Designing for optimum thermal efficiency
(provided by owner, Stu Smith)

The temperate climate of Auckland provides an ideal opportunity for a house that will passively heat and cool with seasonal changes.

Key heating design features for this house include:

• A rectangular footprint; intended to maximise penetration into the building of the low winter sun from the longer northern face
• Floor to ceiling glass (double glazed, Low E gas filled, tinted) on the northern wall and minimal glazing to the southern facade (just sufficient for light and air flow)
• A 125mm polished concrete floor to the ground level, insulated to the underside, with a thermal break to the concrete perimeter wall and fibreglass starter bars connecting to the perimeter foundations
  
• The ground floor slab being divided into 6 zones of hydronic underfloor heating pipes, connected via valves to a central manifold controlled by a Loxone server
• A stairwell (with minimal treads and balustrades) located centrally on the northern wall providing heat transfer to the upper floor

 
The concrete slab thermal mass is used absorb and transfer solar heat. Sufficient mass is needed as a reservoir but not too much mass as that would take too much energy (or time) to heat up.
The Loxone server operates values in a manifold to move heat from warm to cooler floor zones via a low pressure water pump.

Temperatures of the 6 zones (left) and the heat transfer operating (right)

Zone 1 is the northwestern corner and is free from tree shading. This single zone provides about 80% of the heating.  The current setting is for heat transfer to occur to other zones automatically when there is a 2 degree temperature difference between any of the zones.

The underfloor hydronic piping viewed from the south, with Zone 1 at the far left

The 18 solar PV panels on the roof produce a peak of 4.2 kW. The system is currently set so that when 2kW is being generated, the heat pump will firstly heat the hot water cylinder. During winter months the next call is to boost the underfloor temperature (if needed). Any surplus generation not being utilised to run the house is exported to the grid. Since moving into the house in July 2015, the occupant’s experience so far is that most underfloor heating is provided by the sun, and seldom was additional heat from the heatpump needed.

After 7 months of operation it looks likely that imported electricity will closely match exported electricity over a full 12 months.

Key cooling design features for this house include:

• A cool source of air drawn from the southern side of the house over the carport concrete slab which, being shaded by the green roof, receives some insulation from the sun
• Narrow windows and louvres on the south side which can be left open day or night without security concerns
• An ability to transfer heat evenly over the entire concrete floor via underfloor pipes giving a large surface area to be cooled
• An open stairwell with minimal treads and balustrades located centrally that acts as a ‘thermal chimney’ enabling warm air to rise efficiently (passive ventilation)
  
• Raked ceilings at the upper level that follow the roof line to funnel warm air up
• A row of clerestory windows running the length of the house that act as a large exhaust or exit for the flow of warm air up and out

 
The topography of the site protects the house from southerly and westerly winds so there are few cross breezes to ventilate. The design relies on hot air rising to ventilate which often generates its own cool breeze. If needed the slab can be cooled by up to 3 degrees.

To keep costs down, often underfloor water pipes are left out. The owner’s experience has been that the underfloor water piping is a critical component for efficient heating and cooling and so would not consider leaving this out of any design.

In summary, any new house in Auckland should be able to have at least 80% of heating and cooling requirements covered passively by the design. This house demonstrates this ability.

North view - the solar PV panels on the lower roof are just visible below the clerestory windows