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How to Flush a Tankless Water Heater

How to Flush a Tankless Water Heater

How you flush your tankless heater will depend on whether you have a gas or electric model. Either way, you will find it helpful to have a good flush kit

Tankless water heaters are an excellent investment for any homeowner. They are more energy efficient and cheaper to run than traditional water heaters. They also require less space to operate and only need minimal maintenance. However, you will occasionally need to drain your tankless water heater for optimal efficiency.

Hard water can lead to limescale buildup in all water heaters. While the dangers of this buildup are less dramatic than those of tank models, limescale can still cause significant problems for tankless water heaters. The buildup can cause the heat exchanger to work far less efficiently, which can significantly reduce its lifespan. Furthermore, because this component will have to work harder and more frequently to heat your water, your utility bills will increase dramatically. 

How Often Should I Flush a Tankless Water Heater?

You should aim to flush your tankless water heater at least once a year, ideally before winter approaches. Doing so will help you get through the cold months with plenty of hot water at a relatively low cost. Yet if you have particularly hard water, you may wish to flush the water heater two or three times per year. 

How you go about flushing your tankless heater will depend on whether you have a gas or electric model. Either way, you will find it helpful to have a good flush kit, which will include:

  • A submersible pump
  • A large bucket
  • A descaler, such as white vinegar
  • Hoses

How to Flush a Gas Tankless Water Heater

In any situation that requires working with heating equipment for your home, you should first consult the operation manual for your water heater. It may offer specific instructions regarding flushing and descaling.


Before you begin the flushing phase, you should prepare in the following way:

  • Unplug the heater from the electrical socket.
  • Remove the access panel.
  • Close the gas isolation valve.
  • Close the hot and cold shut-off valves. This will stop water from entering the unit during the flushing process.
  • Carefully open the hot water pressure relief valve along the hot water line.
  • Connect hoses to both the cold and hot water service ports.
  • Put the submersible pump into a large bucket.
  • Connect the hose from the cold water port to the pump.
  • Put the free end of the hose from the hot water port into the large bucket.
  • Pour four gallons of white vinegar into the bucket.


Now you are ready to begin the flushing process.

  • Open both service ports, and turn on the pump.
  • Let the vinegar circulate through the unit for at least an hour.
  • Switch off the pump and dispose of the vinegar.
  • Close the cold water port and take off the connected hose.
  • Open the cold water shut-off valve for roughly ten minutes to flush out the vinegar.
  • Close the cold water shut-off valve.
  • Let the water drain completely and detach the other hose.
  • Close the hot water service port.
  • Take out the cold water inlet filter and hot water in-line filter, if applicable).
  • Flush the filters with tap water to wash away sediment.


Once the flush is complete, you can restore your heater to working order in the following way:

  • Put the filters back in place before opening both shut-off valves.
  • Close the access panel.
  • Open the gas shut-off valve.
  • Plug the heater back into the socket.

How to Flush an Electric Tankless Water Heater

Flushing an electric heater is slightly different and may require some disassembly. 


Prepare your heater for flushing by taking the following steps:

  • Turn off the electricity supply at the circuit breaker.
  • Close the cold water shut-off valve.
  • Drain water from the unit by opening taps around your home.
  • Close the hot water shut-off valve.
  • Remove the access panel.
  • Take out the screws that connect wires to the heating elements.
  • Remove the top of the heating element.
  • Check for damage to the elements. If one is cracked, you should replace it before using the unit again.
  • Place the heating elements in the heater’s copper chambers for cleaning during the flushing process.


You are now ready to flush your electric water heater.

  • Fill the copper chambers with white vinegar.
  • Leave the vinegar in the chambers for up to two hours.
  • Drain the vinegar from the chambers.
  • Put the heating elements back into place and reconnect the wires with the screws.
  • Open the cold water shut-off valve to fill up the tank, observing for signs of leakage.
  • Open the hot water shut-off valve.
  • Open several hot water faucets in your home. Allow the water to run for several minutes to flush the vinegar from the line.


Now you can restore your heater to working order:

  • Turn off the taps.
  • Close the cold water shut-off valve.
  • Take out the cold water inlet filter and rinse it with tap water.
  • Put the filter back in place.
  • Close the access panel and turn the electricity back on at the breaker.

Hire Professional Plumbing Services You Can Rely On

The Central Texas plumbing professionals at Indigo Home & Facility Services offer reliable plumbing service for installation, maintenance, repair, and replacement. If you need help flushing your tankless water heater or with any other plumbing needs, call us now and book an appointment with our Austin plumbers at (512) 608-4999.