If available, carefully review manufacturer’s cleaning instructions.
Turn off the power source.
Remove the lid from the water heater (usually just with a couple of screws).
Close off three water valves supplying the water heater.
Slowly remove the purge port valve caps from the purge valves located on each of the cold and hot water valves. (Relieves any pressure that has built inside the valves and will prevent excess hot water from shooting out and coming into contact with your skin.)
Attach hosing lines to each of the 3 valves. These are usually supplied by the manufacturer.
Open the purge port valves, twisting them perpendicularly to the position of the cold and hot valves.
Use 2.5 gallons (9.46 liters) of undiluted white vinegar to clean the tankless water heater.
Flush and drain, following the directions provided by the manufacturer.
Close the purge port valves by twisting the “T”-shaped handles after the flushing process is complete.
Disconnect the hosing lines from each water valve.
Replace the purge port valve caps onto the purge valves. Make sure everything is properly tightened.
Find the filter (should be labeled), remove it by unscrewing it, and clean out the housing and inside screen of the filter by just running it under tap water in the sink. When done, replace the filter.
Refer to manufacturer’s manual for exact instructions on safely restarting the water heater, including replacing the cover and affixing it with the screws you removed earlier.
Slowly turn on the hot water tap in your sink so air passes through the pipe.