Room stat has stopped controlling CH - always on now

[james0478]

Would like some advice, before I finally section myself......

I've lived in house for 5 years and always had HW programmed morning and night, but CH on programmer set on Constant. I would then use stat to control temp along with TRVs.

Noticed in the past few days house has been a lot warmer than usual. Last night I clicked stat down from 21 to 18 and then went to check boiler - still running - where as before it would shut down. The only way to turn off the CH is to turn prog to off.

Having spent hours reading up, I am pretty sure I have a y plan system, 3 port valve.

The stat is a Honeywell T6360. I removed the two wires from 1 & 3, heating still ran, and I notice I have 240v on both cables. My understanding from reading is that when the programmer says CH on, then live is sent to one wire. The other wire is a switched live, so when stat is on it completes the circuit. So I'm confused as to why the switched live is live even when disconnected from stat.

I've followed guides on testing the valve, with power off, it defaults to HW, and moves to all 3 positions, but have read about microswitches failing - could this be a possible cause of the problem? It's a Tower MP322C/H, made by Grasslin dated 2003.

I'll be honest it's the first time I've looked into our heating system, and it is quite a learning curve for me. I'd like to check things as far as possible before calling an expert out to fix it!!

Any help much appreciated.....

 

[Agile]

Could well be a failed stuck motor valve.

Tony

 

[james0478]

Thanks for the reply. Is this the synchron motor part that can be easily replaced?

Cheers

 

[MANDATE]

You have the right idea, power from programmer goes to room stat which is just a switch (if its a two wire stat), the second wire is the switched live which goes to the white wire of the valve.
Power on the white wire has a few functions.
Through a micro switch it drives the motor until the switch is triggered at mid point.
It also feeds a second switch where it powers a PCB to give a modified voltage to hold the valve in mid position if needed.
Power also may or may not be available to feed the grey wire.
The grey wire is important because it takes over from the white at mid point, that of course is if its live.
If it is live and it drives the motor from mid point onwards there is no need to power the PCB, so that switch also gets triggered and now allows power provided by the white wire to exit the valve head to light the boiler.
When the requirement involves HW the cylinder stat provides the power to light the boiler, but it can't do that if HW is not on.
If the switched live at the room stat is live when it should not be and it's dis connected, then it's receiving power in a reverse direction, I'm sure I've heard of this before and it's got to be related to either micro switch contacts being stuck or failed components on the PCB.
I think replacing the actuator head would solve your problem.
Removing of the head allows you to check the valve spindle for stiffness but if your valve is moving to the 3 position then that can't be part of the problem.

 

[bab]

I'd try the motor 1st If you isolate power & remove syncron off the plate if the motor is jammed it should free the micro switch, New motor is a cheaper fix. worth checking out 1st

 

[MANDATE]

He does state valve moves to all three positions, how can it do that with a failed motor.

 

[bab]

If motor moving it would close the port to hot water only and not heat the rads ?

 

[MANDATE]

Hi bab! I think you've misunderstood the problem. The CH does not stop when room stat switched off.
The orange wire should no longer be live when power to the white wire is stopped, but it obviously is. With both HW and CH off the only wire thats live is the grey. That is keeping the valve at CH position as it should but it must also be supplying power to the orange.(which it shouldn't)
If the motor was duff then the valve would not go to either mid position or CH position and there would be no heating at all.

 

[james0478]

Thanks for the replies guys!

Spoke to a local Plumber as well today who owed me a favour from a few years back. He said he would be looking at the 3 port valve as well. He told me how to partial drain system, which I did, and replaced with a Honeywell V4073A.

The physical fitting went well, and the wiring on the old valve used the same colours, so have wired in as such.

Fired both CH & HW up, and bled rads - after 10 mins boiler stops (Oil fired Danesmoor 20/25), Power light off, even with boiler stat fully on. After more bleeding and 1/2 hour boiler fires again, for about 30 minutes.

In this time I turn off HW, and just CH on, goto stat, turn down, and guess what, CH still on and boiler firing - 240V showing on both sides of stat still.

I'm now at the stage where the boiler has cut out again, rads were hot upstairs, cold downstairs - and no further life from boiler - has been a good 1/2 hour now.

Guess I should of had someone in in the first place - but am just baffled as to what is sending 240v down the switched live!!!!!!!!

Headache!!!!

 

[MANDATE]

It's giving me a headache now!
There's not that much that it could be.
Can't be the room stat.
Cylinder stat is a two way switch and can't see how the switch itself could cause it.
The power with HW off and room stat off has got to be coming from the wire from the programmers HW OFF terminal which goes to the valves grey wire. It's the only wire from the programmer that's live.
It's purpose is to keep the motor energised so it remains at the CH position, but it's not supposed to feed the orange wire which is what it appears to be doing.
Of course this wire joins another wire from the cylinder stats satisfied terminal so there two wires going to the grey.
I can only think its wiring problem now.
It's worth checking through the wiring, especially the junction box and the terminals for three wires above (grey to valve/ wire to cylinder stat/ wire back to programmer HW OFF.
I can only think power is getting across somewhere from one these to the orange.
Maybe you should rewire the terminal box to ensure a good clearance between wires.
Don't forget power from the valve (orange wire) and power from the programmer HW ON after going through the cylinder stat will be joined so boiler can be ignited from one of two sources.
Do let me know what the problem was when it gets resolved.