Combi Boiler Water Hot Then Cold

Discussion in 'Plumbers' Talk' started by m623d, May 16, 2022.

  1. m623d

    m623d Member

    Hi, we've got a Baxi Platinum Combi 24HE boiler.
    Recently took out the bath and fitted a new shower, all ok for a couple of weeks but now every tap and shower in the house runs hot then cold/lukewarm then hot then cold etc.
    The boiler is firing up, water temp (on digital guage) goes up to 92, then it shuts down (demand light still on but flame light goes out) temp drops to 72 then if fires up again and the cycle begins again.
    Could this be related to my new shower or is this most likely a coincidence?
    Thanks
     
  2. Mike83

    Mike83 Screwfix Select

    More than likely a blocked plate heat exchanger.
    Easy enough for a pro to remove and clean/replace.
     
    m623d likes this.
  3. m623d

    m623d Member

    How easy on this boiler for a diy'er?
    I've changed a couple of diverter valves on other boilers in the past.
    If not, what's the likely cost?
    Boiler is at least 7 years old so don't want to throw money at an old boiler if it would make more sense to just fit a new one.
    Thanks.
     
  4. Mike83

    Mike83 Screwfix Select

    Very simple diy job.
    Power off.
    Close cold water inlet and open hot tap and wait on water stopping.
    Close flow and return isolation valves then use boilers drain to empty water from boiler.
    Unplug the diverter valve motor and remove the pin holding the motor (this creates a bit space).
    Place towels below the boiler.
    Then it’s just 2 screws holding it in.
     
  5. m623d

    m623d Member

    Do I need any new o rings or seals?
    Can it be cleaned or does it need a new one?
    Thanks
     
  6. m623d

    m623d Member

    Also whilst I was watching the boiler with hot tap running, at one point a fizzing noise started and the central heating pressure gauge was fluctuating
    Like maybe a boiling noise. This was with the tap maybe half open.
     
  7. m623d

    m623d Member

    Anyone pls?
     
  8. dcox

    dcox Screwfix Select

    The water can’t pass through the plate heat exchanger quickly enough so it will be starting to boil then trips the overheat sensor so the boiler turns off until it cools then the process starts again. If you can afford a new one, I’d do that which will come with new seals.
     
    m623d likes this.
  9. m623d

    m623d Member

    Thanks.
    Is it just scale like a kettle gets?
    Could you descale it the same way?
     
  10. plumbpete

    plumbpete New Member

    You should try this product which is used for removing sludge and limescale from central heating systems. Just pour it into the removed Plate Heat Exchanger and leave it for an hour or two.
    *
     
    Last edited by a moderator: May 18, 2022
    m623d likes this.
  11. m623d

    m623d Member

    Thanks.
    Further to the hot and cold, tonight when the Mrs had a shower she noticed a couple of radiators upstairs had got hot.
    I'm guessing this is the divertor valve?
    Could the whole problem be the valve or am I looking at the heat plate AND the divertor valve?

    Thanks again
     
  12. Mike83

    Mike83 Screwfix Select

    Divertor could just be lifting slightly due to plate offering resistance.
    Clean or replace the plate heat exchanger first.
    If your unsure just close the flow isolation valve when using the hot water and see what symptoms you then get.
     
    m623d likes this.
  13. m623d

    m623d Member

    Flow isolation valve? Is that the switch on the front for DHW or CH? Or is there a physical valve to close?
    Thanks
     
  14. m623d

    m623d Member

    Also, what will it tell me, if for example it still heats the radiators?
     
  15. dcox

    dcox Screwfix Select

    Under the boiler there will be a flow and a return pipe which carry hot water to and from your rads. Each should have an isolation valve that can usually be turned by hand. Put your heating on and see which pipe gets hot first - that will be your flow. If you isolate the flow then test your shower/taps again to see if the temperature still fluctuates. If it does, you can disregard the diverter valve as a possible cause.
     
    m623d likes this.
  16. m623d

    m623d Member

    Ok I've got the manual and found the valves, however the flow will not budge. I dont want to force it cause its a plastic handle.

    The manual says to change the heat plate you have to remove a gas ignitor so I guess that means I'll need a plumber.

    If it means professional repair, would it be better in the long run to spend that money towards a new boiler when it's 7 plus years old?

    Thanks
     
  17. Hausfix

    Hausfix Well-Known Member

    Have you considered it may be as simple as air trapped in the system? The boiler would behave in a similar manner if so.
     
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  18. m623d

    m623d Member

    Dont they self purge?
    How would I sort this?
    Thanks
     
  19. m623d

    m623d Member

    Ok I've managed to turn off the flow.
    Strange thing is the pipe still get red hot on both sides and the dhw still fluctuates
     
  20. Hausfix

    Hausfix Well-Known Member

    I’m not expert on Baxi boilers, but I think you have to force to boiler into an air purging mode, from memory, switch off boiler power and wait 10 seconds. Whilst it’s off, turn the central heating temperature dial to the minimum setting, (pointing to 12 o’clock)
    Turn power back on, then as soon as it powers up, turn the central heating temperature dial a quarter of a turn (to 3 o’clock position) then back to zero then repeat this straight away.. (so you spin the dial a quarter turn, turn it back, then a quarter turn and then back again, all within 2 seconds)
    Boiler should then be in purge mode and will take about 10 mins to do this.
     
    m623d likes this.

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