Replace a thermostat

[Jupiter01]

I need to replace this for my folks and would like some advice please.

The current one has four wires by the looks of it and I am looking for a straight forward switch over.

I am looking for an accurate regulator and an indicator on when the heat is being drawn (gas being used). I have a flame indicator on mine and something similar will allow them to easily see that gas is being used.

They don’t need a programmable unit. I would welcome your advice.

Thanks in advance.

 

[CBW]

What about one of these? https://www.screwfix.com/p/danfoss-...eWhZqJF8DdHXEMWx9thoCTxwQAvD_BwE&gclsrc=aw.ds

 

[stem]

I would expect the old thermostat only to have two connections made to 1 & 2, plus the earth connection. - Terminals 1, 2 & 3 are all live connections, so must not have a neutral connected. If it were, a short circuit would be created and a fuse would blow. [Note: A blue wire may not always indicate that it is actually being used as a neutral; it's what a wire is connected to that determines its function, not the colour of its insulation]

When the heating is 'on' the thermostat's 'switch' would be in the position shown in the diagram above (marked "ON") with terminals 1 and 2 electrically connected together.

Terminal 3 is marked "OFF" and becomes live when the thermostat switches off. This would not normally be used in the UK. There are exceptions, one very rare exception (I would estimate much less than 1%) would be when the thermostat is controlling a MOMO valve. Or may be, if the thermostat is used with a cooling system instead of a heating system.

It's difficult to see from your photo where the wires are actually connected, but to me it looks like there may be a wire in terminal 3. (which bizarrely appears to be the middle terminal between 1 and 2 ) If so, that would require investigation as to its function. Especially if you wish to change the thermostat. Because it is rarely used, not all thermostats have the "OFF" connection.

 

[Jupiter01]

What about one of these? https://www.screwfix.com/p/danfoss-...eWhZqJF8DdHXEMWx9thoCTxwQAvD_BwE&gclsrc=aw.ds

Looks good on the face of it. Will it be compatible with the current wiring setup?
There a mention of a "receiver" on the screwfix site. I don't have one currently and assume that this can work without one?

 

[Jupiter01]

Terminals 1, 2 & 3 are all live connections, so must not have a neutral connected

Can I use my Fluke 2 pole tester to verify that all 3 of these wires are live? How does this translate to the wiring of a new thermostat? I am tempted to go with the suggestion from @CBW unless there are any advances on this.

Because it is rarely used, not all thermostats have the "OFF" connection

Can I cap this off at the first instance and use the other two wires if they are live?

I was checking over a honeywell thermostat which only requires two wires and based on the above, I would be inclined to use wire 1 and 2 for that but I am unsure on whether this differs for the Danfoss suggestion.

Look forward to your advice and thanks again.

 

[stem]

The terminals are used for live switching purposes, but only become live when physically connected to a live supply.

If installed properly as per the diagram, terminal 1 should always be live whenever the timeswich or programmer is 'on' [shown as "remote switch" on the diagram] and under that same condition, terminal 2 would be live when the thermostat is 'on' and terminal 3 when it's off.

If that is the case, and there is a wire in terminal 3, something on the other end of it will become live when the thermostat switches off. The question is what is it? Unfortunately the only way to know for sure is to find the other end.

 

[BlueLoo]

I would expect the old thermostat only to have two connections made to 1 & 2, plus the earth connection. - Terminals 1, 2 & 3 are all live connections, so must not have a neutral connected. If it were, a short circuit would be created and a fuse would blow. [Note: A blue wire may not always indicate that it is actually being used as a neutral; it's what a wire is connected to that determines its function, not the colour of its insulation]

View attachment 258575

When the heating is 'on' the thermostat's 'switch' would be in the position shown in the diagram above (marked "ON") with terminals 1 and 2 electrically connected together.

Terminal 3 is marked "OFF" and becomes live when the thermostat switches off. This would not normally be used in the UK. There are exceptions, one very rare exception (I would estimate much less than 1%) would be when the thermostat is controlling a MOMO valve. Or may be, if the thermostat is used with a cooling system instead of a heating system.

It's difficult to see from your photo where the wires are actually connected, but to me it looks like there may be a wire in terminal 3. (which bizarrely appears to be the middle terminal between 1 and 2 ) If so, that would require investigation as to its function. Especially if you wish to change the thermostat. Because it is rarely used, not all thermostats have the "OFF" connection.

View attachment 258580

One of the best posters on the site.

 

[CBW]

Yes @Jupiter01, you can use your 2 pole voltage tester.

• Turn thermostat down, and test between Earth and live - should be 230 vac
• Turn thermostat up and test between Earth and switch live (terminal 2) - should be 230vac

@stem could be correct as in it might only be 2 wires and Earth, in which case don’t go for my linked one. I think the mention of a receiver is just to state it’s not wireless?

If it’s only 2 wires and Earth, then either a Drayton 2 wire stat, or a battery operated one, saves on the neutral not being required.

 

[Jupiter01]

Thanks for the responses guys.
Perhaps I should try and get a better picture of the cables that are currently connected and specifically whether anything is connected to "off". If the "off" terminal does have a wire, can I assume that this has a plausible function given that the existing thermostat - albeit old - has been functioning for many years. If yes, would you be able to recommend a new thermostat that also has the "off" terminal and then I have a like -for-like replacement. This would be easier than tracing the cable and figuring out what it actually does - my current thinking.

Please let me know your thoughts.

 

[ianmcd]

You wont need an OFF connection, your thermostat is a very simple switch the OFF is not used

 

[Jupiter01]

You wont need an OFF connection, your thermostat is a very simple switch the OFF is not used

On this basis, I will place an order for this thermostat unless you have an alternative recommendation:
https://www.screwfix.com/p/danfoss-...l+Content&utm_content=TextLink&utm_term=58258

Also the above states "4 wire installation". I assume that I will only be using my two live wires (1 and 2 on current thermostat)? Hopefully the appropriate terminals on the new thermostat will be obvious once it arrives.

Thanks again.

 

[CBW]

On this basis, I will place an order for this thermostat unless you have an alternative recommendation:
https://www.screwfix.com/p/danfoss-...l+Content&utm_content=TextLink&utm_term=58258

Also the above states "4 wire installation". I assume that I will only be using my two live wires (1 and 2 on current thermostat)? Hopefully the appropriate terminals on the new thermostat will be obvious once it arrives.

Thanks again.

It’ll work, but may not be effective. If you have a neutral wire, then it’ll work fine, but if not, then as I said before, I wouldn’t go for that Danfoss. You’ll need a 2 wire thermostat.

 

[Jupiter01]

I did some further testing on this and took a new picture that I wanted to share.

You can see the earth cable over to the left. You can also see the colour of the wires in each terminal. I appreciate that the colour in itself may not be significant.

Here are the results of my testing.

When thermostat was inactive:
1 - has power
3 - has power
2 - no power

When thermostat was active:
1 - has power
3 - no power
2 - has power

I think that's consistent with what you experts stated. i thought it would be helpful to share this with you.

In light of this, can I replace this with this:
Danfoss RET2001M Room Thermostat | Wired Thermostats | Screwfix.com

If yes, where do the current wires connect to the terminals (has 4 terminals according to the description) of the above thermostat.

Thanks again and I really appreciate your support.

 

[stem]

As you say, the readings are what was expected.

Unfortunately the RET2001-M isn't suitable. Whilst the switching wires COM / NC / NO correspond with the three existing terminals 1,2,3, and terminal 1 is also a live, so could connect to both L and COM at the new stat. However, you don't have a neutral wire at the existing thermostat and the RET2001-M requires a neutral at its N terminal.

Without a neutral, you would need a battery operated thermostat.

I still wonder about the existing connection to terminal 3 though it would be really helpful to know what's on the other end of it.

 

[CBW]

I still wonder about the existing connection to terminal 3 though it would be really helpful to know what's on the other end of it.

Yes, I was intrigued by this too, as they’re not often used.