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Renovating secondary heat exchange unit

Discussion in 'Plumbing and Central Heating' started by RenoMan, 5 Jan 2008.

  1. RenoMan

    RenoMan

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    I live in Reading Berkshire and we have a lot of problems with hard water. I have a Vailant Combimate system installed, and although it works well enough, the Secondary Heat Exchange unit seems to get clogged enough every 2 years and then I have had to get it changed. My brothers have the same heating systems and we have had this problem regularly. The last time (in December), I bought and changed the unit myself. Now I am wondering if there is any way of using some sort of solution to 'refurbish' the heat exchange unit I have so that maybe I can just change the unit every summer and 'refurbish' the replaced one, on the assumption that this will never be as good as getting a new Heat Exchange Unit - but a heck of a lot cheaper!!

    Does anyone know of a chemical solution which will do this job? Does this sound like a good idea, or have I missed something?

    All suggestions welcome.
     
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  3. bengasman

    bengasman

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    Yep, powerflush your system and your heat exchanger will be revived and stay clean.

    Nothing to do with hard water but with shyte in your system. Rads and pipes were not properly flushed when boiler was installed, or it has been leaking.

    Measure ph of circulating water, and you will find around 5, when it should be 8

    Similar question is posted every other day. Modern plate exchangers are bimetal and self cleaning on the dhw side. It is the system side that gets blocked
     
  4. Agile

    Agile

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    Actually in this case it is probably the secondary side that is blocking because the water is very hard and if the boiler's control system is poor it will allow lime scale.

    I cannot immagine a Worcester 24i would work for long.

    They can be cleaned with HCl if you were competent to use it safely.

    Tony
     
  5. JohnD

    JohnD

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    surely would it last longer with a Permutit-type softener to prevent limescale?

    I understand some boilers are unhappy with softened water, but it couldn't get much worse.
     
  6. Agile

    Agile

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    The boiler manufacturers like soft water but dont like the brine from ion exchange softeners!

    Tony
     
  7. billy bob

    billy bob

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  8. ChrisR

    ChrisR

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    RenoMan - which is it, black stuff boiler side or whiteish stuff tap water side?
    Either way, HCl can be used, with reservations. Sulphamic Acid is the classic one to use for limescale (as in Fernox DS-3). Its less aggressive to the other materials. I've used them both, singly and together. (don't bother with Sulphuric Acid)
    SLower but still effective is Fernox DS-40, which is Citric + Malic acids.
    Kamco FX-2, phosphoric acid based, does the job too.

    Hydrochloric acid attacks stainless steels, and the brazing that's used to hold secondary heat exchangers together, but I haven't holed one yet.

    Powerflushing doesn't clean secondary heat exhanger primary sides unless you have the tap running all the time so the jizzm circulates through it. You can use a powerflusher via say washing machine hoses to circulate citric acid for the secondary side.
     
  9. JohnD

    JohnD

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    I've never experienced salty water from the taps. I suspect this is an old wive's tale. Flow through the vessel to the taps is shut off at the start of the recycling process and the resin is rinsed before the connections to the taps are reopened.

    After I installed my first softener, it dissolved and washed away the old limescale from cisterns, bath taps etc.
     
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  11. RenoMan

    RenoMan

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    Wow, thanks all for the responses.

    ChrisR - it was black solid stuff, so I assume that is from the boiler side.

    The plumber recommended that I had a sieve fitted when we changed the boiler, which was supposed to trap any gunk in the old pipes, but each time the sieve has been opened there is no solid stuff in it, but the sieve itself seems to be 'furring up' and I am worried that this reduces the water flow which may stress the pump (?) or other parts of the boier, so thinking of removing this.

    I have a combimate installed on the cold water supply, which is supposed to be a food grade water treatement which should inhibit the build up of limescale on heated water. Not sure how good this is, when I researched it before buying it I read varying reports of how 'excellent' it was and also how 'useless' it was - nothing conclusive I could latch onto. The decider was that in my house over the years, 2 electric showers packed up due to limescale build up, and in the same time, the shower we had installed at my brothers house at the same time as my first one, was running OK - and he had a combimate installed. So I put one in too! But that would not help with the black boiler-side buildup problem I guess. (Incidentally that was about 18 months ago and the electric shower we changed at the time seems to be OK so far - sounds like a positive vote for combimate).

    bengasman, it is possible that the installer did not flush the old system when fitting the new one, although it was discussed as part of the job. I was not around to oversee, and the installer did not fit a thermostat as originally agreed, also left brickwork around the vent unfinished, burn marks on the wall from soldering, untidy pipework, slightly off-vertical boiler .... not one of the better tradesmen as it turned out. But the sieve does not show any bits in it, and I have had to change the Secondary exchanger twice now so I am not sure if I can really hold him responsible for this problem.

    I will look at the combiguard suggestion, billy bob, but again this seems to be on the DHW side so not sure if it will handle the boiler-side issues.

    Water softner might help - but again not with the boiler-side so not too sure if it is worth the hassle.

    You guys seem to have covered most angles here, any more suggestions would be welcome - however could you point me to the best places to obtain the chemicals to recondition the secondary heat exchange unit I removed over xmas, I am not sure if there are any controls over these chemicals or whether I can get them from plumbers merchants (any idea of trade names for them?)

    All suggestions gratefully received, thanks.
     
  12. ChrisR

    ChrisR

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    Well the seive ain't ewworking, so I'd take it out or replace the mesh with a coarser one. Sometimes there are two and you can remove one.

    You might like to look at a magnetic filtering device such as a Boiler Buddy or Magnaclean.

    You might also want to look at powerflushing the whole system to get the rubbish out for good(ish).

    I gave you the product names of all the normal chemicals.
    HCl is available in plumbers merchants under various names including Spirits of Salts. It isn't nice stuff to use though.
     
  13. Onetap

    Onetap

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    The black stuff on the primary side is magnetite sludge, the products of corroding radiators. The system should have been powerflushed before fitting a combi boiler, but even that won't guarantee all the existing sludge is removed. Check the pH, as mentioned to ensure it's not still forming. A Magnaclean filter before the boiler would intercept some of it.

    If the HWS flow rate drops off, then it is limescale forming on the secondary side. An ion-exchange softener or combimate would stop it. There should be no brine discharged from a softener, the salt is only used to regenerate the resin and the resin is thoroughly flushed after regenerating to nesure all the salt is rinsed out. If the latter, you could descale the heat exchanger by pumping compatible descaling solution through it before it blocks up solidly.
     
  14. RenoMan

    RenoMan

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    Thanks guys.

    The last time the Secondary HE bunged up, the plumber told me the same thing as I saw this time, that it was black stuff from the boiler side which formed due to sludge from the old rads getting through to the HE's. Last summer I decided to change all the rads as they were ol single ones anyway, seemed to be cooler than before, and 1 of them started leaking through the top which seemed to be due to rust. At the same time I flushed the system through closing off all rads and opening them up 1 at a time. Note - not a power flush, just flush by opening the cold fill and taking out at the rads. Took me most of the day, and the water eventually ran clean with no bits, but I think that I should have done a bit more and added sludge remover and then repeated the flush after a few weeks or so (hindsight is wonderful!). Also committed the sin of not putting in an inhibitor, kept meaning to but procrastinated. So I was very disappointed that the problem came back again, but I understand it better now.

    From what everyone has said, I will repeat flush this summer but do the last 2 things as well! I will also get some solution to renovate the secondary HE I removed last month, and if the HW starts to run slightly cool again in the future I will whip the HE unit out and change it!

    Last job is to find out details of a filter (magnaclean, boiler buddy or similar) to install on the central heating HW return, and install that as well this sumer (after flush!).

    Hopefully all of the above will resolve the problems of the HW packing up over winter. Of course knowing my luck the boiler will find something else to complain about and fail on!!!

    Thanks to all again.

    .
     
  15. Balenza

    Balenza

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    ChrisR wrote

    It may do if left in contact in a concentrated form however hydrochloric acid is used wholesale everyday by farmers cleaning miles of stainless steel piping that transports milk from the cows udders to the large stainless steel storage tanks.
    The pre-cooling phe will also come into contact with the stuff everyday.
     
  16. JohnD

    JohnD

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    I am just a householder, but I found a Magnaclean, used in conjunction with Sentinel X400 which is a mild non-acidic cleaner that loosens and breaks up sludge deposits, very good for removing black sludge. I was very gratified to be able to take out cupfuls of sludge it had trapped.

    You might think it sounds to good to be true, but it really does work. It will cost about £100 to buy and I found it a straightforward DIY plumbing job to fit. Once fitted no plumbing skills are required to empty it of trapped sludge. You can instal it anytime you like, no need to wait until you are doing the flush - the sooner the better IMO.

    http://www.adeysolutions.co.uk/subprofessional.asp?id=246

    http://uk.youtube.com/watch?v=uDgegChsLvc
     
  17. spudkey

    spudkey

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    HCL is not recomended with stainless steel it will cause hydrogen embrittlement of metal and premature faliure of exchanger. use citric or formic acid
     
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