"Pete" wrote in message
...

Having done some reading into boiler fitted with weather compensation I
can
sort of see the logic of reducing the flow temp as outside temp increases.

I also see that a boiler should only be fired where it can acheive a 10deg
increase in temp to prevent short cycling (rapid on and off to keep the
flow
temp around the desired temp)

This is find when the flow is say 80degC, to fire the boiler once the
return
gets to below 70degC and heat until 80deg.

The problem occurs at lower temps. If the boiler requires a flow of just
25deg, (say outside is 20deg), you cannot acheive the 10deg differencial
so
the boiler would not fire. How do the commercial weather compensators cope
with this? or do they increase the ammount of short cycling?

Commercial systems tend to have a pumped primary loop from boiler flow to
return. Off this the CH and DHW loops are taken. The weather compensator
modulates a 3-way valve to obtain the optimum flow temperature.

Normally a direct acting (acting on the boiler burner) compensator will have
anti-cycling incorporated, based on time and/or temperature depending on the
model. Some boilers also have anti-cycling devices too, so a belt and
braces approach. Even programmer/stats, such as the CM67, have anti-cycling
functions too.

When a weather compensator is a part of the boilers controls, as said below
the boiler comes on at its lowest modulated temperature and maybe a time
factor of an off period is included.


Example of Micromat:

On the Micromat, the CH flow temperature can be set to one of three
preset
levels - 50, 70 and 85 degrees. At low temperatures, say -1,
the boiler will run at its preset temperature. However, at higher
temperatures outside, less heat will be needed, until at 20 degrees
outside, nothing is required.. The end points of the graph can be
adjusted
as appropriate. In practice, there are various effects -
the main ones being that at switch on, the boiler doesn't automatically
go
to full power, and that the power is starting to increase as it's getting
colder outside