Tankless water heater isolation valves explained

If you look under most tankless heaters, you will see two valves with small side ports and handles. They are the reason descaling a tankless heater is a one-hour job instead of a plumbing project.

What is in a typical kit

How they are used for descaling

1. Turn off the heater, disconnect power and close the gas valve.
2. Close both main isolation valves.
3. Connect hoses to the service ports, one to a small pump sitting in a bucket of descaling solution, one returning to the bucket.
4. Open the service ports and circulate for the time your manual specifies.
5. Flush with clean water, close the ports, reopen the main valves and purge air from a hot tap.

Our guide to descale a tankless water heater covers the full process.

If your heater has no isolation valves

Flushing without them means cutting into the plumbing or improvising, which is why many plumbers add a valve kit during the first service. It is a modest one-time cost that makes every future flush easier.

Common issues

• Dripping service port caps. Tighten the cap or replace its washer.
• Leaking valve stems. Snug the packing nut gently or replace the valve.
• A relief valve that drips. Often high pressure or thermal expansion; see our pressure relief valve replacement cost guide.

Exercising the valves

Turn the isolation valves fully closed and open once a year, ideally during the yearly descale. Valves that sit in one position for years can seize and fail to close when you need them.