Upgrade Honeywell ST6400 2 zone & dhw with smart thermostat system

[Andy201]

I have a Potterton boiler controlled by a ST6400C that i was considering upgrading to a smart heating system. I'm struggling to understand due to the way my system is wired what i could replace it with?

I have 2 CM707 thermostats, one for ground floor and the other for 1st and 2nd floors. The DHW seems to be a simple swap for a dual channel smart controller. The Heating though I don't understand. Even though i have a 2 zone system it's controlled by 1 channel on the controller, this then heats either zone depending on the individual thermostat.

With for example a hive system, I'd need a dual channel receiver for DHW and one thermostat and another single channel receiver for the second thermostat. So I could wire the 2 channel as a direct replacement for the ST6400C but then when i look at the second receiver there's not a separate switch wire for that circuit. Could a link be put in from the single channel to the 2 channel receivers so either or both would fire up the heating circuit. Or does the wiring need to be changed to give two independant wiring circuits for the two zones

Considered swapping out downstairs zone and DHW with a 2 channel receiver and leaving the CM707 to control second zone, but then if zone 1 isn't calling for heating the 2nd zone wouldn't be kicking in on that thermostat?

I have three valves in the airing cupboard, for the 2 heating zones and DHW. I was also thinking an alternative could be to link out the second thermostat and turn it into a single zone heating system. All radiators have TRVs except one on each zone.

Any suggestions or advice welcome, many thanks

 

[Andy201]

Although thinking about it more, I'd assumed the hive wired thermostats still talk to receivers and just switch the existing wires, don't think this is the case, so would need to go wireless and connect existing thermostat wires together, which again takes it away from a 2 zone system.

 

[ianmcd]

You dont have a 2 zone system, you have an S plan + system, your last wiring diagram will need a link between L&1 on the single channel receiver

 

[stem]

Because the existing ST6400C is providing on/off time control for both heating zones the zones will have to be split to make them separate for the two Hive's, and the old thermostats decommissioned by taking them out of circuit and bridging the terminals their connections originated from.

Unfortunately it's not an easy exchange. If I was going to do it I would probably disconnect everything and start again from scratch, with the possible exception of the HW wiring including the cylinder thermostat. Probably too difficult to do over a forum.

I'm not trying to be rude, but what you've drawn below is wrong on several levels, and shows you don't understand the basic concept of how the system works. The Single Channel Hive won't be doing anything at all and only one zone of your heating will be working from the dual channel. Therefore I would strongly recommend you engage a professional to do this for you.

 

[stem]

your last wiring diagram will need a link between L&1 on the single channel receiver

True normally, but looking at the OP's drawing, even if the link were added. Both Hives will then be controlling the same zone of the heating, and nothing controlling the other zone.

 

[RandomGrinch]

I dont know what the others think, but an alternative could be to keep the existing controller for hot water (no great loss, for not being smart), and replace the two thermostats?
If they are in good positions already, you could look at something like a single channel hard-wired Tado (you'll need to find a neutral at the wiring centre), or a battery operated Nest Thermostat E?

...although, I agree that the best method would probably be to start again!

 

[Andy201]

OK thanks for the feed back, at least I know what system i have now, an S Plan Plus all makes sense now I'm thinking turning it into a two zone system is not an easy option unless something can be done with any spare cores not being used. Are there any options to replace a S Plan plus with a smart alternative?

True normally, but looking at the OP's drawing, even if the link were added. Both Hives will then be controlling the same zone of the heating, and nothing controlling the other zone.

Yes, realised that when i saw the wired Hive thermostat wouldn't switch my thermostat wires, what i need is a system that can, and then I wouldn't see the harm in commoning up the CH switching of a two zone controller, which in effect would make it more like a true two zone system as each could independently trigger the boiler for their zone when they call for heat.

 

[Andy201]

Wish my connection strip was a bit tidier, but looking at this wiring diagram I'm short of one wire from my boiler to airing cupboard that would let me do what I'd like to do, giving a separate feed to that circuit. I now understand what's needed, hopefully a spare core there that can be used, otherwise back to the drawing board or live with what I've got.

 

[stem]

I'm short of one wire from my boiler to airing cupboard that would let me do what I'd like to do.

The boiler wiring won't change, so I'm not sure why you say it will need another wire. The motorised valves are responsible for controlling the boiler (not the thermostats) so that will all stays as it is.

If you are referring to being a wire short for the single channel Hive receiver, you could always mount it in the airing cupboard adjacent to the wiring centre, provided that it's not out of the operating range of the thermostat. Then you can pick up a L & N for it from there too. But you're getting the idea of why it should be reconnected from scratch. As the old thermostats will need removing and a wiring mod made to bridge the open circuits caused by their removal.

Not sure what you are showing in your modified diagram though. It doesn't bear any resemblance to what has been discussed.

 

[Andy201]

The boiler wiring won't change, so I'm not sure why you say it will need another wire. The motorised valves are responsible for controlling the boiler (not the thermostats) so that will all stays as it is.

If you are referring to being a wire short for the single channel Hive receiver, you could always mount it in the airing cupboard adjacent to the wiring centre, provided that it's not out of the operating range of the thermostat. Then you can pick up a L & N for it from there too. But you're getting the idea of why it should be reconnected from scratch. As the old thermostats will need removing and a wiring mod made to bridge the open circuits caused by their removal.

Not sure what you are showing in your modified diagram though. It doesn't bear any resemblance to what has been discussed.

The wire I'm short of is a separate switch wire for the 2nd heating zone rather than the common wire i have now from the time controller.

My current thermostats act as a switch so i just need to connect the two wires together, the switching would now controlled by the smart thermostats, which would then go straight up to the flow valves and fire up pump and boiler when opened. I can mount the Hives in their place so hide the holes and should look good.

The modified drawing just shows what I'm missing to make it a 2 zone system, basically where I'd have to connect into, taking that through an empty slot in the connection strip, out of 4 and into 9 for example, and connecting the 2nd independent call for heating into 9 too, giving a proper two zone heating system.

I suppose I should simplify it and take the two stat wires out of circuit taking CH1 heating straight into 5 and CH2 heating straight into 7, much cleaner that way.

This could also give me and option to use a Nest system as I could then feed 12VDC to the stats down these wires and then dual and single channel Heat Links at the boiler to replace the existing time controller.

Basically opening the door have any smart thermostat system installed. Any thoughts on the best system, Hive, Nest, Drayton Wiser, Tado or any Pros and Cons of any of them.

 

[Andy201]

Thank you all so much for your help, as they say it's good to talk, it's really not such a daunting or complicated a task as I was thinking it was going to be and can give me the option to basically have any smart thermostat system installed.

Any thoughts on the best system out there, Hive, Nest, Drayton Wiser, Tado or anything else?

..... or any Pros and Cons of any of them to watch out for.

Thanks again

 

[CBW]

Both switch wires from two hive receivers would go to the switch live of the motorised valve, which in turn sends a switch live to the boiler depending on which zone is being used.

 

[Andy201]

The boiler wiring won't change, so I'm not sure why you say it will need another wire. The motorised valves are responsible for controlling the boiler (not the thermostats) so that will all stays as it is.

If you are referring to being a wire short for the single channel Hive receiver, you could always mount it in the airing cupboard adjacent to the wiring centre, provided that it's not out of the operating range of the thermostat. Then you can pick up a L & N for it from there too. But you're getting the idea of why it should be reconnected from scratch. As the old thermostats will need removing and a wiring mod made to bridge the open circuits caused by their removal.

Not sure what you are showing in your modified diagram though. It doesn't bear any resemblance to what has been discussed.

Just dawned on me... what a cracking idea to stick the single zone unit in the airing cupboard, never gave that a thought, why wire it down to where the time controller is when it doesn't need to be there, solves all my problems, thanks stem

 

[stem]

Then you would end up with something like.....

Just be aware that many installers don't follow the wiring centre terminal numbers suggested in this diagram. So, if you went down this route check where the wires are connected first to make sure you know which wire is which by where it goes. You will need to bridge the old thermostat wiring [terminals 4 & 5] and link the single channel Hive terminals L and 1.

If you go for another type of thermostat then it may be different.

 

[Andy201]

Then you would end up with something like.....

View attachment 317697

Just be aware that many installers don't follow the wiring centre terminal numbers suggested in this diagram. So, if you went down this route check where the wires are connected first to make sure you know which wire is which by where it goes. You will need to bridge the old thermostat wiring [terminals 4 & 5] and link the single channel Hive terminals L and 1.

If you go for another type of thermostat then it may be different.

Thanks for this stem, that's excellent. Yes my installer definitely didn't follow this terminal numbering convention, probably a good idea to have it made so, then i can have a wiring diagram that makes sense for future fault finding.

Now just need to decide on a system, was looking at the Hive mini last night and doesn't seem to offer much less than the normal Hive system. Just less functionality at the thermostats and their aesthetics.

On a best smart thermostat guide I saw Hive was ranked 2nd, with Nest 1st. The nest would be a little harder to integrate, so just wondering if the learning element, which seemed to be its winning feature, is worth it, plus it's way more expensive. I also like the idea of having the ground floor thermostat on the stand a moving it to where we are, which i think will also help make us be more energy efficient. I know i could do that with the hive, but it's not wireless.

Anyway a weekend to mull over it and make some decisions.

Thanks again for your input, much appreciated.