Vaillent boiler banging

[whistler]

Have a Vaillant Turbomax combi boiler which is located in an attached garage and of course is exposed to low temperatures.

On a frosty night the boiler starts up (presumably because of the internal frost protection) but after a few seconds there are several banging / knocking noises and it appears to shut down again. This may happen several times on a frosty night.

When it starts up normally via the programmer/thermostat demand, it starts up normally without any banging.

What exactly does the internal frost protection do? and what could be causing the banging.

 

[Timmyturtle]

The frost stat turns the boiler on to prevent freezing of the internal water,when it turns on it creates heat in the boiler and around a bypass circuit.
If its banging there is not enough flow round this circuit,and the water is basically starting to boil in the boiler(just think how a saucepan of water bubbles when it approaches boiling point)

 

[whistler]

If its banging there is not enough flow round this circuit,and the water is basically starting to boil in the boiler(just think how a saucepan of water bubbles when it approaches boiling point)

So does this mean that the pump is not turned on and the hot water just circulates within the boiler? If this is the case I would have thought that it would only take a few seconds for the water to heat up.

Does this indicate a fault in the boiler?