An Example circuit diagram is shown below. This diagrams is schematic and should be used as a guide only. Finally on the programmer what would be the point in providing term 3 to be live when the heating was off.

Designed for use with fully pumped central heating systems, using three separate two-port zone valves. 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. 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]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. 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. PLEASE NOTE - Using this procedure will restore the ChannelPlus to the original factory programme settings, the time setting will also default to 1st January 2005. Lower the front flap of the unit. On third Channel press the ADVANCE(Enter) and SELECT(Blue) buttons together then release the buttons and the programmer will return to the preset factory settings. More manuals of Thermostats Frequently Asked Questions about Horstmann ChannelPlus H37XL Thermostat

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. 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.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? Operating periods per day: Three for Heating 1, Heating 2 and Water (separate daily programme for each Channel) Then, once the common at the Heatlink is connected, both devices will function identically to each other. INSTALLATION AND CONNECTION SHOULD ONLY BE CARRIED OUT BY A SUITABLY QUALIFIED PERSON AND IN ACCORDANCE WITH THE CURRENT EDITION OF THE IEE WIRING REGULATIONS.