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Expansion vessels on hot water cylinder. Are they designed to fail?

Discussion in 'Plumbing and Central Heating' started by Rad1o, 12 Jul 2019.

  1. DP

    DP

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    Rust resulting from fresh water contacting metal wall of vessel
     
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  2. fezster

    fezster

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    This is a genuine question, not an attempt to be facetious. But why can an EV designed for an unvented system not be used on CH if the charge pressure is adjusted appropriately?

    The opposite, as already explained, cannot be used. So aside from the extra expense of an unvented EV, is there another reason it cannot be used?
     
  3. dilalio

    dilalio

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    Think I've mentioned it before as part of a different topic but was called to a leak first thing on a Monday morning to find two flooded floors in a commercial property. The lower floor had just been fitted out on the Friday, ready for business Monday - all brand new workstations, desks, furniture etc...
    Damage in the 100's of 1000's! All down to the wrong EV used on an under counter water heater in gents toilets that failed over the weekend.
     
  4. dilalio

    dilalio

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    AFAIK it's just the extra cost.
    Water in a sealed central heating system is (or should be) conditioned with inhibitor to neutralise mains water so that it won't cause all the different metals, used in the various components, to give up their molecules and lead to corrosion, sludge etc... If an EV designed against this is used, its just one factor out of the equation.
     
  5. bernardgreen

    bernardgreen

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    The more I read about expansion vessels the more I am glad that my vented system has no requirement for pressurised expansion vessels
     
  6. Rad1o

    Rad1o

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    The expansion vessels you mention are for potable and not for potable use. The leaking one was designed and bought for potable, i.e. hot water systems. We also have one fitted to the CH system and that has not caused any problems .... yet.
     
  7. MrBenchmark

    MrBenchmark

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    You can use blue/white ones (potable) on heating but there twice the price of the red. (Heating) non-potable ones .
     
  8. ollski

    ollski

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    Potable water expansion vessels do not let the membrane come into contact with the mild steel body at any point, they are like a hot air balloon shape rubber membrane and thinner than the membrane stretched across the red ones as they shouldn’t have any sort of aggressive chemicals to deal with. If you fit a 3 bar vessel to a system designed to operate at around 1 bar then the water will continue to expand until the pressure gets around equal which is where your air in the vessel will begin to compress but also where the safety valve should trigger. I should imagine if you drop it to 0.7-1 bar it will be fine but not as designed.
     
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