Christian McArdle wrote:

Why use a programmable room stat if there is already a 2 channel
programmer, surely a standard wireless stat will suffice.
I've had one on a combi for the last 3 years so can confirm:
Several reasons. Basically, the advantages of using a programmable stat over
a separate stat and programmer a
1. Controls are in the living area rather than in a manky undersink cupboard
or inaccessible airing cupboard. This enables manual override and
programming to be easily achieved.

Generally the heating gets extra time if we stay up late. We're usually
in the living room in that case. A minor point - if the controls
stayed in the kitchen it would hardly be a problem, especially with
'boost' functions rather than just on/off.
2. Rather than turning off completely, the stat can just turn down the
temperature, to say, 12C, ensuring that the house doesn't get freezing
overnight, or whilst you are out.

or more importantly while you're on holiday. The ACL has 3 settings
each day. By default it comes on in the morning and tries to get the
house to 20C, lets it cool to 16 during the day, back to 21 in the
evening and then down to 7 overnight (from what i remember). If you're
ill or feeling the cold for some reason it's simple to turn it up a
degree or two and it reverts to the normal settings at the next junction.

3. Complex control laws can be used that can predict the house's response to
heat, enabling temperature overshoot to be controlled and enabling the
device to predict how long before the required start period the heating
needs to be on
That's the plan. the claim is that it can save 10% of fuel bills and
with a solid walled, single glazed house you need all the savings you
can get!