Combi Boiler Keeps Filling On It's Own Over Pressure.

Discussion in 'Plumbers' Talk' started by gregory1975, Jun 3, 2015.

  1. gregory1975

    gregory1975 New Member

    We have recently moved into a new house with a Worcester 30cdi Combi Boiler installed 2006. The heating and hot water appear to be working fine. We had a Nest thermostat installed and the installer said the boiler pressure was high, about 2.5 bar nearly in the red. As we have taken out a maintenance contract with nPower we contacted them and they sent an engineer who was baffled. He ordered a new plate heat exchange and pressure gauge. Second engineer arrived to fit parts and was also baffled but fitted the parts anyway. The pressure remains high.

    Pressing the dump valve handle under boiler at the back empties and reduces the pressure via the overflow but the boiler immediately refills to 2.5 bar. The filler key is removed. Turning the mains water off stops the boiler refilling. Turning the mains water partially off allows the pressure level at which the boiler fills to to be reduced, but that is no good as it also means little pressure out of the other taps in the house. Running either hot or cold tap whilst releasing the pressure works until you turn the tap off at which point it immediately rises again.

    I have been told either my hot and cold taps are on the wrong way around! My shower mixer valve has failed! Someone has made a link from the mains water to a radiator pipe in order to fix a leak! I have searched for said link and can not find it yet. Has anyone got anymore ideas before I rip up anymore floorboards? Or do I just ignore the problem as all seems to work ok despite the high pressure, but I don't want to blow the boiler up down the line.
  2. Dave does Gas

    Dave does Gas Screwfix Select

    New house you should not be using your own maintance contract to sort out issues with the fixtures and fittings. You need to get back to the builders to sort it out for you under the new build insurance scheme which should be valid for up to ten years.

    If your taps are the wrong way round you would have cold coming out on the left and hot on the right.

    Shower mixer is a very possible option you can test this by isolating the cold inlet undeneath the boiler (flat blade screwdriver needed) relase some pressure from your system via a drain off valve it should no longer rise or if possible shut off both hot and cold isolation valves ad release the pressure.

    Its posible but unlikely for the cold to be linked into the heating circuit and i would expect the pressure to be much higher than 2.5 bar.

    Try not to use the PRV too much as it an automatic device for pressure release, by using it as a drain off you will damage it and have to have it replaced. Use a drain off point and a hose to release pressure.
    gregory1975 likes this.
  3. gregory1975

    gregory1975 New Member

    Thanks for the reply. Sorry when I said New house I meant new to us it was built in 1924. I have got hot out of the right tap on the bathroom sink mixer but that is the way the tap is labelled when a plumber fitted it so I guess that is ok. The cold water inlet has been isolated by the heating engineer and that does not stop the pressure rising immediately. It only stops when the mains stop cock is turned off or a tap is running.
  4. Blimey! That's a weird one.

    I guess one thing to try is to isolate the boiler using the shut-off valves on the flow and return pipes to the rads. Then dump some of the pressure and see if it rises again with these valves shut.

    Not quite sure what that will tell us tho'...
    gregory1975 likes this.
  5. Dave does Gas

    Dave does Gas Screwfix Select

    Have you had any work done recently?
  6. gregory1975

    gregory1975 New Member

    Thanks Devil's Advocate. I have tried that with the valves shut off to the flow and return pipes the pressure stays constant after I have dumped some of the pressure but as soon as I open one the pressure goes straight back up.
  7. gregory1975

    gregory1975 New Member

    We have had a new toilet and wash hand basin fitted in bathroom and a bath removed, but they were simply swapped by a plumber using the existing pipe work.
  8. Dave does Gas

    Dave does Gas Screwfix Select

    This might sound daft but can you hear water running when all the taps are off and toilet(s) are full. Put a screwdriver on the mains stopcock and your ear to the handle any noise at all
  9. gregory1975

    gregory1975 New Member

    No can not hear water running on mains stopcock and the water meter is not going up either when the taps are off and toilet full. I like the trick with the screwdriver to my ear, wife thinks I have gone mad.
  10. Blimey even more.

    With the boiler isolated from the CH pipes, its internal pressure remains constant? So it cannot be filling from either the mains supply or the PtoP exchanger.

    The pressure rises when you open the boiler to the CH system?

    You are 100% sure this is a 'combi' boiler, and not even a 'system' type or one supplying a Megaflow hot storage tank?
  11. gregory1975

    gregory1975 New Member

    Yes with the CH flow and return off the pressure stays constant. Once I open the CH system it rises in a couple of seconds and you can hear the water coming in. I have not found a tank anywhere one heating engineer even looked in the loft for a tank. I know it is not up there as we are having a new roof put on, and it is only a 3 bed bungalow so not many hiding places!

    I just can not imagine an old filler loop being put in a random room of the house. The kitchen that houses the boiler has a concrete floor so all the pipes are running under the kitchen cabinet plinths or behind them.
  12. Jitender

    Jitender Screwfix Select

    Has the worcester boiler got the internal filling loop fitted? The special key is usually housed on the removable lid under the boiler.
  13. Dave does Gas

    Dave does Gas Screwfix Select

    Yes Jit he mentioned it was removed in his first post
  14. Dave does Gas

    Dave does Gas Screwfix Select

    The big mystery is why only up to 2.5bar and no higher, if it was constantly being filled it would be activating the PRV almost continuously as soon as it hit 3 bar. So from that we can almost eliminate the cold main theory. But you say the pressure drops when you open up a tap and goes back up as soon as you close it, which would indicate that somehow the water and heating are linked. Assuming you have had no major work in the house apart from the bathroom then the fault has to be on the boiler.
    You stated that you have had a new plate heat exchanger, which is one point the two could cross, so lets eliminate that.
    You had a new pressure guage fitted so we must assume its accurate.
    The only other two points on the boiler where the crossover could take place would be the diverter valve or if fitted integral by pass valve.
    May be worth getting your maintanace contract guys back in as to date the problems not been fixed, and it could be one of thoes two I mentioned.
  15. ...except the water appears to be coming from the rad system :(.

    Greg, can you confirm something else for us. You open up the isolating valve to the rad system and you can hear the water come in and the pressure rises to 2.5 bar? Does water continue to come in - can you hear it?

    You know the 'dump' valve you've used to de-pressurise the system? You probably know that's a safety valve designed to open at around 3 bar. I wonder if it's opening earlier - at 2.5 bar? Hence the pressure seemingly stabilising there.

    When you open that dump valve, do you know where the water comes out? It should be via a 15mm copper pipe outside, pointing down at the ground. When you hear water coming in to the boiler, can you see water coming out that pipe at the same time?

    And also to confirm you have no tanks - you turn on a hot tap and the boiler fires up to heat that water instantly? There is no hot water store?
  16. gregory1975

    gregory1975 New Member

    I am assuming that it is only getting to 2.5 bar because the mains water pressure is only that high "Turning the mains water partially off allows the pressure level at which the boiler fills to to be reduced, but that is no good as it also means little pressure out of the other taps in the house." The pressure does not drop when a tap is opened but if you dump some pressure it will stay down whilst the tap remains open once it is closed the pressure rises again.
  17. gregory1975

    gregory1975 New Member

    Once it reaches 2.5 bar the sound of the water coming in stops. The water certainly comes out of the pipe down the outside of the house when I pull the dump handle it is washing the soil away. It fills up so quick I can not see outside at the same time. there may be a slight drip the rest of the time but it is not much. The boiler kicks in as soon as the hot tap is turned on.
  18. Jitender

    Jitender Screwfix Select

    The boiler details that have been provided are for a combi boiler, there is a preheat function available with this boiler, this can be deselected by pressing the ECO button.
  19. Hmm - good possibility. Although 2.5bar is pretty pants (although not unusual).

    Definitely no water trickling out the PRV pipe outside? Def no hot store tank?

    (Soz - you've answered these Qs)
  20. Ok, one more thing to try. Isolate the mains inlet to the boiler too - there's a shut-off valve there. You cannot isolate the DHW flow out from the boiler, but you can ensure no hot taps are on. Also shut off the CH flow and return as before, and then dump the excess to drop the boiler pressure.

    Now see if the boiler pressure recharges only when you open a CH pipe (not the mains inlet).

    If the pressure builds up only when the CH pipe is re-opened, then that must surely indicate that the water pressure is coming from the rad circuit, and not the mains directly into the boiler, and not via the hot water supply (eg from a faulty mixer tap)?

    In which case, someone has done something really weird with a plumbing pipe!

    (If what I'm saying is right...)

    Of course the radiators and circuit will have some 'stored' pressure in them, but surely not enough to have recharged the boiler so many times, and without the top-up loop being used?!

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