Hi everyone, the house design has come along quite a bit and we have floor plan sorted , i used this web site

http://www.design-navigator.co.nz/

to enter all construction details and it gave me a heat transfer of 383W/K. This does not include air infiltration and ventilation so with an HRV i expect up to 750W depending on air in / air out temps.
So with a 20k inside to outside i have heat loss of 7660w + 750w = 8410w.

Average summer temperature is 21.8°C (Night minimum temperature averages 9.4°C below day maximum)

Average winter temperature is 13.0°C (Night minimum temperature averages 7.6°C below day maximum)

Palmerston North has an average 1,730 hours of sunshine per year (40% of possible hours)

The climate is temperate

Annual rainfall is 963mm

Average 121 Wet days (>1mm) per annum

Average 38 days per annum with ground frost (4 per year of -5°C or lower)

Annual average wind speed = 10.6kph (Windiest months are Nov-Jan, 12.5kph; and least windy Jun-Jul, 8.5kph

So it comes back to the question of source, geo or air?
And what to base this on?
thanks Andy