CH only works if HW already on, after replacing 3-port valve

[warney]

Hi.

Plumber recently came round to flush our system as rads weren't working after not being used over the summer. He also replaced the three port valve (I wasn't there at the time!). Now, the heating will only come on if the hot water is also on, otherwise the boiler trips if you just demand CH only.
The single three-port valve appears to be working, in that it correctly switches to top/middle/bottom depending on what's demanded, and if I turn on the HW, then the CH, both circuits heat up fine. However, turn on the CH only, the valve moves correctly, but the boiler almost instantly trips and needs to be reset.

I'm convinced the plumber has messed up the wiring in the junction box, but I'm finding it hard to track down would could be wrong.

Can anyone come up with a mis-wiring that would cause this scenario so I can narrow it down a bit?!

It's a gravity fed system with a Potterton Suprima 30L with in-built timer and Drayton/Invensys ZA3/679 three-port valve.

Many thanks,

Chris


Not of much help, but I've attached a couple of photos. The first shows the airing cupboard connections - black cable is 3-port valve, middle white cable pump and bottom cable HW stat. The second pic shows the connections to the rest of the house (boiler,roomstat etc). Nothing's labelled which is why I'm having such a problem working it out!

Junction Box

• warney
• 12 Dec 2009
• 1

Junction Box

• warney
• 12 Dec 2009
• 1

 

[seco services]

is the pump running when ch only selected ?

 

[twgas]

Do you mean the circuit breaker trips (at the main "fuse box") or the boiler trips (needs the reset button pressing)

 

[warney]

is the pump running when ch only selected ?

Yep, it appears to run even after the boiler's tripped.

Chris

 

[warney]

Do you mean the circuit breaker trips (at the main "fuse box") or the boiler trips (needs the reset button pressing)

The boiler trips (needs the reset button pressing). Although it won't reset until I turn the CH off on the timer.

 

[warney]

Just a quick update.

The wiring diagram for the 3-port valve shows a connection going from the boiler "no hot water" terminal to the junction point of the orange wire. It says this is needed for CH only, although I can't see why.
However, this is not connected on my system (see picture below). AFAIK this has never been connected, but is it possible the previous valve didn't need such a connection, but the new Drayton valve does?
Bear in mind the valve does move correctly depending on what's demanded.

One thing I'd definitely like to know is what would make the boiler lock out as this isn't really explained in the documentation I have? It's not an over temp, or flame issue as it works perfectly unless CH only is selected, so it must be something electrical. But what is the boiler expecting when supplying CH only that it's not getting?

Chris

Boiler wiring

• warney
• 14 Dec 2009
• 1

 

[D_Hailsham]

Check the following:

1. The CH Out terminal of the built in timer connects to room stat Live
2. Room stat Switched live connects to valve white
3. The DHW Out terminal connect to cylinder stat Common
4. Cylinder stat Call connects to valve orange and boiler Sw L
5. Cylinder stat Sat and DHW OFF Out connects to valve grey

 

[glasgowgas]

is it me or is there no Neutral to pump?

 

[aGasMan]

is it me or is there no Neutral to pump?

The neutral for the pump doesn't have to be connected into the boiler as well, just as long as it is connected.

 

[matt1e]

Just a quick update.

The wiring diagram for the 3-port valve shows a connection going from the boiler "no hot water" terminal to the junction point of the orange wire. It says this is needed for CH only, although I can't see why.
However, this is not connected on my system (see picture below).

Boiler wiring

• warney
• 14 Dec 2009
• 1

no it goes to the grey wire Chris not the orange and needs to be connected,yours is not and is the reason you can not have CH on on its own

turn on the CH only, the valve moves correctly,

are you sure about this? if so then there is a problem in the wiring centre as this cannot happen without a connection to the Programmers HW off terminal

what is the boiler expecting when supplying CH only that it's not getting?

a call for heat due to the vavle not being driven fully open and making the micro switch

Yep, it appears to run even after the boiler's tripped.

thats normal on this boiler whilst in lockout
Matt

 

[warney]

Thanks for the replies, guys.

turn on the CH only, the valve moves correctly,

are you sure about this? if so then there is a problem in the wiring centre as this cannot happen without a connection to the Programmers HW off terminal
Matt

I'm pretty sure, yes. The valve moves to the correct position, but the boiler locks out immediately.



So the bottom line is that without the DHW off connection there's no way it can work properly.
I can't see why (or how) the plumber would have removed this, so I wonder whether the original installation was a bit of a bodge job (it was a new build but I guess that doesn't necessarily menay anything!), and when the plumber replaced the valve and wired it correctly in the wiring centre the bodge was removed!

It always appeared to work before, but I wonder whether it was bodged so whenever the CH was on, the HW was as well. That's all I can think of, really, as we probably wouldn't have noticed this.
If I could re-instate this bodge so the HW's always on when the CH is that would do for me. Running an extra cable from the boiler to the airing cupboard so it would all work perfectly would be a real PITA and something I really don't want to do!

Chris

 

[matt1e]

hi chris
it probable it has been a bodge from the start and you always had to have hot water on with central heating
difference now is the plumber who changed the valve has bodged it up even more and has wired it so the boiler is getting "backfed" via the pump live connection and detecting this as a pump wiring fault and going into lockout (he's probably connected the pump, the roomstat return, and the boiler pump connection together)
its hard to see from your photos where the pump core colours are connected can you elaborate? the pump live should be connected to one cable only and that will be none of the valve connections
also where do the two flexs going to the boiler connect to?another connection box? as they must join on to the threecore mains cables coming into the wiring centre somehow

Matt

 

[D_Hailsham]

no it goes to the grey wire Chris not the orange and needs to be connected,yours is not and is the reason you can not have CH on on its own

turn on the CH only, the valve moves correctly,

are you sure about this? if so then there is a problem in the wiring centre as this cannot happen without a connection to the Programmers HW off terminal

It's true that the grey wire needs to be live for the valve to go to the CH only position, but the grey can get its supply from two places: Programmer HW OFF or cylinder stat SAT terminal. The programmer HW OFF is not connected, but the valve can still go to CH only if HW is ON at the programmer and the cylinder stat is in the SAT position.

 

[matt1e]

It's true that the grey wire needs to be live for the valve to go to the CH only position, but the grey can get its supply from two places: Programmer HW OFF or cylinder stat SAT terminal. The programmer HW OFF is not connected, but the valve can still go to CH only if HW is ON at the programmer and the cylinder stat is in the SAT position.

normally yes D but not if the HW is turned off(as in selecting heating only)thats one of the ops problems' he cant select heating only
Matt

 

[D_Hailsham]

It's true that the grey wire needs to be live for the valve to go to the CH only position, but the grey can get its supply from two places: Programmer HW OFF or cylinder stat SAT terminal. The programmer HW OFF is not connected, but the valve can still go to CH only if HW is ON at the programmer and the cylinder stat is in the SAT position.

normally yes D but not if the HW is turned off(as in selecting heating only) that's one of the ops problems' he cant select heating only
Matt

And that's because the grey wire is not connected to HW OFF and cylinder stat SAT.

You seemed to be saying that you had to have HW OFF connected to grey or you would never get CH only.