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hi all,

I know this is a long shot but I'm wondering if there are any M&E contractors who specialise in plumbing that can help me. I think my plumber might have messed up the plumbing for my property by using the wrong pipe size


I've got an 8 bedroom, 8 bath property that I'm refurbishing and want to have piping hot, high pressure showers in the ensuites with a system that can accommodate all showers being used at same time during peak periods. As the property will be used as a boutique luxury hotel - these design requirements are a must.

The property is split into three sections with the boiler being located in the centre - with 4 bedrooms on one section of the house, and another 4 bedrooms on the other section of the house.

I had Bosch do me a system design to meet this design challenge and they proposed an unvented system comprising:
  • 2 x Greenstore cylinders 300l
  • Worcester GB162 50kw boiler
  • Salamander home boost pump
  • 28mm piping branching into 22mm
  • No point double coiling

The inputs they were given along with layout designs were:
  • -!Current standing pressure is 4.2 bar
  • - Over 22 litres per minute flow rate on cold
  • - At 4bar pressure, shower will deliver 17 litres of water per minute
  • - 8 thermostatic showers to be installed at the property and used during peak periods
  • - 8 size heated towel rails (to come on at same time as radiators) - 1,119 BTU per radiator
  • - 15 radiators - average 3,500 BTU per radiator
  • Property is split into two sections with the boiler being located in the centre: section 1 will have 4 bathrooms section 2 will have 4 bathrooms
  • - During peak times (mornings) 8 showers will be used simultaneously!
  • - Heating will be on 8hrs a day, 7 days a week during the winter
  • - Heating will be on 3hrs a day, 7 days a week during the summer


I had work start on section A of the building comprising 4 bedrooms and bathrooms. The plumber re-did all of the pipework for radiators, hot and cold feeds for showers and basins....except he decided to use 22mm pipework branching off into 15mm instead of follow Bosch' guidelines.

I am now worried about the impact this deviation will have - will it still be able to meet the design challenge or does it make no difference?

I did some back of the envelope calcs and wondered if someone can:

a) help me validate if my back of the envelope calcs / assumptions / conclusions are correct? I am not an engineer or plumber.

b) suggest if there's a possible fix to still ensure I can meet the design challenge for the rooms that have already been piped up (luckily they are positioned close-ish to the boiler)

I have a CAD version of the property layout as well las technical docs for all of the proposed equipment to go in.


  • Grohe Grohtherm Cube Perfect Shower Set - 34506000
  • Assuming 11.5 litres per minute flow rate per shower
  • Ave shower time will be 20mins
  • Assuming 8 showers during peak times, that’s 92 litres per minute required
  • 22mm pipe has flow rate of 0.6 litres per second // 36 litres per minute
  • 28mm pipe has flow rate of 1.1 litres per second // 66 litres per minute

Assuming there will be a feed for section A of the property covering 4 bathrooms and another feed for section B covering bathrooms, this means max flow rate required will be 11.5 litres per minute * 4 showers = 46litres per minute needed from each main feed

Conclusion 1. 28mm piping should be used to meet the 46 litres per minute (22mm would be restrictive at max flow of 36)

However, there is also the issue of ensuring the cylinders themselves can supply enough water given peak time demand and then push it through at appropriate flow rate to avoid loss of pressure. To aid this, the spec indicated 2 x greenstore unvented cylinders at 300l each.
  • Greenstore SC300 can re-heat 300l of water in 24 mins = 12.5 litres a minute
  • At 2 cylinders in tandem, this means a re-heat capacity of 25 litres per minute
  • Going back to design challenge of 8 showers on at same time during peak periods:
  • 600 litre total capacity would be used up in 6.5mins (600 litres / 92 litres per min)
  • But in those 6.5 mins, the cylinders would reheat 163 litres of water (25 litres per min reheat capacity * 6.5 mins) = 1.7 mins
  • Total shower time at max demand: 8.2 minutes (6.5 mins from 600 litre capacity + 1.7 mins from reheat capacity)

Conclusion 2. while this doesn't quite meet the 20 mins at 100% utilisation, I think Bosch' proposed 2 cylinders is quite balanced because not everyone will shower for 20mins and it's unlikely that all 8 showers will be used at exactly the same time for exactly the same duration. If one or two people did find cold showers, total reheat time is quite healthy at 25l per minute - essentially covering 2 showers at 11.5 litres per minute flow rate.


I know I've totally neglected pipe runs and the number of bends. I have layout drawings and a CAD version of layout that I can share if necessary.[/LIST]

Super Moderator
2,069 Posts
Hmm, we don't need no stinking hot water or pressure.

The Old (antique) Master
2,246 Posts

With all the metrics your using using in your post. The Zone is in the wrong part of the world. But I do have a question. When you buy a metric socket with various mm sockets -- what size drive do you buy?


And also this link shows 3" and 6" extensions ...
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