Vaillant combi losing pressure

Discussion in 'Plumbers' Talk' started by graceland, Mar 26, 2013.

  1. graceland

    graceland Active Member

    Hi, my dads got a Vaillant combi around 6-7 years old, good condition. When he has heating on the pressure gauge goes up high into the red and when all cools down the pressure is on 0. Obviously losing pressure but only if puts the heating on. Seems to me somethings expanding and letting water out and when it contracts again the pressures gone. I havent checked it yet but told him to check all rad valves for any signs of leaks etc. He reckons all ok. Other thought is it could the pressure relief valve blowing off pressure? Thing is he lives in a mobile home and the pipe goes straight down into the floor and he doesnt know where it terminates and cant check underneath yet as bloody freezing!!!
    Am I on the right trail?
     
  2. petertheplumber

    petertheplumber New Member

    The expansion vessel inside the boiler is not working, it has a rubber diaphragm that may have failed, or the air it it needs re pressurising back to 3/4-1 bar this is done with a foot pump when no pressure in the system, the tube that connects the boiler to the expansion vessel may be blocked with sludge. Those are the possibilities that need investigating.
     
  3. petertheplumber

    petertheplumber New Member

    While waiting for it to be fixed, drain 1/2 the water from a radiator that is not too small and not too important for heat and that will work as an expansion vessel.
     
  4. graceland

    graceland Active Member

    Many thanks for reply Peter. Just trying to get head round why a duff diaphragm would lead to the pressure going way up when heating on then down to zero when cools off? Thanks again
     
  5. You pretty much answered that yourself in your opening post!

    You have a sealed system. Water expands when heated, so the excess needs somewhere temporary to go (unless you want an exploding rad). The expansion vessel is split in two, divided by a rubber diaphragm. On one side is air, which is pressurised to roughly the system's working pressure using a pump (bike or car type works fine).

    On the other side or this rubber thingy is the system's water, joined to the sealed boiler water via a narrow pipe. When you turn on your heating and the water expands, excess fills that vessel, pressing against - and moving - the diaphragm. The air on the other side gets compressed to accommodate this movement. You should find that your system's pressure stays pretty steady throughout all this. When your system cools again, the water contracts and refills the system - the diaphragm moves back, pushed by the compressed air.

    Everything's happy.

    Until either the air is lost (which happens over time) or there's a leak in this diaphragm (necessitating an expensive-ish replacement.)

    What happens then is you top up the cold system to get the pressure to where it should be - around 1 bar. Then it heats up and the water tries to go into the vessel - but it can't, 'cos the vessel is already full (no air - only water in there). So the pressure goes up & up to the red; the rads bulge and the joints creak..until...until...until it opens the PRV (set to about 3 bar) and dumps this excess water out the safety discharge pipe.

    Everything's 'fine' as long as the system is running hot - the pressure will remain high, but it should work ok.

    But then it cools down again, and suddenly the pressure drops to way below the 1 bar it should be at, because the sealed water has contracted, and there's no reserve in the vessel to refill it.

    You check your gauge - it's way too low. You top 'er up to 1bar again, turn her on, and then the pressure shoots up again - repeat ad nauseam.

    If you are up to it, you can take the cover off your dad's boiler and look for the expansion vessel - often painted an 'oxide' red. It should be pretty obvious, but look on line for diagrams. On top you should find an ordinary schraeder valve like on a car tyre. Open this (all done with the system cold, of course) and press the wee thingy in t'middle - like you would to let down your MP's tyres.

    One of three things should happen:
    a) It goes pfffffffffffffffffffffftt. Which means there's still some air in it.
    b) It goes pffff-shrrrr-ppp-ppp--ssrrrrppp-sssst. Which means there's air and water in it (not good - ruptured diaphragm).
    c) It goes                                      . Which means there's nothing in it.

    Ok, get a tyre pressure gauge and shove it on if you had (a) and see what it reads. Bear in mind this should be done with the water side completely depressurised - you'll have to bleed a rad until nothing comes out, or else open the PRV manually. Doing the rad is best. See what the gauge reads. If it's less than 1 bar (which it almost certainly will be) then get a bike pump and get it up to that level.

    Repeat the above for (c)

    Call out a plumber for (b). And raid you savings.


    You can also check the cause of pressure loss by placing a can or bag under your discharge pipe and seeing if water comes out every time the heating is put on. Chances are it will.

    You will also very likely have ongoing problems with your PRV because they don't get activated for years, and then - when they do - they don't shut off properly. Constant drip drip drip. Constant slow drop in pressure... :(




    Well, you did ask
     
  6. graceland

    graceland Active Member

    Devils, many thanks for a brilliant reply. I assume if expansion vessel is ok (Its ok I know what it is, the big red thing inside boiler casing) then the only probs could be what I put in post. I did advise him to put a plastic bag over blow off pipe but, as explained in post, it goes straight down into the floor and he doesnt know where it terminates? When I get over there I'll be able to et underneath and have a look. I cant see it being underneath 'outside' as such as the heat loss would be ridiculous so Im assuming its somewhere between the floorboards and actual bottom of mobile home? If diaphragm is knackered then is it replaceable or the whole vessel got to be replaced? Thanks again for great reply
     
  7. petertheplumber

    petertheplumber New Member

    Yes IF it is the diaphragm in expansion vessel that is split and failed then you can get a replacement expansion vessel, or you can fit and external expansion to any of the heating pipework anywhere on the system that is least obtrusive. Often it is a blocked pipe to expansion vessel or no air in air side of vessel.
     
  8. Good solution from Peter - if the vessel is faulty, you could just leave the existng one and tee into the CH pipe to plumb in an external expansion vessel. No, you can't fix the diaphragm, only replace the whole vessel (afaIk...)

    Where should an external one go, Peter - flow/return/near boiler?

    Gracie, when you check the vessel air pressure, the water system should be fully depressurised. I did this recently for my Nan, and assumed the water pressure would be ok as it was well down. However, the pressure still climbed a fair bit during heating until I did it properly, and fully depressurised the water side first.

    The discharge pipe should be visible outside, but I understand it being too cold at the mo'!
     
  9. petertheplumber

    petertheplumber New Member

    The vessel can go on flow or return it makes no difference. And if it looses pressure it is because the expansion vessel is not working so it will keep discharging every time heating goes on and this also means the pressure relief discharge valve will now not shut off properly so that may well need replacing as well.
     
  10. graceland

    graceland Active Member

    Thanks again Devil and Peter. Latest update. I still havent been able to go and look so have been giving dad instructions over the phone and text. Hes pretty mechanically minded so ok with that. Hes put a gauge on the expansion vessel up to about 14.5 psi, topped the pressure up on the boiler gauge to around 1 mbar (10 oclock on a clock dial on a Vaillant dial) and put the heating on. The boiler gauge pressure went into the red and also the air gauge on the EV went up. Now Im assuming if the diaphragm is buggered then the water has leaked into where the air is (in the EV) and then has nowhere else to expand to, hence the increase in the boiler pressure gauge. I have then assumed that as the air in the EV has been compressed more than usual as the water is leaking through, then it has increased its pressure and thats why the EV gauge has gone up? Is this correct? Water apparently came out of the EV valve after all the air came out first so I assume the diaphragms gone. There was also water coming out of the PRV underneath the floor. Im pretty sure that the diaphragm must have gone but understand that this is quite rare for them to go. My old gas tutor said he'd never seen one go in his entire career? The boiler is a Vaillant eco tec 831. Is it just as easy to replace the EV in it than to add another onto the system?
    One last thing, he has turned the heating off to avoid all these probs etc but is running the hot water if and when they need and this doesnt seem to be causing any probs, maybe a slight raise on the boiler gauge but nowhere near the increase in pressure when the heating is on? Is it ok to still run the dhw as and when they need.
    Many thanks guys
     
  11. graceland

    graceland Active Member

    So can you still run the hot water on a combi when the expansion vessel is knackered?
     
  12. petertheplumber

    petertheplumber New Member

    So can you still run the hot water on a combi when the expansion vessel is knackered?                                                                                                                    Yes you can still run hot water and I did mention above that emptying some water out of a rad will work as an expansion vessel.
     
  13. Take the tyre type dust cap off the exp vessel & put a pin or piece of wire in as if to let down a tyre, make sure there is pressure in the system 1st, if water comes out the diaphragm is shot, if not, drain down & charge ( pump) air side of vessel equal to working pressure of system, in your case 1 bar, then re fill, enjoy
     
  14. Hi Gracie.

    Hmmm, methinks the pressures in your post above are a bit suspect :).

    Your dad didn't really get 14.5 bar reading from the expansion vessel, I hope. You'd have creaking rads at that pressure. Provided they hadn't exploded...

    And he topped up the water pressure gauge to *cough* one millibar...? .  He means 1 bar.

    Ok, ok - I'll stop being cheeky.

    IF water came out of the air top-up valve, then the diaphragm is shot. No other way water can get past. At least that does explain what the cause of the problem is. A quick surf shows that vessels are only £50-odd, and I reckon they'll be an easy fit - a half-hour labour in most cases (unless they're awkward and require lots of dismantling). If you phone a GasSafe who knows Vaillant boilers, they should be able to confirm this.

    There is a fair chance your PRV will also need replacing as they tend to leak slwoly after they've been activated, but you might get lucky on this. Again, tho', they shouldn't cost the earth.
     
  15. Post the exact Vaillant model on here and good chance someone will tell you the likely repair bill.
     
  16. petertheplumber

    petertheplumber New Member

    Yes DA the expansion vessel on a lot of the older ones comes out from the back sliding it up after disconnecting. But there has to be enough room above the boiler to do it the easy quick way, other take it of the wall then put it back, do it in about 2or3 hrs that way.;)
     
  17. graceland

    graceland Active Member

    Thanks again for your replies Devils, Peter, Wilbur etc. Devils my post said 14.5 psi (pounds per square inch) which is apparently equal to around 1 bar. Had a look around for prices from online plumbing merchants etc, only a quick look but cheapest I found was around £109. Then looked on ebay and about £45-50 as you said. Seem ok, not cheap rubbish but why so much cheaper Im not sure? Boiler is an eco tec plus 831 and looking at the manual, an expansion vessel swap doesnt seem to bad? Do I need to isolate the flow and returns and drain boiler before changing? Tank slips in the side of the inside of the boiler.
     
  18. petertheplumber

    petertheplumber New Member

    Yes Grace, isolate the boiler from the water pipes, cold in heating flow and return, isolate from mains disconnect the tube that feeds it with water and bobs your uncle. There will be some water pressure that needs dischrging, best thing is use the drain valve on the boiler.;)
     
  19. graceland

    graceland Active Member

  20. petertheplumber

    petertheplumber New Member

    Very good price well done finding that.lol:eek:
     

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