The size is a very large L shaped building being a long stone cottage like 2 semi or 4 terraced with a massive garage and bungalow attached at the end, around 20 radiators.
But this was my first question to the installer, I require more a commercial boiler rather than domestic as the previous Vaillant fitted, when the heating did work, never supplied heating to more than 12 radiators and rightly so due to the kw output, pump and distance, the installer immediately pointed out the 837 stating small hotels use them with its huge kw output and powerful pump.
Today he removed the heat exchanger and flushed out with acid, replaced and on the digital display the radiator symbol remains on but no flame symbol, previous when the system had been turned off for at least 30 mins I would turn the central heating knob to on and the radiator symbol would appear, it would fire up for 8 seconds then stop and go on to ciculating mode with flashing radiator and flashing C, all pipework remains cold, the pump is working.