CH flow restrictor?

Discussion in 'Plumbers' Talk' started by ant_80, Dec 7, 2018.

  1. ant_80

    ant_80 New Member

    Hello, I am new here. I wonder if I could get some help.

    I had a new combi boiler and central heating system installed and decided to safeguard a future loft conversion by extending the pipes into the loft. This spec unintentionally added dead legs to the system, which are currently capped off. I believe that air trapped in them is causing a slow pressure drop in the system and I also realise now that dead legs are not good practice anyway.

    Capping the dead legs where they tee off is not an option because is not accessible. And it wouldn’t help if I go ahead with a loft conversion at a later date.

    I know that linking the flow and return legs would create a path of low resistance - but would it be sensible to link them with a valve that would restrict the flow and mimic the resistance of an actual radiator? If so, what type of valve should I be looking for? This would - hopefully - eliminate the dead legs without unbalancing the system, by keeping a restricted flow across the loop.

    I will have a plumber doing the work but would like to hear opinions first so I can make an informed decision when they propose a solution. Thanks a lot in advance.
  2. Pollowick

    Pollowick Well-Known Member

    What about installing a small radiator up there with a TRV fitted. Set the TRV to absolute minimum/frost and have the lockshield open just 1/8 turn. A very slight waste of energy but would do what you require and only kick in when extremely cold and even then would take a long time to heat the rad.
    ant_80 likes this.
  3. KIAB

    KIAB Well-Known Member


    Don't know how many radiators you want in future loft conversion,but do use 22mm pipe for main feeds to get up there,can reduce down to 15mm for present radiator suggested & any later onesadded.
    ant_80 likes this.
  4. ant_80

    ant_80 New Member

    Thanks Pollowick - that surely is a good option. The location is a little awkward for a rad (which I missed in my description!) but can be done. That is actually making me wonder if the TRV+lockshield would work just with a pipe between them rather than a rad because the pipes rising to the loft are accessible in the bathroom (inside a cupboard), which already has a rad with a TRV that keeps the room temperature. Or even just a manual valve and lock shield open very slightly to guarantee a minimum flow? Or maybe this is nonsense?

    Thanks KIAB - agreed. The existing pipes are 22mm to feed 2 rads in potential loft conversion.
  5. seneca

    seneca Screwfix Select

    If it's only a temporary measure why not just fit a bleed valve on the end of each dead leg?
  6. dinkydo

    dinkydo Active Member

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  7. ant_80

    ant_80 New Member

    Thanks Seneca - at this stage I don't know "how temporary" this is going to be, need time to save up for the conversion :)

    Thanks dinkydo - I need to read on double balancing valves to understand what they do and where they should go, but from a quick look it seems right for the job!
  8. Vin

    Vin Active Member

    I would personally go with Senecas advice, bleed the air out and that should solve the problem.

    Dead legs aren’t a good idea on potable water but I don’t see it being an issue with a heating system, no different from having a radiator in a spare room and keeping it switched off.
    ant_80 likes this.
  9. terrymac

    terrymac Well-Known Member

    Your problem of pressure loss is unlikely to be anything to do with two dead legs. Air is compressible, and would just sit there ,acting like an EV. ,under the system pressure.
    I assume you are continually adding water via the filling loop ,to counter the pressure loss.
    The question you should be asking is where is the water going
    ant_80 likes this.
  10. Heat

    Heat Well-Known Member

    Manual bleed valves and that is job done
    The Teach, ant_80 and Allsorts like this.
  11. Allsorts

    Allsorts Well-Known Member

    As Sen and Heat say.

    There is simply no need to couple these two pipes together in any shape or form. If they are catching any system air, then fit manual or auto bleed whatsits at their highest points - jobbie jobbed.This will need bleedin' once every - ooh - year or so.

    Once the system has been running for a week or so, any trapped air can be quickly released - and you should have no further issues for donkey's. Being a dead-leg is not an issue as this water is treated (with inhibitor) and doesn't mix with air or anything else. This is just not an issue.

    If you are suffering from a slow pressure drop, then that is an issue but now't to do with your legs. Dead or otherwise.
    ant_80 and Heat like this.
  12. Jimbo

    Jimbo Active Member

    Have a look under the lock shield caps of the existing system for the ominous green deposits.
    ant_80 and Heat like this.
  13. Allsorts

    Allsorts Well-Known Member

    And if you find Dobbie there, let us know.
    ant_80 likes this.
  14. Heat

    Heat Well-Known Member

    Good advice! This is something that confuses people wondering why their system has a very slow pressure drop without obvious leaks.
    The glands on lockshield or wheelhead valves can allow very slight weeps and TRVs can leak at the pin.
    ant_80 likes this.
  15. ant_80

    ant_80 New Member

    Thanks a lot for all your opinions, I really appreciate the advice. By the sounds of it I am tackling the wrong problem.

    I have searched - a lot - for any signs of leaks but cannot find anything. So my hypothesis has been that when I re-pressurise the system the air in those pipes is trapped but slowly starts mixing with water once the CH is working again. I thought that air would eventually be released by an air vent in the boiler, lowering the pressure. But reality is, this problem is driving me nuts and I don’t really know what I’m talking about! (thanks for your patience btw)

    The pressure drops 0.5 bar or so in 2 weeks and at the same time air builds up in the highest radiator in the system (a towel rail). If there is a leak somewhere, can it also draw that air into the system?
  16. Heat

    Heat Well-Known Member

    Yes, air can be drawn into a system if there is a slight leak on return pipe somewhere or on suction side of pump or with an auto vent on suction side.
    First get manual vents on those dead leg pipes and then check all valves glands and any brass compression joints for signs of leaks.
    Also make sure your boiler isn’t getting a too high pressure and discharging through pressure relief valve to outside
    ant_80 likes this.
  17. Allsorts

    Allsorts Well-Known Member

    This is a sealed and pressurised system Heat - are you sure air can be drawn in? I can't see that possibility myself; the pressure reduction on the return side of the pump due to the pump's 'suction' will be minimal, and certainly won't drop it to below atmospheric which is what it would need to allow this. (I don't know this for a fact, but I really can't see it doing this). .

    Ant, you said in your first post that it's a new system? Is it completely new - boiler, rads and pipes? If so, this is a job for the installing plumber to sort - no question.

    It's good to have some ideas about what's going on, tho'. As far as I know, there's only two ways for 'air' to appear inside a pressurised system like yours; the first - and most common - is for the 'air' to actually be the products of corrosion such as hydrogen. Yes, I mean that your rads are 'rusting' on their insides. Ok, this is extremely unlikely in your case since the system is new and it surely has inhibitor added.

    The other source is from a failed expansion vessel. This has a diaphragm in the middle with pressurised air on one side (you'll find a Shraeder valve on it with which to pump it up) and the other side is connected to the system water. The idea is that when the boiler comes on and heats up, the expanded water is forced in to this vessel and presses the diaphragm against the air side. When the system cools down, this water returns. The overall effect is to keep the system pressure fairly constant.

    If this diaphragm is punctured or perished (again unlikely since it's new) then the air can get in to the water.
    Last edited: Dec 7, 2018
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  18. Allsorts

    Allsorts Well-Known Member

    In the former situation - corrosion - you can test this released 'air' with a match... :oops:

    In the latter scenario, the vessel will soon empty and the air situation stop. Before this time, tho', you will be noticing the system's pressure rise quite obviously as the boiler heats up 'cos the EV is no longer doing its job.

    What is the 'cold' pressure, and what happens to this when you turn on the boiler and it heats the rads?

    Obviously the other cause of a pressure drop is a water leak...
    Last edited: Dec 7, 2018
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  19. Allsorts

    Allsorts Well-Known Member

    How many storeys high is your house, and what pressure do you have your system set at?

    Thinking about it more, if it's like 3 storeys, then the system pressure might well be at say 1bar at the boiler's level (ground?) but this would drop by up to a half-bar per storey (I think). So if you have a couple of storeys already, the pressure could be close to zero (ie atmospheric) at the very top. In theory, then, when the pump fires up it could possibly cause the 'sucked' side of the pipework to drop to below atmos p, so could - in theory - draw in air if there's a leak at that point.

    So - possibly - soz Heat :)

    I think it's unlikely, tho'.

    I guess you could prove this by taking the cold pressure up to near 2 bar and see if it now remains constant (but keep an eye on it and let us know if it soars to anywhere near 3 bar when hot).
    ant_80 and Heat like this.
  20. Heat

    Heat Well-Known Member

    Yes, I knew it was a combi system, so sealed, but still think it is possible in theory for a leak to draw air in, if it is on a pipe or at suction side of pump. On open vented it is common.
    But as you say, probably another cause.
    I distrust auto vents as leak or can draw air in
    ant_80 likes this.

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