On Fri, 31 Mar 2006 19:05:45 +0100, "Jeff"
wrote:

Thanks for the input, that last link was very interesting. There is very
little on the 'net about weather compensation which is what prompted the
question here.

So here is my list of current assumptions :-

1. I need to keep the return temp as low as possible below 57 degrees.

That's somewhat true, but don't get the impression that there is some
magic step change in efficiency when condensing commences. What
actually happens is that the rate of change of improvement in
efficiency increases below the dew point.

In essence, the lower the better. This can most effectively be
achieved for more of the time by having the radiators adequately sized
such that they can produce enough output to meet worst case outside
temperatures at 70 degree flow (or lower if you like) rather than the
conventional 82 degree design.



2. A low temp, long burn is more efficient than a high temp short burn

That is true.




and here's the long shot

3. Weather compensation does the same as turning the temp knob up or down
relative to the outside temp.

If point 3 is correct then why have an outside temp sensor, surely you could
just monitor the return temp to keep it constant thus compensating for
different losses due to different outside temps.

I'm still struggling to fit modulation into this

Yes and no. 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.








Regards Jeff


--

..andy