How often to flush a tankless water heater in San Jose

Why tankless units need flushing

When hard water is heated, minerals drop out as scale. In a tankless heater that happens inside the heat exchanger, a set of narrow passages where the water picks up heat from the burner. Scale insulates those passages and narrows them. The unit has to work harder, delivers less hot water, and eventually shows a scale or overheat warning.

Descaling dissolves that layer with a mild acid solution circulated through the heat exchanger, then flushes it out with clean water.

The baseline for San Jose water

Published guidance on San Jose water puts hardness at roughly 10 to 16 grains per gallon depending on the season and the blend of Sierra water and valley groundwater. San Jose Water Company treats anything above 7 as hard, and its data for valley-floor groundwater runs higher still in places.

At those levels, once a year is a reasonable starting point for most households. Large households, homes on harder groundwater, or units that run many hours a day may benefit from more frequent flushing. Most manufacturers also set a maintenance schedule in the manual, and some warranties expect it to be followed.

What the flush involves

The unit is turned off and isolated using valves on the hot and cold lines. A small pump circulates the descaling solution through the heat exchanger via the service ports for about an hour. The solution is drained, fresh water is flushed through until it runs clear, the inlet filter is cleaned, and the unit is restarted and tested.

Units installed without isolation valves are hard to flush without cutting pipe. Fitting a valve kit once makes every future flush a short job.

Signs a flush is overdue

The water takes longer to get hot, or never quite reaches the set temperature. Temperature swings during a shower. Flow at the tap feels weaker than it used to. The unit displays a scale or maintenance code. You hear more noise from the unit than before. Any of these, in a unit that has not been descaled in a year or more, is worth acting on.

Doing it yourself

Handy owners with isolation valves already fitted can descale using a pump, hoses and a manufacturer-approved solution, following the manual. Turn the power off first, close both valves, and never run the burner without water flowing. If the unit has no service valves, or you are not comfortable working near the gas and electrical connections, it is a short professional visit.

What a softener changes

A salt-based softener upstream of the heater removes most of the hardness before it reaches the heat exchanger, which slows scaling dramatically and can stretch the flushing interval considerably. It also protects fixtures and other appliances. A salt-free conditioner reduces scale formation but does not remove hardness, so the benefit is smaller. Either way, the inlet filter still collects grit and is worth checking every year.