Worcester Bosch 30CDi Regular & DT20 Programmer

12 Oct 2011
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United Kingdom

I want to add a DT20 programmer to a greenstar 30CDI regular boiler previously contolled by an external Horstman programmer through a 10 contact wiring centre. I can not find on WB literature site any instruction of how the wired 3 port valve, room stat and Tank stat should be wired so as to work with the built in DT20. Can anyone help?

Many thanks in anticipation
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You can't fit any plug in Worcester controls to a 30CDi conventional. It is external controls only, wired through Ls and Lr.

To be frank, Worcester's controls aren't up to much anyway, being based on Drayton Invensys stuff; you'd be better off using Honeywell parts.

3 port valve is undersized from a hydraulic point of view for a 30Kw boiler, by the way.
Many thanks simond.

I contacted the company who supplied the DT20 today and they said the same and apoligised that the salesman gave me bum info, they have refunded me so will get a Honeywell to replace the faultly Horstman.

Are you able to say what should have been fitted instead of a 3port valve? Should it have been two 2-port valves, one on the water and one on the CH?
S plan layout?

best wishes

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Yes, Mike, the 2 port valves give less resistance to flow.

You must also fit a bypass (such as the Honeywell DU144 or DU145) to allow the boiler pump overrun to work when both valves are closed.

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