Combi boilers and water pressure

Some models might work with a running inlet pressure of as little 0.25 bar. Although the majority are looking for about 1 bar or so.

The primary side whether or combi or not will carry on working even if there is no mains water at all. However eventually the sealed system will needed to be topped up - but probably not that often. A conventional vented system would only need just enough to make up the losses in the loft header tank.

They do require 'topping up' from time to time, but having the mains pressure permanently connected to them is never a good thing, unless you like seeing boiling water running down the outside wall.

Combis don't need mains pressure for the CH to work AFAIK. The central heating water is of course a sealed loop. I believe the combi DHW section senses temperature and flow, but not pressure.

Of course the CH loop is pressurised, probably to stop boiling in the primary loop, but often also as a side effect of supplying radiators that are often in the floor above the combi.

Andy