Boiler timer problem

makfli

I had a new condensing boiler fitted last year. The timer control was an APT analog timer. Worked fine for about 3 months then it stayed on all the time. The orange light remained on no matter where the manual switch (On, Clock, Off)was set. I decided the timing unit was faulty and replaced it. All was fine for a while then same issue again boiler remained on all the time. I then decided APT was no good and bought an Immermat timer. I was about to connect this up today and decided to throw the circuit breaker to make sure there was power and then I could hear the boiler firing even though there were no wires connected. The two brown wires are live when the mcb is on.
Thus the situation is that the boiler is firing even though the two live wires are unconnected.
Really appreciate any thoughts that could help with this one.

Robbie.G

You want to looking at motorised valves.By the sound of things a motorised valve is jammed in the open position.
Take a pic of hotpress your looking for a box connected to pipe work with a cable coming out of it

ptogher14

Sounds like it could be a faulty motorised valve or sticky frost stat. However in either situation throwing the mcb should turn off the boiler. The boiler and all the controls should be fed off the one breaker

Irish Steve

What are you using to determine live or otherwise. I've seen some very strange things happen with things like neon testers, and some digital meters when there are a number of cables running in parallel, there may well be an indication of a voltage when everything is open circuit and disconnected, but that changes when a load connection is made.

That said, proceed with extreme caution, it may not be connected the way you thought it was, and 230 Volts is 230 volts regardless of where it's coming from.

Cogsy88

The two browns are live even with no timer connected???! What sort of meter are you using?

ptogher14

Are you turning off the right mcb Maybe the board is labelled wrong

makfli

Many thanks for all the help with this.
Robbie G and P Togher
I hadn't thought of a faulty motor valve. I had assumed the boiler was controlled directly through the timer. If it is a stuck valve why would the timer behave as if it was faulty?. The mcb kills the power ok .
Irish Steve and Cogsy 88.
I am using a phase tester (neon in screwdriver).

Irish Steve

Phase tester is not a good idea in this sort of scenario, as voltage can be induced in the cables simply because they are running in parallel with another circuit in the roof, or another location, especially when the cable is "floating" at both end, with no connection on it.

One (safer) way to check this out would be to use a short length of flex with a low wattage light bulb on the other end, and connect that to one of the "lives" and the neutrals, and then see if the bulb lights or not with the MCB on and off. You ideally need a multi meter to check for voltage, and can use that to check the voltages with the bulb connected, and that will give a more accurate indication of what's going on, as well as an instant visual indication of the status of the circuit.

ptogher14

The motorised valve will have a permanent feed. It it's stuck open it will give the switch wire to the boiler power no matter what state the timeclock is in. This in turn will back feed the switch wire from the timeclock...... leaving both wires at the timeclock live.

makfli

Many thanks lads that's a great help.
I presume if I leave the boiler on and if it doesn't switch off when room is up to temp, the valve is the problem. If it switches off then valve is ok ?
Boiler is in my mothers house and she has gone away for a while so might be a couple of weeks before I get around to this.