I'm wondering this due to the wiring indication that came with the valve, and also the number of wires in the connecting box. 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. 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] Are you saying that with the heating off the the heat satisfied also has a voltage, in the way terminal 3 has on the Horstmann. I am assuming here that you are referring to a 'Secondary Hot Water Circulator' that circulates the domestic hot water around a ring main so that hot water is available instantly at each outlet, then the answer is yes you will need to move that to another timer. This is the first time I have come across this in a domestic property, but can understand why if you have an unusually long pipe run.

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?Unfortunately, I don't know anything about the valve having never seen your particular type. But electrically speaking, the Channelplus and the Hive are just switches and are identical in their operation, so if your heating works with one, then there is no reason why it wouldn't work with the other. 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. 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]

In reality, the blue wire may not be actually doing anything, but only the original installer would know. However as I said earlier, you are simply exchanging one electrical switch for another and the corresponding terminal at the Heatlink would be (1) Heating Satisfied. Finally on the programmer what would be the point in providing term 3 to be live when the heating was off. Tighten the two 'captive' retaining screws to fix the unit securely, then switch on the mains supply. INITIAL SET-UP I have only ever once come across a system with a 'central heating off' connection that contained a very uncommon type of motorised valve (MoMo, or Motor on, Motor off) that requires this connection to close it.

Manual

I would be tempted to try and trace it to see what it's connected to, for my own curiosity as much as anything, or disconnect it and see if anything stops working.