my boiler keeps on showing the message f 75 .also the heating is on constantly when not required (the radiators are turned off on the boiler but still warm up.

alia., January 2011

Temperature dial broken how can I get inside to turn heating on a,little .

clare, December 2015

The F75 is almost certainly caused by a dodgy pump pressure sensor. A fix to get it going is to give it a quick blast of fresh water. Under the boiler are two valves which will be "off" ie parallel to the front of the boiler. Turn ONE of them on. Turn power off and on at the mains ("reboot") and the boiler will try to start and if left alone will fail 3 times. Turn the other valve on for as short as time as possible. This will allow a quick blast of water into the system and it should get the boiler going.

David, August 2011

You have more than one problem, the F75 is one of three things:
Low water pressure:
Faulty pressure sensor:
or Faulty pump:
(see the response to 'vaillant boiler' 5 questions below your question).

The issue with radiators coming on is a little more in depth. Basically, it's a diverter valve or pcb issue.

If there is a C in the display, the water pre-heat is on, the boiler tries to heat up the domestic heat exchanger, but due to diverter valve being in the wrong position (or passing), the heat generated is sent around the radiators, it keeps going because the DHE never gets satisfied.

As a temporary cure, turn off the preheat (C ) in the display, this involves turning the top left dial (HW) all the way down to the left (minimum), the C in the display will disappear, you can now turn the dial back up to a reasonable position, this will stop the radiators constantly heating up, however, they will still heat up when you run any hot tap until you get the diverter valve/pcb issue resolved.

Carefree, January 2011