Home heating automation

So if your system is as I've described, there is a solution. Systems where the HW always heats, but CH only heats when either a pump or a motorised valve is energised are often referred to as ' 'Gravity' fed. The earliest systems with back boilers or stoves would have an open and rising connection to the HW cylinder, so it would heat as soon as the boiler warmed up. Hot flow to the radiators would require a pump, so by putting the pump on a wall thermostat, you can switch CH in and out. A single timer was often used to fire the boiler. When dual zone timers became popular, it was obvious that two timer outputs, one for CH and one for HW, would not necessarily work when there weren't two seperate zone valves to switch the flow, or even one in a true gravity system.

Many of these dual timers had a mode switch to enable gravity mode. In this mode, you would set both timers, CH and HW. Only the HW timer was connected to fire the boiler, but internally, the controller turned on the HW live out for either CH or HW timed events, so the boiler fired for either. The CH output also went live for CH events only, and this was connected to the CH pump, or zone valve in a pumped boiler system. This way, you could set timed events for either, with the provisio that HW would heat for either timer being on, but CH would only heat during a timed HW event.

Now wiring in a dual timer with a gravity mode swith would require a minor rewiring of a legacy system like yours. The HW output would go to fire the boiler, the CH output would go to the old Robus stat to operate the zone valve or pump. The Nest learning Gen 3 is a dual output system, CH and HW. It doesn't have gravity mode though. The HW relay closes for HW events, the CH ditto.

There is however, a way of wiring the two pole relays to create gravity mode. This effectively sends live to the boiler for either CH/HW events, and live to the pump/zone valve for CH events only. I devised this diagram many years ago for a gravity style system like I suspect yours is. The Nest receiver is connected to the boiler by a slightly more complicated setof linked wires, and the CH live is sent to the old stat and from thee to what I've assumed is a CH zone valve. As the Nest room Thermostat now controls the CH demand, the old Robus is left turned up permanently, or removed and the live in and out bridged. You will need to get someone competent to sort out the wiring. The wiring of a two output controller to operate a gravity style system is not well know among plumbers/electricians, though I've seen since posts where others have figured this wiring arrangement for themselves. Have a look, it's diy doable by a person competent around wiring switches and fixed accessories. My diagram shows a pump, in your case this likely to be a motorised zone valve, but you shouldn't have to connect directly to this, just to the live that is currently being sent to the robus, which is probably already linked to the boiler live in the Nest receiver.

I'm assuming there is a valve, as the boiler contains a pump, which would circulate hot flow to the radiators as well as the cylinder, unless closed by a valve. The fact you can heat HW only by switching off the Robus indicates that you are stopping the hot flow to the rads but not to the cylinder. Was there a timer with the old boiler, perhaps it was part of it. This is a very common old setup. Timer to the boiler for everything, the a stat to operate the valve for ch. It's very likely that the live coming into the rebus is the live from the Nest to the boiler. If you have a phase tested or meter, you can check if live into the Robus goes on and off with the live from the Nest turning on the boiler. You mentioned a back boiler. Did this gas boiler replace another gas boiler or is this your first, and did/do you have a solid fuel back boiler also? If so, how are they operated together. It's really poor if the new boiler and Nest went in together, and no attempt was made to wire it to suit the fact that the Nest had the ability to control zones. The Nest is ideally designed to heat two zones using two motorised valves, CH and HW, but the presence of a back boiler complicates things, as generally you would have to leave the HW circuit always open without a valve if a back boiler was lit, and the cylinder didn't have seperate coils for heating HW by the gas boiler and the back boiler. It doesn't look great under there, you'll have to dig deeper to see what the Robus is controlling.

Not particularly, but you may already have it somewhere, perhaps in the hot press with the cylinder. How else can you get HW only heating. The biggest challenge to fitting zone valves in not the plumbing, it's the wiring from the thermostat receiver to the valves, and in the case of a two valve system, the relays built into the valves have to be utilised to fire the boiler, so each valve has to jointly send a live to the boiler to fire from these relays. This can often mean adding extra cables from boiler location to valve location to hotpess, which is quite a task in a two story house, not so bad in a bungalow.

Yes, from hot press to either Boiler directly, or HW zone valve if you have a zoned system. The HW zone valve then fires the boiler. If you have HW timing on your system receiver or controller, then the HW switched Live from this is connected to C on TR2, not the permanent live link from the power supply live to TR2 as shown in the diagram.

Sorry, I had to look back and find all the posts to have a quick scan/read. If I'm correct, you got two zone valves installed for independent HW and CH, and a Tado wireless stat with extension kit to switch the zone valves on using the app. What you didn't get was the last piece of wiring from the zone valves' relays to the boiler switched live in, so that either zone valve would fire the boiler. The TR21/TR2 kit removes the need for having to conduit cables from the zone valves location in the hotpress to the boiler. So firstly, ignore all my comments in the previous reply, I'd no recollection of your particular system till I reread it, so I was barking up a different tree.

So, you need to send the call for boiler from the joint outputs of the zone valve relays to the boiler switched Live input. Hopefully, these relay outputs are terminated in a wiring box next to the zone valves. The relay outputs are volt free, and the Common and Normally open wires are the Orange and Grey wires in the zone valve cables. Both Orange wires should have permanent live connected, and both Grey wires are joined, and connected to the Lin of TR1. Either valve will then trigger TR1 when the HW or CH look for heat. TR2 is connected to the boiler in place of the old Timer, as per the TR2 Diagram. This is the final step, any timed event on the Smart stat, either for HW or CH, will open their respective zone valve and cause the boiler to fire. Forget my previous comments, I'd forgotten the details, mixing them up with something else.

In the Settings, Does HW list under Rooms and Devices. What controller is showing for the HW. I assume it should be the ext kit. I don't have an extension kit for comparison, but each 'Room' or zone is associated with the device which fires the boiler on its behalf, and can also have a measuring device if temperature control is involved. E.g, a TRV may use its own temperature reading, or the main stat or a wall sensor stat, and it's controller to fire the boiler will be the wired stat or wireless/extension kit relay

Room/device list

And settings for a Room/device

Generic S plan 2 Zone wiring, with cylinder stat.