Replacing a room thermostat - mechanical to digital

[mikefromguildford]

Hi. Any advice on optimal setting for 'cycle rate' and 'minimum on/off' given my boiler type?

Vaillant ecoTEC Plus 418 Open Vent
With new DT90E stat

Assume defaults are good enough but any advice welcome

It certainly clicks more often then previous mechanical stat. Suppose it has to as it's more accurate. I'm just worried about wear and tear on boiler parts with frequent on/off on/off - sometimes only for couple of minutes.. Should I be concerned?!

Thanks

 

[OwainDIYer]

frequent on/offs will also reduce efficiency and increase energy bills.

I'd suggest not operating the boiler for heating for less than 3-5 mins at a time

 

[mikefromguildford]

Thanks. Default is 1 minute. I can increase to maximum 5 minutes. Cycle rate is 6 (default)

 

[Charnwood]

You can fit it in the sense that it will work.
You should not fit it because someone in future, thinking the g/y is earth.....

If the GY is not earth what function is it performing in the current (no pun) set-up?

I could rewire with a 2 core flex, if that's feasible.

I just wanna fit a digital programmable stat!

 

[OwainDIYer]

You can fit it in the sense that it will work.
You should not fit it because someone in future, thinking the g/y is earth.....

If the GY is not earth what function is it performing in the current (no pun) set-up?

I could rewire with a 2 core flex, if that's feasible.

I just wanna fit a digital programmable stat!

G/y is currently switched live (call for heat). This is because your existing thermostat requires (or has been wired with) a neutral, for the accelerator heater inside the stat. The installer has been a lazy bastard and no used the correct triple+e cable.

Rewiring with 2 core flex is no better as (a) flex should not be used for permanent installation (b) all cables require an earth core. In fact it would be worse as a sin of commission rather than omission.

Replacing your existing stat with a battery powered one allows you to use the existing cable as live, switched live, and earth, which is what it's supposed to be.

 

[Charnwood]

You’re saying it can be done using 3 wires, but the wiring diagram for the CM907 shows just 2 wires going into the stat.
If it can be done, what goes where?

 

[D_Hailsham]

You’re saying it can be done using 3 wires, but the wiring diagram for the CM907 shows just 2 wires going into the stat.
If it can be done, what goes where?

The CM907 only needs two wires - live and switched live. It does not need an earth wire and there is no provision for one.

As for cycle time and minimum on time, the standard settings of 10 minutes and one minute are correct for a gas boiler.

 

[Charnwood]

OK, the 6360 has brown to terminal 1, blue to t 2 and g/y to t3.
View media item 52068They are numbered from right to left 2 - 3 - 4 - 1, with an unused terminal to the left of 2 marked with the symbol for earth.

We have a combi, so where should whichever wires go?
View media item 52069
View media item 52072

 

[mfarrow]

You should connect the brown to A and the green/yellow to B. Put the blue in a connector block somewhere safe.

The green/yellow needs to be sleeved with brown so that it's identified as the switch live. Apart from that, assuming everything at the thermostat end is plastic, it will comply.

There's nothing prohibiting using flexible cables in fixed wiring either as far as I'm aware.

 

[Agile]

There's nothing prohibiting using flexible cables in fixed wiring either as far as I'm aware.

I noted that and thought that electricians would argue for ages over that.

The correct wire for fixed installations is the grey PVC insulated flat types including a bare earth. In a metal backing box I think that an earth wire IS required.

Many heating enginers just use flexible cables for everything. Apart from not having an earth wire for the metal box, I dont think there is any reason to prevent a flexible cable being used to connet to a room stat.

 

[Charnwood]

No metal involved in the installation - I'll be screwing the stat direct to the wall.
The cable is currently surface mounted then 'chased' into the plaster where it meets the stat. I aim to prettify it at a later date, but there are more pressing jobs at the mo.

 

[mfarrow]

In a metal backing box I think that an earth wire IS required.

Yes you are correct Agile, I should have clarified as Charnwood's thermostat isn't in one, but many are.

The only real reason electricians use solid cables is because once you get to 2.5mm² stranded cables become a bit cumbersome. Furthermore, stranded cables should technically be ferruled if going into a screw terminal.

 

[Charnwood]

Right, I've bought the 907 and will be fitting it soon.

If there's a flash and a bang that does not seem like fireworks in the centre of England over the next couple of days you'll know what it is!

 

[Charnwood]

Apologies to Mike BTW for hijacking your thread!

 

[Terry7556]

i worked with some pipefitters who claimed the centre of england was newark.