Keston C40 room/weather compensation

4 Jun 2008
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United Kingdom
Some help required please,

Have recently had installed the following:
Keston C40 (opentherm compatible) boiler
Keston Spa 210 unvented cylinder.
It's an S-plan system.

Want to get the most out of the modulating features of the boiler in conjunction with weather/room compensation controls. A Theben Ramses 850 OT controller was provided but not installed because the engineer said it was only for underfloor heating - we do not have underfloor heating - and instead a Drayton wireless programmable digistat was installed. An outside sensor (Honeywell TA1 series) for weather compensation has been installed, though not connected to the boiler at the moment.

As I say, I would like to have a set-up which takes advantage of temperature/weather compensation, but need to know what control to buy if the Theben is not suitable - is it really not suitable?

Also, I have no time control over hot water supply at the moment either. I thought the Theben device would provide this control too. Am I wrong? Was the heating engineer wrong?

To summarise: I want a control/controls to provide temp/weather compensation for CH, plus time control for hot water.

Thanks in advance.
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I am also interested in this. In the operating instructions posted below it looks as if you might need the Ramses 855 ( to operate weather compensation and the motorised valves. Can anyone confirm ?

Thanks for you help
That's the conclusion I have come to: that the only way to control the motorized valves is by having the "OT box" (Ramses 855) as well as the 850 unit. As Xanda asks, can anyone confirm this?

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The 855 box is not available separately but the 855 control center can be bought from then GB rep of Theben:

On further inspection I am not sure that either the box or the control center can control motorised valves but only provides additional sensors to monitor temperatures and to drive pumps.

Can anyone confirm?

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