Installing a central heating thermostat

[Driller]

Any heating engineers about?

In their wisdom d the builders redoing my workplace ripped out the basic dial type thermostat from the gas heating. It looks like it was hooked up to the circulation pump which they've closed the circuit of.

Looking into things I see there are two ways of connecting a thermostat: either to the circulation pump or to a pair of purpose built contacts.

I have the wiring diagram for the heater and it shows the two contacts and says to remove the bridging cable in place to connect a thermostat.

This cable is still in place which confirms that the previous TS was connected to the circulation pump.

Sorry for the long post but what's best: connect a new TS to the pump or go for these ready made contacts?

[Ferg]

Need to know more about the system, really. Any motorised valves?

[Driller]

Hi Ferg thanks for the reply, no motorised valves I can find. The system is from 1988 if that helps.

BTW the boiler is a separate electric one so not involved.

[Ferg]

Driller said:

BTW the boiler is a separate electric one so not involved.

Cylinder?

[Driller]

Ferg said:

confused: Cylinder?

Yes, cylinder, sorry too much time in France!

[Ferg]

Either would do, I'd say.

[Driller]

OK thanks. NOw you come to mention it, what I thought was a pump may be a motorised valve.

It's positioned away from the main unit on the exit pipe with a mains cable going to it.

If the thermostat was connected to this would it make sense?

[Ferg]

Yes. Is it two port...or three?

[Driller]

Ah hah! Two port or three...I could take a photo. There appears to be a three position slide switch on top of it.

Otherwise it has a pipe going into it from the furnace and a pipe exiting it which feeds the rads.

[Ferg]

Two port.
The valve will be opened by supplying power to the brown wire to the valve. this is where the room thermostat would fit.

[Driller]

Thanks very much Ferg.

That all makes sense especially as the cable from said valve goes into a junction box and I can see that the builders have simply connected this directly to a new power cable they installed which also aliments the furnace.

Thermostat will go here.

Cheers!

[Ferg]

The two port valve would normally have five wires:

Earth
Blue - Neutral to motor
Brown - Live to motor

Grey & Orange - These are across a switch which is closed when the valve opens. Normally you would make Grey a permanent live and Orange would feed that live into the boiler to fire it.

[Driller]

Ferg said:

The two port valve would normally have five wires:

Earth
Blue - Neutral to motor
Brown - Live to motor

Grey & Orange - These are across a switch which is closed when the valve opens. Normally you would make Grey a permanent live and Orange would feed that live into the boiler to fire it.

Well there is only neutral, live and earth on the valve but there is a second cable running from the furnace (alongside the main power cable which has two wires in it. I'll go and have a closer look.

[Ferg]

Oh right. Just open it with the brown then.

[Driller]

OK, you got me there

What does "open it with the brown" mean? I was thinking all in series, star style, thermostat, valve and the two cores to the furnace?

Sorry, it's like pulling teeth isn't it and I should know

[Ferg]

Yeah, sorry. Live from the supply (via a timer?) to the stat, then from there to the brown wire. Fire the burner with live from a similar source, I suppose. Thing is....if the valve is closed, but the burner fires, what happens? It has a vent and cold feed? I'm assuming the thermostat on the heater will cut it out.

[vdp1]

Driller said:

Ah hah! Two port or three...I could take a photo. There appears to be a three position slide switch on top of it.

Otherwise it has a pipe going into it from the furnace and a pipe exiting it which feeds the rads.

Sounds more like a pump to me, is it round or square.

[Driller]

I'm going to take a photo tomorrow and I'll post it up and a wiring diagram if you guys don't mind checking back.

Ferg, thanks again for staying the course hope to see you back, vdp1 too.

[Driller]

Right couldn't sleep so went to check: pretty sure it's a pump in fact, it's rounded and it's connected two a three core cable in a junction box coming from the furnace.

There is a constant whirring noise coming from it so it seems to be on all the time.

The furnace also has a separate live feed from the mains.

What I don't understand is that in the furnace instructions it says to connect the thermostat to connections 1 and 20 on the furnace and remove the bridging connector but this is not how it was done: the bridging connector is still in place and it would seem that the old thermostat was connected to the circulation pump.

Edited by Driller on Monday 14th January 03:42

[vdp1]

Right, I'm guessing here and it sounds like a gas fired cylinder, the stay would control the gas valve and the pump will operate once the water is warm.