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Discuss DIY Draining down system in the Central Heating Forum area at

  1. etrader

    etrader New Member

    Hello, I have an open vented boiler with a gravity hot water circuit and a pumped central heating circuit.

    I want to drain the central heating system to remove a radiator and give the system a flush.

    Will the gravity hot water part of the boiler still work as I intend to leave the central heating side unfilled for a few weeks.
  2. Harvest Fields

    Harvest Fields S. Mod Trusted Plumber GSR Top Contributor!!

    Nope. As far as I’m aware
    • Agree Agree x 2
  3. chris watkins

    chris watkins Plumber

    As above, both sides are connected so as you drain the heating the hot water primary's will drain as well.
    I take it you have an F&E cistern in the loft, if you follow the 15mm cold feed pipe down it should connect into the return coming out of the cylinder or go all the way down to the boiler.
    • Like Like x 1
    • Agree Agree x 1
  4. Rob Foster

    Rob Foster Top Contributor!!

    Good luck, some post with a little knowledge and we can help,them along. I my opinion
    the really basic questions you have asked indicates you really need to hire an expert. centralheatking
    • Like Like x 1
  5. Jerry

    Jerry GSR

    Aaaah I loves me a C Plan
    • Funny Funny x 1
  6. Allgoode

    Allgoode GSR

    See if your hot water tank has an immersion heater.

    If so you can drain down central heating system, but make sure you isolate boiler electrically, eg - unplug or pop the fuse out of the switch boiler wire goes too.

    That way the water will be heated electrically without help of the boiler system.

    Just be aware central heating will be out of action until refilled & bled free of air & also the boiler may have a bit of a grumble when it eventually gets refilled/restarted,


    • Agree Agree x 1
  7. Svenedin

    Svenedin Member

    If you do drain down be aware that (depending on the size of the system) there can be a lot of revolting black water in there. You'll need a hose to connected to your drain point (at the lowest point in the system) and to direct the water into a drain. When you fill up again you will need to replace whatever corrosion inhibitor is appropriate for your system and this can work out quite expensive on a bigger system. You need to know where your F&E tank is so that you can tie the ballcock up (or turn off the isolator if fitted) or otherwise the system will try to fill itself as you are trying to drain it. You will also probably need to open some of the radiator air bleeds as the system drains (usually an upstairs radiator). Personally I think it would be better if you did this with a pro at least the first time you attempt this. I'm not a pro but I was shown how to do it by one. It's not unduly complicated but things need to be done in a sequence and sometimes filling back up can be difficult due to airlocks. If your system is very old and sludged up then trying to flush it may have unintended consequences. It could be that the sludge is actually plugging a load of leaks.....Years ago I tired to revive sludged up radiators by disconnecting them from the system, taking them outside and washing them out. It had little effect because the radiators were actually knackered and needed replacement.
    • Like Like x 3
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