Home heating energy use

By - , Build 113

More real-life results from the Papakowhai study reveal the shortcomings of modelling data, which cannot cover the range of choices that inhabitants make.

Two recent Build articles have outlined research into upgrading the performance of nine typical homes in the Porirua suburb of Papakowhai as part of a 3-year study performed by BRANZ for Beacon Pathway. The April-May issue (pages 64–65) provided an overview, and in June-July (pages 70–71), the heating and insulation in these homes were looked at. We now provide some more real-life results, by comparing the modelled thermal performance of the homes with their measured performance.

Thermal modelling of buildings

Modelling has long been used as a low-cost desk-based exercise to optimise or test the outcomes for systems that are difficult to trial in the field. Modelling is good at assessing the impact of various changes to a base case, but the accuracy of any model is dependent upon the validity of the assumptions, the precision of the boundary conditions and the coverage and ability of the model itself.

Table 1: Intervention packages and potential star ratings (interim rating tool used).
Home no. Potential HERS star rating before intervention Intervention Potential HERS star rating after intervention
P01 1.0 Heavy ceiling and underfloor insulation, pellet burner and heat transfer kit 1.5
P02 2.5 Relaid and topped-up ceiling insulation, heavy underfloor insulation 2.5
P03 2.5 Heavy ceiling and underfloor, full wall insulation, double glazing, wood burner, ducted heat pump 5.0
P05 2.5 Ceiling insulation top-up, heavy underfloor insulation, heat transfer system 3.5
P06 1.5 Ceiling insulation top-up 1.5
P07 2.0 Heavy ceiling and underfloor insulation, partial wall insulation, heat transfer system 3.0
P08 2.0 Heavy ceiling and underfloor insulation, minor wall insulation,double glazing 3.0
P09 4.5 Ceiling insulation top-up, heavy underfloor and midfloor insulation, minor wall insulation, heat pump 5.0
P10 1.5 Heavy ceiling and underfloor insulation, full wall insulation, double glazing 4.5

Modelling of the thermal performance of New Zealand homes has provided useful information that has led to changes in the NZ Building Code clause H1 Energy efficiency. Such tools as ALF (the BRANZ Annual Loss Factor software) and HERS (Home Energy Rating System) from EECA (Energy Efficiency and Conservation Authority) have been used to run thousands of models. These models have defined where the best improvements in the thermal performance of the New Zealand housing stock can be obtained and have thereby informed policy.

HERS results

The Papakowhai Renovation Project homes were modelled with the AccuRate tool (used in HERS), both before and after the renovations (see Table 1). The best home achieved a 5-star rating, which is good for a renovation project (where there are fewer things that can be done than in a new build).

For the modelling, it was assumed that all the space heating was from the same resistive electric heater. This allowed direct comparisons between homes and eliminated the variation through different heater efficiencies and fuel types.

The software tool was upgraded after the modelling (late May 2008), and star rating bands have changed, so results may now be different. The tool also now allows the greenhouse gas emissions or ‘carbon footprint’ to be calculated, but did not allow this in 2008 when the modelling was done.

As can be seen in Table 1, all the homes show the same or increased star ratings from the renovation work.

HERS versus measured data

It isn’t possible to compare the actual star ratings with the modelled ones since the behaviour of the occupants alters the actual energy used for space heating. Instead, we compared the modelled annual space heating energy used with the actual space heating energy used over a 5-month winter heating season (see Table 2).

The large difference between the measured and modelled energy use is because of different:

  • baseline assumptions
  • boundary conditions (such as the internal temperatures maintained)
  • user behaviour.

The measured and modelled temperatures were also different. While the occupants of P03 maintained their lounge and bedroom at a comfortable average temperature of 19.5°C, the occupants of P06 maintained an average temperature of 13.7°C in the lounge and a cold 12°C in the bedroom. At one stage, one of the homes kept an exterior door open since their cat door was broken, and another chose not to heat the bedroom at all.

In comparison, the assumptions in the HERS model included a 24-hour heating schedule, with specific set-points for each zone: 20°C for the living spaces, with a thermostat set-back to 16°C from 11 pm to 7 am; 18°C for bedrooms with a thermostat set-back to 16°C from 11 pm to 7 am; and with utility rooms also conditioned.

Rarely are these constant temperatures kept within New Zealand homes. Data for the measured period is only presented for the 5-month ‘heating season’ but the modelling assumed that heating was used across the entire year, when the temperature fell below the set-points. The boundary conditions included an ‘average’ winter climatic condition, which may have been different from the actual 2008 winter conditions.

Occupancy drives performance

The largest difference between the modelled results and the measured results is likely due to occupant behaviour. None of the homes in this study was on a 24-hour heating schedule for the 5-month winter season, and many of them used multiple space heating systems without thermostatic control of the room temperature.

P05 used the most energy for space heating, with its winter evening heating set-point being 20°C on average, yet it used little heating before 5 pm, resulting in an average temperature well below 20°C. P03 maintained an average winter temperature in the living room of 19.5°C, although this fluctuated from 18 to 23°C (even after the installation of a ducted heat pump). P03 did not appear to have consistent 24-hour heating schedules or set-points. This follows typical New Zealand heating behaviour as found in the HEEP study, where only 15% of our homes were found to space condition on a 24-hour basis over winter, and many had indoor temperatures well below 20°C.

Table 2: Measured versus modelled heating energy use for the Papakowhai Renovation Project, in the New Zealand HERS version of AccuRate, current at 5 March 2008.
Home no. Average temperature of lounge/m.bdrm (°C) Measured heating energy post retrofit (MJ/m²/annum – 5-month winter) Modelled temperature of lounge/m.bdrm (°C) (temp setpoint and setback values for each room) Modelled heating energy post retrofit (MJ/m²/annum)
P01 15.8/14.7 29 20–16/18–16 635
P02 16.5/14.2 73 20–16/18–16 428
P03 19.5/19.5 24 20–16/18–16 228
P05 16.5/13.3 207 20–16/18–16 367
P06 13.7/12.0 28 20–16/18–16 567
P07 14.7/13.4 38 20–16/18–16 395
P08 17.1/17.1 72 20–16/18–16 381
P09 16.9/15.5 18 20–16/18–16 207
P10 17.7/17.1 47 20–16/18–16 223

Use of energy modelling

Although it is appropriate to use energy modelling to help develop policy and determine the best cost/benefit options, the comparisons in this article highlight the necessity of using modelled data carefully. Modelling uses assumptions on user behaviour and does not cover the range of choices that occupants make. These choices often have a greater effect on the energy use and achieved comfort levels in homes than any amount of home insulation.

HERS ratings

The HERS scheme was launched in New Zealand on 17 December 2007, and uses the energy modelling software referred to as AccuRateNZ. Energy use of both new and existing homes can be modelled with this tool, thus improving the energy use of new homes at their design stage and reducing the energy use of existing homes through renovations guided by HERS ratings. HERS assessors have been trained through an EECA initiative in association with the Australian government.

HERS uses a 10-star system to rate the energy use of homes, with those receiving a zero star rating needing redesign, an average home being at around the 3-star level and a home having an energy use of zero achieving a 10-star rating.

Although no homes in New Zealand have yet been rated as ‘10-star’, the country’s first 9-star rated home (‘the little greenie’) featured on the Campbell Live programme in April 2009. This Golden Bay hideaway was built by Eco-action Design and Build. The cottage uses passive solar design, an off-grid electricity supply from solar panels and a battery bank, solar hot water, highly insulated walls and windows, and other green features such as a smell-less composting toilet. It also has underfloor heating.

For more

For more information on the Papakowhai Renovation project, visit www.beaconpathway.co.nz.

Further articles on specific aspects of the Papakowhai research will appear in future Build issues.

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Articles are correct at the time of publication but may have since become outdated.

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