On Wednesday, 13 January 2021 at 17:33:06 UTC, Chris Green wrote:

To my mind there are three different types aren't the-

Conventional - with a flue going up a chimney or similar, draw air
from room

Balanced Flue - Both air in and fumes out go via a hole in the
wall, no need for chimney, doesn't draw air from room

Flueless - No flue of any sort, draws air from room, exhausts to
room. Needs careful design to be sure gas fully burnt to CO2,
needs good ventilation.

Yes - that is exactly what I am aware of.

The one I find difficult to understand them not installing is the balanced flue. After all, they are not that different to some boilers.