For a system controlled by a single thermostat to work reasonably it needs to be in a position that cools and heats at roughly the same average rate as the occupied rooms and is not affected by local influences such as point sources of heat, windows/doors being openened, direct sun-light etc. The downstairs hall, preferably near the centre of the house, is a the usual choice. A living room can work okay provided it is not 'sunny' and secondary sources of heat, such as a wood-burner or radiant electric fire are not used, etc. Unfortunately, these constraints usually rule out living rooms.
C.