Separate CH & HW control on an old Worcester 24i RSF..?

Discussion in 'Plumbers' Talk' started by SikeyMikey, Jan 31, 2023.

  1. SikeyMikey

    SikeyMikey New Member

    My son has recently bought a house that’s fitted with a (fairly old) Worcester 24i RSF combi boiler. This has a strange setup for hot water and central heating control which makes no sense to me. When I downloaded the manual, it seems to confirm that there is no electrical connection that would allow separate CH and HW demand. Its either all on or all off.

    If the boiler is ‘on’, then the CH pump runs and regulates to the set circ temp (with TRV’s on the rads), unless the system detects flow on the HW intake when it all flips over to HW (CH pump off).

    I can’t find any wiring that would allow HW but no heating of the CH loop, so no way to fit a proper CH timer/stat to the system (cos you would end up with a cold shower as soon as the rooms warmed-up). It also means you need to plan your showers around the system timer, or leave it heating the CH loop 24/7 for no reason, which right now would lead to stratospheric gas bills.

    Just wondering if anyone on here had come across one of these dinosaurs before..? Is my understanding correct, or did I miss something..?
  2. andy48

    andy48 Screwfix Select

    1. All combi boilers give separate control of heating and hot water.
    2. Hot water is given priority. When a hot tap is opened, the boiler fires (if not already lit) and circulates the hot water around a secondary heat exchanger (instead of round the central heating circuit). The water flowing to the hot tap runs around the "other side" of this heat exchanger and is duly heated.
    3. Central heating may be controlled by a programmer or thermostat. Once heat is called for, then:
    3.1 If the boiler is already heating domestic hot water, nothing happens until the hot water demand ceases. There is then a short (2 to 5 minute) delay, and the boiler fires again and switches the hot water output to the central heating circuit.
    3.2 If there is no current demand for domestic hot water, the boiler fires and sends the hot water to the heating circuit.
    4. Sounds as if there is something wrong with the boiler or its controls. Get a Gas Safe registered engineer to service the system, but explain the problem while booking.
  3. SikeyMikey

    SikeyMikey New Member

    Here it is... Please show me what I am missing... : )

  4. Mike83

    Mike83 Screwfix Select

    Think the diagram could be wrong.
    Boiler will have a permanent live.
    There should be another switched live terminal from an external control for the heating.
  5. Mike83

    Mike83 Screwfix Select

    88F19F6A-DAB0-4532-B9EE-FF569711F921.jpeg These could be the switched live inputs.
  6. SikeyMikey

    SikeyMikey New Member

    Thanks Mike83, but I think the X2 connector you circled is the same one that's at the bottom left of my pic, which has connections for the timer/stat, which then provides mains power to everything (transformer for the low voltage stuff, gas valve, fan, everything). So, it's still all-on or all-off and no separate CH/HW controls. Like you say, either the wiring diagram is wrong, or the CH loop is designed to run for HW to be available.
  7. The Happy Builder

    The Happy Builder Screwfix Select

    What, if any, programmer and room thermostat are connected to the boiler?
  8. The Happy Builder

    The Happy Builder Screwfix Select

    I’m guessing you need to add a room thermostat rather than relying on the thermostatic radiator valves, which could be a programmable thermostat.
  9. SikeyMikey

    SikeyMikey New Member

    The system has a mechanical timer on the front panel, one of those with little sliders for each 15mins, all arranged in a circle... there is no room stat, which to be fair is exactly what they want... my fear is that fitting a room stat will send the HW all over the place, since I can't see how the CH and HW demands are separated...
  10. arrow

    arrow Screwfix Select

  11. arrow

    arrow Screwfix Select

    It is a combi boiler where you have instant hot water, you do not have a cylinder to heat up. Turning on a hot tap gives water priority over heating.
  12. SikeyMikey

    SikeyMikey New Member

    Yes, yes, yes, really want to do exactly that. Problem is that if you follow the wiring diagram, then when the room stat is off, there's no power to the control board, so it will ignore the DHW flow switch and cold showers all round.
  13. arrow

    arrow Screwfix Select

    You said there is no stat, there will be a link.
  14. quasar9

    quasar9 Screwfix Select

    On a combi, the demand for hot water is sensed when a tap/shower is opened. It also takes priority. The heating side is controlled by a thermostat and a timer (programmer)
  15. SikeyMikey

    SikeyMikey New Member

    Yes, Yes, I agree that is how it should be, how everyone very much wants it to be, but my struggle is getting from from where it is to the place where they want to be... : )

    The connector and timer/stat all links up to one place (pin 5 of X2), which is the power to the PCB, gas valve, fan etc etc. The PCB cannot sense the DHW flow when it has no power and the timer/stat control the power. It's all on one circuit, DHW and CH together and no DHW without firing up the CH loop.

    It really makes no sense to power the PCB from the timer (since the board could be permanently on and the DHW should trigger just fine from the flow switch), but that's not how it's wired (in the boiler, not by annoying previous owner) and I can't for the life of me figure out why..!
  16. Tilt

    Tilt Screwfix Select

    If you want folk to help you, you need to get your information correct, at least ........ please.
  17. Mike83

    Mike83 Screwfix Select

    According to the wiring diagram the room stat and programmer are on the permanent live.
    This is what he means. If the programmer or stat is turned off there will be no permanent live for the hot water.
  18. SikeyMikey

    SikeyMikey New Member

    Thank-you Mike83. It's all correct, I promise.

    There is no room stat fitted at the premises. Promise promise. However, I want to fit one. What I am saying is that there isn't anywhere to fit one without messing up the HW.

    If you follow the links and wiring for the timer/stat on X2 then it all links up to one place (pin 5 of X2), which is the power to the PCB, gas valve, fan etc etc. The PCB cannot sense the DHW flow when it has no power and the timer/stat control the power. It's all on one circuit, DHW and CH together and no DHW without firing up the CH loop.

    That's my problem.
  19. adgjl

    adgjl Screwfix Select

    You have correctly shown the incoming mains connected to connector X1, and the timer & roomstat connected to connector X2. However, that is not how X1 and X2 are wired into the rest of the boiler, (or certainly shouldn't be). Your diagram (copied from the WB Installation Instructions) is a "functional diagram" not a "wiring diagram". Whatever a "functional diagram" is, it doesn't seem to show HW demand.

    The connections for the roomstat are shown in section 9 figure 5.
  20. The Happy Builder

    The Happy Builder Screwfix Select

    From the installation instructions
    2.14 OPERATION
    Domestic Hot Water: With a demand for hot water the burner will light at its maximum setting and then automatically adjust its output to maintain the temperature of the delivered water. When hot water is no longer required, the burner will extinguish. The fan and pump may continue to run for a short period to dissipate the residual heat from the appliance.
    Central Heating: With a demand for heating the burner will light at its minimum setting and gradually increase to give a controlled temperature rise. When the required heating temperature is achieved the output of the appliance is then automatically adjusted to maintain the temperature of the system. The output can reduce down to a minimum of 7.5 kW. If the system no longer requires even the minimum output to maintain the desired room temperature the burner will extinguish. The fan and pump may continue to run to dissipate the residual heat from the appliance. The appliance will remain off for a fixed period before re-lighting to automatically meet the system requirements.
    Domestic Hot Water and Central Heating: The appliance will supply heat to the central heating system as required. A demand for domestic hot water at a tap or shower will override the central heating requirement for the period of the domestic hot water demand. When hot water is no longer required the appliance will return to the central heating state and its normal mode of operation. The fan may continue to run to dissipate the residual heat from the appliance as necessary.

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