A tankless hot water heater heats water on demand instead of keeping the warm water in a tank, which typically results in considerable energy savings. Tankless water heater manufacturers recommend that their components be flushed at least once every year to get rid of the accumulation of calcium along with other minerals within the unit — even more frequently in the event that you have hard water. Some units will signal when the unit needs a lot of flushing. You can flush out a tankless hot water heater in about one hour.
Switch off the electric circuit to your water heater at the breaker panel. If the water heater utilizes a gas heater, then rotate the gas cutoff counterclockwise to turn off the gas into the unit.
Remove the cover plate of the water heater unit with a screwdriver to get the unit wiring. Hold the tip of a non-contact electrical tester against the wiring leading to the terminals. If the tester light turns on, continue turning off the main circuit breaker into the house until testing all wires shows that no electricity is accessing the machine. Replace the cover plate.
Switch off the water supply valve that feeds cold water into the water heater. Switch off the main hot-water valve that sends warm water into the local fixture or multiple fixtures in the house.
Connect 1 end of a 6-foot garden hose to the outlet of a pond pump or sump pump, then connect the other end of the hose into the cold-water isolation valve. Connect another garden hose into the hot-water isolation valve. Tighten the garden hose couplers having an adjustable wrench.
Set the pump along with the open end of the second hose into a 5-gallon bucket. Fill the bucket with 5 gallons of clean, fresh white vinegar.
Open the hot and cold isolation valves on the water heater, and turn on the pump in the 5-gallon bucket to circulate the vinegar through the heater.
Allow the pump circulate for 45 minutes, then turn off the pump and let the vinegar drain in the water heater through the seams to the bucket. Close the cold-water isolation valve.
Dump the vinegar from this bucket, then put the open hose back in the bucket. Turn on the cold-water supply valve and let the water flow to the bucket for five minutes. Close the cold-water supply valve and disconnect the lawn hoses in the inlet valves.
Switch off the hot-water isolation valve. Remove the inlet filter screen per the operator’s manual to the water heater, and rub on the screen to remove any sediment. Reinstall the screen.
Turn on the hot-water supply valve, followed with the cold-water supply valve. Open a couple of hot-water faucets in the house to bleed any air in the hot-water lines. Close the faucets once no longer air is in the lines.
Look over the water heater’s connections to be sure there are no leaks. Turn on the gas supply valve, if so equipped, and the circuit breaker for the water heater. Test for warm water at the faucets.