Expansion Vessel - Grant oil-fired combi boiler

Discussion in 'Plumbers' Talk' started by rogerk101, Feb 20, 2018.

  1. rogerk101

    rogerk101 Screwfix Select

    Hi All
    Grant UK seems to have undersized the expansion vessel on their oil-fired combi boilers because they blow far too frequently. I seem to be replacing them every few years, which is not normal.
    The replacement vessels (Zilmet) come precharged at 1 bar, and the water pressure is adjusted to match it at 1 bar. This never changes by much, even when the central heating water is at its hottest.
    1. Does anyone else have this problem?
    2. Can anyone suggest a more permanent fix than simply changing the vessel every few years?
    3. Can anyone see a problem if I put in a bigger vessel mounted outside the boiler enclosure? (I'm thinking of something industrial like a 25 litre vessel.)
    4. Anything to watch out for if I do this?
    Thanks in advance for your experienced ideas.
    Regards
    Roger
     
  2. heatyman

    heatyman Well-Known Member

    For any oil combi or system boiler, replacing the vessel every few years is quite normal. Possibly being so near to the heat source has some effect,; it also curses gas combis. Size up your system and fit a remote vessel. You can then ignore the integral vessel
     
  3. rogerk101

    rogerk101 Screwfix Select

    Many thanks heatyman.
    Does anyone out there know of a big (around 20 litres), relatively flat one that I could fit in under the kitchen base units, and which would be hidden from view by the clip-on skirting boards?
     
  4. heatyman

    heatyman Well-Known Member

    Sorry, but cannot think of any vessel that would fit under a unit.
     
  5. rogerk101

    rogerk101 Screwfix Select

    If these vessels are expected to be replaced every few years, it's pretty poor that GrantUK didn't think to design in an isolation valve to make replacement that much easier.
    I have had to drain down the entire central heating system to replace the vessel.
    Now that I know they don't last, I'll certainly be installing some sort of isolation valve between the radiator circuit and the vessel.
     
  6. The Teach

    The Teach Screwfix Select

    assuming an installation is correctly installed and the initial ev pressure correctly set up and annually serviced they should last a good few years,different makes of vessel have different outcomes.

    Nitrogen charged vessels on the coldest return part of a system,work well :)
     
  7. heatyman

    heatyman Well-Known Member

    Isolation of the vessel with a valve is against regs., but hey ho, its your boiler. 'Delicate' use of mole grips on the flexi, after relieving the pressure achieves the same objective.
     
  8. rogerk101

    rogerk101 Screwfix Select

    What an excellent suggestion ... thanks ... I'll give it a try.
     
  9. BoilermanNI

    BoilermanNI New Member

    Why does no one ever read the manual. The vessel in the boiler is sized for the space left and not the heating capacity of the boiler! Look up the BS for vessels and you will find the average vessel size should be between 15-25 litres not 10 or 12 thrown in by makers. They claim these sizes work in a brand new eco insulated house which 99% of their boilers don't go into and bodgers and diyers never read the manual fully. I price job, get stuffed because I price them right and 3-5 years I get expert whiners on the phone can you come and sort this mess! buy cheap pay twice!
     
  10. BoilermanNI

    BoilermanNI New Member

    Whats more important looks or it doing the job of accommodating expansion. Usual looks before function is why you have a screw up vessel in the 1st place! Find it a proper home!
     
  11. BoilermanNI

    BoilermanNI New Member

    If you read the instructions they advise an expansion calculation be done and if required an additional vessel. Everyone buys on price so never an additional vessel and then they all cry and whine when there is problems. Cheap skates pay twice and whine!
     

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