On Fri, 31 Mar 2006 22:36:56 +0100, "Jeff"
wrote:


"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


This is a simplification, because of course there are all kinds of
complexities if one really wanted to produce an accurate model and
control system.

A temperature sensor is an electronic component costing a few tens of
pence. The electronics on the boiler controller to interface it are
not a lot more. The complexity comes in the algorithms in the
firmware. In one way it is surprising that more manufacturers don't
give their boilers the ability to sense more information and to do so
more accurately - the component cost isn't high and the firmware is
initial development cost (which can be substantial of course.

Nevertheless, I suspect that part of the story is that this strategy
allows manufacturers to have optional features for which much more
money can be charged than the unit cost. I suspect that another part
is that setting up of some of the mopre sophisticated capabilities is
deemed to be beyond what the manufacturer believes the capabilities of
the average installer to be.

If you want to look at more general information, look for references
to industrial control and the techniques used for that. Some of
these are borrowed in simplified form.

e.g. http://en.wikipedia.org/wiki/PID_controller

and links from it is a place to start.




--

..andy