On 27/12/2016 20:37, Roger Hayter wrote:
Fredxxx wrote:

On 27/12/2016 18:02, GB wrote:
On 27/12/2016 15:59, Phil L wrote:

There will be a filling loop under the boiler allowing re-pressurisation,
and a gauge showing system pressure. It will currently be on zero and it
needs to be over 1 bar after all the rads have been bled.

I am surprised that the boiler lights at all if the pressure is that low
that there's nothing in the upstairs system at all.

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.



They might be able to work, as you say, but they have a pressure sensing
valve that is supposed to shut them off if the water pressure is low.