What's this Wiring, new thermostat etc

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Hi guys.

Recently moved into a new place. I have a baxi solo boiler, hot water tank in airing cupboard and tanks in the loft.

Heating and hot water is controlled by a honeywell 9400c.

There is a honeywell Hcw80 room temperature controller and then a HC60NG receiver in the airing cupboard.

Basically the controller and receiver have seen better days.

So I plan on putting in a new smart thermostat and receiver, nest, hive etc.

My question is looking at these pictures would it be a straight swap?

In the first picture brown and blue, live and neutral. But what are A and B?

Reciever wiring
20230117_122218.jpg



Picture show wiring to the receiver
20230119_194332.jpg



Just to add I'm not worried about controlling the hot water. Just the heating from smart phone etc. Want to leave the house. 9400c programmer in place.
 
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If you want the new smart thermostat to control the hot water and the central heating, it would replace the Honeywell 9400c as both the heating and hot water control wires are present here. With some smart thermostats eg. Hive Dual Channel, the wiring connections are exactly the same and would be a straight swap. Other types will use the same wires, but they will need re-positioning. eg. Nest 3rd Gen.

The Honeywell R6660D room thermostat receiver in your photo becomes redundant and is removed. The white and black wires in terminals A and B would need to be connected together to complete the broken circuit caused by its removal.

Alternatively, as you mention, if you are happy to leave the hot water control as it is and not controlled via an app, you could have a simple smart thermostat (eg Nest-e) that does use the wires currently going to the R6660D. In this case, the heating time control at the Honeywell 9400c would be set to be permanently 'on' 24/7 to allow the new thermostat to take full control, but the 9400c hot water channel would still be controlling the hot water.

The wires in A & B are the switching wires that actually control the heating. The other wires in N & L are just to provide the 230V mains for its operating power.
 
Last edited:
Hive Dual Channel, the wiring connections are exactly the same and would be a straight swap.
Maybe but maybe not, as it seems the thermostat in use is using the volt free contacts. Hive and Drayton wiser use the same base, and single channel versions are volt free, but multi channel versions need to have 230 volt control, in the main this is not a problem, as the typical method is for the thermostat/programmer to work the motorised valves, then the valves work the boiler.

There was at one time a 24 volt motorised valve, but most have now been replaced, so unlikely to find one. But to jump in and say you don't need volt free, although likely true, would be wrong as in some cases volt free is required. Nest is volt free for example, however Nest does not link to TRV heads.

This Guide may help, it does depend on house design and boiler design, and nearly any thermostat can be made to work, just some are easier to wire in to others, and the whole reason why I have Nest is only two wires main house to where boiler is, and the Nest thermostat send all info and is powered by two wires. But as far as how well it works, in my house not very well, mainly due to my hall cooling too slowly and not linked TRV heads.

But my last house one thermostat worked A1, being an open plan house and thermostat between the two main rooms. I keep on repeating because it works well with one house, does not mean it works well with all. Oil boilers in the main cycle on/off, where gas boilers in the main modulate (turn down) so gas boilers lend themselves to TRV control far more than oil ones do.

In the main the TRV is king, the wall thermostat is just to turn off boiler in warm weather, knowing that I fitted electronic TRV heads when living with my mother, a house with internal doors, the problem was the anti hysteresis software was OTT, so turning the temperature from 16°C to 20°C at 7 am it was nearly 11 am before at 20°C, so I cheated and set it to 22°C then back down to 20°C at 8 am, this worked well. However it clearly would not work with geofencing.

The Drayton Wiser TRV claims to have cured this using algorithms that work out how long it takes to heat the room, and you can set time you want room at a temperature rather than when you want it to turn on, and it works out when heating needs to turn on, as is claimed to work better than most with geofencing.

But if you don't want geofencing then no reason to get such an expensive system.

Around this time last year we had high winds, and my local EE mast went down, this resulted in Nest turning to Eco mode as it thought I was not home, but each time I approached the thermostat the built in PIR detected me, and turned it back to Comfort mode, it took me nearly 24 hours to realise what was going on, and why house was cool, easy enough to turn off geofencing, but I had not realised that was the problem.

My TRV heads well at least 4 of the 9 electronic fitted can do geofencing, but never used it, they don't link to the Nest thermostat, so I have paid an extra £27 each for geofencing I don't use, the eQ-3 without it were only £15 each in 2019, and they have window open detection, so when I unload the car, it auto turns off kitchen radiator for 20 minutes so I am not heating the street with front door open.

So first question has to be what will smart controls do for you, second question is what boiler and system have you got, I will freely admit for me Nest Gen 3 was a mistake, I was told it would work with my existing Energenie TRV heads, and it does not. However if you don't want to link the TRV heads, then it may work for you.

I have been automating other things in my house, which was why I used Energenie in the first place, same hub does light switches and TRV heads, TCP is the similar, it does not need a hub, but it does do heating as part of a wider control.

Using Zigbee hubs each device can act as a relay for the next, so you light bulb can help the TRV send a signal, to be frank a lot of it goes over my head, but my wife seems to have lost the use of her fingers, rarely uses a light switch now, she seems at 69 to work out these things better than me at 72, but to be frank as to heating, the use of electronic TRV's has helped with a 14 room house, being able to turn down rooms not in use, but the wall thermostat being smart was a waste of time.
 
If you want the new smart thermostat to control the hot water and the central heating, it would replace the Honeywell 9400c as both the heating and hot water control wires are present here. With some smart thermostats eg. Hive Dual Channel, the wiring connections are exactly the same and would be a straight swap.

Maybe but maybe not

Would you like to reconsider that statement based on the below? They look the same to me ;)

ST9400C.JPG


Hive Dual Channel Receiver
Capture.JPG


The Honeywell R6660D room thermostat receiver on the other hand, as you correctly state does have volt free contacts. However in this instance it's connected to the ST9400 which can only supply 230V, so it will be switching the 230V from that anyway.
 
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Would you like to reconsider that statement based on the below? They look the same to me ;)
1674223831333.png

1674223888182.png

I went by the diagram posted, and the wires fitted, the white and black wires seem to be to contacts, no link wire, so from what is shown can't be sure 230 volt control, likely it is, but can't be sure.
 
I think you have missed my point. That's the R6660D. But I was talking about the ST9400 being identical to the Hive which it is. Both are 230V only neither have a voltage free option.
 
Thankyou for your reply everyone how would I go about testing which wire is which with regards to the black and white wire?
 
You don't really need to. When the heating is 'on' the wires in A and B are electrically joined together, when it's 'off' they aren't. It's just a simple switch, on or off.

You need to decide what smart thermostat you want to install and then we can give you definitive info, as they are all slightly different in some way.
 
I am surprised at the post.
Looking at the backplate, is it not self explanatory that A B and C is a switch element?
 
I am surprised at the post.
Looking at the backplate, is it not self explanatory that A B and C is a switch element?
Look at the markings on the backplate, A, B and C are clearly a switch. The contacts you need are the common and normally open, A and B.
 
I think what's confusing things here is the pics are for the wireless stat, there is no picture of the 9400C. The HIVE receiver would replace the backplate for the 9400C and the 6660D would become redundant and would need removed.

@NotThePolice , there will be a bit of wiring removal - rewiring to tidy things up and remove the 6660D but any smart controls being fitted their receiver would replace the 9400C and not the 6660D pictured, the 6660D would be redundant.

ID'ing your system as a Y plan or S plan would also need to be confirmed and then the required wiring would be dictated by that. We need a pic of the 9400's backplate
 
The reason there isn't a picture of the 9400C is probably because......

I'm not worried about controlling the hot water. Just the heating from smart phone etc. Want to leave the house. 9400c programmer in place.

I think I caused the confusion by giving @NotThePolice the option to replace the 9400C and have heating and hot water control, or just replace the 6660D and just control the heating.

I thought that it seemed a shame not to go for the heating and hot water option when it's available and as easy to do as the heating only option, and a more professional job IMHO.
 
I thought that it seemed a shame not to go for the heating and hot water option when it's available and as easy to do as the heating only option, and a more professional job IMHO.
Definitely +1, I would be convincing the client that it's a bit pointless doing it any other way.

....But if needed then a single channel HIVE to replace the 6660D and then just use the current wiring. The HIVE would use the same power and then A & B crosses over to 1 & 3 on the HIVE but definitely a bit weird to go that route.
 

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