Boiler does not fire up on demand - Follow up

Discussion in 'Plumbing and Central Heating' started by Tanel, 1 Dec 2021.

  1. Tanel

    Tanel

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    Hello Group,

    I originally posted this a couple of months ago:-

    https://www.diynot.com/diy/threads/...our-boiler-does-not-fire-up-on-demand.579594/

    and I had some degree of success thanks to your kind help. I changed the jumpers on the controller to the correct setting and I also bled all the radiators throughout the house, and for several weeks I was getting heat on when I needed it.

    Today I have largely been sitting in a cold house, even though the room thermostat is way up and calling for heat.

    One thing I have noticed though is that if I gently turn up the hot water cylinder thermostat (this):-
    [​IMG]

    then the boiler will fire up, radiators will come on and normal service is resumed. This has worked more than once.

    What I think is happening is that the cylinder thermostat thinks there's already enough hot water in the system and is therefore doing nothing. When I turn it up a bit, it calls for more (hotter) water, signals the boiler to come on, and also triggers more (hotter) water to go through the radiators.

    What I don't get is that it's not as if I have some "warm" water in the radiators. When they're not on, they are properly cold, it's seemingly "all or nothing". Therefore, I don't understand why the trigger to push hot water around the central heating system is not happening anyway even with a temp setting of (say) 50 degrees.

    I can live with the minor inconvenience of adjusting the cylinder temperature but it would be nice not to have to do it. I'm keen also not to have to "ratchet" up the cylinder to hotter and hotter temperatures each time, as this would make my hot water way too hot to use.

    Am interested to know, do these cylinder thermostats tend to fail this way after a time? Can they be replaced or are they integral to the cylinder?

    Thoughts welcome and appreciated.
     

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  3. BlueLoo

    BlueLoo

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    Suggest you look at the wiring diagrams and plumbing diagrams for a Y plan heating system. Draw yours out, including wiring and see if anything looks further amiss (noting the incorrect jumpers earlier). If you can't figure out the obvious, call in a heating engineer.

    Your issue reminds me of the failure mode of my ported valves when they die. Valve opens but the boiler isn't getting a call for heat.
    You need to work through each step logically.
     
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  4. ianmcd

    ianmcd

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    you have a motorised valve fault
     
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  5. DP

    DP

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    Look for 240 volts on the grey wire when heating is calling and hot water is off.

    If no 240, you have screwed up the wiring

    By raising the cylinderstat setting, you are putting condition on grey that should be there with HW being off or up to temperature
     
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  6. ianmcd

    ianmcd

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  7. DP

    DP

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    1E3BCC01-98D8-467C-AAB7-B19153FEC17E.png

    When you turn the heating on, the motor in the motorised valve positions the valve to mid position but boiler activation is from the cylinder thermostat “calling for heat” terminal to which the orange wire is connected. So boiler runs the moment you raise the cylinder thermostat from 50 to 55. Heating alone does not work because the grey wire is not connected to N in the programmer. As have already suggested, with heating and hot water in off state, check grey wire- it should have 240 volts on it.
    Looking at diagram above, you call for heat, white wire is now at 240, motor runs. SW1 breaks the drive. If 240 was present here, the motor would continue to drive to operate SW2 to put 240 onto the orange wire thus running boiler and pump

    Not the case- see above

    Again see above. More a wiring issue

    See above

    Normal to set the thermostat to 55 for me anyway.
     
    Last edited: 2 Dec 2021
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  8. Tanel

    Tanel

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    The motorised valve (with the W-M-H on it) has appeared to work and move to all three positions without problem, although I don't know if you mean something more subtle than that. Maybe it's not a question of it mechanically failing, instead a case of it not going where it needs to go, when it needs to go there.

    I'll try and digest DP's reply which refers to the valve too.
     
  9. BlueLoo

    BlueLoo

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    Zone valves work by:

    Powered heating controls send 240v live to thermostat when timer is ON.
    Thermostat is just a dumb switch, so if the thermostat is asking for heat, it just closes its switch sending the live from the controller to the zone valve....
    .....the zone valve receives 240V live from the controller, via the thermostat and energizes the motor, which turns the valve on (normally) or open......
    Once fully open, the zone valve has set of reeds in it which close once the valve is fully engaged (this delay is deliberate). Those reeds have a sperate 240v live across them waiting for the valve to open.....
    Once the valve is opn, and the switch reeds close, that (separate) 240v live is sent to the boiler switched live which tells the boiler to turn on.

    Somehow, this final 240v signal to your boiler has been interrupted. However, because you have a cylinder thermostat working on the same principle (sending a switched live to the boiler once a condition (low temp) is met, your boiler fires up and because the zone valve is open, you get heating.

    Probably either the zone valve is goosed (just the head mind, which can be replaced separately), or, the 240V live to it has been broken (hence the advice here in the thread to check it). Look for a blown fuse, loose wire, etc.

    To recap:

    240V------Controller-----thermostat-------motor.
    240V live-------|-------|---------Zone valve switched live --------Boiler----------whooosh---heat--,etc.
     
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  11. ianmcd

    ianmcd

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    The op doesnt have zone valves he has a mid position valve
     
  12. ianmcd

    ianmcd

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    AS @DP diagram, when the mid position valve moves to any of the three positions, the valve makes a tiny microswitch, one of these has likely failed, it is these micro switches that tell the boiler to come on
     
    Last edited: 2 Dec 2021
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  13. Tanel

    Tanel

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    It's been really useful looking at the wiring diagram and I've read this through about 5 or 6 times now:-

    http://wiki.diyfaq.org.uk/index.php/Three_port_mid_position_valve

    I think it may take 2 or 3 more read throughs but at least it is little by little sinking in.

    This is the valve I have:-

    [​IMG]

    I think the only thing stopping me now getting myself a multimeter and testing the voltage on the wire(s) inside is the fear of damaging it. Did I understand correctly that hot water and heating should both be OFF first before attempting to open this?
     
  14. ianmcd

    ianmcd

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    you dont test the valve inside the valve you test at the wiring centre, have to say I think you would be better getting a pro in, you dont sound very competent to do this
     
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  15. Tanel

    Tanel

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    Fair point, and I will probably do that, thanks.
     
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  16. ianmcd

    ianmcd

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    If the valve is moving between all the correct positions as the controls call for then it is the valve microwswitches that are faulty, the actuator head just clicks off no need to drain anything, you just push the little black release button at the opposite side of the position indicator and it just pops of get a replacement, and get one of these,

    https://www.ebay.co.uk/itm/363561488945?hash=item54a5f41231:g:qwUAAOSwkr1hVZwR

    Turn the power off to the heating system and cut the existing wire and connect to one end and the new valve to the other end and plug them together and turn the power back on, pop the new actuator on , it just clicks on, and job done, the connector will save you going into the wiring centre which I wouldnt recommend you do
     
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  17. BlueLoo

    BlueLoo

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    Same principle.
     
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