Recently, after completing an AccuRateNZ thermal performance analysis on a proposed house, I was asked for recommendations of suitable heating appliances. Using the Modelling Method, I had shown that the dwelling (as designed by others) was compliant with the NZ Building Code clause H1 Energy Efficiency, and that it had passed with flying colours (for a Building Consent the building only has to achieve the NZBC H1 minimum requirements — the matter of heating and cooling units is at the discretion of the occupants).

From the Client's point of view this is a logical question, but it also illustrates that there is not a simple correlation between the base passive thermal performance of the building and the 'best' heating system to be installed. The first is dependent upon fixed physical characteristics of the house — its design, siting, environment, the local climate, etc. The second is dependent upon the requirements of the occupants: their perceived levels of comfort (both for warmth and coolness), financial aspects (capital and running costs), ease of control, reaction speed of the appliance to changing weather, temperature, and humidity conditions, current and future suitability (ie. sustainability), maintenance cycles, ease of updating and replacement, current fashions, urban myths etc. Many of these are subjective and can change with time.

Most of the physical characteristics of the house and its environment cannot be easily changed over time, except maybe windows, insulation, or thermal mass which can be added or deleted to improve the heating or cooling performance of the building's fabric. Usually this is a major operation and so it is best to get the base passive thermal performance as good as possible at the design and construction stages.

Of course, in my opinion, liveability and buildability should not be sacrificed for the sake of 'maximum thermal performance' — whatever that may mean (at the final count each occupant can always adapt their clothing to improve their individual comfort).

Once the dwelling is constructed, it is the role of the heating and cooling appliances to moderate the internal environment to the ever varying weather conditions. These 'appliances' include properly installed curtains (refer to my EBOSS Detailed blog, 'Thermal Drapes: Do They Work As Insulation?' of August 2015). The problem is to select an appropriate system which best fits the lifestyle, budget and comfort requirements of the occupants. There is a wide variety of possible appliances, and a selection of fuels to choose from. Systems could be fixed, such as ducted air or heated concrete floors, or be portable radiation, convection or blown air units. The fuels can be from 'network' or non-renewable sources, e.g. electricity, gas or diesel, or be renewables such as solar-heated water or photovoltaic panels. The choice is wide. In the past electrically heated concrete floors were in favour whereas now heated water is used, and free-standing night-store convection units used to be popular due to the past low electricity price.

Because of this wide choice it is important that new home buyers pause and think through their actual needs and budgets, and consider the base passive performance of the house, before rushing in to buy what is assumed to be the comfort control system to use. Recently I modelled the thermal performance of a new house which was to be constructed with a narrow band of tiled concrete floor in the rooms along the north wall. Yes, the exposed thermal mass would reduce the heating load, but they didn't like the idea of having a bedroom floor only partially carpeted so accepted that a little more heating and cooling would be needed. Liveability was considered to be more important than this particular thermal gain.

Overall the dwelling performed very well except that a significant amount of cooling energy would be necessary to bring the house into the standard comfort range. Because an objective thermal simulation analysis had been undertaken of the original design, it was simple to re-run the assessment with an increase in the proportion of opening windows which showed that most of the cooling energy load disappeared. This is an example of careful consideration of the fixed physical characteristics of the house at the design stage resulting in substantial life-long energy savings.

For another new dwelling the owners wished to have a water-heated exposed concrete floor as the primary heating source. The analysis showed that the passive performance was good, so only a little heating would be needed. Because it cannot be fitted later they have decided to go ahead with the hydronic system, but only to lay the zoned water pipes within the floor slab. They will then live in the house for a year and at that point decide if the heating equipment is needed, and what capacity should be installed. A sensible decision.

Because the software requires an intermediary calculation of the modelled temperature of every space in the building for every hour of every day of the year in order to determine if the building complies with NZBC H1, then this data can be separately extracted to use as an objective temperature brief for the design of an appropriate heating and cooling system to cater to the comfort of the occupants. Most new houses perform much better than people experience in their older homes, so there is a lot of sense in not rushing to the selection of a heating and cooling system before carefully and deliberately developing a Performance Brief for the dwelling which most closely matches the occupant's comfort requirements.