Fault-finding central-heating wiring

We've got a problem with our CH/DHW which I think is likely to be a wiring issue.
The system consists of an (original, 15 yrs old) timer-switch, original 15 yrs old thermostat, original wiring centre next to the (7 yr old) pump, and 6 year old Remeha Avanta 18v boiler.

We found the CH to be OK but never really toasty, whereas the DHW was always HOT. Turns out that the cylinder stat was broken, I recently replaced it myself - easy job. All running OK, CH seems hotter now (as I think the mid-position valve was probably always sending water through the cylinder coils before?).

However there's another weird fault that seems more obvious now that the rest of the system's working well. Sometimes the boiler stays OFF even though the house is cold and the thermostat is indicating demand.

The thermostat does seem to have an effect, as most of the time, once the house is up to temperature the boiler and pump goes idle for a while.

Is it possible that our system is simply wired up wrong? So that perhaps it is coming on only for DHW demand, when it should come on for DHW OR CH?

When it's OFF, the boiler has T1 and T2 readings for inlet and output temperature, these fall from the usual 70C+ running, down to around 30C, as you'd expect, with around 5C difference. There's no error messages on the boiler.

Before I get the CH guy out to look at it, are there some simple checks I should do? According to the boiler installation guide, there are Y, W and S wiring-plans. Without opening it all up, I don't know which plan we have.

Edit: One more thing: Cycling the power off & on at the wall isolating switch, puts the boiler into its usual 4 minute purge cycle, then often it fires up and the CH comes on. Sometimes it's taken two off-on cycles, but it always comes back (so far).

Edit2: The mid-position valve has telltale neons. They are showing correct switching, i.e. CH, HW or both.

Sounds like a "Y" plan.

Have you checked the differential on the room stat?

Tony

There's an audible click from the thermostat with approx 2 deg C difference. So for example - system just switched on with the stat set at 20C. Moved settings back and there was a click at around 18C. There is no neon indicator, so I don't know what's happening with it electrically, and I do wonder whether their contacts can 'stick'.

the stat says "Lifestyle" on the outside, it's original fit by the housebuilders so it's probably a cheap one?

Based on what I found elsewhere on the forums, a modern electronic stat would have other advantages, so if the fault is in the stat then I'd happily go for a better one ..

As the fault happens only sometimes (around twice a week) I could add a 230v AC LED indicator on the stat output (e.g. in the wiring centre) and keep an eye on it. Or else wait for it to fail and measure inside the wiring centre with a meter.

I'm assuming the MV is a Potterton Myson? The neons can't relied upon to tell you the valve is moving, in fact they are almost useless.

To check it is working you run the CH only and check it is getting warm and the port to HW cylinder is cool. Then vice versa.

Worthwhile mentioning 3 port Y plans are often miswired, and cylinder stats are sometimes maked 1,2,3 or C, 1 and 2, which creates problems when they're changed.

A lot of electricians do not understand heating wiring

I've had more of a look this morning. The motorised valve is an EPH Detach B322D. This was a replacement, fitted July 2013. Apparently has a 5-year guarantee.

The wiring in the wiring centre is rather untidy, but the correct signals seem to be present on the grey, white feeds according to this guide: http://www.gasheating.co.uk/Mid-Position-Valve.html
I've cycled the system through various states, and have managed to reproduce the fault, checking both inside the wiring centre and on the end of the MV 6-pin connector:
CH & HW both ON:
Grey: -
White: Live
Orange: Live

Set minimum tank stat temperature to force HW OFF:
Grey: Live
White: Live
Orange: - ..... *incorrect* ?

No noise or movement in the valve (when it should be tracking into the end position, i.e. CH only).
So if the MV follows the principles given in the gasheating.co.uk guide, then the Grey feed is probably failing to actuate the valve to move from its 'mid' position into its 'end' position.
So I think it's the MV that's the culprit.

If you agree with this reasoning, I'll contact their warranty dept, or order a new B322D head unit (unless they are absolute cr*p in your collective opinions, in which case I'll get my local chap to change the entire MV).

Edit: I also noticed that coming into the mid-position the valve sometimes made a thump-thump noise. Not sure if that's from the motor or water-hammer.

1. Is there a wire connecting the programmer HW Off terminal to the valve grey wire? (This is in addition to the wire from cylinder stat SAT teminal to valve grey)
2. If you turn CH On and HW Off at the programmer, do the rads heat up?
3. Green feed? Has the installer used the earth wire to carry voltage? If so, it should be sleeved in brown at both ends to show it is a live wire.
4. 'Lifestyle' products were originally made by ACL (now Drayton) and are at the better end of the market.

D_Hailsham said:

1. Is there a wire connecting the programmer HW Off terminal to the valve grey wire? (This is in addition to the wire from cylinder stat SAT teminal to valve grey)
2. If you turn CH On and HW Off at the programmer, do the rads heat up?
3. Green feed? Has the installer used the earth wire to carry voltage? If so, it should be sleeved in brown at both ends to show it is a live wire.
4. 'Lifestyle' products were originally made by ACL (now Drayton) and are at the better end of the market.

Click to expand...

Will investigate 1 & 2.
3. I was mistaken on - I corrected my post perhaps after you saw it. I meant to say grey + white.

1. Yes there is. It's a short link inside the wiring centre.
2. Yes the system runs under that condition. Voltage is present at the grey wire in the wiring centre, whether the cyl stat is set high (call) or low (sat).

Part of the wiring centre detail:
16: CYL SAT .. two black wires going to the cyl stat (Why two? Looks like a 6-core cable)
15: CYL COM .. brown wire to the cyl stat
:
10: HW ON: Yellow
9: HTG ON: Red
8: HW OFF: Blue
7: Mid Pos valve: Grey
:

7, 8, and 16 have connecting links.
10 and 15 have a connecting link.

Two photos attached.

D_Hailsham said:

.......(now Drayton) and are at the better end of the market.

Click to expand...

Drayton are a LONG way from the better end of the market .............

Looking at wiring centre what is the wires going into terminal 1?also with so many blues surely it needs sleaving on them to work out what is what?

Hason said:

Looking at wiring centre what is the wires going into terminal 1?also with so many blues surely it needs sleaving on them to work out what is what?

Click to expand...

Maybe .. is there a convention to follow there, or are you thinking full-on Dymo name labels?

tuck1s said:

1. Yes there is. It's a short link inside the wiring centre.
2. Yes the system runs under that condition. Voltage is present at the grey wire in the wiring centre, whether the cyl stat is set high (call) or low (sat).

Click to expand...

Then there is nothing wrong with the valve.

The problem is this: when HW is on at the programmer and the cylinder stat has switched to SAT, because the water temperature is high enough, there is no voltage on the grey wire. This could be due to a faulty cylinder stat or a wiring fault.

The cylinder stat can be checked using a meter. Looking at the pic of the wiring centre, the required links (7-8 and 8-16) are in place, but it might be worth checking that all the screws are done up tightly, including the screws in the cylinder stat.

Why so many wires to terminal 16? There only needs to be one from the cylinder stat SAT terminal.

Thanks D_Hailsham, however there is live voltage on the grey wire (as per my quoted reply), when it's sitting in mid-position, cylinder is SAT and CH is Called. I am not a trained CH engineer but I can follow schematics and the posts here: //www.diynot.com/diy/threads/central-heating-programmer-or-motorised-valve.8369/
which are excellent, by the way.

The fault condition is occurring on the 5th diagram in the sequence.
I suspect there's a problem with the microswitch inside the MV, for the following reason.
A gentle push on the manual override on the mid-position valve is enough to get it moving, and it progresses across on the synchronous motor to the end position.
That sounds like a MV fault to me, and I have no reason to suspect faulty wiring at this stage.

The two black wires from T16 are duplicates. The Cyl stat cable has six cores, which seems unnecessary but not actually wrong.
I agree the installer did not make a very tidy job of it.
I will first try a replacement MV. If this does not cure the problem (for £57 inc VAT & delivery .. reasonable price to pay) then I will eat a slice of humble-pie, go through that wiring centre connection by connection, label each wire, test for continuity, and draw up a schematic of what I actually have in that's rats-nest before changing anything else

tuck1s said:

Thanks D_Hailsham, however there is live voltage on the grey wire (as per my quoted reply), when it's sitting in mid-position, cylinder is SAT and CH is Called.

Click to expand...

There are two ways the grey wire gets voltage:

1. From the programmer HW OFF terminal when HW has been turned off.
2. From the cylinder stat SAT terminal when HW is turned on and the cylinder has reached temperature

The test I asked you to do confirmed voltage in the first condition and you say, above, that the there is voltage in the second condition. However HW only comes on under condition 1.

So, if the valve works correctly when the grey is fed from programmer , why doesn't it work when fed from the cylinder stat SAT terminal? The only thing I can think of is the valve sticking. Remove the actuator and try turning the valve spindle by hand. It should be very easy, though it may only move about twenty degrees. If it is at all sticky, you can use a silicon lubricant to ease it.

Don't waste you money on a new valve until you have eliminated everything else.