WHAT IS COMMON ON A WIRING DIAGRAM

[bonedome22]

hi all

hoping to fit a volt free battery operated/powered room thermostat called a drayton digistat 3 and just want to confim what common is on the wiring diagram, is it another term for earth the stat is designed for fixed wiring only, what does this mean assume another term for none mains

there are 3 wire connections in total if i am reading the instructions correctly
1 is common
2 is heating satisfied
3 is call for heat

2&3 as i understand it are straight to the boiler pcb (a link)

would appreciate any feedback and sorry if this is in the wrong forum but thought i would leave the guys on the plumbing and heating forum alone for now, they have been really helpful

 

[HDRW]

hi all

hoping to fit a volt free battery operated/powered room thermostat called a drayton digistat 3 and just want to confim what common is on the wiring diagram, is it another term for earth the stat is designed for fixed wiring only, what does this mean assume another term for none mains

there are 3 wire connections in total if i am reading the instructions correctly
1 is common
2 is heating satisfied
3 is call for heat

2&3 as i understand it are straight to the boiler pcb (a link)

would appreciate any feedback and sorry if this is in the wrong forum but thought i would leave the guys on the plumbing and heating forum alone for now, they have been really helpful

"Volt free" means that it's a switch with no connection to anything except the terminals (1,2,3 in your case). Earth has no meaning in this situation, unless the thermostat is metal and needs to be earthed for safety, but that's nothing to do with the operation of the 'stat. I bet the 'stat body is plastic, so no earth would be needed.

Fixed wiring means it's fixed to the house - you can't use the thermostat with flex to control (for example) a portable electric heater.

As for the connections... the 'stat operates as a changeover switch - in the "too cold" condition the Common is connected to "Call for heat" (in your case 1 is connected to 3, and 2 is disconnected).

When the temperature rises above the level you've set, the switch changes over and connects Common to "Heating Satisfied" (1 is connected to 2, 3 is disconnected).

Since 2 and 3 are never connected together, connecting only those to the boiler would not achieve anything - Common needs to go there too. I'm not sure what you mean by "a link" on the boiler PCB? Are there not a pair of screw terminals designed to connect to the thermostat?

If your boiler wants a contact to be made (connected) when heat is needed, connect 1 and 3 to the boiler, leave 2 alone.

Cheers,

Howard

 

[bonedome22]

hi howard,

thanks for your response. the body is plastic as you rightfully say therefore no earth required. so as i understand it the only wires needed are to the common and call for heat, 1 & 3 and that is it job done with the boiler switching the contacts over when no more heat is required effectively a two wire set up only

the link i refer to is in reference to the piece of wire connected at the two screw terminals. this is in place when no stat is connected. i assumed that wiring the stat to the boiler had the same affect (linking power to & from the two terminals) i apologies for the mis-understanding.

many thanks indeed