Alpha CB28 boiler

Discussion in 'Plumbing and Central Heating' started by BHS, 8 Mar 2010.

  1. BHS

    BHS

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    CB28 overheat problem. The diverter valve diaphragm has been replaced recently, so has overheat thermostat. Boiler trips out with red(A) led flashing red at approx 1 sec interval, i.e. overheat stat fail. Thermistors in HW and Primary circuit check ok, 10k when luke warm, 3.8/4k hot. Pump seems ok, very hot water at pipe to CH 1m away from boiler within 2/3 seconds. Gas flame does not seem to reduce as CH warms up, Very high with hot tap full on, reduces to level of ignition sequence when tap off and CH on. DHW switch checks ok, turns on/off with hot tap, boiler turns on/off in sympathy with no CH on. Average DC voltage across modulating coil of gas valve starts at 5V for ignition, goes to 11V at full HW and drops from 6V to 2V as CH heats up, but flame level does not appear to drop from level at 5V. Modulator coil seems free and diaphragm looks fine. I do not have a gas pressure gauge. Is the gas valve faulty?
    PCB appears ok, no dry joints etc. I am retired electronics engineer.
     
  2. Agile

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    What is the history of work done to the boiler?

    You say that the mod coil voltage is varying as expected but then your posting becomes vague as it says flame size does not drop on CH but you then say it drops on DHW.

    Another test you can do it to turn up/down the CH and the DHW control knobs and watch flame.

    Also to force boiler to modulate down by running the DHW on a minimum flow rate.

    If we are to interpret what you have written then the problem is likely to be a faulty gas valve which is not correctly modulating the output in accordance with the modulating voltage.

    However to be able to do that the gas valve has to have been correctly set up in the first place! They do not usually drift much and any problem is usually as a result of uninformed fiddling. They can sometimes be sticky but I have usually only encountered that on newish valves.

    We cannot give detailed information on adjusting the gas valve or any gas work. The only advice has to be to get a competent registered gas engineer.

    Tony
     
  3. BHS

    BHS

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    The only work on the boiler is to replace the fan relay three years ago, the PCB was then replaced with a recon unit two years ago due to spark transformer failure and in Jan. this year the diaphragm in the diverter valve replaced. The gas valve has not been touched other than to remove the modulating coil and check for free movement.
    Turning CH or HW controls causes the boiler to turn off burner until water temperature drops, then re ignites, with water getting very hot very quickly.
    With CH off, a hot tap will give a full height flame and as the flow is reduced the flame reduces to some extent (similar to ignition setting) and the the burner cuts off, restarting as temperature drops. The boiler does not seem to fail when in HW mode only. If this is gas valve failure I wil get a "gas safe" friend to check burner pressures and if required, change the valve.
    Many thanks for quick response.
     
  4. Agile

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    The only advice I can give is that you you get a competent registered engineer to check all the settings on the gas valve.

    Tony
     
  5. BHS

    BHS

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    Thanks for advice. My Gas Safe friend has been round, all pressures seem to be correct as listed in the service manual for this boiler. About 11.5/8 Mb at full hot water flow, dropping to 2Mb when CH hot. So gas valve seems to be ok. The only conclusion we came to is that the pump, although apparently working is not producing enough output, there does seem to be a larger temperature difference between the flow and return pipes than there should be. A new pump has therefore been ordered. Although this is a Grundfos 15-60, it is not a standard pump, having a different casting to accept an automatic air vent adjacent to the output fitting. Time will tell.
     
  6. Agile

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    Often you can just replace the pump head from a standard pump.

    Before you buy anything I would have said remove the pump head and check the impeller for a build up of dirt!

    If clean do the "finger test" on the pump, see the FAQ.

    Tony
     
  7. BHS

    BHS

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    The pump has been removed and the impeller checked. There is no dirt or sludge visible, the only deposit is a fine dark brown/black film on wet surfaces. The pump rotates freely, no noticeable wear in bearings. We did consider changing body with a standard version but the ratings on the end label were different, so decided not to risk problems and go for the genuine version. The boiler has only tripped once in last two days, possibly water is colder in return as weather is very cold.
     
  8. Agile

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    You should still have done the finger test in case the pump is lacking torque.

    If the impeller is clean and has plenty of torque then the pump is fine.

    Boilers usually use the larger size of domestic pump like a 15-60 if it was a Grunfoss.

    Tony
     
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  9. BHS

    BHS

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    The saga continues.
    A new pump was fitted on Thursday, I tried the "finger test" before the new one was fitted as before I had no reference, it is difficult to be objective with one sample. I could feel no difference between the two, they both burnt my finger before they could be stopped.
    The system has worked for two days, but it would do this before, and has today tripped out with the same indication, overheat thermostat.
    I have run out of suggestions from friends in the heating and gas business and Can find no hard evidence for the failures. The water circulating in the CH still seems to get very hot. With the front control on 3, the water still gets very hot, but the boiler does not come on again until the water cools down to warm. It would appear that the control board allows the temperature to rise well above that set for some time before turning off. Maybe there is an intermittent fault in the PCA. I know that this is the most difficult thing to find, having been in the electronics industry all my life.
     
  10. Agile

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    All these "getting very hot" do not enable us to reach any conclusion based on scientific evidence.

    We really do need proper thermal evidence!

    What you last mentioned almost indicated a scaled up temperature sensor as that would allow the temperature to exceed the set value before the sensor responded and it would cause the reverse hysteresis on cooling.

    Before I rushed into changing parts I would monitor the flow temperature at turn off and turn on during CH. Thats normally set by the circuitry at about 5°.

    Your symptoms may also be as a result of a boiler that has not had the CH output set to match the system. Is it still set at maximum?

    Tony
     
  11. BHS

    BHS

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    At the beginning of April I had a friend who is a heating engineer with a laser guided thermometer spend a lot of time measuring all the temperatures in the system.
    What I thought was very hot, was in fact 60 c, the feed line, after a few minutes to stabilise, remained around 60 c and the return was varying around 48 - 50 c. All radiators were checked, most had about 10 c gradient, two were a lot more as they are turned down to control temperature in those rooms.
    We then set the boiler to max, the output temperature rose to 90 c, should limit at 82, but the boiler did not trip out !! There may be a slight problem with the primary sensor, but as it regulates to 60 c under normal conditions it was decided to leave it alone.The settings were then returned to normal, 5 on the CH and the boiler output to 80% of output, considered correct for the size of house and no. of rads.
    Following this session the boiler has worked perfectly for about 5 weeks, even though nothing was changed, but has now tripped out three times in the last week. Although the flashing red led code says that the failure is due to overheat, I am sure that is not the case. Looks as if we are back to an intermittent fault on the PCB.
    Many thanks for your input, this has given me a lot of help and will hopefully solve problems quicker in the future.
     
  12. Agile

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    Laser guided?

    An infra red thermometer is very inaccurate if used to try rad balancing as its readings depend on the emissivity of the surface.

    A contact thermostat is required.

    There is so much information, much of it conflicting, that its very difficult to come to any conclusion about whats wrong.

    You say the output goes up to 90° which in itself would indicate something wrong IF thats an accurate value.

    Tony
     

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