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Boiler & System Pressure Problem

BHDougalBHDougal Member Posts: 5
I have a 10 year old Valiant Eco Max 646 Wall Hung Sealed fan assisted boiler. It uses an external pressure vessel as it does not have an internal one. I have 30 radiators in my house, total output 42KW 470 litres. The pressure vessel capacity is 18 litres. We moved here 5 years ago. About a year ago I noticed I was having to top up the boiler from the cold feed fairly regularly in the mornings ( about every 10 days). (After the 10 days, during the night when everything was cold the boiler pressure dropped to zero on the boiler display and I then had to re-fill to 1 Bar.) However, after filling to 1Bar cold pressure once the boiler was up to temperature ( 65c ) that pushed the boiler pressure over the red 3 Bar limit and it vented outside. I thought that I maybe had a leak somewhere in the system. No signs of a leak anywhere even though we have concrete floors. After many weeks of fiddling, I discovered that if I set the standing pressure ( no heat) of the boiler to 0.25Bar and the max boiler temp to 60c the system worked within the boiler pressure guidelines and did not lose water. It did not drop the pressure overnight and only needed a minor top up every couple of months which I think proved I did not have a leak. However, I knew this was not ideal and reckoned the problem was the pressure vessel. So I got a local installer to review my problem. He agreed and a few weeks ago he change the pressure vessel. He said the old one had water on both sides of the diaphragm & so was clearly broken. However, the same problem has returned and I now have to top the boiler to 0.25bar max cold and set a max operating temp for the radiators of 60 DegC . This works ok just, but in very cold weather I would prefer to run the boiler at a higher temp. The installer says he can't help me further. I have bled all the radiators regularly, checked the capacity of both the boiler and the P Vessel for my system, and they are both ok. I would be grateful for any thoughts or ideas from your experiences. I am not an installer but I worked for a radiator company for 35 years so have a good working knowledge.


  • ZmanZman Member Posts: 5,870
    Do you have some pictures or a drawing if how your system is piped?
    I think you want to rule out the pressure relief valve as the source of problem. Sometimes they blow off when you are not watching. Putting a bucket under the valve will tell you if that is the issue.
    Pressure testing the distribution system at 3 or 4 bar with the boiler isolated should expose a leak on that side of things.
    Although rare, you could have a pinhole leak in the boiler heat exchanger. Does the condensate line drip steadily when the boiler is not running?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • hot_rodhot_rod Member Posts: 13,499
    Was the pre charge pressure adjusted on the new vessel?
    Is the fill valve turned off tightly?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • BHDougalBHDougal Member Posts: 5
    Hi ( sorry - had the flu I think!) No - I do not know if the pre-charge pressure was adjusted on the new vessel. I will need to ask the installer and advise. I believe this should be adjusted to the system pressure. (2 Bar). Can you advise how this is done as maybe I could do this myself? The fill valve is definitely turned off tightly and not letting through. Now, when the boiler turns off, the pressure gauge on the boiler does not drop - it stays at just below the red ( 3 Bar). Thanks for your help.
  • BHDougalBHDougal Member Posts: 5
    Thanks Zman - Yes - I already put a basin under the pressure relief pipe and it only lets water out at 3bar. None during standard operating pressure or when the boiler is off - so no drips which in turns hopefully means the heat exchanger is ok. I think the problem lies with the new pressure vessel and how it was installed.
  • BHDougalBHDougal Member Posts: 5
    Spoke to the installer. He said he did NOT adjust the Pre Charge pressure - just installed it as it was. ( it is an 18 litre Vessel with a 3 Bar capacity.) He said that was standard practice? I looked it up on the Web and your comment is very important. It should be adjusted every time to suit whatever the system needs are. My max operating pressure in the green on Boiler gauge is 2 Bar. So I think the Pre Charge should be 1 Bar which is the desired cold pressure. I don;t have much confidence in my installer so will get someone else to leek this and hopefully resolve it. Thanks for your help.
  • Jamie HallJamie Hall Member Posts: 12,947
    You are correct -- the precharge should be 1 Bar, the cold pressure.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • nicholas bonham-carternicholas bonham-carter Member Posts: 8,139
    Precharge pressure adjustments should be done when the tank is not connected to the system.
    Some incorrect piping will allow the pressure from the circulator to over pressure the tank, which complicates things-(see “pumping away“ on site).—NBC
  • BHDougalBHDougal Member Posts: 5
    Thank you all - very helpful. Piping is ok - I had that checked and it also worked fine for 10 years. Problem is just since the old PV rotted through and the new one was fitted that I've had this problem. I will have it disconnected and Pre-Charged to 1 Bar and that will hopefully resolve the matter. Many thanks
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