Boiler permanently on

Discussion in 'Plumbers' Talk' started by Graham price, May 31, 2011.

  1. Graham price

    Graham price New Member

    I have a sealed fully pumped system.  Boiler is a Worcester Bosch 19/24CBi.  Programmer is a Honeywell St699.  The installation is about 5 years old.  The boiler is now ignoring the programmer and  both the water and CH  thermostats and therefore runs permanently.  It can only be switched off via the main switch. The system has been regularly serviced but I can't get the installer interested in fault-finding ( too time-consuming ? )  All ideas welcome.
  2. Hi Graham. When you say the boiler is permanently 'on', what's it supplying - the hot water or the radiators?

    The only thing I can think of is the 2-way motorised valve which directs the boiler's output to either the hot cylinder or radiators (or both) as required. Do you know where this is located? I suspect you'll find that it's stuck in one position and is therefore directing the boiler to stay on and supply whatever its stuck position is asking for!

    You should find a lever on the side of the metal body which can be moved by hand. Turn off the boiler at the mains and try moving the lever from one end to the other and back again a few times. Make sure you don;t 'lock' it in a far position (there's a 'notch' on the side of the lever slot where the lever can be 'parked' - make sure it doesn't stick in there and is free to move.)

    Then get someone to turn the boiler power back on and see what the lever does...
  3. Graham price

    Graham price New Member

    Ahh..............interesting.   It was in the middle.  I pushed it to the "manual" end a few times and allowed it to return to the middle.  Then when I switched the main switch on,  the pump ran for a second or two then stopped, which sounds to be what should happen.......................?

    To clarify my earlier note, it was the hot water which came on alone. If I used the programmer to also switch the heating on they both came on together, even with the water "off" on the programmer.
  4. Graham price

    Graham price New Member

    Sorry Devil's Advocate, I didn't quite answer your question.  The lever stayed in the middle where.  I then turned the water on using the programmer but when I turned that off an hour later the boiler carried on.  The trick with the lever didn't work after that.
  5. Ok, try this; power off, and move the lever from one extreme to t'other a few times. What does it feel like? Does it move smoothly against spring pressure? Both ways? Does it make a faint 'whirring' sound both ways? Where does it 'settle' when released?

    Then get someone to switch on demand for the hot water and tell us what the lever does. There should be a very obvious 'whirr' from the unit follwed by a light 'click' as the little microswitch inside is activated. The boiler should then turn on. (You may need to gently move the lever each way to see where it's 'gone' ie: the mechanism inside the unit doesn't always 'pull' the lever along with it, but you should find the lever has gone loose if the thingy inside has moved.)

    Ok, now turn off the HW demand. What happens to the unit - does it whirr again as the lever goes back to centre position? Now turn on CH demand. Report...

    If the motorisied valve seems to behave as it should but the boiler doesn't turn off afterwards, I'd suspect a sticking microswitch inside the motorised valve.

    Finally, if the boiler comes on with the hot water demand as it should, but doesn't turn off again when asked, try giving ther motorised valve body a gentle thump with a shoe heel or similar and see if it clunks and clicks and turns the boiler off.
  6. leswood

    leswood New Member

    Hi I have come across this type of fault before and twice it was the motorised valve and once it was the programmer.  You realy need a multi meter to test if you have voltage going at the programmer when the system is off.
  7. sam spade

    sam spade Active Member

    But the motorized valve of a Y Plan is not energised when HW is called. The boiler lights straight from the cylinder stat.
  8. Graham price

    Graham price New Member

    The case has MAN marked at one end and AUTO at the other.  The lever settles in the middle.  There is resistance when I push it to the MAN end. The lever is then loose and it whirrs as it returns to centre.  I then get resistance again if I move it back to MAN again.  Between centre and the Auto end it's always completely loose.  I did this a couple of times this morning.
    Then with both CH and HW  "off"  on programmer I put main switch "on."  The pump went for half a second then shut off; the boiler did not fire. I turned on the HW and pump ran.   No sound at all from the valve and the lever stayed as it was, resistance towards the MAN end only.  Switched off the HW.  The pump carried on, presumably boiler also firing. Knocking the valve case had no effect. I switched the main switch off.  I then switched it back on again and the pump just ran for half a second.
    Switched on the CH.  Pump ran and the valve drove towards the MAN end.  Switched off the CH and the lever stayed where it was with the pump running.  Switched off at the main switch and lever returned to centre.
  9. sam spade

    sam spade Active Member

  10. Graham price

    Graham price New Member

    Thanks Les.  I have a multi meter and I'm reasonably competent at electrical stuff but I have little understanding of what signals what and when when it comes to central heating systems.
    My response to Devil's Advocate may give you some more clues.
    Thanks again.
  11. leswood

    leswood New Member

    Hi Graham sounds to me from your discription that the programmer is at fault.  You could check this in the wiring centre. Trace the wire that comes from the central heating motorised valve to the wiring centre then create the fault by turning the central heating on then off at the programmer. then test between the brown and blue wire which is going to the morised valve.  If this is live with the programmer showing central heating off then the fault is the programmer. Or the wiring has a fault.  I guess if you were to turn the room stat down the heating should go off even with a programmer fault if its wired correctly.
  12. Graham price

    Graham price New Member

    Les.  We had to use the heating a couple of nights ago.  The CH definitely stayed on with the programmer "off" even when the thermostat was turned down. Just the same with the water, the thermostat is ignored.  Given that the system has been fine for several years it wouldn't seem to be a wiring fault.  
  13. (First apologies - I didn't realise; as Sam Spade says, Y plan doesn't have a switch in the diverter valve for HW. That's a cracking link!)

    Graham, the motorised valve wiring - and what happens inside - is a lot more complicated than I thought! There's apparently a resistor/diode thingy going on which holds the valve in the CH positon even when the CH is turned off. The resistor is then meant to introduce enough of a resistance to turn the pump and boiler off. Yeah, I don't understand that either.

    However, what is of direct interest to me - and you struck on this yourself, Graham - is that the programmer in CH mode supplies the motorised valve via the room stat. This suggests to me that even if the programmer is faulty - sending a constantly on signal when it shouldn't - the mororised valve still shouldn't get a signal if the room 'stat was turned down. The room stat should surely act as a break in the wiring and the boiler should go off when the room stat is turned down?

    So, my money is still on the valve - even tho' I don't fully understand it :(

    Worth taking off the cover and having a looksee underneath? Look for burnt components/wiring. (With power completely off, of course.)
  14. Graham price

    Graham price New Member

    You can see why I asked for some help !     There is one other clue.  With CH and HW off at the programmer provided I don't let anything get warm I can use the main switch and the pump only runs for half a second as it is switched on.   Once anything gets warm if I switch it all off at the main switch ( and CH and HW off ) and then put the main switch back on again the pump runs and runs.  It sounds like something sticking ( either electrically or physically ) once it gets hot.  I wonder if it could be anything in the boiler itself which is why it ignores the stats ?
  15. sam spade

    sam spade Active Member

    The resistor/diode is what holds the valve in mid-position. If you want heating only the valve just motors over completely, just like a zone valve.

    The purpose of the second resistance (R2) is to prevent 240V getting onto the boiler (and lighting it) when CH is satisfied but the valve is still held in the CH only position. R2 also prevents damage to the valve motor when it is in the CH only position.
  16. Graham price

    Graham price New Member

    Thanks everyone for all your help.  I tried another experiment.

    I left the HW on for an hour ( we need hot water ) then switched it off at the programmer.  The pump ran on but the boiler burner went out.  I think pump over-run is normal ?  I then put on the CH.  The valve drove across and the burner came on.  I then turned the stat down but the valve stayed where it was and the burner kept going too.  I could only stop it by using the main switch.  The initial fault happened in the middle of the night quite spontaneously, the main switch being on but the CH and HW off.
  17. Thanks, Sam. Could this resistor (R2) be affected so's its resistance is lowered? Usually when a resistor becomes damaged - overheated, etc - the resistance increases, but I wonder if something could have happened in this case to lower the value to the extent it keeps the boiler/pump running after the valve moves? Nah, I don't think so either... Just thinking aloud :(

    Graham, where does the valve cable go to - is there a wiring box? If so, I guess the room and cylinder thermostats, programmer and mains supply also go in there. Have a looksee in there (power OFF...) and check for loose terminals or wires (tug gently on each one).

    Can you work out which wire feeds back to the boiler to control it? (Sorry - can't help as I have a combi system and wiring.) If so, if you can isolate this wire when the system has been running and is still hot, I think you should be able to dismiss the boiler as being the cause; disconnect this wire, and the boiler should shut off. If it doesn't, then the boiler is at fault, I guess. If you're brave, you should be able to hold the wire by its insolated part and use and insulated screwdriver to slacken the terminal and pull the wire out while the system is running. Got to be your call, of course. Rubber gloves would make sense too.

    If we don't hear back... :'(
  18. Graham price

    Graham price New Member

    I should be shot.  I owe all you guys a big apology.  I have been assuming that because the pump in the airing cupboard has been running that the boiler is firing.  Today I had something to do in the garage and I heard the boiler cutting in and out so it's obviously obeying the HW stat.  That said, the programmer was off for both CH and HW.  The pump was running continuously.  Given that the valve seems to behave as expected this is beginning to say "programmer " to me.  Once again, my apologies for being an idiot but your views on the problem  would be welcome;  I suspect your views on me would not be !!
  19. Graham price

    Graham price New Member

    Addendum.  With the main switch and programmer off I pushed the valve  to the CH end then switched on the main switch.  The valve stayed where it was.
  20. Hi, no bullet loaded - yet.

    Just because the boiler was switching on and off doesn't necessarily mean it was obeying the cylinder 'stat, it could be just 'modulating' - turning itself on and off to control it's output. (As the cylinder heats up, so will the hot water return. So the boiler needs to provide much less heat to this water before it sends it around again. Ergo the boiler may need to shut itself off for a short while. In this case, the pump would keep running.)

    I still suspect the valve because: (a) even if the programmer was faulty, the room 'stat would still control the boiler as it's connected in series. From what you say the boiler behaves funny even if the room stat is turned down? (b) Why should turning the main switch and programmer off make the valve 'behave' when the power is turned on again? You are suggesting that the programmer works better after having the power turned off? How could that be?

    Tbh, we are going to struggle to take this any further unless you are prepared (and capable) to get a grip with the wiring and get your meter stuck in. Both lit and met'ically.

Share This Page