faucet in a nearby sink or tub. This lets air into the system so the tank can drain freely rather than forming a vacuum.
4. Attach the garden hose to the drain valve at the bottom front of the tank. Run the other end to a floor drain, a utility sink or outside to an area where hot water won’t damage plants, a lawn or a septic system’s drain field. Remember the drain end needs to be lower than the valve.
5. Open the drain valve. Turn the handle or the slotted stem counterclockwise. A 40- or 50-gallon tank typically takes 20 to 40 minutes to empty, depending on sediment and water pressure.
6. Check the water. Every so often, fill a bucket from the hose. Expect cloudy water, sand-like grit or white flakes at first.
7. Stir up the remaining sediment. Once the tank is empty, open the cold water supply valve for 10 to 30 seconds at a time with the drain still open. The rush of water churns up sediment at the bottom. Repeat several times until the water from the hose runs clear.
8. Close the drain valve firmly by hand and disconnect the hose. Have a towel ready; a little water will spill.
9. Refill the tank. Open the cold water supply fully. Leave the hot faucet open until air stops sputtering and you get a steady stream, which means the tank is full. Then close the faucet. Running other hot taps briefly will purge air from those lines.
10. Check for leaks at the drain valve and fittings.
11. Turn the gas control back to “On” and set it to your normal temperature. If the pilot went out, relight it following the instructions on the heater’s label. Most newer heaters have a push-button igniter and a sealed combustion chamber.
12. Wait for the tank to reheat, usually 30 to 60 minutes for a gas unit.

How to flush an electric water heater

The process is almost identical to a gas unit, with one critical difference: an electric heater’s elements must never be powered when the tank isn’t full of water. A “dry-fired” element burns out within seconds.

1. Turn off the breaker for the water heater at the main electrical panel. It’s usually a double-pole breaker labeled “water heater.” Don’t rely on the heater’s thermostat setting.
2. Shut off the cold water supply to the tank.
3. Open a hot water faucet nearby to break the vacuum.
4. Connect a garden hose to the drain valve and run it to a drain or outside.
5. Open the drain valve and let the tank empty.
6. Pulse the cold water on and off with the drain open to stir up and flush the remaining sediment until it runs clear. Electric tanks often build sediment around the lower element, so take your time here.
7. Close the drain valve and remove the hose.
8. Refill completely. Open the cold supply and leave the hot faucet running until water flows with no air at all. This step is essential on electric units: the upper element must be fully submerged before you restore power.
9. Turn the breaker back on.
10. Check for leaks and allow time to reheat. Electric tanks recover more slowly than gas, often 1 to 2 hours or more for a full tank.

Tip: if your hot water is still short after flushing an electric tank, the lower element may already have failed because of sediment. Element replacement is a relatively inexpensive repair. See our water heater repair cost guide.

Quick flush vs full flush

You’ll see two approaches recommended:

• Quick (mini) flush: With everything left on, put a bucket under the drain valve and open it for a few seconds to let a few gallons out. This removes loose sediment at the very bottom and is a useful habit every few months on a tank you already maintain. It isn’t a substitute for a full flush.
• Full flush: The step-by-step process above. Do this at least once a year.

How to flush a tankless water heater

“Flushing” a tankless water heater really means descaling the heat exchanger. You’re not draining sediment; you’re dissolving mineral scale with a mild acid. The steps below follow Rinnai’s published 13-step procedure, which is similar for most brands (Navien, Noritz, Rheem, A. O. Smith). Always check your own manual, as some models have a dedicated flush mode or specific limits.

1. Turn off the power to the unit (unplug it or switch off the breaker).
2. Shut off the gas supply to the heater on gas models.
3. Close the water valves: the cold (blue) and hot (red) isolation valves on the service kit, and the main water valve feeding the unit.
4. Slowly remove the purge port caps on the hot and cold isolation valves to release any trapped pressure. Keep the small rubber sealing discs inside the caps safe.
5. Connect the hoses. Attach one hose from the pump outlet to the cold-side purge port. Attach a second hose to the hot-side purge port and put its open end in the bucket. Place the pump in the bucket.
6. Open the purge valves by turning their handles perpendicular to the main valve handles.
7. Add the solution. Pour about 4 gallons of undiluted white vinegar, or the manufacturer’s recommended descaler, into the bucket. Rinnai specifically warns against using harsh chemical cleaners.
8. Run the pump to circulate the solution through the heat exchanger and back into the bucket. Rinnai says the process can take up to about 45 minutes. Many manufacturers suggest 45 to 60 minutes; a heavily scaled unit in hard water may need longer.
9. Rinse the unit. Turn off the pump, dump the vinegar, and with the hot-side hose still draining to the bucket or a drain, open the cold isolation valve for a few minutes to flush clean water through and rinse out the vinegar.
10. Close the purge valves and disconnect the hoses.
11. Replace the purge port caps, snug but not overtightened.
12. Clean the inlet filter. Most tankless units have a small mesh filter on the cold water inlet. Remove it, rinse it and reinstall it (consult your manual for location).
13. Restore water, then power and gas. Open the hot and cold isolation valves (handles parallel to the pipes) and the main valve, then restore gas and power.
14. Purge air. Open a hot water faucet slowly and let it run until water flows steadily without spitting. Rinnai says this can take 2 to 3 minutes.

If your tankless unit throws a scale or flow error code, loses hot water at low flow, or makes unusual noises, it likely needs descaling, regardless of when it was last done. Learn more about running costs in our tankless water heater cost guide and how maintenance compares in tankless vs tank.

Choosing a tankless water heater flush kit

A basic tankless flush kit is a simple product, but a few details matter:

• Pump type: a submersible utility pump of roughly 1/6 horsepower is typical. Avoid underpowered aquarium-style pumps.
• Hoses: two lengths of washer-style hose with standard 3/4-inch garden hose thread fittings that match your purge ports.
• Bucket: 3.5 to 5 gallons, ideally with a lid to limit vinegar fumes.
• Descaler: some kits include a descaling concentrate. White vinegar works for most homes; heavy scale may need a manufacturer-approved product.

If you have very hard water, a scale-reduction cartridge or a water softener upstream of the heater can significantly reduce how quickly scale forms. Check your manufacturer’s guidance first, because some recommend specific treatment systems.

How often should you flush a water heater?

These intervals reflect guidance from Rheem, Rinnai and The Home Depot. Your manual is the final word, since some warranties require documented maintenance. If you don’t know whether you have hard water, check your city’s annual water quality report or use inexpensive hardness test strips. White crust on faucets and showerheads is a strong hint.

How much does a professional water heater flush cost?

Figures are national ranges from HomeAdvisor and Angi. A professional flush often includes useful extras: testing the T&P relief valve, inspecting the anode rod, checking the venting and gas connections, and catching early leaks. If your heater hasn’t been serviced in years, that inspection alone can be worth the price.

When NOT to flush a water heater

Flushing is routine maintenance, but there are situations where a DIY flush can do more harm than good:

• The tank is old and has never been flushed. On a heater that’s 8 to 12 years or older with years of built-up sediment, the sediment can be the only thing “sealing” thin, corroded spots. Disturbing it can expose a pinhole leak. Rheem suggests calling a professional for units in this age range. Many plumbers will advise you to skip the flush and plan a replacement instead. Find your unit’s age with our water heater lifespan guide.
• The drain valve won’t open, is cracked or leaks. Forcing an old plastic valve can snap it off, leaving you with a tank that can’t be closed. Call a plumber.
• Sediment has hardened. If almost nothing comes out of an open drain valve, the valve may be clogged with sediment. Specialized tools or a valve replacement may be needed.
• The tank is already leaking. If the tank body is leaking, flushing won’t help. See water heater leaking: what to do.
• There’s heavy rust or corrosion on the fittings, the tank bottom or the gas valve.
• You’re unsure about the gas or electrical steps. That’s a good reason to hire a pro.

If you’re facing a likely replacement anyway, get prices from our water heater replacement cost guide and check what size water heater you need before you buy.

Common flushing problems and fixes

• Water drains very slowly or not at all: Sediment is blocking the drain valve. Try closing the valve, turning the cold water on briefly to “backflush” through the hose, and reopening. If that fails, you may need a plumber or a full-port drain valve.
• Drain valve drips after flushing: Sediment is stopping it from seating. Open and close it a few times to flush the seat, or fit a brass hose cap. Replace the valve if it keeps dripping.
• Pilot won’t relight: Air may be in the gas line if the gas supply was fully shut, or the thermocouple may be weak. Follow the label instructions; if it won’t stay lit, call a pro.
• No hot water from an electric unit after flushing: The element may have been dry-fired if power was restored before the tank was full, or the high-limit reset button may have tripped. Turn off the breaker and call a professional if you’re not comfortable working on 240-volt components.
• Sputtering faucets: Air in the lines. Run each hot tap for a minute.

Other maintenance to do while you flush

1. Test the T&P relief valve. Put a bucket under the discharge pipe, lift the lever for a few seconds and let it snap back. Water should flow and then stop. If it keeps dripping, replace it.
2. Inspect the anode rod every few years. If it’s worn to less than about 1/2 inch thick or you can see the steel core wire, replace it.
3. Check the temperature setting. The DOE and CPSC recommend 120°F for most homes.
4. Look for corrosion on fittings, the drain pan and around the burner.
5. Keep the area clear of stored items, especially flammables near gas units.

FAQs

How often should I flush my water heater?

Once a year for most tank water heaters, and every six months if you have hard water or hear rumbling. Tankless units should be descaled at least once a year, and more often in hard water areas. Check your owner’s manual, because the manufacturer’s schedule takes priority.

How long does it take to flush a water heater?

A typical 40- or 50-gallon tank takes about one to two hours from start to finish, including draining, rinsing and refilling. Add reheat time: around 30 to 60 minutes for gas and one to two hours or more for electric. A tankless descale takes roughly an hour to an hour and a half.

Do I need to turn off the gas to flush my water heater?

You need to stop the burner from firing. On most units, setting the gas control to Pilot or Vacation does this. Some manuals recommend turning it fully off. Never let the burner run with the tank empty.

Can I flush my water heater without turning off the water?

You can do a quick mini-flush by opening the drain valve for a few seconds with the water on, but for a full flush you need to shut off the cold supply so the tank can drain completely. You’ll turn the cold water back on in short bursts to stir up sediment at the end.

What happens if you never flush your water heater?

Sediment builds up, which can lower efficiency, cause rumbling noises, reduce hot water capacity, burn out electric elements and overheat the tank bottom. All of this can shorten the heater’s life. Many tanks still last years without flushing, but they tend to fail sooner.

Is it too late to flush my old water heater?

It may be. If the tank is 8 to 12 years old or more and has never been flushed, disturbing heavy sediment can expose corrosion and cause a leak. In that case, have a plumber assess it, or consider budgeting for a replacement.

How much vinegar do I need to flush a tankless water heater?

Rinnai’s procedure calls for 4 gallons of undiluted white vinegar. Most flush kits use 3 to 4 gallons in a 5-gallon bucket. Don’t dilute it and don’t use harsh chemical cleaners unless your manufacturer approves them.

How long should vinegar run through a tankless water heater?

Rinnai says up to about 45 minutes. Many manufacturers and plumbers recommend 45 to 60 minutes of circulation, with longer times for heavily scaled units. Rinse with fresh water afterward.

Does flushing a water heater make it last longer?

Regular flushing removes sediment that causes overheating and corrosion on the tank bottom, which helps a heater reach the upper end of its expected life. Flushing works best alongside replacing the anode rod and controlling water pressure.

How much does it cost to have a plumber flush a water heater?

About $75 to $150 for a typical tank, rising to $180 to $250 or more for 60- to 80-gallon tanks. A tankless descale usually costs $150 to $250. Prices vary by region and whether other maintenance is included.