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I want to have a switch on the wall by my bed that is wired to my combi boiler so that I can switch the central heating on without having to get up and go to the boiler if I wake up earlier than the switch-on time of the boiler's programmer. I have drawn a diagram of the initial intended set-up. The switched line from the programmer will go via my intended SPDT switch rather than be directly connected to the switched line screw terminal of the boiler and there is a link from the boiler's line terminal to it as well. Other than that, everything remains as professionally installed. This means I will be able to switch the central heating on and off regardless of whether the programmer is calling for heat or not, or leave things to the programmer and its time settings, all at the flick of my switch. However, I then thought how much better it would be if there was an additional switch that could duplicate these functions in a similar way to how two two-way light switches can independently control a lamp. So, for example, if I were to flick the bedroom switch to 'manual on', the heating would come on but I could turn it off again not only by the same switch but also by the one at the boiler. In other words, no matter which of the two switches I flicked and regardless of what either had been set to, it would change the state of the system to my choice. Another thing is that there would have to be two indicator lamps at each switch so that I could see at either one whether the boiler was currently under manual control, off or automatic. But I can't see a way to do it. Hope this all makes sense.

Diagram of initial intended modification to combi boiler central heating control. The switched line from the programmer to go to one ON terminal of my intended ON-OFF-ON SPDT switch rather than be directly connected to the switched line screw terminal of the boiler, a link added from the boiler's line terminal to the other ON terminal of the switch and the COMMON terminal of the switch wired to the switched line screw terminal of the boiler.

An alternative would be to have two pairs of two-way switches. But this is a lot of wiring. Again, indicator lights would be needed.

Two pairs of two-way switches to control central heating.

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    \$\begingroup\$ your question is not really about electronics design ... it is a better fit at diy.stackexchange.com/questions \$\endgroup\$
    – jsotola
    Mar 18, 2023 at 0:01
  • \$\begingroup\$ @jsotola It's actually about the circuit logic. In the first diagram, when the switch's toggle is in the up position, the programmer is bypassed, thus the boiler fires. The toggle's centre position disconnects the boiler, so the boiler is off (not firing). The toggle's down position puts the programmer in-circuit, thus that is then controlling the boiler. However, I now want an additional switch that duplicates the first's functionality, similar to how two-way light switching works, provisioning identical control from two separate locations. \$\endgroup\$
    – user599851
    Apr 6, 2023 at 4:29
  • \$\begingroup\$ what may be doable from the perspective of electronics design, might be against electrical code in use \$\endgroup\$
    – jsotola
    Apr 6, 2023 at 5:54

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So simple. An ON-OFF-ON DPDT in each of the two rooms, the switches wired in reverse of each other. Thus, two-way-light-style switching that also switches between two circuits. Only thing now is to include a pair of indicator lamps to each switch so it can be seen whether the boiler is under manual control (override) or auto control (programmer controlled) or off.

enter image description here

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Including status indicator lamps at each switch requires an upgrade to two ON-OFF-ON 4PDT switches:

Circuit schematic of upgrade to two ON-OFF-ON 4PDT switches.

How to wire up:

Wiring schematic of upgrade to two ON-OFF-ON 4PDT switches.

That's two five-core flexes (no earth) between switches (or one of those and a five-core-and-earth)!

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