Room Stat faulty?

[Dave.S]

Hi there,
My heating system is a 'W' plan, hot water priority.
The HW heats up no problem, in fact, recently turned cylinder stat down slightly as water was nearly scalding out of the tap.
My problem is this:- When the heating comes on (after HW is satisfied) the rads dont get very hot at all, certainly not too hot to touch. And after a while seem to begin to cool down, not to cold, just to warm. If I try to fire the boiler up by turning up the room stat, nothing happens.At some point the boiler will fire up, and the rads heat up again, but as before, not to very hot, just to very warm, and so it goes.
The room stat clicks at about setting number 3 (it goes to 5). If I turn it down below 3 the boiler will go out, and will fire up again if I turn it above 3, but that seems to be the only control I have using it.
My mother's system works as I imagine mine should, ie.if the boiler is off and you want the room warmer, you just nudge the room stat up and the boiler fires up, and presto, the rads get hot (too hot to touch if need be)
Does this point to a faulty room stat, or something else?
My motorised valve is not the std 4044 diverter, but a honeywell 4073a mid position unit wired so that the white and grey wires are joined and connected to the brown wire from the cylinder stat, and the orange wire is islolated.
I hope some knowledgable member will be able to help me. Thanks, Dave

 

[JohnKay]

Hi Dave.S
The fact that the stat clicks is some indication that it may be working. Does the valve open fully when the room stat is calling for heat, and cylinder stat satisfied ?
As you have a V4073A, why is it not wired as a Y plan ? I think the best course of action would be to rewire the system correctly, so eliminating an electrical fault. If you go to Honeywell UK site, you can download a diagram. Take great care to get it exactly as the diagram, I've known a lot of faults with the cylinder stat being incorrectly wired, and the valve grey, and white getting confused.

 

[Dave.S]

Thanks for the reply JohnKay.
The main reason I havnt wired it as a Y plan is that my controller is a potterton mini-minder which is a mechanical device, and it doesnt allow CH to be on without HW. I intend to get another controller, but ATM thats what Ive got.
You mentioned cyl stat wiring. Some time ago I fitted a new one which came unwired, so I connected the wires what I THOUGHT was correctly. The boiler fires if the stat is turned up, and goes out when stat is turned down. The HW temp also corresponds to the stat settings.
Could it still be wired up wrongly?

 

[JohnKay]

Now, I could be wrong here as I'm only going on memory, and it's a long time ! but I thought the Mini-minder had a switch on the back that would separate the supplies. I can't really comment on your cylinder stat wiring - I need to be there with the test instruments.

 

[Dave.S]

Ah, ok, I hope your memory is correct. I'll check later. Off to hit tesco right now.....

 

[Dave.S]

You're right John, there is a little thingy that you can turn with a screwdriver, that allows either slider to be operated independently of each other! The problem now is, there are only 3 wires going into the unit, 2 red and a black.there are 6 connectors, so I'm guessing I need to run an extra cable from the controller to the wiring centre, yes? I'm no electrician, but I can follow instructions

 

[JohnKay]

Pleased I got that bit right. It can't have ever been correct if there's only three wires. There's a line and neutral on the left, and I would expect a wire on terminal 3 for hot water, and one on terminal 4 for heating. You need those even for the V4044C valve.
It would seem the best thing would be to replace the existing cable (if i can call it that) with a 6 core heat resistant flex, obtainable from heating suppliers. You need 6 as, with Y plan you need a hot water off connection from terminal 1 or 2 (can't remember, but there's a little diagram on the back)
You don't need an earth, but bad practice to use green/yellow for anything else, so leave safely tucked away, but connect at wiring centre.
You can of course add an extra cable if that's easier than replacing, choice is yours, just as long as you can do this safely.

I feel that I'm rambling on just about a bit of wire, but hope it makes sense !

 

[Dave.S]

Ramble on as much as you like John, and it does make sense, at least in theory. This is good news if its just a wiring fault! The heating has always been lousy, and hopefully, youve cracked it!
Will get back to you asap, as not on pc atm. Thanks and goodnight John