Error F4: electronic anode fault on the heat pump water heater

What it means

These water heaters have an electronic anode, also known as an impressed current anode: a part that protects the tank from corrosion using a small electric current.

F4 indicates that this circuit is open, and the most likely cause is the anode or its connection. The manufacturer uses the same code when the tank is empty, so it's best to rule out a lack of water first.

Most likely causes

1. The electronic anode or its connection. This is the most common cause.
2. Lack of water in the tank, for example after a water supply cut-off or with the inlet valve closed.
3. A leak that has partly emptied the tank.

What you can check yourself

Without tools and without opening the unit.

1. Check that you have water at home: turn on a cold tap.
2. Check the cold water inlet valve on the water heater: with the handle in line with the pipe, it's open.
3. If there's been a water cut-off, wait for the supply to come back, run a hot tap until the water comes out without any air, then switch the water heater off and on again.
4. Look for water under the water heater or around its connections.

If there's water and F4 is still showing, book an engineer: it's most likely the anode. It's best not to leave it like this, because the tank is left without its corrosion protection. And if you see water under the water heater, turn off the inlet valve.