Manual Override Switch Conundrum

Switching between Manual (Override) and Automatic (Programmer) Control of Central Heating from Two Locations, Two-Way-Light-Switching-Style

I have now solved this; read on.

I have a combi boiler, providing hot water for the taps as well as the central heating radiators, installed in my home. The boiler has a programmer, a time switch, which can be set to turn the central heating on and off at various times each day. However, because I don't wake up and get up at the same time each day, this is not ideal as it means either the house is cold if I get up earlier than the time the programmer is set to fire the boiler, or it wastes gas by coming on while I am still asleep in bed. I also don't go to bed at the same time each night, so there's a similar problem there.

My initial proposal to solve this was to have a simple on-off switch, just like a normal light switch, by my bed so that when I wake up I can simply reach over to it and switch on the central heating and then remain in bed for the five-to-ten minutes it takes for the house to warm up and then get up.

However, wiring such a switch to the boiler presents a couple of problems:

• wiring the switch in series with the programmer means flicking the bedside switch to 'off' would turn off the boiler no matter whether the programmer was calling for heat or not; ie: at and during the time the programmer had been set to fire the boiler at. But flicking the switch to 'on' would only fire the boiler if the programmer was calling for heat;
• wiring the switch in parallel with the programmer means the programmer would fire the boiler regardless of whether the bedside switch was 'on' or 'off'; only useful when heat is wanted but the programmer is not calling for heat.

The solution was in the form of using an ON-OFF-ON, single-pole, double-throw (SPDT) switch. When the switch toggle is in the up position, power bypasses the programmer, just like using an on-off, normal, light switch wired in parallel with it; however, when the toggle is in the middle (centre) position, the boiler is switched off, regardless of whether the programmer is calling for heat or not, and when the switch is in the down position, control of the boiler firing is returned to the programmer. It's wired as follows:

Great! But then I decided to get a little bit more sophisticated. How about a second switch in another location with the same functionality? Dual control, just like with two-way lighting switching. Well, I didn't think that could be done using two ON-OFF-ON, SPDT switches but it could be done using two pairs of such switches, a pair at each of the two locations. One switch at one location and its counterpart at the other location bypasses the programmer; the other switches switch the programmer in- and out-of-circuit, allowing control of the boiler by the programmer. As the switches operate just like two-way-lighting switches—which, after all, is what they are— there is no way of telling just by looking at any of the switches whether the boiler is under bypass control or programmer control; to solve that, status indicator lamps would need to be provided at the switches. Wired as follows, without consideration, and therefore without inclusion, of the lamps:

But this arrangement is not ldeal: at either of the two locations: for example, flicking the bypass switch 'off' only turns off the boiler if the switch for the programmer is 'off', otherwise the programmer will be in-circuit and thus will fire the boiler if it's time for it to do so according to its time settings; so attention needs to paid by the person operating these switches. I then realised that I could use just two ON-OFF-ON, SPDT switches, one at each of the two locations, wiring for each a little different from the other. Here's the circuit diagram, thermostat not included as it's not relevant:

However, there is no way to wire in status indicator lamps to reliably indicate whether the boiler is under manual or programmer control, as well as the problem of they themselves being a bypass.

I finally solved all these problems, which is by using two ON-OFF-ON, four-pole, double-throw (4PDT) switches, one at each location. As before, there's no way of telling by just looking at the switches whether the boiler is under manual or programmer control, but that is determinable from the status indicator lamps at either switch as they are on their own dedicated poles thus circuits. Here's the circuit diagram:

And here's the wiring diagram: