S Plan Vented converting to Unvented in Restricted Space

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Kingston, London
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I have a vented S Plan with a System Boiler stacked over the DHW Cylinder in a tight airing cupboard. The Cylinder is 1150 High and over it is a Vaillant Boiler. All is working perfectly with flow to a first floor shower just acceptable and we have plenty of stored hot water. But we are about to add a single story extension with shower. The architect says we should convert it to an Unvented Cylinder.

So these are the measurements. Flow is 20 litres per minute. Static pressure is 1.8 Bar (Dynamic 1.0 Bar). Seems to be borderline. The present shower is fed from a high mounted tank directly overhead but probably is not greater than 0.3 Bar.

Feed from the road is 15mm copper to a stop tap below the airing cupboard

I had considered a whole house pump but it would have to be located in the airing cupboard in front of the cylinder. I'm sure the Vaillant engineer would not be that keen on having to lean across the installation to get at the boiler.

I'm wondering about putting an Accuboost Cylinder and Pump in the loft. Taking the rising main and delivering output down to the Unvented Cylinder. The instructions say the unit should not be greater than 5 metres from the stop tap. I see that Salamander do a mains boost which they restrict to Combi Boilers?

I like the idea of an additional cylinder of water to cope with the frequent loss of supply. Thames Water say the supply is failing due to clay soils so we lose it about twice a year. They are installing a new poly pipe about a mile away which may reduce the failures.

Anything else I should be considering?
 
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