Horstmann CentaurPlus C27 replacement for Landis & Gyr RWB2: wiring problem

[F]

Our central heating is 20 years old. Central heating is pumped and controlled by an Eberle programmable thermostat.

DHW is gravity fed and controlled by a cylinder thermostat and a motorised valve.

The whole lot has been switched by a Landis & Gyr RWB2 timer.

To enable separate switching of DHW and heating there is a relay in the circuit to the boiler.

I have now replaced the Landis & Gyr RWB2 with a Horstmann CentaurPlus C27.

On the Landis & Gyr there are 4 connectors other than live and neutral. According to the label on the back of the timer:

1 is HW off 2 is HW on 3 is CH off 4 is CH on 2 was not connected.

On the C27:

1 is HW off 2 is CH off 3 is HW on 4 is CH on

On that basis I have moved the connection that was on 2 on the Landis & Gyr to 3 on the C27.

With both HW and CH set to off on the C27, and the hot water cylinder thermostat calling for heat, the motorised valve opens, the boiler fires for a second, and then the motorised valve closes and the cycle repeats.

With the C27 set to provide heating and hot water, and both thermostats calling for heat, the central heating works fine. However, the hot water fails to work. The motorised valve remains closed.

If I then set the central heating to off, but leave the hot water calling for heat, the motorised valve opens and the boiler runs. Turn the central heating back to on, and the motorised valve on the hot water remains open and the boiler fires. If I turn the programmable room thermostat down at this stage so that it stops calling for heat, the boiler stops firing but the HW motorised valve remains open. Turn CH off at the C27 now and the boiler immediately fires, turn the CH back on at the C27 and the boiler stops firing. As I turn the CH off on the C27 I can hear the relay click.

It would seem that when the central heating is on on the C27 then something is holding off power to the motorised valve and to the boiler through it.

Anyone able to offer a solution?

TIA.

[Dave Liquorice]

As you only mention 2 not being connected, is it a safe assumption that there where wires in 1, 3 & 4?

So what has happened to the wire from 3 on the Landis & Gyr (CH off)?

Check your wiring... Without digging out the relevant diagram and working through the logic I can't say that this "missing" CH off is the cause of your problem. That wire is certainly used to feed mains to the HW control side which could lead to strange things happening if it doesn't...

[F]

Just tried replacing the RWB2 to check that I hadn't disturbed any wiring. The RWB2 works fine so the problem must be in the way the send its signals?

[F]

Thanks! I checked the wiring diagram on the back of the RWB2 for the umpteenth time, drew a labelled diagram to compare the two connections, and realised I had misunderstood their version of a 'double negative'.

I should have left the wiring as it was.

In my defence, there is an earlier posting on uk.d-i-y which comes up with a search on Google which refers to swapping a wire over.

Apologies for any time wasted...

Doh!