Wiring to replace old room thermostat to new digital

I’m replacing a tatty old mechanical Drayton room thermostat with a sleek digital Flomasta 7635G.
Could a wise wizard please help decipher the code known as the wiring diagram in the pictures attached.
Theoretically, given the ‘standard’ wallplate, I should just remove the old and pop on the new. But the diagrams got me concerned especially since the wiring is T&E but should be 3&E(?).
My understanding for the new kit is this: brown (L) to COM (1), green/yellow (SL) to ON (3) and blue (N) to (4) as it isn’t used.
BUT, on the old unit, blue looks to be SL(?) and g/y looks to be Neutral(?)
Could anybody clarify.
DIY jobs should never be so difficult. I’ve done washing machines/dishwashers/microwaves/vacuums, but this!!!
Thank you all in advance.

You need a cable with 3 cores plus earth.
1 live
2 neutral
3 switch live
4 earth

SufiMaster said:

DIY jobs should never be so difficult. I’ve done washing machines/dishwashers/microwaves/vacuums, but this

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some where on the information sheet ,consult a spark .

buzzlightyear said:

some where on the information sheet ,consult a spark .

Click to expand...

Thanks buzz, guess that’s why I’m here
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James the Spark1976 said:

You need a cable with 3 cores plus earth.
1 live
2 neutral
3 switch live
4 earth

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Thanks James, was kinda expecting that answer.
Next question then, where does the SL wire get connected at the other end in the fused connector, see picture.

SufiMaster said:

Thanks buzz, guess that’s why I’m here
[automerge]1568828225[/automerge]

Thanks James, was kinda expecting that answer.
Next question then, where does the SL wire get connected at the other end in the fused connector, see picture.

Click to expand...

Actually skip that, just realised it wired directly to the combo boiler. This fast snowballing. Would a wireless be any simpler?
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SufiMaster said:

Thanks buzz, guess that’s why I’m here
[automerge]1568828225[/automerge]

Thanks James, was kinda expecting that answer.
Next question then, where does the SL wire get connected at the other end in the fused connector, see picture.

Click to expand...

Actually skip that, just realised it wired directly to the combo boiler. This fast snowballing. Would a wireless be any simpler?
Source URL: Wiring to replace old room thermostat to new digital - https://www.electriciansforums.net/threads/wiring-to-replace-old-room-thermostat-to-new-digital.178683/#post-1574814
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SufiMaster said:

Thanks buzz, guess that’s why I’m here
[automerge]1568828225[/automerge]

Thanks James, was kinda expecting that answer.
Next question then, where does the SL wire get connected at the other end in the fused connector, see picture.

Click to expand...

Actually skip that, just realised it wired directly to the combo boiler. This fast snowballing. Would a wireless be any simpler?

Can you show a bit more of the original thermostat photo.
In particular is that twin and earth and is the earth being used as a live conductor direct into the boiler.

Agree with above scrap this thermostat and get a wireless one. Much simpler.

SufiMaster said:

Actually skip that, just realised it wired directly to the combo boiler

Click to expand...

now we are getting some where, have you have a programmer in the boiler yes ?

Existing programmer integrated with boiler is a manual dial clock type, where you depress buttons to select the duration for which you require heating. Nothing digital, after all it’s a 20 yr old Worcester Bosch 28cdi.
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Paignton pete said:

Can you show a bit more of the original thermostat photo.
In particular is that twin and earth and is the earth being used as a live conductor direct into the boiler.

Agree with above scrap this thermostat and get a wireless one. Much simpler.

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Thanks Pete. Yes to T&E, by the looks of it, E being connected to N. Really bizarre.

SufiMaster said:

Existing programmer integrated with boiler is a manual dial clock type, where you depress buttons to select the duration for which you require heating. Nothing digital, after all it’s a 20 yr old Worcester Bosch 28cdi.
[automerge]1568830903[/automerge]

Thanks Pete. Yes to T&E, by the looks of it, E being connected to N. Really bizarre.

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Blizzard and wrong. Even though it functionally works.
You won’t be able to wire your new thermostat as is.

Wireless is the way forward I’m afraid.

Paignton pete said:

Blizzard and wrong. Even though it functionally works.
You won’t be able to wire your new thermostat as is.

Wireless is the way forward I’m afraid.

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No diy should be this testing. Thanks Pete.

Traditional bimetal room stats required a neutral to operate the accelerator resistor, essentially a tiny heating element inside the thermostat to help it stay thermally in step with the room as it warms. The current from these is small, a milliamp or two, so people bodged them by using the bare or G/Y core of T+E as the neutral, although 3c+E would have been correct for an installation where only COM, N/O and N were to be connected. This bodge was more prevalent where a 3-position motorised valve was in use that required both the N/O and N/C contacts of the stat, as there were then 4 live conductors (COM, N/O, N/C and N) and this was achieved with 3c+E again using the bare core as N. This was never compliant with regulations as all live conductors, including neutral, must be individually insulated.

Mains-powered electronic stats and timeswitches also need an N to power the electronics so the same requirements exist. You need 3 insulated cores L, SL & N and T+E does not give you that. An alternative to fully wireless would be a battery-powered stat / programmer, that does not need a neutral as its electronics are powered from the battery. That will require just L & SL which your existing cable will provide, thereby avoiding possible wireless connectivity problems.

Paignton pete said:

Blizzard and wrong. Even though it functionally works.
You won’t be able to wire your new thermostat as is.

Wireless is the way forward I’m afraid.

Click to expand...

There are loads of battery powered wired thermostats that don't need a neutral, but wireless I only know of one, Nest e is wireless and battery powered, but don't know of any others. The Flomasta 22199SX is two wired battery powered programmable thermostat at £35 the Nest e is £179 if you want OpenTherm or internet connections or to link to thermostatic radiator valves then yes Nest e is good. But just to get two wire working that is going OTT.