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Discuss Pressurised Heating System Not Draining Completely in the Central Heating Forum area at

  1. Technomorph

    Technomorph New Member

    I am having problems draining my pressurised heating system. I am the home owner, and not a plumber. The property is a new build, three storey house, 16 rads, s plan, 3 zones. The issue is that the rads on the middle floor (8 rads) are not draining. The top floor and some of the ground floor rads have drained correctly, 3 and 5 rads respectively.

    Heating turned off. All TRVs and lockshields have been fully opened. There are three drain-offs - all opened. All 2 port valves latched to open (3 zone pl HW). There are a couple of bleed valves in the airing cupboard - opened. Pressure is reading zero. I thought this was all I needed to do, but perhaps there's something else.

    Any advice appreciated.

  2. Chuck

    Chuck Active Member

    You've tried opening the bleed valves on the affected radiators? If so, the most likely explanation is the flow and return to the middle floor are dropped down from the floor above so those radiators are in the bottom of a 'U'. If so, the easiest fix is to fit an additional drain valve on that section. You can replace a lockshield with a combined "lockshield with drain off". E.g.

    Pegler Yorkshire Lockshield with Drain-Off | TradePoint

    Lockshield 15mm Angled Radiator Valve with Drain Off

    I'd go for a Pegler Yorkshire branded valve as first choice but the generic one is an easy straight swap in a lot of cases.

    Several assumptions here, photos and/or a sketch would help.
    • Like Like x 1
  3. king of pipes

    king of pipes Trusted Plumber GSR

    As above make sure you lock open your motorised valves loosen air bleeds from top floor downwards , if any pipework is on a drop and cant drain that section will need a additional drain point fitting cheers kop
  4. Technomorph

    Technomorph New Member


    Is it really possible a drop would be installed with no means of draining? I know it happens, but surely if you're designing a system from scratch in a new build, you just wouldn't do it?

    Could it be an airlock? Is compressed air ever used to get things moving?
  5. scott_d

    scott_d Plumber GSR

    Why are you draining?
  6. king of pipes

    king of pipes Trusted Plumber GSR

    No compressed air is not the answer a wet and dry hoover is what to use with a flatish tray to catch the splashes cheers kop
  7. snowhead

    snowhead Well-Known Member

    If it was a National house builder and it's about price / time then yes it could have been installed without drainoffs.

    Compressed air won't move the water if it's got to go uphill first.
    A wet vac is more usefull.

    A plumber /heating engineer wouldn't have commented on the lack of drainoffs, it's an everyday occurance.
    Last edited by a moderator: Aug 21, 2017
  8. Technomorph

    Technomorph New Member

    Thinking about it, each floor is an independent zone controlled by a motorised 2 port valve. The valves are on the top floor in the airing cupboard, so given that the middle floor is effectively isolated it will never drain as the water would need to go up hill right? Strictly speaking therefore shouldn't each floor/zone have a drainoff? I'm just trying to understand what's going on. If this is the case I'll just fit one one of those combination lockshield/drainoff valves.
    • Useful Useful x 1
  9. Chuck

    Chuck Active Member

    That's the 'flow' path and, as you say, the system won't drain through that branch. What path does the water returning from the radiators take as it makes its way back to the boiler?
  10. Technomorph

    Technomorph New Member

    Okay, makes sense. Don't know about the route of the return, although there is a drainoff right next to the boiler which is marked 'return'. I've had this open but nothing else is coming out. I have a vacuum pump used for bleeding brakes which I'm now thinking about using to see if this gets things moving. The boiler's in the garage, so it shouldn't end to badly even if there is a sudden gush.
  11. king of pipes

    king of pipes Trusted Plumber GSR

    You really are over complicating things use a wet vac isolate the valves crack a valve tail on the rad and just suck the water out job done, empty the water out then you can do the same on the pipework and any other rads connected . cheers kop
    • Agree Agree x 1
  12. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Are you sure the port valves are open ?
  13. Chalked

    Chalked Plumber GSR

    Yes Shaun. I'm still on here.
    • Winner Winner x 1
  14. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

  15. Technomorph

    Technomorph New Member

    Kop, I take your point. I've since just drained the individual radiators in situ and done the work I needed to do. Being my 0wn home I still want to understand why the system did not drain completely. If I need to do further work in future or want to flush the system in it's entirety, I don't want to faff about.

    Looking in the airing cupboard again it seems that the returns from all three zones come back to the airing cupboard and join with the hot water on a what appears to be a common return. I've attached an image. It's not that clear, but behind each zone valve, you can just about see a what appears to be a return. Is this normal?

    Airing Cupboard.jpg
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