On 12/11/2017 14:02, Roger Hayter wrote:
Tim Watts wrote:


No - I read the installer's manual and rang WB tech. They also confirmed
that the CDi fancy "Wave" IIRC programmer could not accept an external
control input - so not very fancy.

That's a shame, I thought that it was one of the big selling points of
condensing boilers that you could do this.

Some you can easily. Others may require a bit more intervention / invention.

You can do stuff with a controlled blending valve on the output of the
boiler, that mixes some of the return flow back into the output before
its gets reheated. (much as some of the UFH mixers do), but in this case
use the external temp to control the mix (when low temps result in less
mixing, and hotter effective flow temps)

Not that there appears to be
any choice about getting a condensing boiler!

There are a limited set of cases where you can still legally fit a non
condenser.

I suppose an alternative
would be to adjust the flow temperature according to the return
temperature, but it probably varies too rapidly for this to work. How
does the fancy controller do it without any external informaion?

In the case of the one on mine, via an external temperature sensor. You
select a response curve on the programmer based on the heat loss
characteristics of the house, and then the programmer picks a flow
temperature on that curve based on the current external (and possibly
current internal) temperature.

One
thing I don't think is common in oil boilers is modulation of heat
ouput, It would I think be dfficult to achieve without multiple burners.

IIUC modern oil boilers do support *some* modulation - but not nearly as
wide a range as with gas ones.

I don't know how modulation in gas boilers is controlled without
external temperature - speed of temperature rise or something?

Usually based on the return temperature and the flow set point
temperature. As they start to see the return temp start to rise toward
the flow temp, they modulate the power down so as not to exceed the flow
maximum set temp. If they reach minimum power output and still the
return temp is too close to the flow, then they cycle off much like a
fixed output boiler would on its stat.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/