Replacing CentaurPlus c21 with Nest 3rd Gen Heatlink

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I was hoping to get some information from the forum users if possible.

My Current Heating setup is I have a Programmer and a Central Heating Boiler which just heats the Radiators with gravity fed Hot water Tank heated by the same feed as the Radiators. Currently there is no thermostat.

C21 - 4 Terminals used only - N L 3 4

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Screenshots from c21 Manual.

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I'm confused by the number 3 wire since that is shown as HW ON. the boiler does not heat HW on its own??? or am I wrong???? Maybe just terminal 4 used like that however??

Anyway...

Can someone confirm the Programmer can be replaced by the Heatlink, or will the Heatlink need to be put next to the Boiler (Boiler and Programmer in different locations)

I would prefer the Programmer to be directly replaced by the Heatlink in this situation as it can then power the Nest Thermostat through a studded wall.

Any thoughts or comments would be great. I intend on getting someone to fit this but just need to know if the way I would like it can be done.

Thanks for any help.
 
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What boiler do you have? Never heard of a boiler that can't do HW without having the CH on, sounds like a fault on the system to me
 
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What boiler do you have? Never heard of a boiler that can't do HW without having the CH on, sounds like a fault on the system to me
its more likely I dont know the exact system ;) I'm not in the house at present so cant answer the make of Boiler as yet.
 
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Be careful with the term 'gravity'. In the circuit diagram you have posted 'gravity hot water' refers to a system where the hot water circulates around the boiler and hot water cylinder by natural convection (ie it is not pumped) using the principle that warm water rises cooler water falls. These systems stopped being installed about 40 years ago and I'm not sure that the Nest has the capability of controlling them, because you can't have the heating on, without the hot water being on also.

On the other hand, if by gravity, you are meaning that you have a tank in your loft supplying the hot water cylinder, then gravity is the wrong term for you to use, and the diagram you posted is irrelevant. The correct term for what you have is 'open vented' not 'gravity' most of these systems have a pumped supply from the boiler, with flow control achieved by motorised valves.

Assuming your system is the latter, then regardless of the boiler, then yours is very simple to swap to make, especially as you don't have an existing room thermostat to remove. Looking at the diagram for your existing programmer then the wires can be identified as:

Neutral = N
Live = L
Hot water on = 3
Central heating on = 4

They simply need to go to the corresponding terminals that do exactly the same function at the Nest Heatlink

Neutral = N
Live = L
Hot water on (or call for heat) = 6
Central heating on (or call for heat)= 3
Then you need to link the two common terminals 2 and 5 to the L terminal

Finally the Nest Thermostat can be powered by wiring it to terminals T1 & T2 of the Heatlink, or using a separate 'plug in'power supply.
 
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