peter green wrote:

>> Really?  I would think that happens only if you have pure electric
>> heating, which isn't particularly efficient (nor quick to change your
>> mind about what temperature you wanted.)  Gas or oil heat tends to have
>> a central furnace, and it's moderately difficult to get more than even
>> a single thermostat or "zone."
>
> here in the uk (where wet central heating is the norm and aircon very rare
> in homes) we use thermostatic radiator valves for that, a little slow
> responding but normally adequate.

That's what I was talking about. This is also the norm in Germany. Usually
the not too old houses have a two-staged control: one central control takes
care of the water temperature (which can be lower at night or eg. during
work days when nobody is at home), and individual radiator valves
(sometimes with remote sensors) take care of the flow through each
individual radiator. 

> i can't see any reason why hot air couldn't be controlled in a similar way
> though the size of the pipework may make designing the valves tricky.

I think one difference is that possibly forced air flow heating doesn't
work with hot air that heats up the rest of the air in the room, but with
air that is at the target temperature and gets forced around the house. I
don't know. If the latter is the case, you'd need a controlled air mixer
(hot and cold air) at the inlet of each room, which shouldn't be impossible
either.

The "valves" could be something like rotating shutters. 

Gerhard

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