Y Plan problems

[ARWadsworth]

The problem is that when only CH is called the boiler will not switch off when the programmer switches off. The same happens when both HW and CH are called for by the programmer and the HW is satisfied before the CH.

The wiring seems to be fine and the mid position valve moves to the correct position at all times and is correctly controlled by the room stat and the cylinder stat.

Any ideas.

The only idea that I can come up with is that orange wire (which stays at

90v when there is no power to the valve after calling for CH only) is somehow tricking the boiler into staying running and I suspect the boiler PCB is faulty.

Adam

[geoff]

In message , ARWadsworth writes

How long have you been a regular here ?

How many times has someone been called a numpty for not saying what boiler they have ?

[ARWadsworth]

Worcester 24Ri

Adam

[geoff]

In message , ARWadsworth writes

I'll check tomorrow

[ARWadsworth]

Cheers

Adam

[YAPH]

Greenstar? Fit 0.47uF (275V AC X2 rated) capacitor across switched live.

[ARWadsworth]

Yes a Greenstar.

Worth a shot. Cheers for that.

Adam

[geoff]

In message , ARWadsworth writes

Common problem John ?

[ARWadsworth]

Which end of the wiring? The boiler end or the airing cupboard end?

Adam

[ARWadsworth]

I fitted one today. The boiler now works correctly.

Thanks

Adam

[YAPH]

Pretty obscure actually - it was about 3rd time calling W-B tech support before I found out about it. Basically it seems the small current passed by Honeywell-type mid-position valves (presumably by that 680K or whatever resistor?) can fool the electronics of these boilers (and maybe some other models? & other makes even?) into thinking they have call-for-heat at their switched-live input.

The boiler I came across would jitter on an off with a relay inside chattering away until it gave up and went into lockout. I'd changed the motorised valve and W-B had been out and (as I recall) changed it again before I eventually found someone there who knew of this issue.

I'd actually tried hanging a resistor across switched-live thinking that might cure it and was quite surprised at the capacitor suggestion, but it did the trick and the system's been sweet as a nut since then. Odd thing is the system had been up and running happily for a couple of years before the problem showed up (and it was one of my installs so I'm pretty sure nothing had changed in that time).

[Roger Mills]

Can you please explain "across switched live". What does the other side connect to, and how does it fix the problem?

[geoff]

Interesting reading

cheers

[ARWadsworth]

Connected across to neutral. The switched live stayed at 90V when the last call was from the CH.

The cap that Geoff so kindly posted to me dropped the voltage to 3V (readings made with a DVM) after the CH was no longer called for.

As to how it fixes the problem then you need Geoff or John to answer. It is long time ago that I did electronics.

Adam

[Roger Mills]

Yes, I see - thanks. Whatever the exact electronics explanation, it apparently reduces the switched live voltage to a very low value so that the boiler is no longer fooled into firing when it shouldn't.

[YAPH]

I guess it simply attenuates the voltage seen at the SL terminal caused by the high value (680K?) resistor in the motorised valve. It's too late in the night (or early in the morning!) to ask my brain to do the sums especially seeing as wot they has reactance at 50Hz in them and it's a looong time since I did that stuff, but maybe some brainy sort will be along soon to help us out here. :-)

[ARWadsworth]

My post on the 7th suggested that it did:-)

In case you missed that post, then Thank You.

Adam

[YAPH]

Yes, just checking that I'd read your post correctly. Brain in neutral and all that :-)

Glad it helped. And to put one over on that know-it-all in Watford ;-)

[geoff]

Makes note to add 10% smugness excess on next order

[YAPH]

Curses! :-(