Vaillant Turbomax no hot water when cold outside

[denandy]

Can anyone help please - I am new to this forum.
I have a Vaillant Turbomax. I have read the other posts relevant to this boiler but can't see the same problem.
When the outside temperatures are very cold I turn on a hot water tap but the water remains cold. However the radiators get vey hot. The boiler is on an interior wall so not exposed to outside temperature.
Could this be a problem with the diverter valve? If so does anyone know where I can get one and at what price? Is it straight forward to replace?
Thanks for any help.

 

[ALCPlumbing]

Whilst running the hot tap if you touch the pipe that connects under the boiler to the left hand side. There is six copper pipes connecting to the boiler. The far left is the flow to the radiators. If this is passing heated water down it whilst you have the hot tap running the you do have a problem with the diverter valve. This however wouldn't be connected to the outside temperature. Just a coincidence I think.

 

[jackthom]

I'm not clear whether the boiler is actually firing when the hot tap is running and the heating is turned off.

 

[Agile]

I can work out a connection with the outside temperatures.

Tony

 

[Terry lambert]

Is it one of those secrets that you could tell me, but then you would have to kill me ?

 

[Dean744PiN5]

try turning your tap down to slow the flow down until the water comes through hot, incoming main is much colder in the winter and needs to pass through the boiler slower than it does in the summer to reach the same temperature