Honeywell CM907 room 'stat swap out

[Stuie]

Hi All,
First post here so go easy on me ! I have an old, broken, Potterton PET 1 room thermostat that I want to replace with a Honeywell CM907. I have, hopefully, attached a picture of what the current 4 wire set up, and want to know where the wires go in the new Honeywell unit. I have also attached a wiring diagram from the Honeywell installation guide. I assume the colours are - Brown (Live), Black (Switched live), Blue (Neutral) and Earth in the old set up. Do I just connect brown and black across A and B in the new unit and terminate Blue and Earth ?
Thanks in advance

Cheers

 

[jackthom]

Yes it's as simple as that.

Along with making sure the unused Earth & Neutral wires are safely covered up.

 

[Stuie]

Thanks. I will get the screwdriver out !!!! Just for my own knowledge, why does the old one appear to have the Neutral linked to terminal 1 (Black wire, switched live) via a resistor ? Surely when the unit switches and calls for the boiler the N is joined to the switched live ?? = Bang !?

 

[JohnD]

it is probably a small heating element or "accelerator"

the idea is that it starts warming the thermostat while the radiators are warming the house, to prevent an overshoot while the warmth finds its way into the thermostat housing.

 

[Stuie]

Thanks.
OK, new unit fitted, batteries in , time set, PROBLEM ! The unit switches the boiler on, but doesn't switch it off...... I hear the relay click, the symbol of a flame in the display, all running ok. If I then turn temperature down, again I hear the relay click and the flame symbol goes away, but boiler still runs.... even if I set the 'stat to frost mode it keep running How is this possible ? By the way, its just running the CH, not the H water.

 

[D_Hailsham]

1. You say the Potterton was broken; what were the symptoms?
2. Was the Honeywell new?
3. The " resistor" in the diagram represents the boiler. The Potterton thermostat does not need an anticipator.
4. If the boiler is not a combi, how many motorized valves do you have?

 

[Stuie]

Hi D_Hailsham, The Potterton's LCD display would randomly go blank, new batteries didn't help. Pressing the reset button with a pin would wake the unit up, but again the display would be random and sometimes show the time of 33:33 and a room temperature of 99 degrees ! The Honeywell is brand new. The boiler is a glow worm, its so old it still has a pilot light in it ! There is a 3 speed pump in the boiler "housing". There is a neon switched fuse that isolates the boiler. I.E - turn the switch off, boiler goes off. I guess the room stat either runs the pump, which in turn fires the boiler up, or it opens the gas valve in the boiler, then the pump runs.... So in answer to point 4, I guess I have no motorised valves. Boiler is for CHeating only, immersion in cylinder for hot water. Cheers

 

[Stuie]

Just another slant on this simple swap..... I thought maybe the relay in the new Honeywell may be knackered, hence the reason for not switching off the boiler. So I've rigged it up to light fitting, neutral to one side of light, live into position A on the Honeywell and out of B. Clicks on and off (bulb on and off) a treat, as I change the temperature, so the relay does work.

 

[D_Hailsham]

Turn the power off
Remove Honeywell from backplate
Link A and B together with a bit of wire
Turn power on
The boiler should light

If it does, there's something wrong with the Honeywell.

If it does not, you need to find the other end of the cable connected to A and B (at the boiler) and link those rwo terminals together.

Boiler lights = cable problem
Boiler not light = boiler problem

Motorized valves look like this:

This one has three ports. You can also get valves with two ports

View media item 5946
Are there any pipes connected to the side of the HW cylinder, or just one at the bottom and one at the very top?

 

[MrTherm]

Also check for the correct wiring to the old glow worm boiler, many models have a pump overrun incorporated into the boiler stat, this would have needed another live wire (bypassing the room stat) to overrun the pump, this live would normally be wired directly to the boiler isolation spur.

 

[Stuie]

Yeah, just tried that, I joined the brown (live) and Black (sw live) together and the boiler fires up. Separate them and boiler stops. This was done with the cables coming out of wall at room stat end, but this is the same as you suggest when saying join A to B with a piece of wire. Kind of points at the Honeywell me thinks now...... Is there any signal needed in Honeywell to confirm boiler / pump is running, some sort of safety thing ?

 

[St0rmer66]

Faulty thermostat by the sounds of it. You've done all the right checks.

 

[Stuie]

Guys, thanks for all your re-assurance and advice. I have returned the unit, and ordered a replacement of the same type. I will post the results when it arrives so we can all learn from it.
Cheers