24v at switched side of room stat (230v) possible backfeed ?

[weep]

hi all, having an issue at work where the heating wont turn off. wiring is all ok and been tested, the stat is switching fine although it is reading 24v at the switched live at the stat when its turned down which im thinking is a backfeed from the boiler of some kind.
Have replaced a few stats now and they appear to work fine for a bit. boiler does take a time though to kick in tho when calling for heat.
it also appears to be going into syphon mode sometimes when switching also.
soz but im **** at explaining things lol. its a worcester combi boiler with danfoss controls (programmer and stat).

Anyone had any backfeed issues with these stats/boilers before.

Many Thanks

 

[Marvo]

Ignore the 24volts on the switched live, it's probably just a ghost or coupled voltage that your oversensitive tester is seeing

 

[davesparks]

Is it a new boiler? Some of the most recent Worcester boilers do really take their time to for after getting the signal.

 

[weep]

have been on the worcester website and it mentions this on the fault chart but offers no solution "some older stats (containing capacitors) may give a low voltage signal to st10 when the stat contacts are open, check there is no permanent live on Lr from another source"

its obviously a fault if it mentions it , only thing i can think of is a backfeed off the 24v side of the boiler somehow?

 

[weep]

im 98% sure its a greenstar 28i junior

 

[Darkwood]

We need to know the wiring set-up ie :-
1-The stat is forming part of an external timeclock circuit and is 230v switching. (short feed and switch out to conform operation).
2-The stat is wired directly to the boiler connections and is just using a monitoring voltage SELV (in this case shorting then opening the stat terminals at the boiler terminals will prove operation).
3-The stat is direct to the boiler again but 230v switching 'see boiler instructions' (Again test as in 2 with caution of the voltage present).

Many other scenarios can give similar symtoms like failed limits on the changover valve, loose connections, pressure low etc etc .....get a boiler engineer on board and work together to source the issue if you struggle, too many brands and generations of boilers out there to specify a definite issue, read the instructions and usually it will have both the internal wiring set up of the control and the external wiring options listed to operate boiler.

And agree above the 24v is probably a floating voltage or induced voltage but would expect the boilers pcb to be galvanically isolated anyway.

 

[weep]

meant to mentio its a danfoss rmt230 stat

 

[weep]

its the usual combi wiring. fused spur next to boiler, 4 core flex from spur to boiler with a connection made in the spur with the switched live coming back from the stat.
3 core from spur to programmer, 3 core from programmer to stat.
live to stat from 4 on programmer with the black return connected with the black from the 3 core from the spur ( which is connected to Lr on the boiler).
greys obviouslu used as a neutral.

 

[davesparks]

The 28i junior doesn't suffer from the extremely slow start issue, that problem is limited to the new 'compact' range in my experience.
Have you looked at/tested at the boiler terminals? If so is this switching arrangement connected between Ls and Lr? Ls should be the live supply going out to the controls and Lr the switched live to fire the boiler. (If it doesn't have the Ls terminal then it should be the perm live in to the boiler used to supply the controls)

What type of tester are you using to measure the voltage?

A quick google shows that the roomstat is a basic mechanical one rather than a fancy electronic jobby, so a quick continuity test with a megger should be fine to see if it working correctly or not.

 

[weep]

tested at boiler terminals, 230 on Lr when stat up and calling for heat, 24v when stat down, tested at stat and boiler, was a fluke voltage tester.
we just take a feed from the load side of the spur to feed the programmer which in turn feeds the stat when programmer is on. switched side of stat is conected back to the boiler in connector strip to Lr on boiler.
Its switching fine, just strange where this 24v is coming from.

 

[Darkwood]

Sounds like a floating voltage then if at terminals too (assuming wires disconnected), just a hiccough of some designs, as its ELV its not a worry and probably why its not been designed better.

 

[davesparks]

Ok, so is there not an Ls terminal on the boiler? I can't remember if the Ri combi has it or not?

If you disconnect the wire from Lr (and having secured it safely in a connector) power it up does the 24V remain on the Lr or is it on the wire?

 

[weep]

Ok, so is there not an Ls terminal on the boiler? I can't remember if the Ri combi has it or not?

If you disconnect the wire from Lr (and having secured it safely in a connector) power it up does the 24V remain on the Lr or is it on the wire?

this is going to be my next step, surprised i never thought out it at the time but where on the rewires and times money lol

 

[weep]

there is an Ls on the boiler but ive never used it.

 

[davesparks]

Better hope that it's not a boiler fault that Worcester Bosch get called out to fix under warranty then!
They will void the warranty as it has not been wired according to their instructions.
I have seen it happen, and had to put right someone else's work before they would repair one boiler.

Next time try reading and understanding the manual before jumping in!

 

[weep]

lol, yeah yeah, been wiring boilers like this for years

 

[davesparks]

And I've been wiring boilers for years too, but what does that prove? It only proves that the way we learned to wire boilers is out of date, it doesn't prove anything about our competence or knowledge.

 

[weep]

i know what your saying but in all honesty, nearly every spark i know wires them this way and very very rarely do we get any issues. still doesnt solve the problem at hand, will check boiler terminals with switched live on Lr disconnected (safely of course ) tommorow.

 

[Pondy]

Think Marvo hit the nail on the head in post two

 

[davesparks]

i know what your saying but in all honesty, nearly every spark i know wires them this way and very very rarely do we get any issues. still doesnt solve the problem at hand, will check boiler terminals with switched live on Lr disconnected (safely of course ) tommorow.

If lots of people do something wrong then it must automatically become correct?

Everybody I know wires them according to the boiler manufacturers instructions, there are a few manufacturers who specify that they be wired this way.
The point is to ensure that if the boiler is switched off or a fault takes the internal fuse out then the whole system goes off. As you have wired it at the moment the Lr would remain live and so be feeding a live into the boiler despite the fuse having blown.

It's perfectly logical and sensible if you take a second to think about it, and I'm sure they have a good reason for doing it.