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Is my wire too thin? Moving room thermostat for central heating from one room to another using thin wire

Postby tkmax » Fri Jan 15, 2021 4:12 pm

We have recently moved into quite an old house and the thermostat for the central heating is in a very bad place. I have moved it from the hallway to the sitting room where we spend most of our time but I have done it using thin single strand copper wire (probably about 1/2 mm Diameter). I have just twisted the wires to each com lead coming out of the wall where the thermostat used to be, whipped it with electrical tape and passed the wire through an air vent into the wall into the sitting room and reattached the thermostat to the wires in the sitting room. It now works just fine but I was just a little concerned that the wire I've used may be too thin? Am I right in thinking that the wires going to the thermostat are just com wires anyway so its not actually carrying much amperage so doesn't matter?

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Postby Mr White » Fri Jan 15, 2021 8:36 pm

You did the right thing by asking.
In your instance there will be little current flowing when things are working, the problem arises when there is a fault, the wires will be too thin to carry the fault current and overheat and cause a fire.
Also insecure joints (such as you have made) can and do become high resistance, again causing overheating at the joint.
Another problem is that "singles" only have 1 layer of insulation, and as such can easily be damaged.
Cables should always be clipped and never left like "a washing line" as they can become an entanglement hazard or if lower down a trip hazard.
The two cables also do not have earth protection, the idea being if the cable is cut one of the cables will be the earth, so it too is cut the current can flow to earth (with a bang) The earth cable is connected to any metallic part to earth said part. (It has to be connected to earth at the supply end too)

And finally for now, the sitting room is the worst place to put a thermostat, as that means the rest of the house will get cold, because the thermostat says it is warm.

I would suggest you put the thermostat back where it was and turn it up slightly and turn the sitting room radiators down.
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Postby ericmark » Fri Jan 15, 2021 11:17 pm

If you have oil or solid fuel central heating I can see why you want to move thermostat, however Mr White is correct, there could be danger in what you have done.

There are two reasons for the wall thermostat.
1) Control room temperature.
2) Switch off boiler when not required.
There are 230 volt thermostats and 24 volt thermostats and 0 - 40 volt thermostats. Also likely others. But the modern gas boiler has two ways it can be controlled.

1) Thermostat connected to the ebus for example OpenTherm in which case wall thermostat is either really a hub or on wall of main room.
2) Return water temperature, each room is controlled by a thermostatic radiator valve (TRV) and the boiler modulates (turns down) based on how much hot water has by-passed the system through the by-pass valve.

In both cases the idea is the boiler adjusts output to suit demand.

However with the water based there is a problem, once the boiler turns off, it does not know when to turn back on again, algorithms can help, but really it needs some thing to tell boiler it is a warm day turn off.

So we use a on/off thermostat, on the ground floor, in a room kept cool so it will not fire boiler if likely to be a warm day, with no alternative heating, and no outside door.

However the room rarely exists, so a compromise is called for, often this is the hall, but you want hall to reheat fast when outside door is opened, but not to turn off wall thermostat before rest of house is warm, so the hall lock shield valve and TRV settings are critical to rest of house staying warm.

Note with a non modulating boiler this is not the same, I am talking about a gas boiler deigned to extract latent heat.

So a TRV set to around 3.25 will start closing at 17.5°C and be fully closed at 19.5°C they are not that accurate, but in the hall that works out well, so open from door and it reheats rapid to 17.5°C then slows down, so a wall thermostat set to 19°C will only switch off on days when boiler is not required, but will allow boiler to modulate on cooler days.

Although the TRV does have a wide range, in the main we set the lock shield valve to 20°C differential in pipe to out pipe, so the TRV is only correcting the flow and will work well, except in rooms with alternative heating, the main problem is the sun, with South facing windows morning sun can cause rooms to heat up well over the setting of the TRV because they are so slow, but fit an electronic head, and instead of setting to 3.5 or other random number you set to 20°C.

In the hall we can also use electronic TRV heads, but depends on distance radiator to wall thermostat, and hight of wall thermostat, in this house the TRV records around 3°C lower than wall thermostat in Winter, but the same in Summer. One has to allow for that, but using programmable wall thermostat and programmable TRV I can still set the hall temperature to run boiler as required for the other rooms.

My problem is hall cools too slowly, so any over shoot and other rooms get cold, so I set the thermostat to raise 0.5°C per hour in the morning so hall heats up gradually, but rest of rooms controlled by their out programmable TRV so they heat up fast.

I have upper bathroom not controlled, radiator always on, then 9 programmable TRV heads and 5 standard TRV heads, two wall thermostats one in the hall and one in the flat kitchen, so 12 rooms, 4 not used, so 8 rooms controlled with programmable TRV heads and one wall thermostat, and in the main it works, there is no need to have wall thermostat in main room in most houses, that's what the TRV is for.
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