"Andy Hall" wrote :-
External monitoring provides for increased comfort
because both the temperature as well as the rate of change of
temperature outside become known to the controller more accurately
than by the other means. think of it this way. Changes in outside
temperature (eventually) affect the heat loss through the building
surfaces and air changes. However, the boiler, if it has return
temperature sensing only learns about this indirectly as a result of
TRVs adjusting flow rates through radiators. In most systems the
only additional monitor is an on/off room control.

There are room thermostats which will do things such as optimised
starting to start the boiler based on rate of temperature rise during
previous heating periods. However, these can't be as accurate as
being able to measure the actual conditions.

If the boiler has that information, then being able to adjust the heat
output finely and continuously will produce far better control
tracking leading to greater comfort and saving of fuel.


Bingo !

So, you can do it by monitoring return temp, but with outside temp
monitoring the system is more accurate and responsive

Regards Jeff