the most energy efficient and reliable boiler

We are just completing a self build and I want to install the most energy efficient and reliable system boiler. The house is approx 200 square metres and the conservatory will add another 16 sq m. i am getting conflicting advice about the boiler size. We have 4 showers so I was advised to go for a system boiler rather than a combi.
What size boiler do I need - does it matter if it is oversized? and what makes would people recommend?
Thanks

Vaillant ecotec plus 600 series and unistor cylinder, thats the best if you can afford it.

Every installer will have his own favourite make of boiler and will recommend that particular make. A boiler is a machine and will eventually break down. Of the mainstream stuff, Worcester or Vaillant are both pretty reliable, but the reliability of any machine depends on servicing..

You ask for the most efficient boiler and yet are willing to pump your heat to the great outdoors through the thin glass roof of the conservatory.. If you want efficiency, ditch the greenhouse..

cjpax said:

What size boiler do I need - does it matter if it is oversized?

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The Recommended Boiler Size Calculator on the Sedbuk website will tell you what size boiler you need.

As your house will, hopefully, be up to current standards for insulation etc, you will be surprised how small a boiler you need. The calculation only allows 2kW for water heating so, as you have more than the usual number of showers it might be sensible to add and extra 1or 2kW.

There is no point in oversizing the boiler.

What would be be more sensible would be to oversize the radiators by about 25%. This would enable you to run your boiler with a flow temp of 70°C (OK for hot water at 60°C) and a return temp of 55°C, which will ensure that you boiler is in condensing mode nearly all the time. The actual flow and return temps may be different, depending on what the boiler mfr recommends. The return needs to be below 60°C to ensure condensing and the flow needs to be higher than 65 to give hot water at 60°C.

This Radiator Output Calculator will tell you how the output of a radiator varies with temperature, so you can feed in the watts required and the flow and return temp and it will tell what size to buy.

For example say you need 1000W to heat a room. The calculator shows that a 1000W radiator will only give 809 watts at 70°C/55°C. So you will need to buy a radiator which is 1000/809 X 1000 = 1236 watt nominal output.