Horstmann H37XL Channelplus Electronic Central Heating Programmer Series 2 - 3 channel, 7 day

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I am wondering if the stand-off comes away from the main unit. I can pull the main unit away from the stand-off enough to get a screwdriver into the gap but it's not coming away consistently - only where I exert pressure so much that the plastic deforms.

This will also be the case with the "similar Horstmann programmer" in your last post. The reason common terminals are provided outside of the unit, rather than connected to 230v inside, is so that they can be used to switch other voltages (some boilers use 24v) if required]Finally at least one radiator in the room where the Nest Thermostat is located should not be fitted with a TRV, otherwise it could interfere with the operation of the Nest and prevent it working properly. More manuals of Thermostats Frequently Asked Questions about Horstmann ChannelPlus H37XL Thermostat By pulling outward and generally downward I can clearly see the single wall box because I am dragging that towards me too. It is not separating from the main unit or the stand-off. Finally on the programmer what would be the point in providing term 3 to be live when the heating was off.

The reason I say this is because I connected the blue wire to this as your original instruction and the Nest never worked. I wondered if this only went live when the desired temperature was reached and it received a signal from the thermostat?You will have to link the terminals L, and the two 'commons' 2 & 5 together at the Heatlink. [This link is made internally in the Horstmann] What I really want to do is to install a NEST in place of the two thermostats, and a single timer for the water but I don't yet understand what wiring my system employs and how I might convert that for NEST. 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.

Of course the Nest will not only switch off when the time goes off, but also switch off when the correct temperature is reached, but the wiring doesn't know that, as far as it is concerned it's just going off.

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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. I also don't really understand how each of the two-way valves operate - whether the timer "job shares" between the two zones and whether two NEST thermostats can replace that functionality. Or if each thermostat demands heat and the timer is on for each zone, both valves just open and they get served with hot water equally.