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Discuss Worcester Bosch Greenstar 30 CDi Conventional Boiler in the Central Heating Forum area at

  1. Philip Bartleet

    Philip Bartleet New Member

    Hi All,

    I have the above mentioned boiler linked to a Nest Thermostat. The problem I have is when the boiler is set to No.5 which heats the water to circa 67 degrees, it reaches temperature, the gas light goes out and it cools down to circa 30 degrees then heats back up again. This continues for the whole time that the the Nest is calling for heat. The issue is the house struggles to get to temperature.

    If I set the boiler to between 6 and maximum (heating the water to circa 79 degrees) the boiler stays on constantly until the Nest Thermostat reaches the desired temperature.

    I've spoken to Worcester and Nest but neither can understand why the boiler would be doing this.Ideally I want to run the boiler on the lower temperature which is sufficient to heat the house and water.

    The boiler is approx 8 years old and has recently been serviced which did not identify any problems. The engineer did suggest there could be air in the system which I've checked and all seems fine.

    Approx 2 years ago when we were having some work done, the builders drained the heating system which unfortunately meant that significant debris from the F&E tank got sucked down into the system amd meant the water could not flow from the boiler and it was cutting out. We had an an engineer at the time reverse flush the system and the boiler came on but could this have caused the problem? Could there be damage to the heat exchanger? This all happened when we had recently moved into the house in the summer time so I'm unsure how the boiler was functioning before.

    Any suggestions would be gratefully received.
  2. Jones82

    Jones82 GSR

    Sounds like its working as intended, the boiler is running constantly as the house is cold, its not unusual to run the boiler at higher temperature during cold weather
  3. Philip Bartleet

    Philip Bartleet New Member

    Quick update on this, last night with the boiler set to between 6 and Max and the Nest set to 18 degrees, the boiler starts cycling on and off at about 17.5 degree and the house then lost temperature to 17 degrees.

    I agree with regards running the boiler at a higher temperature in colder months but surely it should run constantly whilst the Nest is calling for heat whatever temperature the boiler is set to.
  4. Jones82

    Jones82 GSR

    When you adjust the boiler your adjusting the maximum temperature that the heating pipe work gets too. For example most boilers max temp is 82 degrees. The boiler thermostat senses the boiler has reached 82 and shuts of the burners, but still runs the pump, so the water is being circulated around the system. This circulation cools the water and it returns to the boiler at a lower temperature, the boiler senses this and then fires the burners to obtain the boiler thermostat temperature you have set. Rather than reading "1-6" most modern boilers tell you the temperature in degrees. So if you set the boiler to 3-4 aprox 65 degrees the pump will run but the boiler won't fire as often.

    The nest controls the room temperature, the boiler thermostat controls the boiler, radiator and pipe work temperature. Usually ranging between 40-82 degrees, or "1-6"

    In winter you may have to run the boiler at about 75-80 degrees to keep up with all the cold air and heat loss
  5. Rob Foster

    Rob Foster Top Contributor!!

    From what I learned at a meeting with some senior figures in domestic heating industry the other day there are big problems with nest. Worcester Bosch quality has also declined with 300 engineers on the road waiting times are now up to 3 weeks from next day once upon a time.This Is surely the builders liability, esp. if the heat exchanger is blocked. Take the nest out and revert to simple controls to begin progressive fault finding from simple known variables. Rob Foster aka centralheatking
  6. Philip Bartleet

    Philip Bartleet New Member

    Thank you Rob. I agree with your suggestion and will revert back to simple controls to establish what is happening.
    • Like Like x 1
  7. Eyes

    Eyes New Member

    Excellent advice above, also have checked whilst reverting to simple controls that the external pump is electrically supplied from the boiler as per MI and most importantly if it is a smart pump that the polarity is correct, just a quick simple check for your engineer while he/she is in there.
  8. Chuck

    Chuck Well-Known Member

    Insufficient circulation rate can give the symptoms you describe. A partially blocked heat exchanger is a strong contender with a failing pump or sludged-up pipework also in the running and an incorrectly set ABV as an outsider.

    If the behaviour is the same for both CH and HW it points to a problem inside the boiler (or the common pipework). If the cycling is absent when heating the HW cylinder it points to sludge in the radiators or a faulty valve.

    A competent heating engineer should have little difficulty diagnosing and fixing the problem.
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