Unvented Cylinder in loft.

Discussion in 'Engineers' Talk' started by LEE EDWARDS, Sep 11, 2019.


    LEE EDWARDS New Member

    Hi all. I hope you are well. This is my first time here so please point me in the right direction if this is not the correct place to post.

    I would like to install a 300 litre unvented cylinder in my loft (I'm a plumber) but need to figure out how to make the platform to support it.

    I live I a Bungalow from the 1930's which has 100 x 50mm joists at 400mm centres. The span from binder to load bearing wall is 1.9 metres and then from binder to wall plate 1.5 metres.

    The dead load of my tank I believe is 1400 kn over an area of 0.28m2. Would I be able to span a few joists with larger timbers/ double up the joists etc?

    Any advice given I will gladly take.

    Regards Lee
  2. KIAB

    KIAB Well-Known Member

    Unvented cylinder around 350kg (777lbs) full, with my rough sums,might vary abit depending on make of cylinder.
  3. Hans_25

    Hans_25 Well-Known Member

    350Kg is quite a weight, I've got a 70 gallon tank in the loft with a similar construction, it actually holds nearer 55 gallons which is about 250Kg. There is a big structural timber right underneath it and its right next to the gable wall too where this timber is supported, I wouldn't want to put 350Kg just on joists even if they're sistered up or load spread. You're looking at the weight of 5 people. Ideally you want something solid underneath it or have it as near as possible to something solid like an outside wall.

    Is it an option to add height to the 2x4s and screw/glue another 2x 4 timber on top? Doubling the height of the timbers increases the stength 8 fold. If not, perhaps look at screwing in long steel plates to the joists, screwing 25mm ply across many joists etc.

    LEE EDWARDS New Member

    Hi, thanks for the reply. Unfortunately I cannot get it to be close to an outside wall due to it's height. I can span 2 brick walls of a room below which is a 2.4 metre span. In the opposite direction of the joists. The tank would probably be in the centre of this span though.
  5. Severntrent

    Severntrent Active Member

    1400 kn is 140 ton I think you are over estimating
  6. Mike83

    Mike83 Well-Known Member

    If concerned about the weight just fit a smaller capacity tank.
    300 litres is a fair amount.
  7. Hans_25

    Hans_25 Well-Known Member

    More like 140,000 tonnes. 350Kg is 3.5Kn.

    LEE EDWARDS New Member

    Sorry all. I was meant to say 14kn m2.

    I will have 3 showers running at the same time with 18 litres of flow per shower so need the big tank.

    Thanks for the replies

    LEE EDWARDS New Member

    So area of 600mm cylinder is .28m2
    400kg/0.28 =1,429kg/m2=14kn/m2

    Hope that is correct
  10. KIAB

    KIAB Well-Known Member

    What about using a slimline unvented cylinder on ground floor somewhere, Megaflo,Gledhill,Vaillant, etc all do them.
  11. Severntrent

    Severntrent Active Member

    and 1 ton =10kn hence 1400kn = 140 ton(ne)
  12. Severntrent

    Severntrent Active Member

    350 kg centrally located on a 2.4m span, 2nr 200 x 50 joists should suffice. Get 3 mates to stand on 1 joist and see if that supports them.
  13. Severntrent

    Severntrent Active Member

    Done a calc (to involved to put on here) but one SC24 50x200 joist would be man enough giving a 5mm deflection so 2 joists would eat it.

    LEE EDWARDS New Member

    So here is my set up at home. Hope it makes sense

    Oh wow, thank you so much. Is this spanning the 2.4 metre walls then and then plywood on top.

    I don't mean to be rude but would it be possible to lower the height and use a thicker timber as I need to keep as much height as possible due to the cylinder being 2 metres high.

    Again, thank you for your time spent helping out.
  15. Heat

    Heat Well-Known Member

    You could use 2 smaller unvented cylinders to achieve same storage.
    That would let you put the higher base and spread the weight greatly.
    They would need piped together in a simple arrangement.
    Also have the loft roof (between rafters) insulated around the cylinder and ideally the entire roof
  16. Severntrent

    Severntrent Active Member

    4nr C24 47 X 97 would do the job, deflection would be 7.7mm but as with all things structural this will be erring on the safe side. Calculations based on 350kg load and effective span of 2.5m i.e. actual span plus 100mm bearing either side, all in accordance with BS5268-2:2002. You could have even lesser depth joists but obviously the number would increase, I think 4nr is the optimum solution.
    Last edited: Sep 15, 2019

    LEE EDWARDS New Member

    No way. Thank you so much. I had an engineer in and he said the same. Recommend though to put 50 x 150 x 3 across the span.

    Will show the calcs later once I have learnt how to reduce the size of the image
  18. Severntrent

    Severntrent Active Member

    Yep, 150 deep also works and stresses in wood are less, but to be expected as joists contain 64% more wood. Also contain more wood than original suggestion of 2nr 50 x 200 joists. More than one way to skin a rabbit but depends how much head room you are trying to save?

    LEE EDWARDS New Member

    Thanks again for your help. Really is good to have a second opinion. In your scenario, is this just going off the top of the existing joists at this span or onto the walls? If I did go with the lower height joists as you and the engineer said is a possibility, would doubling up 3 be ok? It seems like it would.

    Thank you again kind sir for your help.
  20. The Teach

    The Teach Well-Known Member

    Over the years,have fitted many water tanks & cylinders in lofts :)

    Usually it was the idea to install suitable-safe loft floor access from the loft hatch to the tank/cylinder in 18mm (graded plywood,yes structural but cheaper ;)) then under the tank/cylinder the 18mm ply was doubled up.

    Plywood all screwed down and made safe & suitable.

    horizontal unvented cylinders are an option to spread point loading :)

    Thats a lot of water ;)

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