Are programmable digital thermostats only for combis?


Postby callowg » Thu Nov 06, 2008 9:24 am

I have a conventional oil-fired boiler with gravity DHW and pumped rads (Sundial C-plan - motorised valve in gravity circuit).
There is a mechanical 3-wire room-stat in one room and TRVs in all other rooms.
I want a better stat (e.g. Drayton Digistat 3+) to give accurate control and 5/2 programmability.
The manuals for most of these stats seem to assume they are being used with a COMBI boiler. I've even found someone on eBay selling a Digistat because his plumber said it couldn't be used with his conventional boiler.
I can't imagine why a Digistat could not be used in a C-plan, simply replacing a basic stat. The HW channel of the old timer/programmer would still fire up the boiler independently whenever the cylinder stat wanted hot water, and the CH channel could be set permanently ON, so that the Digistat would take control.

Can anyone tell me if there is a problem and ,if so, what it is?
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Postby plumbbob » Thu Nov 06, 2008 6:50 pm

There is no reason why a programmable room stat cannot be used in a "C" plan, or indeed in any plan.

Only one problem can occur on old style plans similar to the "C" plan. Specifically gravity systems that require the hot water to be on before the heating can be activated. When using a programmable thermostat, the heating timer must be left set to 24 hrs as the new stat may call for heat at any time. It won't stop you using it, just the heat won't come on until the water is on.

If your existing timer acts independently for hot water and heating, you will have no problem.
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Postby callowg » Sun Nov 09, 2008 10:40 am

Thanks Plumbob - it's good to know I wasn't being stupid!
My C-plan is wired sensibly: the HW and CH are truly independent -so I should have no problems. Thanks again.
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