Swapping room stat for digital one

[Mentalguy2k8]

Hi,

Just had new combi boiler installed, want to replace the old analogue wall thermostat with a digital one (one that has different heat programmes for different times of day). Analogue seems so "vague", I reckon there's at least 4 degrees either side of chosen temperature that it reaches before it'll click on or off.

Firstly, I've seen some that claim "universal UK backplate fixings ", so will the new one just screw in to the existing backplate?

Secondly, should wiring the new unit be a case of simply removing wires from old stat and inserting into new? I suppose best answer would be to check old unit first and match it up with what new units are available, but are there many different types of wiring for stats?

Any pitfalls?

Thanks very much for any help.

[Dave Liquorice]

If the backplate is the same physically and the pin designation for function you just remove the hold stat and put on the new...

Honeywell (I think) produce (or did) a pretty comprehensive cross model wiring leaflet. Have a dig about on their site or the web.

[Paul D Smith]

...snip...

...one that has different heat programmes for different times of day.

Interesting - I have a simple "one temp" model - what one have you found that has different temps for daytime? It would be nice to be able to leave the heating on gently overnight in cold winters.

Paul DS.

[Dave Liquorice]

JFG "programmable thermostat" there are loads out there. I like the Danfoss TP7000 but others swear by the Honeywell CM67. They are nearer the top of the range for programmable stats there are cheaper, perfectly adequate, ones about as well.

[hugh]

What do they mean by "volt free connections"?

[hugh]

CM67 has now been replaced by CM907

[Dave Liquorice]

That any pole of the switch(s) are not connected to any supply. If you need to switch a supply you have to arrange for it to be on the appropriate switch pole.

[Mentalguy2k8]

The ones I looked at work as "normal" stats where you can turn the heating up or down manually, but they also had a programmable clock under the flip-down cover (similar to a central heating/boiler clock) so that in my case, I could have the same temperature all day and night but set it to go up a couple of degrees from 6am - 9am to make getting out of bed and in the shower more comfortable. I like the house cool at night but warm in the morning, but at present I only have the option for heat on or off. I did see one for 20 quid that had 4 separate daily programmes, that would be enough for most people I reckon.

[hugh]

I don't understand. Surely the object of the exercise is to switch a supply?

[Dave Liquorice]

Ultimately yes but is that supply mains to drive a motorised valve or an LV one from a boiler to tell it to begin its startup sequence?

[Mathew Newton]

1:59:03 +0000, hugh wrote:

It is, but the key aspect of a volt free contact is that it will switch the supply that is provided to it.

For example, you might have a programmer that runs of its own 6v battery. If it were a volt free switch then you could supply it 240v AC to one pole of the switch and it'll will switch that to the other pole as required.

It basically means you can use it to switch anything (within defined limits of voltage/current ratings etc) and are not limited to using a switched output from its own supply.

Mathew

[hugh]

Thanks I understand it now.

[hugh]

So what are you saying about the definition of volt free connections. In both your examples volts are present. In the particular specification I was quoting the thermostat is described as being suitable for S and Y plan wiring which involves the switching of mains level voltage by the thermostat.

[Mathew Newton]

Only those that you feed into it to be switched.

The contacts of the switch/relay in the stat itself are volt-free i.e. it just sits there opening and closing until you feed something into to be switched back out.

That's right. The key benefit of a volt-free switch is that the voltage being switched is completely independent of the supply in the stat itself hence why our 6v battery powered stat is able to switch mains level voltage (rather than just toggle a 6v output on/off).

Mathew

[hugh]

My current stat is electro-mechanical with an "anticipator" resistor built in, which I have just got working, as the installer originally hadn't wired in a neutral. Are the digital stats better than this? I note the Honeywell one is 2 wire so presumably no anticipator.

[dennis]

These are cheap and are OK.

[hugh]

You bet. Unbelievable the difference.

I will change to digital at some stage. The way I got a neutral in was a bit of a bodge - not something I would like to leave behind if we ever moved.

[Dave Liquorice]

Something involving the CPC?