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How Do You Descale A Tankless Water Heater?

water heater repair new hampshireDo tankless water heaters need to be flushed, even though they don’t have a storage tank?


Limescale builds up on the heat exchanger of a tankless water heater just as it does on a conventional storage-type water heater. Without intervention to remove the scale, it will gradually reduce the efficiency of your equipment until it negates the advantages you paid for by going tankless!

While removing scale might sound like a complicated process, it’s actually pretty easy to do – something you might want to tackle yourself (don’t worry – if you’re not comfortable with that, we can do it for you).

Here are some pointers:

Flushing Your Tankless Water Heater

Please note: Before attempting to flush your tankless water heater, always read the manufacturer’s instructions first! While the procedure detailed below is fairly standard, there might be slight but important variations based on your particular model.

Here’s what you’ll need:


  1. Before you begin, set your temperature to 150°F and let hot water run through the heater for about a minute.
  2. Shut off all the power to the water heater.
  3. Close the incoming and outgoing water isolation valves. If your unit doesn’t have isolation valves, then you can buy some and install them yourself.
  4. Have a bucket ready to collect water as it drains. There will be about a liter of water or so in the pipes of the tankless heater.
  5. Drain the unit and connect hoses to the drain ports of the isolation valves “A” and “B”.
  6. Dump this bucket or have another bucket ready to go with a solution to be pumped through the heater.
  7. Fill an empty bucket with a mix descaling solution and water. About a half a bucket of water will do the trick.
  8. Open the isolation valves A and B.
  9. Turn on the circulation pump. Let the solution circulate through the tankless water heater for at least 30 minutes, but an hour is better.
  10. Drain the descaling solution from the heater and flush fresh water through the heater by closing valve “B”.
  11. Open up valve “A” with a hose leading to an empty bucket.
  12. Open up the inlet water valve to let fresh water flow through the unit and empty into the empty bucket to get all of the solution out of the heat exchanger.

Descaling is not the only maintenance your water heater needs! Be sure to schedule maintenance a minimum of once every two years for a conventional water heater, and once a year for a tankless water heater, to keep it running at its safest and best.

Contact Pemi River Fuels today to learn more about water heater service in Grafton, Belknap and Carroll County, NH!