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Reverse boiler flush? (Viessmann Vitodens 100)

Discussion in 'Plumbing and Central Heating' started by MikeAustin, 20 Dec 2015.

  1. MikeAustin

    MikeAustin

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    This is temperature measurement - not flow. I would expect it indicates flow problems in most cases, but there may be exceptions where it relates to a very long system path.
    That is useful to know. As a matter of interest, how far away is your most distant radiator? Mine is about 18m - because the boiler is in the garage at the end of the house. Does your system get up to full temperature - and quickly?
     
  2. Agile

    Agile

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    I dont see why you seem fixated on the boiler responding to the return temperature.

    It seems pretty obvious to me that the boiler is turning off after a very short burn because it reaches a high flow temperature of 70 C. It then stays off for the anticycling time.

    My solution to that would be to increase the flow rate which may not be possible because of flow resistances or to reduce the boiler maximum power to encourage it to burn for longer.

    But that is based on heating your cylinder which will be a smaller load than the heating.

    For fun why not reduce the maximum boiler power and repeat that test?

    Tony
     
  3. Phill Peck

    Phill Peck

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    Not an option on this model Tony.

    OP you really need to get someone in who has experience and knows what they are doing. I appreciate u have some knowledge here. But you're getting nowhere.
    These problems can't be resolved over the Internet.
     
  4. Agile

    Agile

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    The OP first told us that its a 100 C

    But the Viess website shows their boilers as 100-W with the OV as WB1B. Presumably he has the earlier WB1C ?

    Yes, with a boiler that decides on what power output it is going to give, that limits what can be adjusted to the external aspects of flow rate as determined by the pump and pipework resistances.

    Having said that generally the Viess boilers do seem to make quite a good job of choosing what power to run at. With a normal system which is not blocked and with a working pump then they are very good.

    Tony
     
  5. Phill Peck

    Phill Peck

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    The 100 range has been labelled as....
    Wb1a
    Wb1b
    Wb1c
    C being the latest version but there is no OV in the wb1c range. The OV is still a wb1b.

    But we have already established this is a wb1a from 2005
     
  6. simond

    simond

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    This is using a lot of space up on the internet and I'm worried we could run out.

    I can tell you, Mike, that the worst people to do boiler repairs for are the ones who think they know the answer. They tell you the possible causes rather than the symptoms.

    You have a boiler that might be faulty but you are not sure. Your temperature graphs have not given you the answer.

    We, as I said at the start, together with Hooting Owl I believe, have seen problems with this boiler where junk gets caught in the spiral long HE. From the pictures it looks like a WB1A open vent, we fitted around 40 of these.

    When the boiler is installed, mains water has to be circulated through the boiler to clear it of air, this is normally done with an inlet and outlet garden hose, simples, but a pain in the arsse. If you run the hose till the air bubbles stop, the job is done. (Whatever Tony thinks).

    Once the boiler has a few years under it's belt, and because of the high flow resistance, dirt can get trapped in the spiral long HE. But unlike Vaillants etc with the Giannoni multi spiralled HE, you can easily flush it out at mains pressure using the two hoses directly on the boiler flow and returns. Obviously, there isn't a problem with air after doing this (Tony, take note).

    If we have one that is giving odd ramp up times and keeps cycling, we connect the hoses as described, get a healthy flow of water through the thing.....and turn it on. The boiler will be heating the hose outlet and you can slightly vary the flow rate and see if the boiler fires normally. If it does, then it is a system flow problem.

    And you can use the opportunity to wash the car.
     
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  7. MikeAustin

    MikeAustin

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    Hehe, it is the boiler that is fixated - not me! I am just trying to fathom out the boiler's fixations. Apart from the user controls, it seems the return temperature and flow switch (simple on/off sensor) are the only sensors that it responds to. Or is there another sensor somewhere? It does not know the flow but uses the temperature to give it some idea of whether it is sufficient or not. I am questioning the validity of the assumptions it makes for all situations.

    In this case, HW only, it is true that the cylinder thermostat was set at 60C. The measurements I took just allowed me to calculate the flow rate by observing when the heated water returned.

    I have done that before but there is little if any difference. There is a power dial on this boiler (WB1A) on the front panel. I normally have it at max (80C) but there is an 'efficient' setting (60C, I think). I plan to try HW again on Monday with the cylinder stat at 80C. I don't know how much more of this 'fun' I can take!

    Actually, it is (see above). In addition, behind the control panel, there is a pot to limit the power.

    Yes, a professional is coming to service and survey on Tuesday. Because of the possibly large sums involved in fixing this, it is essential that I have an understanding of what is going on. For example, if the way the boiler treats the return temperature can be 'fixed', I may not need to consider a new boiler.
     
  8. MikeAustin

    MikeAustin

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    Oh, I did not realise there was a possible space problem here. Because of your experience with this particular boiler, your posts are always helpful. Could I email you via the address on your website? I would like to ask some specific questions about what you have said here.
     
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  10. Phill Peck

    Phill Peck

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    Turning the thermostat down and rescuing the output are very different things.
     
  11. Agile

    Agile

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    As far as I recall your WB1A OV boiler only has a FLOW temperature sensor and so that's all that can responded to.

    Its clear from your graph that it turns off when the FLOW reaches 70 C. That's a well defined temperature on your graphs so why do you think it only turns off on return temps?

    Under normal conditions it would modulate back to its minimum power input.

    But you seem to have rather limited flow through the boiler on HW only. I am not sure why that should be because most hot water heating coils and connection pipework are 22 mm and usually that is lower resistance to flow so gives the highest flow rate.

    Tony
     
    Last edited: 16 Jan 2016
  12. MikeAustin

    MikeAustin

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    Yes, that is right. It is just a switch. It does not measure flow - only confirms that there is flow of some sort.

    On that particular graph, posted to show how I estimated the actual flow in the HW, the cylinder was set to 60C. So that is the user's limitation. On my other CH graphs, with thermostat to maximum, there is essentially no user limit. All the boiler has to go on is the return temperature.

    The cylinder is indeed 22mm. The boiler HE is a coil of about 18m of 20mm x 40mm section pipe with a wall thickness of 3mm. The flow rate I calculated was 323L/hr (equivalent to 29cm/s through the boiler HE) for HW only. Because I don't have the experience, I don't know if this is high, low or normal. But, whatever the case, the boiler has no way of knowing the flow and thus will have no inbuilt limiting values for it.
     
  13. Agile

    Agile

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    Oh dear, you are describing water flow rates and rather confusing them with the flow temperatures.

    Then adding to the confusion by including external controls as if they were part of the boiler. The cylinder stat measures the average cylinder temperature and not the heating coil flow temperature going into it.

    I am very familiar with the construction of the Viess boilers and its a very good heat exchangers and as a one loop so easy to clean.

    Your boiler has a flow SWITCH which is just a basic interlock to prevent it firing if there is no water or no water flow.

    It monitors the flow temperature with a temperature dependent resistor.

    Depending on the exact model it may or may not have a similar temperature sensor on the return to the boiler. I don't think that your model had a return temperature sensor.

    Tony
     
  14. Agile

    Agile

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    Are you sure of those sizes?
     
  15. MikeAustin

    MikeAustin

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    I don't think so. The boiler only has the return temperature to assess whether the flow is adequate. I suggest this may not be adequate for systems with a long circuit. Of course, it also monitors it's own temperature.

    Again, I don't think so. I only said that my last HW run was limited by my setting of the cylinder stat. My CH runs were done with the room stat at maximum, so it was not a limitation.

    Yes it does. I have seen it, and it is in the service manual. It is one of my current investigations (theoretical - I am not tinkering with it!) whether something could be done electrically to make the return appear to be warmer. It seems to be a semiconductor device with a logarithmic resistance-temperature dependency.
     
    Last edited: 16 Jan 2016
  16. HootingOwl

    HootingOwl

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    Mike...why not try putting a fixed value or variable resistor in place of the return sensor. You can then model the return temp and see if this has any effect on the firing profile.
     
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