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Discuss Central Heating System losing pressure despite replacement parts and multiple visits from plumber? in the Central Heating Forum area at

  1. Silverfox2306

    Silverfox2306 New Member

    Good afternoon,

    Our system is losing pressure on a daily basis. Despite repeated top ups. I have listed the key points below.

    1. When motorised valve on the heating side is switched to automatic.
    System loses pressure overnight. Guage on top of expansion tank drops to zero.

    I then top the system up to just over one bar and reset/switch on the boiler. The heating will work fine, however later the same day or overnight the pressure drops back down to zero.

    2. When the motorised valve on the heating side is switched to MANUAL, the pressure is maintained, however the heating is on 24/7 but NO loss of pressure whatsoever :)

    Now, our plumber has fitted, new valves next to the Megaflow where most of the pipework and pump are. Fitted a new expansion tank. Put a sealer into the system.

    Still losing pressure on automatic but not manual.

    Now, our plumber is telling us there must be a leak in the system under the floor downstairs. No sign of leakage upstairs. When I asked, why does it not lose pressure when the motorised valve is set to zero. He said, when the water is hot, it won't leak, it will leak when the system cools down. EH?

    Could someone please tell me if this is true. I would have thought that hot water under pressure would leak out of a pipe quicker?

    Before we start ripping the floor up to see if one of the pipes has a leak which will be no mean feat as we have 18mm solid Oak tongue and groove on top of 18mm OSB, I thought I would post on this forum to see if there is an alternative solution?

    Thank you very much

    • Useful Useful x 1
  2. Rob Foster

    Rob Foster Well-Known Member

    I don’t think you need to panic just yet, there are loads Of experienced fellas on here including me, when they come home and switch on you will get plenty of ideas . Now my advice is to isolate the boiler via valves underneath and re pressure and see if the system still goes down , if yes it’s the boiler if it does not the leak is in the system somewhere. I am assuming you are reading pressure of a gauge on the boiler. Do this and report back by which time my colleagues will be on line and full of more advice ...Rob Foster
    Aka centralheatking
  3. Silverfox2306

    Silverfox2306 New Member

    2018101114211479924.jpg IMG_20181011_141630.jpg IMG_20181011_141606.jpg IMG_20181011_141649.jpg Hi Rob,

    Many thanks for taking the time to respond to my post.

    I probably should have clarified the type of boiler we have. It is a Rayburn Heatranger 480CD gas fired that does the heating, hot water and cooking. A mains fed system with a Megaflow tank for hot water storage.

    I have attached some photographs of what the set up looks like to give you a better idea.

    The first photo is the motorised valve on the central heating side. Notice the valve is set to MANUAL. The second photo is of the valve I am looking at which is on top of the expansion vessel. The third photo is the spaghetti junction of pipes we have in the tank cupboard. The last photo is of the pump.

    There are no leaks visible above ground floor level. The radiators have been checked that they are full. There were a couple of the upstairs radiators that had a very small amount of air in them, no more than a pssst! There are no signs of water on the floor under the boiler.

    The other thing I noticed today was after I topped the system up to 1bar+ I went downstairs and pressed the reset button on the boile. It was only when I went back upstairs I noticed that I hadn't switched the lever on the motorised valve to MANUAL. I also noticed that the light wasn't on on the pump. As soon as I flicked the lever to MANUAL the light on the pump came on and hot water immediately started to flow through the pipe on the other side of the valve and down through the system.

    Not sure how I can isolate the boiler as you can't get into it. Unless there are valves in that cupboard beside the hot water tank.

  4. AMGasServices

    AMGasServices GSR

    As rob stated isolate the boiler from the flow and return for 24hours if the pressure is fine then there no internal leak in the boiler

    After that I would isolate the coil inside the cylinder as it may have a pin hole leak in it.

    Tbh I’ve not heard of your boiler manufacturer.
    • Like Like x 1
  5. snowhead

    snowhead Well-Known Member

    Don't get hung up on this "Only leaks when it's in auto"

    The system is loosing pressure, therefore you have a leak / loss of water.

    When the expansion vessel was changed was the precharge pressure set correctly?
    Was the Pressure relief valve above the expansion vessel changed at the same time?
    Has the Pressure relief valve on the boiler been changed?

    Has anyone checked whether there is obvious discharge from the PRVs by putting a balloon over each discharge?
  6. Silverfox2306

    Silverfox2306 New Member

    First off, I am not a Plumber. I am just the home owner. I am certainly no expert when it comes to central heating systems. The PRV was changed when the expansion vessel was replaced. I cannot access any valves in the boiler as it is a Rayburn and I would need to remove a metal plate to gain access. There are valves beside the Megaflow tank as per the photograph I posted. Can the boiler be isolated here? I don't think there is a leak in the boiler as I would have expected to see water underneath it on the floor?
  7. bacon_sandwich

    bacon_sandwich GSR

    Its loosing water, so where is it going ? Need to logically rule out one, by one. As mentioned above. Last two i went to where equally bizarre. Never leaked when i was there and boiler on, it would leak overnight when it cooled down, was a poorly drain cock, fortunately it did leave a trace of water overnight, second was even more bizarre, again no loss when i was there and boiler hot, would loose pressure when cooled down, I eventually trace it to a gate valve that had been used to supply an old conservatory, conservatory gone but valve was just closed not capped, frost had caused a split in the valve body.
  8. dancinplumba

    dancinplumba Trusted Plumber GSR

    Can't / shouldn't be isolators on this boiler, it will turn into a bomb, will then be isolated from exp vessel
    • Like Like x 1
  9. Silverfox2306

    Silverfox2306 New Member

    I had the motorised valve on manual. Central Heating working fine. Flicked it back to automatic and within a couple of hours this evening the pressure was down to zero.

    I have had my ear to the floor to listen for the sound of water escaping but can't hear a thing. I reckon (although it's a guess TBH) I'm putting a couple of pints of water in when I re-pressurise the system. So is it possible to lose that volume just with a drip over a period of a couple of hours?

    As I said earlier in this post, my plumber is telling me it won't leak when the water is hot, only when it is cold? To be honest, I'm starting to think he doesn't know what the problem is and he's hanging onto this theory. Bear in mind these guys have been at my house five or six times trying all sorts to get to the route cause. I'm certainly not suggesting they don't know there stuff, but you can appreciate how I'm losing confidence, especially with the HOT/COLD theory?

    Thanks to everyone who has responded. Very much appreciated. Bit like a Doctor, really wanted a second opinion before I start tearing my floors up.
  10. Gasmk1

    Gasmk1 GSR

    different leaks when hot pipes valves joints expand leak stops when cool pipes valves contract thus leaks hard to find as eveything still warm so it evaporates
    • Agree Agree x 1
  11. Silverfox2306

    Silverfox2306 New Member


    So that might narrow it down to a valve somewhere. The only valves I am aware of are in the tank cupboard (see photo)

    It all seems to be dry in there although I'm not sitting in the cupboard for hours watching the pipes

    If it's a joint under the floor then presumably it's going to be fairly difficult to trace. The plumber who invents a contraption that can see through floors will become a millionaire overnight
  12. Silverfox2306

    Silverfox2306 New Member

  13. Silverfox2306

    Silverfox2306 New Member

  14. dancinplumba

    dancinplumba Trusted Plumber GSR

    thermal imaging may help
  15. Silverfox2306

    Silverfox2306 New Member

    There was a hot and cold feed to what was the old kitchen now an extended sitting room. The pipes were a heavy looking black PVC/Plastic material. They were capped off using some sort of end caps, Brass with large Brass nuts. Is there a possibility that these have given way after two and a half years?

    The guys who installed the system capped these off, however they were leaking and we had to lift the floor then. It was easy to know they were leaking as you could hear the hissing sound. We had another plumber, a guy who is one of the best in our area come in and cap them off properly. That's not to say they wouldn't go again as nothing is 100% foolproof. I have had my ear to that part of the floor again but can hear nothing?

    BTW I will nip out to the shed and get my Thermal imaging camera in the morning ha ha.

    Amazing I can keep my sense of humour under such stressful circumstances :-D
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