Boiler thermostat wiring help
Discussion
I've got an ageing Vaillant VCW GB 221 boiler with a Danfoss 103 time switch. I want to add a new thermostat/timer. At present the time switch has the neutral and live feeds and a single black wire which presumably controls the boilers operation. The boilers wiring diagram shows three terminals for a thermostat:
From a quick browse in Screwfix two wire thermostats seem to be the norm, with only a handful stating two or three wire compatibility. Do I need to find a three wire thermostat? Or would my boiler be compatible with a two wire model?
From a quick browse in Screwfix two wire thermostats seem to be the norm, with only a handful stating two or three wire compatibility. Do I need to find a three wire thermostat? Or would my boiler be compatible with a two wire model?
Edited by Accelebrate on Tuesday 7th September 10:03
You can attach a two-wire thermostat across terminals 3 and 4
cut and paste from another forum said:
A standard room stat is just a switch but there is generally an extra bit in there. It's a small resistor tucked in somewhere near the bi-metal strip and it's job is to make the thing cycle on and off. Here's how:
The room is too cold so the switch closes. The heating comes on. That resistor is connected between the load side of the thermostat and neutral so it heats up the bi-metal strip. The switch opens. The resistor cools down and so does the bi-metal strip. The switch closes. And so on.
This cycling on and off happens slowly. The proportion of closed time to open time varies with room temperature and so you get smoother control. It's a form of pulse width modulation.
With no neutral connection you have nowhere to connect that resistor. If you leave it floating, the stat will still work but your room temperature will swing up and down, possibly more than you would like. Note that this will also happen if you wire a stat in the wrong way round with the resistor on the supply side.
There may be stats that can do the job but I've never seen one. Maybe somebody else on this forum can help.
The room is too cold so the switch closes. The heating comes on. That resistor is connected between the load side of the thermostat and neutral so it heats up the bi-metal strip. The switch opens. The resistor cools down and so does the bi-metal strip. The switch closes. And so on.
This cycling on and off happens slowly. The proportion of closed time to open time varies with room temperature and so you get smoother control. It's a form of pulse width modulation.
With no neutral connection you have nowhere to connect that resistor. If you leave it floating, the stat will still work but your room temperature will swing up and down, possibly more than you would like. Note that this will also happen if you wire a stat in the wrong way round with the resistor on the supply side.
There may be stats that can do the job but I've never seen one. Maybe somebody else on this forum can help.
Edited by rsv gone! on Tuesday 7th September 10:07
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