Combi boiler cutting in and out

Discussion in 'Plumbers' Talk' started by Silver45, Mar 22, 2020.

  1. Silver45

    Silver45 New Member

    Hi guys, I was wondering if anyone can help

    I have a ravenheat csi150t combo boiler. I know not the best of make but it’s actually been a great boiler for well over 4 years. I’ve kept it serviced. All I’ve had to replace is the timer.

    What it’s doing is when you demand central heating the boiler isn’t coming on fully for the period until you turn off the demand or it reaches its target temperature. The boiler and pilot light is cutting in and out throughout the entire period of the central heating demand being on. Typically the boiler will fire up for just 10-20 seconds but then it goes quiet and the pilot light display remains on for a 3-4 minutes without the boiler firing. The pilot light display will then go out for typically 3-4 minutes and then it all cycles again and repeats itself so every 6-8 minutes all I get is 10-20 seconds of the boiler actually firing up. The pump is running at all times

    My initial thoughts were it’s an overheating problem. I replace a clip on pipe thermostat which didn’t do anything. I now see there’s a screw on overheat stat. Is that the likely culprit?
    My only other thought is it could be the automatic air vent. I’ve bleed all the rads but I wouldnt be surprised if there’s air in the system. My pipes run over 3 floors and I’ve had problem with air in the system in the past

    Could anyone offer any help or advice please? I’m pretty competent in replacing parts once I know what to order

    Thanks for your help it’s appreciated

    Sent from my iPhone
  2. itchyspanner

    itchyspanner Member

    and the hot water runs fine without cycling hot and cold?
  3. Silver45

    Silver45 New Member

    Yes the hot water is absolutely fine
  4. Silver45

    Silver45 New Member

    Can anyone else advise what part they suspect I may need to swap? I’d really like to get it ordered and try

  5. PandA3

    PandA3 Member

    If you can see and get at the overheat stat then test that it gives continuity through it..
    Or better still, if you are competent, then bridge it out with a suitable wire/cable/connector, and try a CH run.
    I would think if the pump is running then the external controls are calling for heat, but switch the boiler to manual/max anyway.
    If no change with above, I would start to suspect the pipework, blocked or low pressure? what is the pressure gauge reading?
  6. PandA3

    PandA3 Member

  7. PandA3

    PandA3 Member

    p.s don't leave it running for too long with stat bridged... just long enough to establish it stays on longer than it normally does before cutting out.
    Maybe better to test electrically that the stat isn't cutting out.. but you'll need a multi-meter for that.
  8. Silver45

    Silver45 New Member

    Thanks I really appreciate your reply.

    The stat is very easy to get to. I’ve uploaded a pic. If I remove either of the 2 wires when central heating is being demanded the boiler will fire up into action immediately but pilot light goes out. I don’t know if this tells anything.
    To bridge it shall I just remove both wires and bridge the 2 connectors? I can do this no problem.
    Pressure is at 1.7 bar. Thanks

    Attached Files:

  9. Silver45

    Silver45 New Member

    I do have a multi meter but I’ve never been good with one to be honest lol.

    What exactly am I looking for andvill give it a go? Thanks
  10. PandA3

    PandA3 Member

    I don't think it is a simple stat switch (on off).
    I believe that is a thermistor that will give a resistance to an intelligent controller.
    need to check what output/reading that component is designed to give at certain temps.
    Where we find that info I'm not sure, but I expect most makes are similar, so some googling needed.
    Alternatively, change it anyway as its less than £20.
  11. PandA3

    PandA3 Member


    Set your multimeter to Ohms, disconnect both leads and test for approx. 12,500 Ohms (12.5k Ohms) at room temp.
    If it is ok then take it off and put it in a saucer and pour hot water in and watch the reading DROP to 1500 Ohms (1.5kOhms)
  12. Silver45

    Silver45 New Member

    I think I’ll order a new one and swap and see if that works. It seems a logical place to start..... I’ll report back once I know either way. I really appreciate your advice. Thanks :)
  13. PandA3

    PandA3 Member

    Ok.. I noticed while looking for the resistance values for your boiler that a few comments were referring to checking the wiring loom, so look at both ends of the cables for condition.
  14. Silver45

    Silver45 New Member

    Ok I will do. Thanks. I ordered the part. It was only £15. Hopefully it will be here Wednesday

    I ordered an auto air vent before posting up here as I read it could be air in the system. Again only a cheap part so no big deal if that’s wasted. I think the thermo is most likely.... fingers crossed

    Thanks again. I’ll report back my findings
  15. Silver45

    Silver45 New Member

    I fitted the thermistor overheat stat today. Unfortunately no change. On first demand of central heating it only lasted 30 seconds before pilot light went off on the display and it started to cycle. Gutted. I checked all the wiring as suggested and that looks perfect.

    You suggested before of a potential blocked pipe. Is there any trial and error things I can do to try and find the problem? Any thoughts that it could be air in the system? I read this in my user manual. I got the automatic air vent part through but no point in changing if it’s not a real suspect.

    Thanks again. The advice is really appreciated
  16. PandA3

    PandA3 Member

    It does sound linked to the flow in the pipework. either blocked or air or maybe even Pump not performing as should.
    If the HW side of the system is working fine then I cant see why it would be Gas related, or ignition related, or fan etc.
    I'm reluctant to suggest to bypass flow/pressure switches to see if it runs longer, but that's what I'd do.
    Low water pressure is where I'd look first.. Take connectors off and link them together. Only do it for long enough to establish if it changes anything.
    If this is beyond your competence, then I'd strongly recommend getting someone in.
    (have you bled the pump? by unscrewing big middle screw?)
  17. Silver45

    Silver45 New Member

    No I haven’t bled the pump. I didn’t know you could. I’ve only bled from the rads and boiler. I will try this now and also go and check the rads again. It’s been a few days now. I think the pump seems to be running fine, it seems to get the water around the system quickly.

    I’d get someone in if it wasn’t for this Covid 19 and being on lockdown. What connectors would you take off and link together? I assume you don’t think it’s worth fitting the auto air vent?

    Is there any value in shutting some rads off and running the pump? Maybe opening each rad one at a time? In the past I did have some issues getting heat to some rads. The system has been treated with chemicals although no flush. I just have a magnatec

    Thanks again :)
  18. Silver45

    Silver45 New Member

    I just attempted to bleed the pump. It wasn’t what I expected. Just a trickle of water when untightening the screw, nothing pressurised and no air. Is there something I’m not doing there?

    All the rads were checked again and didn’t need bleeding

  19. PandA3

    PandA3 Member

    No.. it should just trickle... was it running?
    If you can see the 2 brown wires that go from the control card to the low pressure water switch then link them out.
    If its easier at the controller end just bridge the 2 terminals. If you are at the pressure switch end, pull the 2 connections off and put them together (maybe a paperclip into each connector, or something metal that fits in the connector)
  20. Mike83

    Mike83 Well-Known Member

    Sounds like a circulation issue.
    Are the boiler and any filters isolation valves open?
    Could maybe be the diverter valve.

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