On Tue, 27 Dec 2016 20:37:05 +0000, Roger Hayter wrote:

Hmm. I know a Vaillant that will light up, once to the detriment
of the heat exchanger.

As long as the water level is above the top of the boiler, why
should it not work perfectly well in hot water mode with no CH
circulation? I know mine (an old oil-burning cast iron combi)
does, although I suppose it is possible modern ones need a CH
circuit to dissipate excess heat.

Yes these 38kW Vaillant boilers push out a LOT of energy, it has to
go somewhere and if it can't the boiler is likely to crash.