Siemens RDD10 Thermostat wiring

Hi - can someone advise me?

I'm intending to replace my existing mechanical room stat (3-wire) with a Siemens RDD10 digital stat. The RDD10 comes in a mains version and a battery version as far as I can tell. If I get the battery version, I know I'll just need 2 of the 3 wires. However, I'd rather have the mains version if I can to avoid the need for battery changes etc.

Does anyone know how many wires are needed for the mains version? My guess is that it will need 4 - a permanent live & neutral, and the feed & return from the switched contact - am I right? I haven't been able to find a connection diagram online.

Also, can you advise - should it be earthed? My current room stat isn't earthed.

Thanks.

Hope this helps

RDD10 Instructions

The three wires of your old mechanical stat will be: 230v supply, Neutral, switched 230V.

The siemens stat has volt free terminals so you will have to link L to Lp to mimic your existing stat. Q14 is the switched feed.

Thanks for that. Just one point though. The 230V 'supply' to my stat is switched by the main central heating timer.....so if I join the 2 'live' connections as you suggest, then when the timer sets the heating to 'off' there will be no supply to the stat. Won't that make the stat forget its settings? That's why I wondered if I would need a permanent live (L), a neutral (N), the switched live from the controler (Ln) and the switched 'output' from the stat (Q14).......... which would mean I'd ned a 4-core cable.

Any thoughts?

Thanks.

agmalias said:

Thanks for that. Just one point though. The 230V 'supply' to my stat is switched by the main central heating timer.....so if I join the 2 'live' connections as you suggest, then when the timer sets the heating to 'off' there will be no supply to the stat. Won't that make the stat forget its settings? That's why I wondered if I would need a permanent live (L), a neutral (N), the switched live from the controler (Ln) and the switched 'output' from the stat (Q14).......... which would mean I'd ned a 4-core cable.

Click to expand...

You are quite right; I had not thought about that.

The alternative, if you don't want the hassle of running a four core cable, is to use the battery operated version. The battery life should last more than a year. In this case you will only need two wires: feed from timer; switched output back to boiler etc.

The stat is supplied with preset temperatures which means that the data is stored in a non-volatile chip, so they are not lost when the power fails. The battery just powers the LCD display, which turns off when not required.

Thanks - I think I'll go with the battery version as you suggest.