Thermostat Wiring

[Larches]

I'm planning to replace my old dial thermostat (Honeywell T6360) with a programmable one.  I've been considering the Danfoss TP5000, Honeywell CM907 and Salus RT500.  The Honeywell is pricey but looking favourite in terms of features and reviews (any views on that welcome though).  I have a conventional boiler.

What I'm looking for is a bit of help in getting my head round the wiring.  I understand it's generally quite a straightforward swap, and in fact Salus specifically categorise their model as DIY rather than pro installation, but as far as the Honeywell goes, I'm not sure exactly how I'd map over the connections.

The current stat has terminals labelled and arranged as follows (left to right):
> Earth: yellow & green wire
> 2: blue wire (presumably neutral)
> 3: yellow wire (what's this?)
> 4: empty connector
> 1: red wire (presumably live)
> 5: no connector
> 6: no connector

Seems like a mix of pre & post 2004 colours, so not sure what that's all about.

Looking at the installation PDF for the CM900 series (http://goo.gl/s5J0Hp), it shows various wiring layouts including one for a "basic boiler" - presumably meaning conventional/open vent/heat only..?  The terminals are just A, B & C, with live to B, although I have to confess I don't fully understand the diagram, and have no idea about placement of the yellow wire as noted above, or the earth which doesn't seem to have a terminal (even for parking).

So, is anyone familiar with this layout and can advise me on how to translate old to new? 

Thanks in advance...

 

[ProDave]

Red will be live in. blue will be neutral, yellow will be switched live out.

Some digital thermostats are battery powered and don't need a neutral,

Looking at the CM900 link you gave, your red goes into terminal A and yellow into terminal B. There is no need for neutral so just park the blue wire into a choc bloc terminal which you will probably have to provide.

one VITAL point, if you are considering a battery powered one, is that it MUST retain it's set program when the batteries go flat (ask me how I know)

 

[Larches]

Thanks ProDave, makes sense.  I expect I'll need a terminal for the earth as well, as AFAIK the CM907 doesn't provide anywhere to park it, unlike some.

About the battery power, I've not been able to find a mains equivalent to the CM907... unless I'm missing something, it looks like Honeywell don't do one, not sure why.  But the good news is, the spec sheet for the CM907 says "Minimum battery life of 2 years with low battery warning indicator", and - even better - "EEPROM memory holds the user program indefinitely".

BTW (to get it off my chest), just as I thought this was settled, I've found a load of reviews (http://goo.gl/WFV3Ef) suggesting the LCDs are prone to failure on the CM900 stats, and I could do without forking out 60-70 quid only for it to fail (outside warranty) after a few years.  I suppose with all the bells & whistles it's not likely to soldier on as long as a simple old dial stat, but I'd still expect a good number of years out of it for that kind of money.  So not so sure now.   Real shame, as functionality-wise it seems like a real winner.

 

[Sharpend]

If it's any use, we fit the danfoss tp5000 to all new builds and have done for many years with very few comebacks?

 

[Larches]

If it's any use, we fit the danfoss tp5000 to all new builds and have done for many years with very few comebacks?

Yes that's useful to know, thanks... seems people only have good things to say about this stat, and I see there's a TP5000M mains version although bit dearer and not so widely available for some reason.  Not quite as many features as the Honeywell (e.g. backlight, overrides, timed holiday mode especially useful) but leaning towards this one now as no reports of dodgy screens!

 

[Murdoch]

Simply changing a room stat is highly unlikely to bring out about the changes you indicate. Are you sure that nobody has been meddling??

 

[Larches]

Simply changing a room stat is highly unlikely to bring out about the changes you indicate. Are you sure that nobody has been meddling??

Umm... not sure I'm following!?  Which changes did you mean?

 

[Murdoch]

Umm... not sure I'm following!?  Which changes did you mean?

As I previously wrote, if you just changed the room stat, why would the central heating and water not work properly?

 

[Larches]

As I previously wrote, if you just changed the room stat, why would the central heating and water not work properly?

No I wasn't saying the CH and water don't work properly.  Just that I've decided to replace my old room stat with a programmable one (for various reasons), so I was just after a bit of advice about wiring up the new stat, which ProDave kindly advised on (before I found out about the Honeywell display issues and had second thoughts about which model to buy...).

 

[Larches]

Red will be live in. blue will be neutral, yellow will be switched live out.

Some digital thermostats are battery powered and don't need a neutral,

Looking at the CM900 link you gave, your red goes into terminal A and yellow into terminal B. There is no need for neutral so just park the blue wire into a choc bloc terminal which you will probably have to provide.

one VITAL point, if you are considering a battery powered one, is that it MUST retain it's set program when the batteries go flat (ask me how I know)

Right, after all that deliberation, I've gone for a Danfoss TP7001.  Not a bad price, and there are just too many reports out there about failed displays on the CM907.

I'd be grateful for a quick sense-check on the wiring for this one... I know this is simple stuff but I'd rather be sure I'm not going to fry my nice new stat.

http://heating.danfoss.co.uk/PCMPDF/TP7001.pdf

 

[Rob_the_rich]

Right, after all that deliberation, I've gone for a Danfoss TP7001.  Not a bad price, and there are just too many reports out there about failed displays on the CM907.

I'd be grateful for a quick sense-check on the wiring for this one... I know this is simple stuff but I'd rather be sure I'm not going to fry my nice new stat.

http://heating.danfoss.co.uk/PCMPDF/TP7001.pdf

Make sure the power is off first.

red into terminal 6 and yellow into terminal 7.

I can't see if there is an earth terminal on the backplate for the earth wire so it might need a 5 amp choc bloc terminal

The blue neutral will need its own choc bloc terminal too as ProDave said.

Don't worry about 1,2,3 and 4 unless you have bought the accessories, you need extra wiring for them anyway.

Looks like it can remember programs after battery replacement

What time management do you have already? Does the boiler have a clock? This would need to be disconnected or linked out for the programmable thermostat to work properly since it acts before the thermostat circuit. TIME -> TEMPERATURE -> ACTION

 

[Larches]

Thanks Rob_the_rich, much appreciated.  Yes it has battery backup which lasts "for the life of the product" (although it's a Li button cell in a standard clasp holder on the PCB so looks like it could be replaced if needed). 

No earth terminal on the backplate so will choc block that too... I've got some spare 15A blocks I can use, assuming no reason not to use a higher-rated block.

Current setup is a Potterton EP2002 programmer, which I was planning on setting to 'Always On' for the CH and keeping it on the timed programme for HW.

 

[Larches]

Job done.    Seems a good unit, plenty of features, pretty logical and straightforward to program.

I just need to get used to the cycling on & off for just a minute or so at a time, which I gather is intentional behaviour and does seem to maintain quite a stable temperature, even though the rads sometimes barely get warm.  Probably a gain in energy efficiency, although I do wonder about what extra wear and tear this approach might put on the boiler, pump etc.

Anyway, thanks all for your help - much appreciated.