System replacement following unvented cylinder burst

Discussion in 'Engineers' Talk' started by Randolfus, Apr 10, 2023.

  1. Randolfus

    Randolfus New Member

    Dear all,


    My OSO Unvented Hot Water Cylinder sprung a major internal leak on Easter Sunday. Water pouring out from behind the circuitry plate! Yeah, ca. 250 litres straight down from loft to living room. Managed to isolate the cylinder and get the cold supply back on to taps and WCs.

    Obviously no rads or hot water. Electrics to the damaged areas are isolated, thanks to a decent main board with split zone RCDs. Can cook, do laundry and get online….and watch the darts on TV! I’ll be getting a certified electrical professional via my insurer to check the affected zones and reinstate in due course.

    Can get a shower at my gym, so a complete disaster is mitigated to that extent, plus the fact that it isn’t the middle of winter. I can boot up an open fire if it gets chilly.

    I’d like to move away from a big tank in the loft and whilst being without running hot water is a real pain, I’m reluctant to jump straight in with a like for like set-up as this incident presents an opportunity to improve things.


    Large late 1980s 5-bed domestic house, insulated and double-glazed, 15 rads, 2 bathrooms, 4 WCs, 1 kitchen, 1 utility. No airing cupboard space. Currently on gas with a 37kW Vaillant EcoTec Plus 637 condensing boiler. OSO 250 Ltrs Unvented Cylinder (now presumably scrap). Hot water demand is actually quite low (just two adults) and a system change won’t be seen as an investment for whoever has the house once we’re pushing up the daisies.


    Before I even ask anything, I know that there will potentially be as many opinions and recommendations as there are members reading this.

    However, that’s no worse than pulling a name out of the local Check-A-Trade leaflet – i.e. I respectfully expect the advice to be based on the experience and preferences of experts, but the key advantage of posting on here is to see where common ideas coalesce.

    I’m not keen on getting rid of my gas-fired system despite the pressure to swap to a greener solution. If I was in my 20s, I may think about it.

    Q. Other than rigging up a cold-feed electric shower off the mains, am I missing anything obvious with getting a temporary hot water solution? Most options appear to be aimed an industrial and commercial markets. I’ve double-checked that my boiler ID is what I have said (so to keep it, I need a tank to store hot water...I can’t use it as a temporary solution as a kettle just for the taps/showers).

    Q. I’m not against getting rid of the current boiler, The space for a replacement would need to be no bigger than the footprint of the EcoTec Plus 637, (720 x 440 x 406mm) give or take a few inches top and bottom. So I presume I’m looking at a Combi? Based on the operating environment, recommendations would be helpful. I’m assuming that as the current system is “pressurised” and my pipework would therefore withstand a Combi...but this assumption isn’t based on anything other than my own hunch.

    The required BTU output was accurately calculated a few years ago at in excess of 115k. So, at the very least a replacement boiler needs to be from 35kW upwards to allow for normal heat loss.

    Thanks in advance for ideas and recommendations. The responses I have had in the past through these forums have always been spot on and very much appreciated.


  2. Kiaora oap

    Kiaora oap Active Member

    My view it to install a new unvented cylinder and keep the quality boiler you have ,

    It’s the best system to have, especially in a larger home,

    good luck,
  3. Teki

    Teki Screwfix Select

    OSO have a 25 year warranty on their cylinders as do most unvented cylinder manufacturers. Have you contacted them about the leak?
  4. Randolfus

    Randolfus New Member

    Hi guys, thanks for your replies.
    I've been weighing up options. Replacing the cylinder with another one in the loft is a situation I'd like to move away from. So much so I'm considering two boilers...keep the Vaillant system boiler for just the rads and a big W/B combi for hot water which could be converted for rads and hot water when the Vaillant gives up, even just to get by with.

    On the OSO warranty, yes, waiting for a call back on the warranty (? due to Easter holidays) but most components are just 2 years, with 25 on the stainless steel drum. I dare say I'd have to pay for a lab examination to determine the cause of the failure. I may not pursue it because I'd rather move away from any storage tanks at all if possible.
  5. terrymac

    terrymac Screwfix Select

    If you don't want a hot water storage cylinder,and your not bothered about future limitations of a combi in a large property.....
    Fit a combi boiler.
  6. Kiaora oap

    Kiaora oap Active Member

    I installed a combi in a large house, was a new build, the customer had two bathrooms, and en-suite
    The combi supplied the kitchen and en suite
    The combi also, via a separate zone valve, heated the unvented cylinder, that was used if guests/families were visiting ?

    Personally , I have a combi for my kitchen and en-suite, and all electric unvented cylinder, (cheap rate electric) for the rest on the house, when I have visitors

    Best of both worlds,
  7. Dr.Bob

    Dr.Bob New Member

    Hi there,
    I have had a similar failure. Did you have any luck with the warranty? Was there corrosion of a brass internal part of the tank?

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