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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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    But I think you are more disposed to explain the reasons why rather than just dismissing my comments.

    I don't know how I could better explain. In the reverse flow flush, there is just the length of flow pipework from the pump to the boiler and then out from the drain just outside the boiler return. I will not be disconnecting the boiler to flush it directly. With the forward flush, the boiler us not included. A valve on its flow is turned off. The flush goes through the pump, through at least one rad, through the Magnaclean and out the drain at the boiler return. I hope this makes sense, but I could draw a diagram. If you follow what I have said, do the flow rates that I measured make any sense?

    What are the "three possible loads"?

    I assume the flow temperature is directly related to the temperature on the LCD. Then, according to the Viessmann engineer, the working temperature difference is 20C. So it seems that flow and return temperatures are linked via software. The Viessmann engineer said that is why the boiler was cutting in at 41C and cutting off at 60C. Was he correct?

    Yes, that is possible - although it did not happen in the previous six occasions. What I now need to do is run the system again with and without the extra lagging to prove this.

    If the return is 55C (I.e. 75-20), it can
     
  2. Steelmasons

    Steelmasons

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    With a correctly functioning system (no restrictions/blockages) it's not uncommon for delta T to be around 2c in some instances (75/73).
    If this is not the case then the system has a blockage.
     
    Last edited: 9 Jan 2016
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  3. Agile

    Agile

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    My recent postings have been specifically to get you to flush the boiler HE using mains water and a flow rate of at least 15 li/min if possible.

    That would normally be done most effectively with direct connections onto the boiler!

    That is for the sole purpose of ensuring there is no air left in the HE which would reduce the heat transferred into the water.

    Part of your reply above seems to have concerned flushing the system without the boiler in circuit.

    The three boiler use conditions are:-

    CH only

    HW only

    CH + HW together


    Tony
     
  4. Agile

    Agile

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    How is it getting on Mike?

    Tony
     
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  5. MikeAustin

    MikeAustin

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    Thanks for asking, Tony. The system is limping along, taking a long time to get to a working temperature below the boiler's maximum. It is 'livable' until I get the professionals in, starting with a service and survey next Tuesday.
    My main interest now is the length of pipework I have. This reduces the return temperature. At the start, a low return temperature makes the boiler think the flow is not good enough, so it stops burning. That is why I was trying everything to keep the return warm.
    What I have done today - and I find it quite interesting - is to measure the temperatures for HW only. After about 10 minutes, I notice an increase in temperature about 2 minutes after the boiler stops burning - see below. This is a repeating pattern. I take it to indicate a warm 'packet' of water has returned to the boiler. Using this time period, calculating the length and volume of the piping (including boiler HE and cylinder coil), I get a flow rate of 323L/hr - equivalent to 29cm/s through the boiler HE. Of course, if the CH were switched in, this flow would increase.
    So my question now is how does the boiler know if it is a blockage or simply long pipework? After all, it has no flow meter and only uses the return temperature to check this. Furthermore, I think the boiler has a minimum fire up period of 4 minutes (maybe less, I don't know) after the last burn to avoid short cycling. It seems to me that this behaviour severely limits its performance as it tends to get into a negative loop.

    [​IMG]
     
  6. Phill Peck

    Phill Peck

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    The flow will not increase if u open the htg circuit.
    The boiler doesn't know if it has a blockage or a long return time that the point of why it fires low.
    Imagine u could tell the boiler it had a very long circuit and would therefore take say 20mins to Se a return it temp.
    It would the fire at max output.
    If a blockage then happened it would over heat
     
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  7. MikeAustin

    MikeAustin

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    That is what someone else told me - but they could not explain why opening up more pipework does not allow more flow (not speed, volume flow). Can you explain?
    That makes sense - but it does not help someone like me who has a long circuit and no major blockages (as far as I know). Does it mean that my boiler is not the right one for this circuit. Indeed, could it mean that there is no domestic boiler suitable?
     
  8. Steelmasons

    Steelmasons

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    Mike , I thought you solved the problem when you insulated the pipework? Did you not say this in a previous thread?
     
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  10. MikeAustin

    MikeAustin

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    It improved it a bit - and I still have the 'lagging' there - but I also removed it and tried the system again. It had improved things a bit, but not as much as I originally thought. The idea was to get the return up to as high a temperature as I could so that the boiler fired. I was grasping at anything. I may even yet put a nightlight under the return temperature sensor!
    I think there are some unpredictabilities in this system. For example, the figure that I just posted seems to indicate 'packets' of warmer water going round the circuit. This is probably not so noticeable on CH. Should these packets sync with the boiler, it could make a difference whether it fires continuously or not.
     
  11. TCCHeating

    TCCHeating

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    Hi Mike. I'm sure I read a post on here recently where someone suggested that a manual bypass valve (only very slightly open) should be put between the flow and return pipes on these boilers to warm the return straight away so the boiler ramps up quickly
     
  12. MikeAustin

    MikeAustin

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    Yes, that's right. I responded to it asking for advice. Today I spoke to Viessmann support and the representative did not seem that keen on the idea.
    Basically, all that needs to be done is to trick the boiler into firing by making the return temperature seem to be higher than it is. So I wondered if some resistor could be added to the return temperature sensor. Any ideas out there?
     
  13. roguetrader

    roguetrader

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    I always get a terrible sinking feeling when I read...

    'It's ok, I'm an engineer myself'
     
  14. HootingOwl

    HootingOwl

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    One of my clients has the same problem. He got Viessmann in. They diagnosed a blockage in pipework or heat exchanger. Having seen it myself, I agree. I repaced the pump with my powerflush unit. Boiler went on to full firing rate and stayed on. Powerflush improved matters but not fully resolved. Going back to start replacing return pipework.
     
  15. MikeAustin

    MikeAustin

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    Do you know how they diagnose blockage in the pipework? The boiler seems to assume that flow rate is represented by return temperature. Your powerflush would have got the hot water back to the boiler quicker and kept it running. I don't think that sort of flow rate is what one would operate with. So, after the powerflush, one is back where one started (removal of deposits aside). Am I understanding correctly?
    Above, I calculated a flow rate in my system. I hoped it might be a figure that could be assessed. If the boiler is temperature driven, I suppose that figure means nothing.
     
    Last edited: 15 Jan 2016
  16. HootingOwl

    HootingOwl

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    There was a definate improvement after the flush but still not good enough to heat some of the most distant radiators on the circuit.
    I can tell there is a restriction from the noise that the pump is making. The Viessmann engineer could.tell because he measured differential temp and had seen it hundreds of times before.
    You can hire.a powerflush unit from PHC in St. Werbies and use it to ram some fx2 through the system. Much cheaper than calling a mob in to do the job for you.
     
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