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Discuss Open-vented system with no expansion pipe in the Central Heating Forum area at UKPlumbersForums.co.uk.

  1. SlimD

    SlimD GSR

    Can anyone tell me if there is any situation where, on an open vented system, you wouldn't have an expansion pipe on either the cold water cistern or the F&E tank?
    I came across it today. Looked like a decent install otherwise but it struck me as odd.
    The owner said he'd been told by the installers that it didn't need one.
    I only went there to put a radiator back on that I'd taken off the week before so they could decorate. I offered to check the ball cock was working properly after refilling the radiator but noticed there were no F&E pipes.
    Anyone any ideas why?
     
  2. Best

    Best Trusted Plumber Top Contributor!!

    Open vent hot cylinders need to have a vent pipe going somewhere.
    The combination cylinders, (with the little tank on top) will have their own built in feed and vent.
    Heating systems that are open need to have a feed and vent, although some have them combined and some have only a feed pipe with the "vent" as only an air release manual or auto vent, often done to avoid pumping over, but definitely not ideal.
     
  3. SlimD

    SlimD GSR

    Thanks. I didn't look at the rest of the system too closely and I never saw what type of hot cylinder they had but I've just not heard of an OV system not having a vent pipe. Hence the question.
     
  4. Best

    Best Trusted Plumber Top Contributor!!

    I really don't like an open vent system that doesn't have a vent pipe.
    Obviously the feed pipe acts also as a vent, but systems need a vent pipe to allow for easier point of expansion and extra safety should a feed pipe block.
    I find systems without a vent seem to absorb air and also have magnetite corrosion.
    But last thing you want is a system pumping up as it corrodes rads and everything in literally a few months, so first choice best system is sealed system on gas or oil, or if it has to remain open vented, then the 'close coupling' method of joining the teed vent and feed close together and on the negative side of the pump is the prefered way, although tbh I am not fond of it.
     
    • Like Like x 1
  5. Sparkgap

    Sparkgap Active Member

    Back when Noah was a lad I came across a number of domestic systems which relied on the cold feed for both feed and vent for whatever reason. Some instead had a vent coming off the cold feed just below the tank. I believe -but stand to be corrected- that this was common where you had a direct HW cylinder off a back boiler and no heating.
     
  6. Best

    Best Trusted Plumber Top Contributor!!

    Obviously we are now talking about the domestic hot water, not heating systems.
    But normally the direct flow & return from a wee back boiler were just rising up to the cylinder on gravity circuit and directly connected in, no vent or feed needed obviously. Cylinder has its own feed and vent which will serve both the cylinder and the back boiler.
    Exception would be if the gravity flow was going up above and then back down to the cylinder, like on a bungalow. Then the flow would need its own vent pipe.
     
    Last edited: Jan 23, 2017
  7. Chriswareham

    Chriswareham GSR

    there are sill some primatic cylinders out there not a lot !
    but they fill through the hot water cylinder no header tank no vent just an air bubble that traps air and separates the two from mixing
    may be some one that nows how to down lode from the internet can uplode a single feed (primatic cylinders)
     
  8. rpm

    rpm Trusted Plumber Top Contributor!!

    Pretty sure this came up recently with pictures if somebody has the time to go digging.
    Something about being Tee`d into another pipe ?
     
  9. Chriswareham

    Chriswareham GSR

    think i have done it

    image1.PNG
     
    • Informative Informative x 1
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