[OpenTRV-dev] OpenTRV install problem

[Damon Hart-Davis]

Hi,

An odd one: I wonder if any of you can offer any optinions or solutions (and it’s something that OpenTRV is likely to deal with in other boiler installs).

On an installation today, now with 5 OpenTRV-controlled rads and a separate unit only doing boiler control:

Boiler: Valliant ecoTEC plus 837
  http://www.vaillant.co.uk/mediaarchive/2013/09/ecoTEC-plus-installation-service-manual.pdf
Controler boiler end: Drayton Digistat SCR (part of Digistat RF system)
  http://www.draytoncontrols.co.uk/WorkArea/DownloadAsset.aspx?id=28360

There is a problem in that though the SSR indicator LED operates appropriately, eg in response to call for heat (so the hub is working), the boiler never seems to go off as if the zero-switching SSR (crydom D2W203F-11) does not ‘release’.

There are some off voltages seen across the ‘call for heat’ contacts (with the SSR and normal controller removed, boiler on):

DMM VDC: 0V
DMM VAC: ~200V (cf mains ~235V)
30kOhm/V analogue MM: ~220V

So it seems to be AC and near but not quite mains.  (Not the 24V alternative that can be used with the boiler.)

I wonder if thie boiler is being ‘clever’ and either the circuit is high impedance (too high for the SSR to power itself correctly) or some such.

Any ideas?

If for some reason I cannot use a normal ZC SSR there, I may have to use a real old-fashioned relay (I can’t find an SSR that provides the volt-free 24VDC—240AV volt-free switching that the Drayton SCR unit manages), driven somehow from the logic 3.3V output from the OpenTRV unit.  Is there a neat unit that I can use to do that, small and low power and that I can stick on the wall in place of the SSR box?

Rgds

Damon