How to Flush the Heat Exchanger on a Rinnai Tankless Heater

A Rinnai tankless water heater is designed to be efficient, but this only remains true when they’re properly maintained. When the time comes to flush the heat exchanger on a Rinnai tankless water heater, follow these steps to get things clean.

Supplies Needed

• 4 gallons of undiluted, virgin, food-grade white vinegar
• 5-gallon bucket
• Submersible circulating pump
• Two washing machine-style hoses or service hoses
• Access to hot and cold isolation/service valves
• Towels or a small container for drips
• Gloves and eye protection

Step-by-Step: How to Flush a Rinnai Tankless Water Heater Heat Exchanger

• Step 1. Turn Off the Water Heater – Turn the power off at the controller, then unplug the unit. 
• Step 2. Close the hot and cold water shutoff valves – Make sure the hot and cold water lines are shut off.
• Step 3. Connect the pump hose to the cold water service valve – This allows the hose to send the vinegar solution from the bucket into the water heater.
• Step 4. Connect the drain hose to the hot water service valve – Connect the second hose to the hot water service valve and the other end of the hose into the bucket.
• Step 5. Add white vinegar to the bucket – Add four gallons of undiluted, food-grade white vinegar into the bucket.
• Step 6. Open the service valves – Open both the hot and cold service valves to circulate the vinegar throughout the heat exchanger.
• Step 7. Keep the pump running – Run the pump for at least one hour to give the vinegar time to dissolve lime and scale inside the heat exchanger.
• Step 8. Turn the pump off and drain the vinegar – Once the flush cycle is done, turn off the pump. Remove the free end of the drain hose from the bucket and place it into a sink or floor drain.
• Step 9. Rinse the heat exchanger – Close the cold service valve and open the cold water shutoff valve to let clean water flow through the water heater for about five minutes.
• Step 10. Clean the filter – After rinsing the unit, close the cold water valve and allow the unit to finish draining. Remove the in-line filter and clean out any residue. When done, put it back in place and reopen the cold water valve.
• Step 11. Disconnect the hoses – Make sure the service valves are closed, disconnect all hoses, reopen the hot and cold water shutoff valves and restore electrical power to the Rinnai water heater.