Gravity Hot Water Airlock?

Discussion in 'Plumbers' Talk' started by screwfox, Sep 9, 2004.

  1. screwfox

    screwfox New Member

    I have a gravity hot water system, I drained the CH to replace a rad, when I filled the system back up the water does not get hot (the C/H does), there are no bleed valves on the hot water side, how does one normally bleed the system?
  2. tomplum

    tomplum New Member

    hi
    there will be two pipes going into your cylinder from the boiler, usually 2 thirds way up the cylinder. undo the top one BUT NOT ALL THE WAY. it will be tight, and when you have slacked it the pipe won/t come out readilly. so you will probably have to tap it outwards to break the seal. once the seal is broke it will hiss, then hot water will follow, retighten and you have vented the cylinder,i will stress again noy to back the nut off all the way cos when the water comes it will be too hot to handle and you will be in deep water if you cannot retighten the nut

    my advise call a plumber but
    good luck anyway
  3. screwfox

    screwfox New Member

    Worked a treat, thanks.
    The only problem was that the cylnder is behind a shower wall, accessible from a small cupboard, the boiler connections are on the far side! Ended up lying on top of the cylinder dangling over the other side to undo the connecton, who'd be a plumber?
  4. matty68

    matty68 New Member

    Would this method not just vent the primary water side of the system and not the hot water cylinder?...assuming your loosening a primary flow to an indirect cylinder.
  5. Dan The CORGI Man

    Dan The CORGI Man New Member

    Yes, and that was screwfoxs' problem - an airlock on the primaries - obviously the vent is not in the correct place.
  6. matty68

    matty68 New Member

    my mistake...i thought it was a hot water airlock...good tip though.
  7. Dan The CORGI Man

    Dan The CORGI Man New Member

    It is, but you;d be brave to do it on some of the ancient tanks we come across... Makes one wince to think of what might happen... :-s
  8. matty68

    matty68 New Member

    yes..with the temp of that water i wouldn't try it. An automatic air release valve being a better option.
  9. screwfox

    screwfox New Member

    Bleeding the bleeding thing was bad enough, fitting an auto vent, no chance!
    The whole system is being replaced later this year anyhoo, so it shouldn't be a problem again, unless I have to drain down for any reason.
  10. daz9643

    daz9643 New Member

    think i have the same problem. A colleague had also tried to undo the large nut on his cylinder and the cylinder wall buckled. Not sure now whether to attempt undoing the nut on my own cylinder or fit a bleed valve.
  11. matty68

    matty68 New Member

    go for tomplums method..with proper care its an effective way to vent.
  12. daz9643

    daz9643 New Member

    Managed to cure it a different way. I didn't just have no hot water, the upstairs rads weren't getting hot either. Got a tip to close all the downstairs radiator valves so that the water is forced to the upstairs rads. Worked a treat, now have working rads and hot water.
  13. BodgerRobbie

    BodgerRobbie New Member

    I know this is a very old thread, but I've just had this problem and I visited here to get suggestions, so hopefully this will help someone out, I tried pretty much everything (other than boiling it out as has been suggested, but this does essentially the same thing without the risk of damaging the boiler)
    I ran the garden hosepipe (this needs to be mains pressure) up into the loft to the feeder/expansion tank.
    It's a good idea to have drained the f/e tank down as you may end up adding a lot of water to the tank doing this and overwhelming the overflow pipes.
    Also, on my system I needed to bung up the vent pipe that comes up over and into the f/e tank (I used a rubber stopper, you want it to release under pressure, not seal it completely!). Otherwise if the airlock in the coil is large the water from the hose will take the easy route around the rest of the system and simply flow back up the vent pipe, essentially you need to "pressurise" the system to force the airlock out of it's current location to a position where it can escape up either the vent pipe or up the cold feed pipe (ie back to the f/e tank). There are only two ways for the air to go!

    I then held the end of the hosepipe up to the f/e tank bottom outlet (at the bottom of the feeder tank) and got a helper to turn on the hosepipe, slowly at first.
    Hey presto, after a few seconds the bung blasted out of the vent pipe. Lots of gurgling and bubbling followed indicating that a large amount of air had been dislodged.
    I ran the boiler (hot water only at first) and then checked the top pipe at the cylinder coil was getting hot. It was, however the bottom pipe wasn't getting as warm as expected, suggesting there was still a restricted flow. However the water tank was heating, although it took a couple of hours to get a decent tank of hot water.
    I was going to repeat the process, but as it was working (albeit slightly unsatisfactorily) I decided to leave it to see if it cleared (the 28mm gravity flow pipe back to the boiler still wasn't getting very hot when running the boiler on hot water only but I was unsure exactly how hot this should get - pretty hot as it turns out)
    After a day it still didn't seem to be working to full efficiency so I turned up the boiler to max and this seemed to do enough to "boil" out the last of the airlock.
    So in short, if you try the hosepipe process above, make sure you check the bottom coil outlet is warming up, otherwise repeat it again. And if your system is configured like mine, bung up the vent pipe too. I couldn't find any mention of using the bung on the vent in any of the forums I visited trying to sort out the problem.

    Hope this is of use to someone.

    PS If sticking a mains pressure hosepipe up into your loft isn't particularly appealing (it wasn't to me) I did consider trying a simliar method but using a pump (ie a drill pump may work) to pump straight from the f/e tank to avoid the risk of overflow. However I couldn't find anywhere selling a drill pump that didn't have to be delivered.
    Last edited: Feb 20, 2014

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