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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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    In my open vent system, I have a drain on my boiler return, upstream of which is a Magnaclean. I have valves on my F&E and vent pipes with a valved mains loop into the vent. I want to flush the boiler alone. My plan is to close the pump gate on the upstream (low pressure side), close the Magnaclean, close the vent pipe and open the boiler drain. This should allow a reverse gravity flush of the boiler. Then I plan to close the F&E pipe and open the mains loop. This should reverse flush the boiler, and nothing else apart from the flow pipe. The boiler is a Viessmann Vitodens 100C. I suspect it has some muck in it. Because of the small part of the system being flushed, I will not lose a significant amount if inhibitor.
    Is there any problem with this proposal?
     
  2. 45yearsagasman

    45yearsagasman

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    You sound like you know what you're doing with valves on your feed and vent and a mains connection into the vent. Go for it. House insurance up to date?
     
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  3. simond

    simond

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    The Vitodens heat exchanger likes filling up with debris, we fitted a lot of them and even with a dirt filter and a full powerflush the long spiral heat exchanger can get partially obstructed by the smallest amount of muck.

    So I think you are on the right track, however the sarcasm above is noted. Us tradesman have done this type of thing before and we can normally tell a story about when one went wrong. If you are not familiar with heating systems this probably isn't the job to start on.
     
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  4. simond

    simond

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    PS: there's a paddle flow switch and there is no benefit otherwise in reverse flushing.
     
  5. MikeAustin

    MikeAustin

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    Thanks for the info, simond. I recently ran X800 for a day or two and then mains flushed (in a forward direction). You seem to have some particularly useful experience here. I was about to give up entirely with this boiler. What would you say to running the boiler and nearest rad with FX2 for a few hours? Or have you another suggestion?
     
  6. simond

    simond

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    I would disconnect the flow and returns to the boiler (they may have 22mm comps on them already) and connect a mains hose pipe into the flow and another out of the return.

    It is usually only a small bit of debris so if you want to know if you've caught anything put the outlet pipe into the kitchen sink or similar and leave the plug in.

    The best way to know whether it's the sensors (commonly misdiagnosed on this boiler) is then to fire up the boiler with mains water running through it (in the right direction) and see if it keeps ramping up and then slowing down. If it's clear, the modulation indicator and temp reading should stay consistent while the water is running.

    Then put it all back together.
     
  7. Cgas

    Cgas

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    Looks like everyone missed or ignored your point that theirs a valve on his open vent......why not just add isolation valves on the flow and return out of boiler also go all out.
     
  8. MikeAustin

    MikeAustin

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    Thanks a lot! I need to get the bits and pieces together to do this, so I will have a go after Christmas. The nearest mains water is my washing machine hose in the kitchen. The boiler is in the garage just outside. The problem is indeed that it ramps up and down. The installer told me it needs a new PCB for £380, and maybe sensors as well. The same installer replaced the PCB 6 years ago when I had the same problem - but it cost £193 then! My other option is £2K to install a new boiler.
     
  9. MikeAustin

    MikeAustin

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    Nah. I already have an isolation valve on the boiler flow and can isolate the return via the Magnaclean. I am fully prepared with a hard hat at the ready, but there is not much to protect in the head that it goes on.
     
  10. HootingOwl

    HootingOwl

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    The open vent Vitodens has a habit of fouling the heat exchanger - particularly the 26kW model. You can hear it start sizzling when it ramps up the burner. You want to put some fx2 into the exchanger but make sure you neutralise it and flush it out of the system afterwards. Why not get someone in to see if it can be converted to a sealed system (with the correct safety devices fitted) and get them to powerflush the exchanger at the same time?
    A new boiler of the same type could be fitted in less than a day and would cost a lot less than 2K. More like 1200 with five years of warranty. Five years of BG boiler cover at say, £23 a month is £1380.

    You need to resolve the cause of the problem else you will be back here again in twelve months with the same problem.

    Loads of these boilers were slung in during the boiler scrappage scheme circa 2010. The heat exchanger has a high resistance and this has caused lots of problems to start appearing a few years down the line.
     
  11. MikeAustin

    MikeAustin

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    I have now mains flushed my boiler (Viessmann Compact 100C) six times over 3 days, restarting between each flush, using my reverse flush method above. I thought the mains flow was strong enough not to have to directly connect the boiler to the mains. Successive flushes brought out a small amount of magnetite, but none on the last flush. I measured the temperatures today as shown below.


    I started with CH only and all rads fully open (i.e. unbalanced), room thermostat on max, boiler on max (80C) and pump on max (>3 on Wilo Yonos Pico). The boiler ran continuously up to about 60C after which it settled down to cycling between 41C and 60C. When I switched on the HW, it fired continuously up to 73C, dropped to 41C again, back up to 73C and then down to 49C and eventually stayed at a constant 73C. The boiler seems to be clear, but I note that it never fires at more than 2 or 3 out of 5 bars on the LCD. The pump is running all the time.

    When I checked the rads, the downstairs were cold (because all rads started fully open and the upstairs rads hogged the heat). I balanced them to known settings and the maximum boiler temperature dropped back to about 60C and stayed there. I think there was no more demand from the HW at this point. It seems to me that I could cheat the boiler by adding a rad to the HW loop just to ensure constant HW demand and keep the boiler firing. Bonkers, eh?

    So my question is to those who are familiar with Viessmann boilers (e.g. simond), is this correct behaviour? Should the CH be limited to 60C and always drop back to 41C? Should the boiler only fire at 2-3 of the 5 bars?
     
  12. HootingOwl

    HootingOwl

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    x800 will not touch the crud that builds up in these things. you need to use FX2 or DS40 and run it through hot, preferably with a powerflush machine.

    is it a 26kw model? does it make sizzling noises when the burner is on full output?
     
  13. Phill Peck

    Phill Peck

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    What kW rating is the boiler and what is the heat requirement of the system.

    Restriction is more likely on connections to the heat exchanger which mains will rarely move.

    Last one like this I had. Was a 30kw (edit it was possibly a 26)ov with about 12kw of rads connected. And partially blocked. Showed similar symptoms to what you are describing.

    The Viessmann ov will fire at around 2/3 bars (20-40% output) until the return warms and confirms circulation.
    If the boiler is over sized it will never fire at maximum.

    Given that you shouldn't be removing the cover and playing about inside id recommend u get a pro in
     
  14. MikeAustin

    MikeAustin

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    It is an 18kW model and there is no sizzling that I can detect - but that would not be on full power (5/5 bars) because it usually on reaches 2-3 and, very rarely, 4.
     
  15. HootingOwl

    HootingOwl

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    it doesn't take much of an obstruction to cause these boilers to behave in an erratic manner. i would suggest a powerflush using some strong chemicals to improve the situation.
    but if it is working, radiators getting hot and boiler not making a racket, then i would just live with it.
     
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