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How do you descale a tankless water heater?

Updated August 2026

60–90 minutes · Moderate DIY

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Circulate 3 to 4 gallons of white vinegar through the heat exchanger for 45 to 60 minutes, once a year, using the two service valves and a small submersible pump. Setup and rinse bring the job to 60 to 90 minutes. Scale is what kills tankless performance: a mineral film on the exchanger walls forces longer burns, throws error codes, and cuts your usable flow rate. Most manufacturers also write annual descaling into the warranty, which makes a $15 jug of vinegar cheap insurance on a unit that should run 20 years.

What you need

Before you start: Two hazards here: gas and 120–140°F water. Close the gas valve at the unit before you open anything, and never let the burner fire while vinegar is circulating. Give it 20 to 30 minutes to cool, because water trapped at the service ports comes out hot enough to scald. Work with a door or window open, and put nothing but vinegar or descaler in that bucket: acid mixed with a bleach based cleaner produces chlorine gas. If you smell gas, leave the house and call the utility from outside.

Steps

  1. Turn the unit off at its power switch and close the gas valve serving it. An electric tankless gets its breaker opened instead.
  2. Let it cool for 20 to 30 minutes, or run a hot tap until the water goes cold, so you are not opening ports on 130°F water.
  3. Close the hot and cold isolation valves on the service valve kit under the unit. If your install has no service valves, stop: adding a kit costs $150 to $300 and a plumber has to do it.
  4. Put the bucket underneath and open both service port caps slowly. A cup or two of hot water comes out of each one.
  5. Run one hose from the cold service port to the pump sitting in the bucket, and a second hose from the hot service port back into the bucket. Pour in vinegar until the pump is well covered.
  6. Open both service ports, start the pump, and let it circulate for 45 to 60 minutes. The vinegar goes cloudy and grit settles in the bottom of the bucket. That grit is the scale you came for.
  7. Shut the pump off, dump the bucket, and pull the pump hose. Open the cold isolation valve and flush clean water through the ports for 5 minutes to rinse the vinegar out.
  8. While the ports are open, unscrew the cold water inlet filter screen and rinse it. A plugged screen mimics a scaled exchanger and takes 60 seconds to clean.
  9. Cap the ports, reopen both isolation valves, restore gas and power, and run a hot tap until the flow is steady and free of air. Clear any stored error codes per the manual.

Or hire it out

A plumber charges $150 to $250 for an annual tankless descaling and usually folds in a burner and venting check. A DIY kit is about $150 once, so it pays for itself on the second flush.

Done. Now the part everyone skips: doing it again on schedule. Log it in OnOtto and the next reminder for tankless descaling sets itself.

Log this task, get the next reminder

Frequently asked questions

How do I know the unit is overdue for a descale?

The symptoms arrive in this order: hot water takes longer to show up, the temperature swings mid-shower, the flow rate at a single tap drops, and finally the unit throws ignition or exchanger error codes and shuts down. A rumbling or whistling noise during a burn is scale boiling against the walls. Water above roughly 7 grains per gallon of hardness produces all of it faster. The rest of the yearly service list is on the tankless water heater maintenance checklist.

Vinegar or a commercial descaling solution?

Both work. Household white vinegar is 5 percent acetic acid, costs $15 to $20 for 4 gallons, and needs the full 45 to 60 minutes of circulation. Purpose-made descalers cost $20 to $30 a jug and generally finish in 20 to 30 minutes because they are stronger. Never substitute muriatic acid or a bathroom lime remover: those attack the exchanger metal and can void the warranty outright. Check the manual, since a few manufacturers name a specific product.

Do I still need to descale if I have a water softener?

Less often, but do not skip it. Softened water can stretch the interval to every 2 to 3 years, though many warranties still require documented annual service regardless of water treatment, so read yours before betting on it. The inlet screen and the combustion air filter stay on a yearly schedule either way, since those catch dust rather than minerals. See how often to clean the tankless air filter and how often to replace a tankless water heater.

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