Switching from EPH CP4 to Hive Thermostat

[griolfc]

Hi. I can see similar discussions but none have the same wiring as mine - so apologies in advance for posting something similar!

I have a Main combi boiler - installed in 2020 - with an EPH CP4 programmable RF thermostat and receiver and have decided to upgrade that to Hive for the convenience of using an app. I had done some research ahead of ordering and thought it would be straight forward but have hit a bit of a glitch on the wiring and hoping for some guidance here.

The EPH reciever (pictured below) has 4 wires. Live, neutral, nothing on number 1, a black cable to number 2 and a grey cable to number 3. There does appear to be an earth in the casing but snipped short and not connected (should I be concerned with that?).

I assumed it would be a simple switch to the Hive receiver (pictured below) and connected them to 2 & 3 (as per the way the EPH was wired). However, in this state, the boiler didn't seem to fire up when I pressed Boost on the thermostat or set a schedule. I then Googled and found a similar thread and the suggestion was to wire to 1 & 3 - which seemed to work. However, the boiler then wouldn't go off.

Logic now says to connect to 2 & 3 (as the wiring diagram above suggests that they are for heating on and off) and leave 1 empty but I don't know what "common" really means. In the end I played it safe and put the EPH back in place for now, as per the original picture and looking for some guidance from here!! Pretty frustrating and I don't really want to have to pay for an engineer to come and do something that will take a couple of minutes and be charged for it!!

Any help/guidance will be greatly appreciated!

 

[RandomGrinch]

First, silly question, but have you got a Hive for combi (single channel), with one button on the receiver?

Secondly, your EPH had the black wire connected to COM and the grey wire connected to ON.
The correct terminals to use are 1 (Common) and 3 (Heating on), on the Hive receiver backplate.

 

[RandomGrinch]

and leave 1 empty but I don't know what "common" really means.

Most receivers like the Hive, or EPH contain relays, this is basically a switch that joins together two wires, to complete a circuit, firing your boiler.

Usually a relay has three terminals on its output side. A common, a normally closed (N/C, in this case labelled Off) and a normally open (N/O, in this case labelled On).

There is a coil in the relay that when powered, move an armature that will connect the COM and N/O terminals together.
When power is removed from the coil, the armature is sprung back, connecting the COM and N/C terminals.

So, in the case of the EPH, your black and grey wires were connected to the COM and N/O terminals, when the call for heat was made from the thermostat, the coil was powered and those two terminals were connected together. When the thermostat reached it's set point, the coil would be turned off, the armature would spring back, connecting together COM and NC and breaking the circuit to the boiler.

And it's nice to see the relay with the correct markings labeled on the EPH PCB...

This is what we would hope to replicate, by using the Hive receivers terminals 1 and 3.

Hopefully that makes some sense!

 

[griolfc]

Thanks for the replies - bit pushed for time today, so will have another closer look at this tomorrow. Picture below of the Hive receiver - pretty sure I ordered the combi version!
So you said above, "This is what we would hope to replicate, by using the Hive receivers terminals 1 and 3." - that's what I had in my picture above on the Hive settings but the boiler didn't seem to be switching off - hence me thinking something would be needed on T2 for heating off.

 

[RandomGrinch]

You have a single channel Hive receiver, good.

T2 for heating off

As I tried to explain, it isnt a heating off terminal.
You need to connect the grey and black wires to COM (1) and NO (3). The heating will turn off, when the relay breaks the circuit.
How long did you wait for the heating to turn off? Your boiler is likely to have a pump overrun that will keep the pump going for a few minutes, after the call for heat has ended.

Otherwise, the Hive receiver may be broken, or have a sticky relay.

 

[griolfc]

Not sure - will give it a go tomorrow when I have time and give it some tests. Cheers!

 

[griolfc]

You need to connect the grey and black wires to COM (1) and NO (3). The heating will turn off, when the relay breaks the circuit.

@RandomGrinch - many thanks for your help.

I reinstalled the Hive receiver this morning, wiring as per your suggestion above.

I initially pressed the "boost" button on the thermostat and the boiler switched on and the radiators started warming. Using the Hive app, I stopped the boost and observed the boiler switch off. I then set a schedule in the app for the heating to come on via schedule - which it did. I set if for an hour, but had to go our. However, after coming back in the bolier is off.

So. it looks like all is well. Many thanks again!

 

[RandomGrinch]

So. it looks like all is well. Many thanks again!

Well done, and thanks for letting us know the outcome