CH constantly slightly on, DHW working fine (y-plan)

[gwinkless]

Hi

About three weeks ago our DHW suddenly started coming out steaming hot - to the point where you couldn't put your hands under the water. I did some reading (including these v helpful forums!) and read that the most likely cause was the cylinder stat; replacing this seems to have fixed the DHW overheat (so thanks for all the posts!).

Ever since though the CH has been misbehaving; either not coming on at all or being constantly slightly on (ie the rads are warm, not hot). I assumed (since it has failed before) that the 3-port motorised valve (it's a Honeywell Y-plan system) was failing so yesterday I replaced the motor. I also replaced the room stat since it was slightly dubious (if you pushed the front slightly it would click off).

Today CH seems to be on permanently (frustrating since this is the first day of the year when we could totally have done without it!) unless we turn off both CH and DHW. DHW is still fine.

Does it sound like the 3-port valve itself (rather than the motor) is fried? It fails in fully open state, yes?

The only thing that makes me think that's not the root cause of the problem is that turning up the room stat doesn't make the boiler kick in; that seems weird because even if the valve were in the wrong place the stat should call for hot water and the boiler should heat, yes? Is there something else I should be checking?

Actually to make sure I got the stat right...

I took the wiring from the old Honeywell room stat (it had Blue->2, Yellow->3, Red->1) and, from looking at the diagram on the inside, it looked like Blue=Neutral, Yellow=Switched, Red=Live, so I wired the Grasslin 103 stat (£8.95 from toolstation!) Blue->4, Yellow->2, Red->1, does that seem right?

My photo of the Honeywell diagram here (it's old, so bits are obscured, sorry, that's not the pic, that's how it is!)

Untitled

• gwinkless
• 6 Feb 2011
• 1

Diagram of the 103 here:

Untitled

• gwinkless
• 6 Feb 2011
• 1

(taken from http://www.grasslin-controls.co.uk/assets/graphics/static/Thermio 103 Spec.pdf )

Have I messed that up? Or is the boiler not kicking in for some other reason?

Any suggestions appreciated, I'm a bit lost here - I'm electrically capable and I'm handy with a multimeter but I've no CH experience.

Thanks!

 

[mysteryman]

It sounds as if there are other problems with the valve. You can change the complete head unless it is more than about 25 years old. However, the fact that it is letting by to CH suggests that here are hydraulic problems with it. If you disconnect it electrically and run the pump and boiler, the valve should deliver only to the DHW port. If it flows from the CH port as well, it is faulty. You can get replacement arm-plus-ball kits.

 

[Agile]

I think you need to check the way the cylinder stat is wired.

Unfortunately there is no standard for colour of wiring and often a green/yellow earth wire is used as in your system.

You need to see which each wire is used for by checking each end.

Tony

 

[twgas]

It does sound as though the 3 port valve is playing up - I would suspect the system is dirty, which is causing the valve to stick and/or pass to CH when no call.
A 3 port does not have a "fully open" position, it is always open, whether thatbe to HW, CH or both.

 

[gwinkless]

It sounds as if there are other problems with the valve. You can change the complete head unless it is more than about 25 years old. However, the fact that it is letting by to CH suggests that here are hydraulic problems with it. If you disconnect it electrically and run the pump and boiler, the valve should deliver only to the DHW port. If it flows from the CH port as well, it is faulty. You can get replacement arm-plus-ball kits.

Thanks all for the posts.

I turned off the CH and (with a nudge from the manual/auto lever) the spring moved the head to position A. I left the heating stat turned down and two hours later the rads are still hot; I'd say it's safe to assume that the valve itself is faulty. That's the point at which I give up, I think - proper plumbing needs a plumber

I'm still not sure that the motor's wired up right; when I replaced it I ended up pulling the blue wire off its soldered position (just while trying to strip the other end, grrr) but I checked the diagram here http://www.diyfaq.org.uk/plumbing/controls/midpositionvalve.htm and assumed that it needed to be attached to the point between the large resistor and the diode, ie here...

Untitled

• gwinkless
• 6 Feb 2011
• 1

does that make sense? Staring at the diagram I can't see anywhere else it should go, but the motor isn't doing what it should (even if the valve is broken).

 

[gwinkless]

I think you need to check the way the cylinder stat is wired.

Unfortunately there is no standard for colour of wiring and often a green/yellow earth wire is used as in your system.

You need to see which each wire is used for by checking each end.

Thanks Tony; I'm pretty certain I wired the cylinder stat exactly as the original was done. There's three ports - C (common) 1 and 2; what I found was that both 1 and 2 are live when the stat is in the "on" position - that seems broken, no? It's certainly not what's described here:

http://www.salus-tech.com/upload/download/CT100.pdf

or would both be live if the CH was on (can't remember what state it was in at the time, to be honest)?

 

[mysteryman]

There are wiring diagrams etc on the Honeywell website. In spite of what Agile Tony wrote, there are industry-standard wiring colours, and you will only find different schemes on very old installations such as the blue Switchmaster valves. Quite often there is no earth wire, but everything else is pretty well standard nowadays.

 

[gwinkless]

I'm still not sure that the motor's wired up right; when I replaced it I ended up pulling the blue wire off its soldered position (just while trying to strip the other end, grrr)

I'm an idiot; of course it goes on the back end of the 13k resistor. I'm not quite sure what I was looking at at the time, I think maybe I was trying to persuade myself that it had come from the obvious point (where it looked like the wire might easily have come from). Or maybe I was just overtired

The motor and the CH stat are now working as they should. The valve, unfortunately, is still completely hosed. Now I have to decide whether I'm more stressed about the £200 it'll almost certainly cost me (and which I don't have) to get a friendly heating engineer to drain down the boiler and fit me a new valve or about the fact that if I do it myself I don't know what I'm doing with draining the thing and compression joints have never been my strong point anyway (I tend to get there eventually...).

Thanks again for all the posts.

 

[gwinkless]

The motor and the CH stat are now working as they should. The valve, unfortunately, is still completely hosed. Now I have to decide whether I'm more stressed about the £200 it'll almost certainly cost me (and which I don't have) to get a friendly heating engineer to drain down the boiler and fit me a new valve or about the fact that if I do it myself I don't know what I'm doing with draining the thing and compression joints have never been my strong point anyway (I tend to get there eventually...).

Finally had the time (easter long weekend) and the courage to try it. 3 hours all-in and everything looks like it's working. Thanks again everyone who posted.