rogue voltage problem

[ambrad]

Hi All

Hope you can help. I have wired a biomass boiler on an s plan and the only existing wiring that has been used is the fused spur. We had a problem with the boiler not turning off so did all the relevant checks on the cabling and everything was fine. ie switching through the clock, room stat and cylinder stat all working fine. We got the maker out and thay could not find an issue. When i re checked the system i had a rogue 10v on the switching wire to the boiler at all times. The only time this went was when i turned off the fused spur. I tried disconnecting things individually but this made no difference. I am a bit stuck. Can anyone help.

As a foot note i have one idea in my head that i am considering but i might be barking up the wrong tree. The plumber has fit a magna clean about 500mm away from the wiring centre. The cables to the boiler run behind the magna clean. This thing gives off a very strong magnetic field. Could it be inducing a current that is stopping the boiler being switched.

Any ideas or similar problems please.

thanks

 

[HandySparks]

...We had a problem with the boiler not turning off...

Does a biomass boiler really 'turn off'?

When i re checked the system i had a rogue 10v on the switching wire to the boiler at all times.

What instrument did you use to measure this voltage and between which two points?

What voltage is specified for the boiler switching wire? (230V AC?)

As a foot note i have one idea in my head that i am considering but i might be barking up the wrong tree. The plumber has fit a magna clean about 500mm away from the wiring centre. The cables to the boiler run behind the magna clean. This thing gives off a very strong magnetic field. Could it be inducing a current that is stopping the boiler being switched.

Not unless either the wires or the MagnaClean are moving about fairly rapidly.

 

[ambrad]

The boiler shuts down if there is no calling from the thermostats or the clock. I am having a meeting with the maker tommorow but i want some info to take with me. There shouldnt be any voltage on the switching wire when the clock is off for both water and heating. I just cant trace the source. The cabling is new and wired corect. Something is stopping the boiler turning off and this is the last issue i can think is the problem.

 

[applemac]

if you unscrew the magna clean lid the mag rod will be removed so you can retest to aliminate magnetic problems..

 

[HandySparks]

The boiler shuts down if there is no calling from the thermostats or the clock. I am having a meeting with the maker tommorow but i want some info to take with me. There shouldnt be any voltage on the switching wire when the clock is off for both water and heating. I just cant trace the source. The cabling is new and wired corect. Something is stopping the boiler turning off and this is the last issue i can think is the problem.

If you disconnect the switch wire from the boiler does it turn off?

Have you tested whether this voltage is coming from the controls or from the boiler. i.e., if you disconnect the boiler switching wire from the boiler and measure at both the boiler terminals and at the controls wire what voltage do you get?

And how about answering the other questions if you want help?

 

[ambrad]

Hi
Sorry for not being clear. The biomass boiler shuts down and goes through a cooling down cycle. It is a 230v ac s plan system. I tested the switch wire at the boiler control wire to neutral. I then tested it in the wiing centre to neutral. I got 10.7 volts. I then disconected the switch wire in the control centre and there was 8v on the orange control switch cable. I then turned the clock off on both hot water and central heating and got the same voltage. When i turned the fused spur off the voltage on the control wire dissapeared. I have een told by a heating engineer that small amounts of current can pass to the control wire when zone valves are used. I have established that a 230 to o relay can be used to stop the voltage passing to the boiler. This current doesnt turn the boiler on but appears to stop it turning off. The thing that has baffled me more is that the boilers we are using (trianco 250 indoor outdoor boilers) dont appear to be affected the same. Some work fine even with the rogue voltage and some wont turn off. I hope this is more of an indepth explanation and you can offer me some advice based on this.
thanks

 

[HandySparks]

OK, if you are measuring this voltage with a high impedance instrument, then it could well be a phantom voltage induced into the switch wire from adjacent live cores. You may be able to fix this by separating the cable runs. The switches in 2-port zone valves should be electrically isolated from the valve motor supply, so I don't think that's the cause of the voltage.

If this phantom voltage is affecting the boiler, that suggests that the boiler switched input is also of high impedance. You could try putting a small load across the switched input to the boiler i.e. in parallel with the boiler input. This would provide a low impedance 'short' for the phantom voltage. An ordinary low wattage incandescent lamp would do as a trial. Alternatively, using a relay to isolate the boiler wiring from the control system as suggested should do the job.

Edit: Just re-read your last post. Presumably, with heating or hot water on and the relevant thermostat closed, you were getting 230V on the orange switch wire from the motorised valves?

 

[Lucien Nunes]

I think Handy is bang on with all of this, but would add that I remember a piece of kit, not a boiler, that had similar troubles a while back. Some units would correctly respond to a switch input that had a bit of ghost voltage on it from the wiring runs, others not. I opened one up and found they were using an unsuitable resistor across the input that would often go open-circuit and make the input much too sensitive to stray pickup.

PS Applemac the magnet / wiring is a red herring, fixed magnets do not induce voltages in anything, (although they can upset reed relays etc if very close)

 

[alanl]

Have a look at the internal wiring diagram for the zone valves. You'll see that there is a circuit board in there, and, in certain positions, there can be a low voltage on terminals that should be dead.
Honeywell list this in their instructions..
See the bottom of page 25 of this:
http://www.honeywelluk.com/documents/all/pdf/wiring guide issue 16.pdf

That is the best wiring guide out there. Honeywell do a printed version, ring them and they'll send you one. If you havent done one before, they also do a one day training course which is excellent, see their site for details.
Go through the fault finding list, and you'll soon narrow the fault down.
There is likely to be at least one wire terminated incorrectly, or, one of the valves could be faulty. I recently had a faulty micro-switch on a valve which gave all sorts of strange results, but, when doing the Honeywell fault finding, soon isolated the fault.

 

[HandySparks]

Have a look at the internal wiring diagram for the zone valves. You'll see that there is a circuit board in there, and, in certain positions, there can be a low voltage on terminals that should be dead.
Honeywell list this in their instructions..
See the bottom of page 25 of this...

The OP refers to the system being an S plan. So all valves should be 2-port, where this issue doesn't arise (the back feed issue is for the 3-port V4073A).

To the OP: what type of valves are you using, and how many?