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Discuss Feed and Vent fun :DDDDD in the Central Heating Forum area at

  1. fixitflav

    fixitflav Member

    Why would it pump over, if the headloss between open vent and cold feed is < height of vent loop above F/E tank TWL? Which it must be anyway to avoid over-pumping, however the system is arranged.
  2. fixitflav

    fixitflav Member

    If the F/E tank and pipework are existing, why go to the expense of changing to an expansion vessel? You would still need to decide where to connect it (ideally between boiler and pump IMO)
  3. Best

    Best Trusted Plumber Top Contributor!!

    Difficult to say Shaun what you should do. If you combined the feed and vent I think would be wrong as I find that causes systems to corrode.
    Pity you can’t install new feed & exp tank and new vent and feed pipework at boiler end.
    One solution would be neutral point installed where the feed and vent could then come off it. But am sure you don’t want to do that.
    I am actually having the same problem trying to decide how I convert a gravity hot water/pumped rads to all pumped on a very old system, without pump over or corrosion.
  4. Best

    Best Trusted Plumber Top Contributor!!

    If you are right, then that is his answer just to go with vent and feed ahead of pump. I would wonder if most of zones are closed, would it still be ok.
    Where possible I would seal systems because the cost is very little (if vessel size required is not large).
  5. oilmark

    oilmark New Member

    Open vented system must have vent from flow on boiler before any other connection.
  6. Best

    Best Trusted Plumber Top Contributor!!

    Virtually all open vent systems (except gravity to cylinder) I know of in my area are done with the vent off the flow at cylinder and feed off the cyl return.
    Pump is at boiler. They usually work fine, but some factors make it likely to pump over if cylinder is close to boiler and pump on a non zoned system, or where many rads and zones turn off.
  7. Ric2013

    Ric2013 Plumber Top Contributor!!

    What if you reverse the flow and return under the pump in your diagram and have the pump pushing into the boiler? Also reverse the feed and vent so vent remains the first tee after the boiler? Negative pressure system, and microleaks at rad valves will let air in etc, but would be safer than your diagram.
  8. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    ive decided to put the pump on the return and keep the feed and vent to spec if it does pump over will give him the option to leave or seal it
    • Like Like x 1
  9. Ric2013

    Ric2013 Plumber Top Contributor!!


    An old boy told me the pump will last longer if it is running on the cooler return water, hence the old way of doing it with the pump on the return. Don't bleed the rads with the pump running, though ;)

    Nos da
    • Like Like x 1
    • Agree Agree x 1
  10. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    cyfaill hen nos dda
  11. 64degrees

    64degrees GSR

    I’d go with a robokit and pressurise the system, never seen it cause the pipework to fail, and if it stays open vent the air in the system will cause bigger problems in time.
    • Agree Agree x 1
  12. king of pipes

    king of pipes Trusted Plumber GSR Top Contributor!!

    Seen that happen Shaun quite a few times bud i had the same issue with the Worcester classic system I posted on here couple of months a go pipework was poorly installed cast your mind back I sealed it but as I said the working pressure was set to a bar not much more than a open vent system worcesters dont work on a pressure switch so you can run it on low system head low as 0.5 bar worth considering bud good luck kop






  13. Best

    Best Trusted Plumber Top Contributor!!

    I do a lot of plumbing repair jobs, and you are correct about pumps lasting on the return much longer. I noticed over the years that pumps on oil boilers fitted to the return where lasting 20 or more years and their rubber washers weren’t perishing.
  14. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Nice rads kop
    • Agree Agree x 1
  15. fixitflav

    fixitflav Member

    That wasn't one of your original options, but that would be OK.
    Should then be open vent - cold feed - pump - boiler. You definitely don't want the cold feed on the discharge side of the pump or there's a good chance there will be negative pressure somewhere, if pump DP is 8m.
    As I'm sure you know, the open vent and cold feed should be no more than 150mm apart to avoid risk of over-pumping, but where they are in the circuit relative to other kit is irrelevant.
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