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Discuss Should I attempt to mains flush my CH system? in the Central Heating Forum area at

  1. Vincent UK Plumber

    Vincent UK Plumber New Member

    I just moved into my new flat and the boiler is probably around 20 years old. It's a sealed non-condensing Valliant Turbomax VUW 242 and on it's last legs. Just got the circuit board replaced with a refurb off eBay while I save some pennies.

    The system is a mix of 15mm and microbore copper pipe with a bit of sludge in the system. Got a few cold spots on the bottom of a few rads but nothing excessive. I don't think a powerflush will do anything to an old microbore system other than cause leaks.

    I was thinking of adding some chemical cleaner, running it for a little less than the recommended time (incase of pinhole leaks), isolating the flow on the boiler and using the quickfill to do a MW back flush. I haven't found a drain point yet so was planning on removing the bathroom rad and running a hose from the flow pipe to the WC from there.

    Then (someone correct me if I'm wrong) I could attach a hose to the flow pipe of the removed rad (boiler still isolated) and do a forward flush that way to remove anymore crud in the system.

    Or will a magnetic filter installed with the new boiler achieve the same result after a while and MW flushing a waste of time?
    Last edited: Apr 9, 2019
  2. scott_d

    scott_d Plumber GSR Top Contributor!!

    If you add chemicals and circulate you will just send the sludge through the boiler
    I would add a filter if you can
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    • Informative Informative x 1
  3. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Not worth it

    Get it done when the boilers changed
    • Agree Agree x 1
    • Creative Creative x 1
  4. Allgoode

    Allgoode GSR

    Other option drain down, put in specific drain point's with heavy duty type drain cocks, remove rads (remove fluid with wet vac) & flush each rad through with hose pipe outside.

    Also gain access to attic F&E tank & either replace it if it's minging inside or wet vac out & wipe clean.

    Not much else you can do with system as Shauncorbs says.

    When replacing boiler look at replacing rads & valves with new as prob same age as boiler or older & then give a good power flushing through.

    Some good deals on rad & TRV packs out there so you may be surprised.

    False economy to not do it right when changing boiler or giving system a good refurb/overhaul,

    Hope it helps,

  5. Last Plumber

    Last Plumber Trusted Plumber GSR

    I would dose with Fernox F3 and leave it in system a couple of weeks prior to the Boiler change and then get it as hot as possible and drop it out.
    If some of the rads need removing to flush out sludge then you'll have to do that too.
    I wouldn't bother doing this until you're ready for the new Boiler. Once the new Boiler is in, give it one more hot flush before dosing it with F1.
    • Like Like x 2
  6. moonlight

    moonlight Plumber GSR

    Vincent UK Plumber has a sealed system.
  7. Rob Foster

    Rob Foster Top Contributor!!

    You can also get a decent magnet and roll up up and down the radiators whilst flushing this will loosen up the ferric sludge, some use a vibrating tool to loosen sludge. I would isolate the boiler whilst flushing the system out. Connnect end to mains incoming and far end out to outside and watch the outflow run clear. Then go round switching rads on and off in some coherent planed manner.
    spend ages at after all water is cheap, diy time free. Afterwards
    fill up system check for leaks then add inhibitor . Centralheatking
    • Like Like x 1
  8. Last Plumber

    Last Plumber Trusted Plumber GSR

    I use a rubber mallet. That works too.
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  9. Vincent UK Plumber

    Vincent UK Plumber New Member

    In an ideal situation, I would like to get all the microbore pipe ripped out and all the pipework replaced with 15mm copper or even PVC but the amount of work involved in that isn't something I'm prepared to do myself or can afford to pay someone to do. Maybe much later down the line when it's time to redecorate and I have the time / money.

    The rads are mostly old but they heat up quite quickly. They don't heat up the room as fast as I'd like but hopefully removing some of the sludge and new boiler will make that a little better. If I ever get round to changing the pipework, I'll change the rads at the same time. Last thing I want to do just now is add plastering to my DIY list.

    Good point on leaving the cleaner and flushing till closer to the replacement date, I'd be in trouble if I blocked the boiler before I can afford to replace it. Unfortunately the return pipe runs up behind the boiler and into the wall so there's no access to install a magnetic filter in the current setup. Will definitely get one installed along with the new boiler.

    The bathroom rad is a new addition and on 15mm pipe so I'm planning to use that pipe connection as my back flush drain and forward flush inlet as it's close to the WC and a mains water tap. It's probably the 2nd rad off the boiler, so not end of line but my flat is on the 3rd floor so a hose from last rad on the system out the window will probably not be much appreciated by the neighbours.
  10. Last Plumber

    Last Plumber Trusted Plumber GSR

    Why are you keen to lose the microbore?
    Is it undersized?
  11. Vincent UK Plumber

    Vincent UK Plumber New Member

    Not sure how I'd figure out if it's undersized but all the rads heat up relatively quickly without one lagging notably behind. I just think it looks ugly and due to the age and sludge in the system, I'm guessing the microbore will be getting clogged up too.

    Wouldn't a 15mm pipe make the system more efficient too or do I have the wrong understanding of bore sizing? I'm thinking better flow, less heat cycles
    Last edited: Apr 11, 2019
  12. Last Plumber

    Last Plumber Trusted Plumber GSR

    No, not necessarily. The flow rates through microbore can be higher than larger bores, although on a domestic system I don't find it makes a noticeable amount of difference so far as debris is concerned.
    The other reason is that the corrosion tends to take place inside the radiators and that is why they end up with a collection of sludge. The sludge can of course pass into the pipework and cause blockages but if your rads are adequately supplied, that may not have occurred in your case.
    Aesthetically, it depends on the skill of the installer. Can it not be neatened up where it is visible?
    • Like Like x 2
  13. Vincent UK Plumber

    Vincent UK Plumber New Member

    Aesthetically, it can definitely be neatened. They come out the floor and an angle and make wiggly bends to the rad. They just need straightened and better routed. Will need to check but I think there's enough length to do so.

    If there's no real need or benefit to replacing microbore, then music to my ears. I'll gladly just get it neatened and leave it alone.
  14. Last Plumber

    Last Plumber Trusted Plumber GSR

    In addition to what I said previously, pipe work with a good flow will normally clean with a flushing chemical pretty well.

    A Plumber (unless you're competent to do it yourself) could cut and connect on where they appear and run them in new micro-bore.

    Copper work hardens and may be difficult to straighten successfully without annealing.

    If I use it I tend to bring the two up together and connect one into the nearest valve and run the other along the top of the skirting (or just level with the base of the rad so it is out of sight) to the other end. It can be very neat if some effort is made.

    If you decide to replace rads you need to be aware that different pipe size and lengths will carry different flow rates. There is little point in buying larger rads if the flow requirement cannot be met. That is something which needs assessing prior to buying new radiators. It may be that if you require more heat the micro bore could be undersized. Going of what you have said, the Heating - I would expect - will be less than 30000 Btu's/hr (8.8 Kw)?

    Newer Vaillant Boilers with a better Hot water performance can modulate lower than yours when in Heating mode. They also have modulating pumps and anti-cycling delays for the period when everything is up to temperature so you may find that a good flush and new boiler along with balancing and the right controls, solves the problem.
    • Like Like x 1
  15. Vincent UK Plumber

    Vincent UK Plumber New Member

    I'll probably get a plumber to look at the microbore in that case, it'll probably take me at least 5 times as long as a professional to do it. I've learned the hard way to know your limits lol.

    I wasn't planning on getting bigger rads, just newer ones of the same size if the new boiler and flush doesn't heat the room as well as I want since there's every chance the rads are as old as the boiler.

    I have no idea how many BTU's this flat will theoretically need. Any tips on how to calculate it? I tried from a quick google guide but I ended up with near 100,000 BTU, which must be wrong.

    Looking at the instruction manual of my current boiler, it says the CH output range is 32,000-81,900 BTU/h or 9.6-24.0 kW.
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