air lock in CH system..

Discussion in 'Plumbers' Talk' started by aak, Jan 5, 2019.

  1. aak

    aak Member

    hello all,

    novice looking for some pointers from a kind soul.

    heating has been sporadic on and off the last couple of weeks, seems ok most of the day and then when called at night for an extra hour, from time to time I get:

    1) banging from the pipework around the boiler
    2) central heating not working
    3) pump very hot but only luke warm attached pipework
    4) noticed today some water leaking from the valve in the pic below

    the radiators have all been bled - there was no air in them. I switched the system off and unscrewed the central pump nut to bleed it - lots of air escape, no water at all, and the pump seems to be rotating freely. Checked to see if the valve was rotating and engaging the microswitch to trigger pump and boiler when calling for heat - havent yet got a multimeter out in the connection box to confirm, but it seems to be working in order, free movement.

    im assuming that all means an air lock - any help on how to resolve would be much appreciated!


  2. aak

    aak Member

    ps on a later perusal it seems there is an issue with the motorised valve - don't know if this was happening before, or only happened once I played around with it to see if it was working, but it seems to stay locked on open, even when I switch the central heating off, and even when I switch the power to the programmer off.

    ive released it by opening it up and taking the actuator off, and have WD40'd it until I get some feedback here and/or can make it to Screwfix in the morning to get a replacement.

    many thanks in advance, and apologies is these are dumb questions.
  3. Jimbo

    Jimbo Well-Known Member

  4. aak

    aak Member

    Thanks Jimbo - am I correct in assuming then that if the vents are working correctly there should be no air locks and no need to bleed the system anywhere?

    I’m guessing it’s that asynchronous motor on the CH motorised valve that was stuck in the on position that caused the problem.
  5. Jimbo

    Jimbo Well-Known Member

    I'm only a DIYer but I would replace the two air vents and the stuck valves and go from there.
    aak likes this.
  6. Allsorts

    Allsorts Well-Known Member

    Hi Aak.

    Excellent informative post by the way :)

    I think it's fair to assume that f both auto air vents are working properly then your system shouldn't have air in it, so that would make the left-hand one a bit suspect (since the other one is dripping water so presumably is not full of air).

    There are vids and info on the interweb about these vents and how they work, and hopefully how to service them too, tho' they are so cheap it would probably make more sense to replace them (but the system would need draining down to that level first).

    It would obviously also be worth determining if your 3-port valve really is faulty (although I cannot see how this would, in itself, allow air to be drawn in).

    You are familiar on how to remove the motor head, so if you remove this again you should be able to monitor its movements... First test the valve part on the pipe - does that move smoothly and is it without leaks? (The actual arc of movement is very small, but the spindle should be moveable by hand, be smooth and end with a firm 'clunk' at each end).

    Now set the spindle in the mid position so water can flow easily, and have someone switch the system to 'CH' and then off and then DHW and then off and then CH & DHW together again and then off - or whatever combination you feel like.

    What should happen (provided the room and cylinder stats are also calling for heat) is that the valve head's spindle recess should whirrrr over to the end and there should be an audible 'click' as the microswitch is triggered (you might hear another click just before this too).

    Now turn the CH off. The valve should whirrr back - at this point I forget if it'll go all the way back or stop half-way. I think half-way if the system is still powered - but proper plumbers will come along and confirm :oops:.

    Ok, turn on the DHW - the valve should stay where it is (if it's already at the far other end) or whirr back from the middle position to the end otherwise. Cool. Now also turn on the CH - it should go to the middle position. Try turning the DHW off - it should go right over to the far end as in the first test.

    Basically, does it move smoothly and without major grindings from position to position?

    If there's any doubt, then buy a whole new one - likely you'll be draining the system down anyway...

    You clearly have air in there, so I think - if you want to fix this yourself? - you should look at replacing the two air vents and the valve at least (if it's provably dodgy).

    How to test the pump? Not sure - although I have wired them up and dipped one end in the sink before (careful to avoid electrics...). It should whooooosh impressively. Or you could split the casing to see if the impeller is corroded - it might be spinning ineffectually.

    SEe the LH air vent? Want to risk slowly slackening off that 15mm brass compression nut on the bottom? Don't remove it, just slacken and move the vent about - see if air comes out.
    aak likes this.
  7. Mike83

    Mike83 Well-Known Member

    A few more pictures may help.

    Is the system sealed or tank fed?

    There may be a lack of water in the system.
    aak likes this.
  8. aak

    aak Member

    thanks so much for going to all the trouble of writing this up - very kind of you.

    I have two two port valves, one for CH and one for HW - replaced the HW motor last year when it lost the strength to engage the microswitch and its been fine since. On the CH valve, I haven't manually checked the valve part on the pipe - thanks for instructing how to do that, ill check later. the motor was moving sometimes when called by the CH, but last night when the CH was switched off, it stayed in the open position. today it seems be acting normally, open when the temperature is below set, closed when it hits temperature an heat call expires. not sure whether last night was an anomaly or whether the WD40 helped it along a bit. Have just bought a replacement asynchronous motor so can replace if/when needed now.

    will monitor but am ready to look ot replace the items yo mentioned! might call in a professional if I do need to drain down given I live in a flat - don't want to take undue risks which might damage other people's property.

    will also research on how/if I can service/test the auto air vents. interested to know how they work!

    couple of questions I have remaining:

    1) is it known for 2 port valves to remain in the open position if there is a weakness in the asynchronous motor?
    2) the boiler does have a pump overrun. could that in combination with the valve remaining open even when the programmer has tried to switch the system off (end of day heating programmed to switch off at 10pm) explain why the pipework around the boiler was scalding hot (burned a finger touching it), and the pump body was boiling hot, but the rest of the pipework was not?
    3) would this also explain the banging in the pipework?
    4) whats that red valve - is that another automatic air vent?
    Last edited: Jan 5, 2019
  9. aak

    aak Member

    its tank fed. stuck my hand in the feeder tank and heard drips of water although was to high to check water was in it. suspect its ok though. today so far the heating has worked fine, no noises, and pump not making weird noises either.
  10. Allsorts

    Allsorts Well-Known Member

    1) AfaIk, these valves return to closed when the power is off - they do this under the power of the return spring. You will see a 'manual' lever on the head's side? Check that that isn't catching and keeping the valve open.

    You can get some idea of the valve's condition by gently-but-firmly pulling that lever to the manual position and releasing it - it should move and whirrr smoothly in both directions with no slips, clunks or grating. Ditto when you actually power it on by 'calling' for CH or DHW - it should whirrrr smoothly to the open position (with a wee 'click' when it arrives) and return just as smoothly when powered off.

    NB I don't know if the pump overrun feature would also keep that valve open for a while too until the boiler's heat is dispersed; it would make sense for it to do so or else the hot water wouldn't be shiftable by the pump. So very likely what you noticed the other night - the valve remaining open - is intentional. These valves have microswitches in them at the end of travel. The boiler's controller/programmer/timer/stats first tells the valve to open, and then the valve goes 'click' and this turns on the boiler. So I wouldn't expect the pump to run unless the valve(s) is also open.

    2) As above. The valve being open shouldn't cause any issues in itself, certainly not scalding hot pipework. I can only guess that the scalding hot pump coupled with medium hot pump pipes was down to the air in these pipes (which there was). The pump MUST always have water in it or else it'll become very hot (and also risk wearing its bearings).

    3) 'Banging' is almost always down to the boiler literally boiling - ie it cannot disperse the hot water fast enough. The causes can be many - failed pump, air lock, blocked pipe, closed valve - anything that doesn't allow the water to flow quickly enough out of the boiler.

    I would tackle this issue systematically based on what you do know - there was air in the pipe around the pump, so suspect these auto vents (espec the LH one). Check this by slackening off the comp nut as mentioned before - see if air comes out. Do that first, and then consider other parts like the pump itself and the valves (you know how to test them).

    The original cause of air getting in might be down to something else like a partially blocked pipe (sludge), but I'd check the obvious first.
    aak likes this.
  11. Mike83

    Mike83 Well-Known Member

    With a a system that’s tank fed when the cold feed becomes blocked the system can start to run dry.
    Usually if pipework around the pump and boiler is scalding hot but everything else cool this is caused by a faulty microswitch inside a valve. Usually when the switch is faulty the valve remains closed.

    If when turned off the rads get hot the zone valve is usually jammed open. This lets water flow to the rads and the microswitch is made.
    Is the pump fitted higher than all the rads and boiler?
    aak and Allsorts like this.
  12. aak

    aak Member

    excellent - thanks, sounds like a good plan of action for me!
  13. aak

    aak Member

    ok great, thanks for that. ill bear it in mind. the pump is fitted higher than the rads but about the same height as the boiler. now I know theres a problem, ill try to keep a closer to eye on what the behaviour patterns are to better diagnose.
  14. Mike83

    Mike83 Well-Known Member

    When the pump overrun is on the zone valves are closed.
    A bypass is provided if a boiler has a pump overrun.
    Allsorts likes this.
  15. aak

    aak Member

    ok thanks.

    have noticed that air release valve on the right is constantly weeping now - so looks like its done for. it is missing the little red cap - whats that for? im guessing it can be tightened to stop weeping in the case of failure?

    also just tried putting up the CH thermostat - everything fired up as expected - set it to 20.5 degrees. however, the next bizarro thing that happened is that everything switched off at 20 degrees. on the programmer is said: set to auto - temperature 20 - thermostat 20.5 - and yet the system had stopped. when I set the programmer to permanently off and then back to auto, it fired up again. its driving me nuts. its a pretty modern programmer, one of the Drayton wireless thermostat ones I put in a couple of years ago.

    after a few minutes and before getting to temperature it switches off again. and then a few minutes later switches on again. so the programmer is getting a signal to switch off from somewhere. either that or its faulty. perhaps the boiler stat?
    Last edited: Jan 5, 2019
  16. Jimbo

    Jimbo Well-Known Member

    Which Drayton stat? If it has TPI enabled that is the design behaviour. Once the measured temperature is within 1 degree of the set point it starts to fire the boiler in intervals to avoid overshoot.
  17. aak

    aak Member

    aha. I thought I was a half decent diy-er, but seems I know a lot less than I thought. its a MiTime T720R. I have now hit the right temperature so thats consistent with your diagnosis!

    ok another question if I can squeeze it in - I figure when I get someone to drain down and replace the air release valves (are these a standard size??) ill finally have a magnetic filter put in. its a small flay and so I only need a small filter like the MagnaClean Micro 2 kind of thing. However, the pipework around the boiler is all 28mm, and the pipework immediately before the pump is 28mm too. The only accessible 22mm bit is either after the CH 2-port or the HW 2-port - would a 22mm filter work in those kinds of locations, or will I need to overkill with a 28mm filter?

    IMG_9853.jpg IMG_9856.jpg IMG_9849.jpg
  18. Jimbo

    Jimbo Well-Known Member

    What’s the bit over the funnel? Looks like white a complicated set up. Magnaclean is best on the return at the boiler but will do it’s job anywhere in the system, all the water gets continually mixed up anyway.
  19. Jimbo

    Jimbo Well-Known Member

    aak likes this.
  20. aak

    aak Member

    cool, will see if I can shove it in on one of the 22mm sections after the CH valve.

    the stuff over the funnel I think is a main pressure valve, the input is from the mains, and the output goes into the hot water tank. presumably the funnel is an overflow.

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