Horstmann H37XL to Nest 3rd Gen - HELP

[Anton87]

Hi Everyone,

I’d like to replace my current Horstman Channelplus Series 2 H37XL with a Nest 3rd Gen Thermostat.

I’m really confused with the wiring and wondered if somebody on this forum could explain to me what to do please?

Thank You

 

[stem]

The Channelplus is a 3 channel programmer which provides control of two separate heating zones and stored hot water (i.e. a hot water cylinder) The Nest only controls one heating zone and hot water. So you would need two Nest's to replace it like for like.

1. One Nest for one heating zone and the hot water (if utilised) and
2. One nest for the second heating zone.

It's difficult to see the wires properly in your photo, but some of the wire connections appear somewhat unconventional. There seems to be a link between terminals L, 2 and 5 and it's unusual to have links between L and any of the other terminals (eg L, 2, 5) and also to have connections to the CH off. (eg terminal 4)

You don't give any details about the heating system that the Channelplus is connected to, so to give any further information would involve some guesswork.

 

[Anton87]

Thank you for the prompt response Stem.

My house has one thermostat which is downstairs, and one hot water cylinder which is upstairs.

Does that help?

Cheers

 

[Anton87]

Dohhh, sorry i’ve been a bit of a wally!

uploaded the incorrect wiring diagram for the horstmann unit.

please see the correct image attached.

 

[stem]

As I said, the Horstmann is capable of controlling two heating zones (eg upstairs and downstairs) so there would normally be two room thermostats. Looking at your wiring I suspect that the 3 channels are connected as follows:

CH Zone 1 is the central heating
CH Zone 2 is being used to control the hot water
HW isn't actually doing anything at all

It would be really helpful to know what is the final destination of the white wire in terminal 4 (the one marked CH Zone 2 off)

Can you confirm that?

 

[stem]

Dohhh, sorry i’ve been a bit of a wally!

Yes you have! That makes more sense.

 

[CBW]

I think @stem the Op is confused which programmer they have and sent you on a bum steer, it’s a H27XL as per photo.

 

[Anton87]

Why would “CH OFF” not have a wire connected to it? When “HW OFF” has a wire connected to it?

 

[stem]

So the wires move from the terminals of the Horstmann to the Nest Heat link terminals that have exactly the same function.

N, L and E are self explanatory:
Horstmann 1 Hw On = Nest 6 Hot water call for heat
Horstmann 3 HW off = Nest 4 Hot water satisfied
Horstmann 4 CH On = Nest 3 Central heating call for heat

Then link Nest terminal L to 2 and 5

The old room thermostat should be set to max, or if you want to remove it the switching wires would need to be bridged.

Why would “CH OFF” not have a wire connected to it? When “HW OFF” has a wire connected to it?

Because you have a 3-Port motorised valve that needs a 'hot water not required signal' to close off the hot water port. The central heating port is closed by a spring, so no wire required to close it.

 

[Anton87]

Ahhh that makes sense, thanks for that, you’re good at explaining!!

One final question please, where would the existing thermostats wires be? I’m assuming i need to located them and terminate them to T1 and T2 on the Nest Heatlink to power the Nest Thermostat.

 

[Anton87]

Actually thinking about it, the hot water cylinder and pump is upstairs, so the thermostat wiring should be up there?
If so, my boiler and Horstmann is downstairs. How will i connect the thermostat wiring upstairs to the heatlink downstairs without routing a cable?

 

[stem]

The old thermostat can't simply be disconnected as this would leave the heating wiring 'open circuit' and the heating would not operate; the integrity of the circuit needs to be maintained.

1. Look at the wiring diagram for your make/mode of room thermostat that you are removing. If you don't have the instructions for it, a diagram is usually printed inside the lid. This will show which are the two switching wires, and which is the neutral wire. (If there is a neutral present. Not all thermostats need or have one fitted) unfortunately you can't rely on the wire colours.

2. Trace the thermostat cable back to its origin.

3. Take take a photo, or make a note of where the switching wires are connected at their origin.

4. Disconnect all of the wires going to the old thermostat.

5. Insert a wire link between the terminals where the two switching wires have just been removed from, so that they are now electrically joined together.

Only once the old thermostat cable has been completely disconnected from its connections to the 230V mains circuits can it be used to connect T1 and T2 at the Heat link to T1 and T2 and the Nest thermostat. But you will probably find that the existing thermostat cable runs from the thermostat to a wiring centre and not the programmer. If so it would need to be rerouted to the Heat link. Often this can involve more work than running a new cable between the Nest thermostat and its Heat link.

 

[Anton87]

Yes, sounds like the heatlink needs to be installed upstairs by the boiler and pump. I think I will have to pay for someone to come in and do it now as its beginning to get a bit complicated.

 

[stem]

Probably a wise move if you don't feel confident to DIY

 

[Terrywookfit]

The Channelplus is a 3 channel programmer which provides control of two separate heating zones and stored hot water (i.e. a hot water cylinder) The Nest only controls one heating zone and hot water. So you would need two Nest's to replace it like for like.

1. One Nest for one heating zone and the hot water (if utilised) and
2. One nest for the second heating zone.

View attachment 250905

It's difficult to see the wires properly in your photo, but some of the wire connections appear somewhat unconventional. There seems to be a link between terminals L, 2 and 5 and it's unusual to have links between L and any of the other terminals (eg L, 2, 5) and also to have connections to the CH off. (eg terminal 4)

You don't give any details about the heating system that the Channelplus is connected to, so to give any further information would involve some guesswork.

The H37xl has volt free contacts.