Air Locks in Central Heating !!!

A prob with an air lock !!

I've just changed a rad in the bathroom. There was no drain valve on any of the system, so I shut valves and cracked open to drain enough from the system so as to take valves off and change also, and refit new rad.
When i tried to refil there was a problem !!! ... only a few upstairs rads got hot, one was cold, and bathroom one got hot up to halfway.

Was told i had to fully drain system and then refil, venting apropriately.

I cut the pipe and fitted a drain valve on the rad in the hallway - don't know if this is the last one in the run. You can't really tell?! but does it matter? It is just one of the four rads downstairs.
I then drained the system ( I think!), opened up all bleed valves and flushed it through. ... clean water flowed eventually.

Then refilled and bled rads before switching the CH back on.

There's just a pump above the boiler, to which the pipe and pump itself got hot. .... although there is no sound from the pump at all ?! ..... is this normal, or should there be something to tell the pump is working ?

Some rads upstairs got hot although one still cold, so I shut the rads off that were hot, then the other upstairs rad got hot too, so all upstairs rads were hot, although none downstairs ! And no hot water !! Must still be an air lock but don't know what to do now to get rid of it.

The lack of domestic hot water i guess is due to no circulating CH water through the coil in the cylinder ?!

I feel like i ought to fit another drain valve on the other rad at the other end of the downstairs rads to make sure I'm getting the system fully empty before refilling .... is this pointless ?

It's a gravity fed system, with a feed and expansion tank in the loft. A boiler in the kitchen.

....................................I HATE AIR LOCKS !!!!!!!!!!!!!!!!

Any advice / comments / help would be much appreciated ...... before I rip every radiator in the house of the wall !!!!!



  • malamutemalamute Forumite
    710 Posts
    Drain the system fully again and now follow the refilling procedure


    Close the drain !!!! and all the air bleed valves which were opened to help drain the system. Turn the water supply to the cistern back on.
    bleed_order.jpg Now, open and bleed the downstairs air bleed valves first, then the upstairs ones. Once all this has been done, the cistern will stop filling as the ballcock closes off the water. The level of water should be just enough to float the ballcock. More than this, and there will not be enough room for expansion as the system heats up. If necessary, adjust the ballcock.
    Now Start and heat up system check for leaks if all ok remember you need inhibitor in to stop corrosion.
    You will need to bleed all the radiators again over the next few days to get all traces of air out of the system (only bleed when the pump is not running as you can draw air into the system otherwise)
  • EliteHeatEliteHeat Forumite
    1.4K Posts
    It sounds to me like you have done everything right, especially installing the new drain-off valve on one of the downstairs rads.

    Some heating systems can put up a bit of a fight before they start working properly, but they are usually easy to sort out.

    First of all it is likely that the downstairs rads will cause the majority of the problems so these need to be sorted out first.

    Bleed them all again, just to be sure and then turn off of all of the rads upstairs and downstairs bar one of the downstairs rads – this will be the first one that you are going to get working.

    Switch just the heating on and confirm that the boiler comes on and stays on. After a short while you should be rewarded with some distinctive gurgling sounds from the one open radiator and heat will surge through very, very quickly. Leave it until you are certain that it is heating evenly and consistently and then open up the next downstairs radiator. When this is open, close off the first rad. Remember to bleed all of the rads in turn as the heat starts to come through. You repeat this process until all of the rads start to work. Then, open up all radiators and they will all work simultaneously.

    Now turn the heating off and the hot water on. The hot water cylinder should have two 22mm pipes readily accessible. One near the bottom of the cylinder and one about a third of the way up. The uppermost one should be very hot and the lower one should be significantly cooler but still warm. If this is the case then the hot water circuit is working properly.

    If this isn’t happening, look for a manual air vent in the airing cupboard and open it until water comes out if you find one.

    You don’t say what sort of layout you have, but some of the old gravity hot water systems can be difficult to start, but hopefully you will not have one of these.

    Anyway, try doing what I suggest and let us know how you get on.
  • IgolIgol Forumite
    434 Posts
    Re your pump, you should be able to feel some vibration if its working. When mines free of air its totally silent, no bubbles to swish around, otherwise I would of thought it'd make some noise.
    What about the bleed screw on the front of the pump as well? I get some heat transfer along the pipes and rads from the HWC when the pump is off so it could explain your random heating.
  • malamute wrote: »

    only bleed when the pump is not running as you can draw air into the system otherwise

    Why and how? How will air get into the system if the header tank has water in it?

  • EliteHeatEliteHeat Forumite
    1.4K Posts
    wildmandy wrote: »
    Why and how? How will air get into the system if the header tank has water in it?


    The pump should be installed in the most neutral point of the system. Immediately after the cold feed and open vent and on the boilers primary flow pipe.

    This is the best configuration to use as the majority of the sysyem is under positive pressure from the pump and it is unlikely to ever draw air into the system.

    Many systems, especially older ones, were not configured like this and were installed in such as way as to make the majority of the system under negative pressure with the pump effectively pulling water around the system instead of pushing it.

    Under these circumstances it is possible that bleeding the pump or bleeding the radiators for that matter, whilst the system is running, will result in air being drawn (or as it is technically known entrained) into the system.

    Applied thermal dynamics (or something like that) in action ;)
  • Thanks everybody for you replies and advice.
    I've been tussling with the system now and just when I thought it was the pump that had packed up, i was advised to turn the thermostat up ... and hey presto, the pump kicked in !! I was then able to open up rads and bleed where needed bleeding...... piping hot rads throughout !!!! hooray !! Although, the pipes for feed and return into the cylinder for the coil weren't warm at all. I was advised that it would take a while for these to heat up and therefore to get hot water...correct ? ...I hope so, I've left it now and so hopefully will have hot last !!!
This discussion has been closed.