I have replaced an old programmer with a newer model.
From programmer (downstairs in pantry) to junction box (upstairs under floorboards) is a cable, 3+E.
Cable is:
Earth (bare)
Neutral
2 x switched live.
For the new programmer to work correctly I need one more switched live.
It would be a major headache to run a new cable.
Is there anything I can do using existing cable?
Example:
Take Earth from another under floor cable to free up a core (Earth core is bare so probably dangerous - it wouldn't be double insulated, only the outer grey sleeve).
Take a Neutral from another under floor cable to free up a core (is it safe to have live & neutral feeds in different cables?)
From programmer (downstairs in pantry) to junction box (upstairs under floorboards) is a cable, 3+E.
Cable is:
Earth (bare)
Neutral
2 x switched live.
For the new programmer to work correctly I need one more switched live.
It would be a major headache to run a new cable.
Is there anything I can do using existing cable?
Example:
Take Earth from another under floor cable to free up a core (Earth core is bare so probably dangerous - it wouldn't be double insulated, only the outer grey sleeve).
Take a Neutral from another under floor cable to free up a core (is it safe to have live & neutral feeds in different cables?)
It all sounds 'very non regulations' to me 
and only you know the full set up and what everything does.
Does the programmer get its mains supply via this cable or is this cable purely an 'output' feed. From how it reads it is just an output. So what is neutral doing? is that a common neutral for the two switched live feeds load return?
Hazy thinking, you would have to draw it out, but could the neutral be used and any load returns connect to another neutral near the load.
All just hypothetical without seeing how it all connects and as always, safety has to be paramount and I'm sure it breaks every rule in the book
and only you know the full set up and what everything does. Does the programmer get its mains supply via this cable or is this cable purely an 'output' feed. From how it reads it is just an output. So what is neutral doing? is that a common neutral for the two switched live feeds load return?
Hazy thinking, you would have to draw it out, but could the neutral be used and any load returns connect to another neutral near the load.
All just hypothetical without seeing how it all connects and as always, safety has to be paramount and I'm sure it breaks every rule in the book
Second Mooly's comments. I think you would have to explain and ideally draw out what cables go where. The programmer provides line voltage to heating components, not low voltage? Which cables are the connection to mains and which cables are "inside" the heating system? If you were to fully isolate an already existing cable from the mains and use it for the heating system that would likely be safe.
When I replaced my heating controller I had a similar issue. I used a wireless solution from my local plumbing merchant that had a suitable relay to switch the mains. It has been working well for years at this point and requires no strange practices wrt to wiring codes.
Could that work for you?
Could that work for you?
No idea if this applies, but I replaced my down unit recently. Not enough control wires. I found a TSTat called honeywell prestige IAQ which uses radio to communicate with a base unit mounted in the air handler. The TStat only needs 24VAC for power. It is pricey relative to other types but not crazy expensive and much less than running new wires. So you may be able to find some TSTat that either communicates over radio. I stumbled on the prestige, which is not carried at big box stores.
You can all stop worrying.
I am not going to inflict any dodgy wiring on my Mum.
The old controller was a simple timer and when active the boiler would heat HW, CH or both depending on the thermostats.
The new timer allows separate timings for CH & HW but is jumper set to both same timings to replicate the old timer.
The extra wire allows CH only.
The only issue without it is if the 'Boost' button is pressed for 1 hour CH.
It is inoperative unless HW is also pressed.
It has operated like this for a couple of years.
I am tempted to add the extra wire and have figured I can remove a piece of plaster coving in the kitchen.
The kitchen is overdue for a redecoration anyway.
This would allow a wire to be dropped from under the upstairs floorboards, routed along behind the coving and into the pantry.
I would use a new length of 5 core cable direct from the programmer to the under floor junction box.
Incidentally, the old timer was electro-mechanical, 24hr dial with movable arms for on/off and easily understood by Mum.
The new timer is digital, programmed by endless button presses, timings cannot be seen without entering 'settings' and is a complete pain to program.
I am not going to inflict any dodgy wiring on my Mum.
The old controller was a simple timer and when active the boiler would heat HW, CH or both depending on the thermostats.
The new timer allows separate timings for CH & HW but is jumper set to both same timings to replicate the old timer.
The extra wire allows CH only.
The only issue without it is if the 'Boost' button is pressed for 1 hour CH.
It is inoperative unless HW is also pressed.
It has operated like this for a couple of years.
I am tempted to add the extra wire and have figured I can remove a piece of plaster coving in the kitchen.
The kitchen is overdue for a redecoration anyway.
This would allow a wire to be dropped from under the upstairs floorboards, routed along behind the coving and into the pantry.
I would use a new length of 5 core cable direct from the programmer to the under floor junction box.
Incidentally, the old timer was electro-mechanical, 24hr dial with movable arms for on/off and easily understood by Mum.
The new timer is digital, programmed by endless button presses, timings cannot be seen without entering 'settings' and is a complete pain to program.
Tekmar is one of the companies that makes these
https://www.watts.com/our-story/brands/invita-wifi-thermostat-564
Attachments
I have a Nest because the house I bought came with one. Frankly, the thermostat I designed for my dad back in the early 1990s was smarter and had more useful features. Specifically I would have liked the Nest to have better graphing capabilities to allow for better tracking of energy use. It would also be nice if the inside temperature could be (weakly) dependent on the outdoor temperature to take the edge off on really cold days. It would be nice if multiple schedules could be stored.
For a "smart" thermostat it is remarkably dumb. It doesn't seem very smart to turn the heat on five minutes before the schedule says to turn the heat off for the night, for example.
The learning feature is basically useless for those of us who work from home. I just set a schedule and have the thermostat stick to it.
That said, it is a nice looking thermostat that's pretty easy to use. I just wish it was a bit smarter. I do like that I can program each day individually. That's often not the case with programmable thermostats.
Tom
For a "smart" thermostat it is remarkably dumb. It doesn't seem very smart to turn the heat on five minutes before the schedule says to turn the heat off for the night, for example.
The learning feature is basically useless for those of us who work from home. I just set a schedule and have the thermostat stick to it.
That said, it is a nice looking thermostat that's pretty easy to use. I just wish it was a bit smarter. I do like that I can program each day individually. That's often not the case with programmable thermostats.
Tom
The common "Add-a-wire" gadgets use a pair of diodes to steer positive and negative 24VAC pulses to control two relays over a single pair of wires. I expect that the furnace box has DC relays that then switch the furnace AC relays. However there do exist 2-wire to 4 and 5 wire adapters. I installed one for my daughter and the part that attaches to the thermostat had positive and negative diode circuits for heat and AC plus a resistor plus diode circuit for the fan (w/o heat or AC). You could make your own, but the commercial units come in nice boxes with appropriate color-coded leads, and code certification. I remember they came from a HVAC web site, which may be the second link below. Here in North America, all HVAC use a standards 24VAC transformer for control circuits. I'm not sure what you have there?
https://www.supplyhouse.com/Venstar...ping&msclkid=173503d987e51696aeea658bc228c2e0
https://www.supplyhouse.com/Fast-Stat-Wiring-Extenders-29621000
https://www.supplyhouse.com/Venstar...ping&msclkid=173503d987e51696aeea658bc228c2e0
https://www.supplyhouse.com/Fast-Stat-Wiring-Extenders-29621000