How to Flush a Tankless Water Heater: The Annual Descaling Guide

To flush a tankless water heater, shut off the power and gas, close the hot and cold isolation valves, connect a small submersible pump and hoses to the service ports, and circulate 3–4 gallons of white vinegar through the unit for 45–60 minutes. Then rinse with fresh water, reconnect everything, and restore power. The whole job takes about 90 minutes and needs roughly $15 of vinegar. In Tampa Bay’s hard water it should be done every year — every six months if you don’t have a water softener.

Flushing matters because a tankless heater pushes water through a narrow heat exchanger, and hard-water minerals bake onto those passages every time the burner fires. Scale insulates the exchanger, so the unit works harder, uses more gas or electricity, delivers less hot water, and eventually throws an error code and shuts down. Most manufacturers require regular descaling to keep the warranty valid.

Signs your tankless heater needs flushing now

• Hot water that’s less hot than it used to be, or that fluctuates
• Lower hot-water flow than the cold side
• The unit takes longer to deliver hot water, or hot water cuts out mid-shower
• A maintenance code on the display — for example, Rinnai’s LC codes, Navien’s E-series codes, or Noritz’s 920/921
• Popping, rumbling, or clicking sounds from the unit
• It’s been more than a year since the last flush (or you don’t know when it was)

What you’ll need

• 3–4 gallons of plain white distilled vinegar (or a manufacturer-approved descaling solution)
• A submersible utility pump, 1/6 hp or so, with a 3/4-inch garden-hose thread outlet
• Two 3/4-inch washing-machine hoses (or a tankless flush kit, which includes the pump, hoses and bucket)
• A 5-gallon bucket
• A screwdriver for the inlet filter, a towel, and a second bucket or drain for the rinse water

Isolation valves are required. Look at the pipes under the heater for two valves — one on the cold inlet, one on the hot outlet — each with a small capped service port. If your heater was installed without them, you can’t flush it without cutting the plumbing; have a plumber install an isolation valve kit, which makes every future flush a DIY job.

How to flush a tankless water heater: step by step

1. Shut it down. Turn off the gas valve (or the electrical breaker for an electric unit) and unplug the heater or turn off its power switch. Let it cool for a few minutes.
2. Close both isolation valves. Turn the handles on the cold-inlet and hot-outlet valves to the closed position so the heater is cut off from the house plumbing.
3. Connect the hoses. Remove the caps from the two service ports. Attach one hose from the pump’s outlet to the cold service port, and one hose from the hot service port running back to the bucket.
4. Set up the pump. Put the pump in the bucket and pour in the vinegar — enough to keep the pump fully submerged.
5. Open the service ports. Turn both service-port valves to open. (The main isolation valves stay closed.)
6. Circulate. Plug in the pump. Vinegar flows up into the cold side, through the heat exchanger, and back out the hot side into the bucket. Let it run 45 minutes for a yearly flush, or 60–90 minutes if it’s been years or the unit is showing symptoms. Check that the pump stays submerged.
7. Drain the vinegar. Unplug the pump. Close the cold service port, disconnect the pump hose from it, and let the vinegar drain out the hot-side hose into the bucket. Dispose of it down a drain.
8. Rinse. Point the hot-side hose into an empty bucket or floor drain. Open the cold isolation valve (house water now flows into the heater) while keeping the hot isolation valve closed. Let 4–5 gallons of fresh water run through and out the hot service port to flush out all vinegar and loosened scale. Close the cold isolation valve.
9. Clean the inlet filter. Most units have a small mesh screen at the cold-water inlet. Unscrew it, rinse off the debris, and reinstall it.
10. Reassemble and restart. Close both service ports and cap them. Open both isolation valves. Open a hot faucet in the house until water runs steadily with no air. Then restore gas/electric power and turn the unit back on. Run hot water for a minute and check for leaks at every connection.

How often should you flush?

• Soft water (under 3 grains per gallon) or a whole-home softener: once a year.
• Moderately hard water (3–7 gpg): every 6–12 months.
• Hard water (over 7 gpg): every 6 months. Much of the Tampa Bay area’s municipal supply falls in this range, and private wells are often harder.
• Any unit showing a maintenance code: now, regardless of schedule.

If you’re flushing twice a year and still getting scale codes, a water softener or a scale-inhibiting filter ahead of the heater will save you money and extend the heat exchanger’s life — replacing a heat exchanger costs a large fraction of a new heater.

Mistakes that damage the unit

• Using anything stronger than vinegar or a manufacturer-approved descaler. Muriatic acid and CLR can damage the copper exchanger and void the warranty.
• Running an electric unit with the water off — even briefly.
• Forgetting to rinse. Leftover vinegar corrodes internal parts and gives your hot water a smell.
• Skipping the inlet filter. A clogged screen mimics a scaled exchanger.
• Ignoring the condensate drain on a condensing unit — if it’s clogged, clear it while you’re there.

When to call a plumber

Have a professional handle it if the unit has no isolation valves, if the error code doesn’t clear after a flush, if you see any leaks or corrosion at the heat exchanger, or if you’d rather have the annual service done in one visit along with a combustion and venting check on a gas unit. A plumber can also test your water hardness and tell you whether a softener would pay for itself.

Frequently Asked Questions

How often should a tankless water heater be flushed?

Once a year in soft or softened water, and every six months in hard water. Most of Tampa Bay has moderately hard to hard water, so twice a year is a safe rule unless you have a softener. Flush immediately if the unit shows a maintenance or scale error code.

Can I flush a tankless water heater with vinegar?

Yes — plain white distilled vinegar is what most manufacturers recommend. Circulate 3–4 gallons through the unit for 45–60 minutes using a small submersible pump, then rinse thoroughly with fresh water. Avoid harsher acids and general-purpose descalers unless your manufacturer approves them.

What happens if you don’t flush a tankless water heater?

Scale builds up inside the heat exchanger, reducing hot-water output and efficiency, causing temperature swings, and eventually triggering a shutdown error. Over years, heavy scale can crack or corrode the exchanger, which is one of the most expensive repairs and may not be covered by warranty if maintenance was skipped.

How long does it take to flush a tankless water heater?

About 90 minutes total: 15 minutes to set up, 45–60 minutes of circulation, and 15–20 minutes to rinse, clean the filter and restart. A heavily scaled unit may need a longer soak.

How much does it cost to have a tankless water heater flushed?

A professional flush typically costs $150–$300, often bundled with an annual inspection. A DIY flush costs about $15 in vinegar once you own a pump and hoses (a flush kit runs $100–$150 and lasts for years). Adding isolation valves, if you don’t have them, is a one-time plumber job.