Unvented Cylinder Piping Requirements

3 Sep 2015
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United Kingdom

I'm thinking of having an unvented cylinder installed. I know the flow rate is good over 20 litres per minute. I'm getting the pressure measured tomorrow.

My question is on what size piping is sufficient to get good performance from showers and if changes need to be made.

Mains into house looks like 17-18mm.
Mains to loft / unvented cylinder is 15mm (being upgraded to 22mm)
Unvented cylinder to showers could be 15 - do they need to be upgraded to 22mm?
Anything else?

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Pipe sizing is not the £6M question.

Pressure and flow is.
Generally, you would size the DHW pipework from the cylinder in 22mm, and branch off this to outlets in 15mm. Cold water to bathrooms (especially for showers) should be taken off after the pressure regulating valve in 22mm, and tee'd off to outlets in 15mm. This will ensure a balanced feed to the showers.

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