As Trilogy has posted, you don't offer enough info about your build to be able to advise on the exact spec for boilers for your house.
If you are building a new house, you will be required to obtain a BER certificate for it. The best route to take is to:
1. Appoint a BRE consultant who is experienced with low energy, highly insulated houses.
2. Get the consultant to prepare a preliminary BER cert based on the specs you hope to achieve with regard to insulation levels, airtightness etc. This involves the consultant inputting the specs for different elements into the software and calculating the end result.
3. This preliminary cert will tell you, amongst other things, what your heat load for the house is going to be. You can then work out what size boilers you need to meet this heat load.
4. It is pretty easy to examine different options with regard to insulation levels etc. once the basic info has been inputted into the software. You should take the opportunity to examine different options for the insulation levels of elements, what savings different solar hot water options will give you, is it worth using triple glazing in certain areas, etc.
It is important to note that the heating/hot water/airtightness aspects of a house are interrelated, and a good consultant will be able to advise you on the most efficient way to go about obtaining your required results.
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