Intermittent under pressure fault on a Vaillant Combi

Hi everyone,

I've done some frantic googling with no luck on this one. I have Vaillant TurboMax Plus 428. This installation is very bizarre, the ho water part of the boiler isn't connected, instead the central heatin circuit goes through an external 3 way valve to a cylinder heater.

Anyway, the problem is that I'm getting intermittant "under pressure faults for the heating circuit (code F22). Now I know the pressure i the circuit is more than adequate as it's up at 1.4 bar (according t the pressure guage on the front of the boiler, anyway). I've even ha an F22 when the guage reads 1.6 bar.

Resetting the boiler seems to work, but sometimes it gets stuc producing an F22 immediately after reset. I then have to turn th thing off and turn it on again - and it'll work quite happily fo another couple of days.

Does anyone have any ideas? The only thing I can think of is that th internal under pressure sensor is either faulty or scaled up o something, but I've no idea if you can get replacements for these

-- tomf

With some manufacturers their pricing structure might make fitting a combi cheaper than the system boiler. However this is surprising with the Vaillant where the combi is about £75 (IIRC) more than the 6xx series system boilers. It might be that the combi was available cash & carry whereas the system boiler was a 'special' order.

The one and only Turbomax that I have fitted with a fault out-of-the-box was an intermittent F22. This was cured by replacing the NTC temperature sensor. How trouble with the NTC led the control program to report F22 is a deep mystery to me...

OK there are three NTCs in a T/max+. There are no primary pressure or flow sensors. As a wild but educated guess the control uses the fact that the NTCs (I and II) on each side of the primary heat exchanger should read the same when the pump starts before the heat is applied.

HTH.