EXECUTIVE SUMMARY

AccuRate is a software tool used for the calculation of annual space heating/cooling requirements of residential buildings. It is currently used for compliance with the building code to regulate energy efficiency of new houses. AccuRate provides a useful tool for optimising energy efficient house designs for Australian local climates.

AccuRate includes a graphical user interfaces (GUI) written in Delphi and a simulation engine written in Fortran for thermal heat flows in the house. The simulation engine, known as the Chenath engine, and its previous generations has been developed over six decades by CSIRO. The methodologies, algorithms and rules implemented in AccuRate and the Chenath engine may have been published in various journal or conference papers, in reports or technical notes over the years. However, a single repository is required to list the algorithms and methodologies used. Commissioned by the former Department of Climate Change and Energy Efficiency, this document forms a single repository for AccuRate and the Chenath engine. This document will be updated regularly whenever adequate new information is available to CSIRO.

1. BACKGROUND

The Australian Government continuously supports residential energy efficiency and low carbon emission strategies and schemes, demonstrated by its substantial investment in various energy efficiency initiatives, in particular, the Nat ionwide House Energy Rating Scheme (NatHERS) (https://www.nathers.gov.au/ ). During the last three decades, NatHERS accredited software tools such as NatHERS, AccuRate, FirstRate and BERS house energy rating tools have been successfully adopted and played important roles in energy efficient house designs in Australia. These tools contribute to gain in-depth understanding of house energy use in Australian climates and influence green building practices.

Served as the benchmark tool for the NatHERS scheme, AccuRate includes a graphical user interfaces (GUI) written in Delphi and a simulation engine written in Fortran for thermal heat flows in the house. The simulation engine, known as the Chenath engine, and its previous generations has been developed over six decades by CSIRO. The methodologies, algorithms and rules implemented in AccuRate and the Chenath engine may have been published in journal or conference papers, in reports or technical notes over the years. However, a single repository is required to list the algorithms and methodologies used.

Commissioned by DCCEE, CSIRO initiated a repository for the collection and documentation of the methodologies, algorithms and rules implemented in AccuRate and the Chenath engine. This document forms a single repository for AccuRate and the Chenath engine. The document will be updated regulatory whenever adequate new information is available to CSIRO.

2. HISTORY OF ACCURATE AND THE CHENATH ENGINE

AccuRate's Chenath engine and its predecessors CARE, STEP and ZSTEP have been evolved and enhanced over the last six decades. CSIRO's research and development in building energy modelling went back to 1950's. Dr R.W.R. Muncey and his co-works were among the pioneers of computational building thermal modelling [1-4]. In 1950's, Muncey at CSIRO Division of Building Research developed a single zone harmonic analysis/synthesis program CARE. In early 1960's, Muncey developed the STEP program for modelling one zone building using the matrix method which established the core methodology of today's AccuRate Chenath engine [5,6]. The STEP program was subsequently refined by J.W. Spencer. In 1977, STEP was extended by P.J. Walsh to handle multi-zoned buildings and was named as ZSTEP [5,6].

From 1984 to 1986, A.E. Delsante and his colleagues developed the house thermal modelling software CHEETAH (Cooling and HEating Energy Tool for ArcHitects) using ZSTEP as the modelling engine and a TurboPascal (DOS) User Interface [7]. In 1990, the 1st generation nationwide house energy rating tool NatHERS was released with the Chenath engine (improved ZSTEP) and an ObjectVision User Interface. In 2003, NatHERS was further upgraded to the 2nd generation NatHERS tool - AccuRate with an enhanced Chenath engine particularly the inclusion of air flow modelling to account for natural ventilation and a Delphi User interface [8].

From 2008, D. Chen led the further development of AccuRate and the Chenath engine. AccuRate has been upgraded to AccuRate Sustainability which includes energy and CO₂ emission calculations for lighting, hot-water system and the water usage modules. In 2022, AccuRate Homes was developed to include whole of house assessment modules for the compliance with the building code.

3. FUNDAMENTALS

The Chenath engine is one of the first building simulation tools which couple a frequency response multi-zone thermal model and a multi-zone air flow model for energy requirement calculation for residential buildings. Taking into account the local climate and building fabrics, the Chenath engine automatically switches the building operation between mechanical air conditioning and natural ventilation operation whenever natural ventilation satisfies occupant thermal comfort. The Chenath engine can calculate hourly heating and cooling energy requirement over a period of one year or multiple years.

3.1 Matrix Method

The matrix method of the Chenath engine was based on the pioneer work by Muncey published in 1953.

Muncey, R.W.R., The calculation of temperatures inside buildings having variable external conditions, Australian Journal of Applied Science, 4(2), 1953, 189-196.

3.2 Multi-Zone Thermal Modelling

The Chenath engine uses response factor method for the thermal modelling in multi-zone buildings. Detailed descriptions of the multi-zone response factor calculation and the treatment of internal walls, radiative heat transfer, the calculation of heating and cooling requirements can be found in Walsh and Delsante [9].

Walsh PJ and Delsante AE. Calculation of the thermal behaviour of multi-zone buildings, Energy and Buildings 1983; 5: 231-242.

3.3 Multi-Zone Airflow Modelling

The existing Chenath engine v3.22 in AccuRate uses the MIX 2.0 (Multi-zone Infiltration and eXfiltration) air flow model developed by Li et al. [10]. This model was found to diverge when model some building designs with permanent openings. An improved air flow model was later developed and implemented in the Chenath engine after Chenath engine v3.13 [11].

Li Y, Delsante A, Symons J. Prediction of natural ventilation in buildings with large openings. Building and Environment 2000; 35: 191-206.

Ren Z.G. and Chen Z.D., Enhanced air flow modelling for AccuRate - A nationwide house energy rating tool in Australia, Building and Environment 2010; 45: 1276-1286.

4. INPUT, BEHAVIOURAL SETTINGS AND ASSUMPTIONS

4.1 Thermal Properties of Building Materials

A list of the materials and their properties can be found in the document " Material Properties Used in NatHERS Software Tools ".

4.2 Climatic Data

AccuRate and the Chenath engine uses Reference Meteorological Year (RMY) weather files. RMY weather data are specially selected so that it represents the range of weather phenomena for the location and gives annual averages that are consistent with the long-term averages for the location in question. RMY weather files currently used in AccuRate for the compliance with National Construction Code (NCC) 2022 were derived from the Australian Bureau of Meteorology weather data for the period from 1990 to 2015 [12]. RMY weather files for 69 locations in Australia are available in the existing AccuRate software as listed in Table 1.

Nationwide House Energy Rating Scheme (NatHERS) - Software Accreditation Protocol .

Table 1. ACDB climate zones

ACDB Climate Zones	Location	Longitude	Latitude	Postcode	State
1	Darwin	130.9	-12.4	800	NT
2	Pt Hedland	118.6	-20.4	6721	WA
3	Longreach	144.3	-23.4	4730	QLD
4	Carnarvon	113.7	-24.9	6701	WA
5	Townsville	146.8	-19.3	4810	QLD
6	Alice Springs	133.9	-23.8	870	NT
7	Rockhampton	150.5	-23.4	4700	QLD
8	Moree	149.9	-29.5	2400	NSW
9	Amberley	152.7	-27.6	4306	QLD
10	Brisbane	153.1	-27.4	4000	QLD
11	Coffs Harbour	153.1	-30.3	2450	NSW
12	Geraldton	114.7	-28.8	6530	WA
13	Perth	115.9	-31.9	6000	WA
14	Armidale (old Tamworth)	151.7	-30.5	2350	NSW
15	Williamtown	151.8	-32.8	2300	NSW
16	Adelaide	138.6	-34.9	5000	SA
17	Sydney RO (Observatory Hill)	151.2	-33.9	2000	NSW
18	Nowra	150.5	-35	2541	NSW
19	Charleville	146.3	-26.4	4470	QLD
20	Wagga	147.5	-35.2	2650	NSW
21	Melbourne RO	145	-37.8	3000	VIC
22	East Sale	147.1	-38.1	3852	VIC
23	Launceston (Ti Tree Bend)	147.1	-41.4	7250	TAS
24	Canberra	149.2	-35.3	2600	ACT
25	Cabramurra (old Alpine)	148.4	-35.9	2629	NSW
26	Hobart	147.5	-42.8	7000	TAS
27	Mildura	142.1	-34.2	3500	VIC
28	Richmond	150.8	-33.6	2753	NSW
29	Weipa	141.9	-12.7	4874	QLD
30	Wyndham	128.1	-15.5	6740	WA
31	Willis Island	150	-16.3	4871	QLD
32	Cairns	145.8	-16.9	4870	QLD
33	Broome	122.2	-18	6725	WA
34	Learmonth	114.1	-22.2	6707	WA
35	Mackay	149.2	-21.1	4740	QLD
36	Gladstone	151.3	-23.9	4680	QLD
37	Halls Creek	127.7	-18.2	6770	WA
38	Tennant Creek	134.1	-19.6	860	NT
39	Mt Isa	149.2	-21.1	4825	QLD
40	Newman	119.7	-23.4	6753	WA
41	Giles	128.3	-25	6438	WA
42	Meekatharra	118.5	-26.6	6642	WA
43	Oodnadatta	135.5	-27.6	5734	SA
44	Kalgoorlie	121.5	-30.8	6430	WA
45	Woomera	136.8	-31.2	5720	SA
46	Cobar	145.8	-31.5	2835	NSW
47	Bickley	116.1	-32	6076	WA
48	Dubbo	148.6	-32.2	2830	NSW
49	Katanning	117.6	-33.7	6317	WA
50	Oakey	151.7	-27.4	4401	QLD
51	Forrest	128.1	-30.8	6434	WA
52	Swanbourne	115.8	-32	6010	WA
53	Ceduna	133.7	-32.1	5690	SA
54	Mandurah	115.7	-32.5	6210	WA
55	Esperance	121.9	-33.8	6450	WA
56	Mascot (Sydney Airport)	151.2	-33.9	2020	NSW
57	Manjimup	116.1	-34.2	6258	WA
58	Albany	117.8	-35	6330	WA
59	Mt Lofty	138.7	-35	5240	SA
60	Tullamarine (Melbourne Airport)	144.9	-37.7	3020	VIC
61	Mt Gambier	140.8	-37.8	5290	SA
62	Moorabbin	145.1	-38	3189	VIC
63	Warrnambool	142.4	-38.3	3280	VIC
64	Cape Otway	143.5	-38.9	3220	VIC
65	Orange	149.1	-33.4	2800	NSW
66	Ballarat	143.8	-37.5	3350	VIC
67	Low Head	146.8	-41.1	7253	TAS
68	Launceston Airport	147.2	-41.5	7120	TAS
69	Thredbo (Village)	148.3	-36.5	2625	NSW

4.3 Infiltration

In AccuRate, the infiltration rate, in air changes per hour for each zone, is specified as A + B*v, where v is the hourly wind speed (m/s) from the AccuRate weather files multiplied by a terrain factor. For details of the infiltration calculation, please refer to the following document [13]:

Download:

Chen D., Infiltration Calculations in AccuRate, 2022

4.4 Internal Heat Gains

The heat loads in Tables 2 to 4 are for a 160 m² dwelling with two adults and two children, with a floor area split of 80 m ² for all the living areas and 80 m² for all the bedroom areas. The load is for the one hour period up to the time stated, i.e. a time of 1:00 am indicates the period between midnight and 1:00 am.

Table 2 - Internal sensible and latent heat loads - for living spaces, including kitchens [12]

Time	Sensible heat load (Watts)				Latent heat load (Watts)
Time	Appliances and cooking	Lighting	People	Total	Latent heat load (Watts)
1:00 am	100	0	0	100	0
2:00 am	100	0	0	100	0
3:00 am	100	0	0	100	0
4:00 am	100	0	0	100	0
5:00 am	100	0	0	100	0
6:00 am	100	0	0	100	0
7:00 am	100	0	0	100	0
8:00 am	400	180	280	860	400
9:00 am	100	180	280	560	200
10:00 am	100	0	140	240	100
11:00 am	100	0	140	240	100
Noon	100	0	140	240	100
1:00 pm	100	0	140	240	100
2:00 pm	100	0	140	240	100
3:00 pm	100	0	140	240	100
4:00 pm	100	0	140	240	100
5:00 pm	100	0	140	240	100
6:00 pm	100	300	210	610	150
7:00 pm	1100	300	210	1610	750
8:00 pm	250	300	210	760	150
9:00 pm	250	300	210	760	150
10:00 pm	250	300	210	760	150
11:00 pm	100	0	0	100	0
Midnight	100	0	0	100	0

Table 3 - Internal sensible and latent heat loads - for living spaces that do not include a kitchen [12]

Time	Sensible heat load (Watts)			Latent heat load (Watts)
Time	Lighting	People	Total
1:00 am	0	0	0	0
2:00 am	0	0	0	0
3:00 am	0	0	0	0
4:00 am	0	0	0	0
5:00 am	0	0	0	0
6:00 am	0	0	0	0
7:00 am	0	0	0	0
8:00 am	180	280	460	140
9:00 am	180	280	460	140
10:00 am	0	140	140	70
11:00 am	0	140	140	70
Noon	0	140	140	70
1:00 pm	0	140	140	70
2:00 pm	0	140	140	70
3:00 pm	0	140	140	70
4:00 pm	0	140	140	70
5:00 pm	0	140	140	70
6:00 pm	300	210	510	105
7:00 pm	300	210	510	105
8:00 pm	300	210	510	105
9:00 pm	300	210	510	105
10:00 pm	300	210	510	105
11:00 pm	0	0	0	0
Midnight	0	0	0	0

Table 4 - Internal sensible and latent heat loads - for bedrooms [12]

Time	Sensible heat load (Watts)			Latent Heat load (Watts)
Time	Lighting	People	Total
1:00 am	0	200	200	100
2:00 am	0	200	200	100
3:00 am	0	200	200	100
4:00 am	0	200	200	100
5:00 am	0	200	200	100
6:00 am	0	200	200	100
7:00 am	0	200	200	100
8:00 am	0	0	0	0
9:00 am	0	0	0	0
10:00 am	0	0	0	0
11:00 am	0	0	0	0
Noon	0	0	0	0
1:00 pm	0	0	0	0
2:00 pm	0	0	0	0
3:00 pm	0	0	0	0
4:00 pm	0	0	0	0
5:00 pm	0	0	0	0
6:00 pm	0	0	0	0
7:00 pm	0	0	0	0
8:00 pm	100	0	100	0
9:00 pm	100	0	100	0
10:00 pm	100	0	100	0
11:00 pm	100	200	300	100
Midnight	0	200	200	100

4.5 Thermostat Settings

Cooling Thermostat Settings

The NatHERS Scheme requires that for an energy assessment, all conditioned spaces must be maintained within a certain range of thermal comfort. The cooling thermostat temperature in each Climate Region is indicated in Table 5. These settings represent an assumed thermostat trigger point for the operation of space cooling appliances. Baharun el al. [14] gave a brief description on how these cooling thermostat settings are derived.

The cooling thermostat for each climate zone is set equal to the neutral temperature in January for the corresponding climate zone defined in Eq. (1), up to a limit of 28.5°C, above which both the neutral temperature and the cooling thermostat are taken to be 28.5°C. The low limit of the thermostat is set at 22.5°C. If the neutral temperature in January is below 22.5°C, both the neutral temperature and the cooling thermostat are taken to be 22.5°C.

T_n = 17.8 + 0.31T_m (1)

where T_n is the neutral temperature in January and T_m is the mean January ambient air temperature.

Heating Thermostat Settings

In the NatHERS scheme, the heating thermostat settings are as follows:

For living spaces (including kitchens and other spaces typically used during the waking hours): a heating thermostat setting of 20 ^o Celsius.
For sleeping spaces (including bedrooms, bathrooms and dressing rooms, or other spaces closely associated with bedrooms): a heating thermostat setting of 18^o C from 0700 to 0900 and from 1600 to 2400; and a heating thermostat setting of 15^o C from 2400 to 0700.

Table 5 - Cooling Thermostat Settings

Climate Region	All conditioned spaces ( ° C)	Climate Region	All conditioned spaces ( ° C)
1	26.5	36	26.0
2	27.0	37	27.0
3	27.0	38	27.0
4	26.0	39	27.0
5	26.5	40	28.0
6	27.0	41	27.0
7	26.0	42	27.5
8	26.0	43	27.5
9	25.5	44	26.0
10	25.5	45	26.5
11	25.0	46	26.5
12	25.5	47	25.0
13	25.5	48	26.0
14	24.0	49	24.5
15	25.0	50	25.0
16	25.0	51	25.5
17	25.0	52	24.5
18	24.5	53	24.5
19	27.0	54	25.0
20	25.5	55	24.0
21	24.0	56	25.0
22	23.5	57	24.0
23	23.5	58	23.5
24	24.5	59	22.5
25	23.0	60	24.0
26	23.0	61	23.5
27	25.5	62	24.0
28	25.0	63	23.0
29	26.5	64	23.0
30	27.0	65	23.5
31	26.5	66	23.5
32	26.5	67	23.0
33	27.0	68	23.0
34	26.5	69	22.5
35	26.0

4.6 Operational Behaviours

In the NatHERS scheme, the schedules of indoor and outdoor adjustable settings are as follows:

Indoor adjustable shading

Closed at 1800, open at 0700.
Closed if all the following three conditions satisfied:
- Outdoor temperature exceeds Cooling thermostat setting + 2.5°C;
- Incident solar irradiance on glazing exceeds 200 W/m², and
- Outdoor blind non-exist or cannot be drawn.

Outdoor adjustable shading

Closed if outdoor temperature exceeds T, where:
- T = Cooling thermostat setting - 0.5°C, except
  - If Cooling thermostat setting - 0.5°C > 26.0°C, T = 26.0°C
  - If Cooling thermostat setting - 0.5°C < 24.0°C, T = Cooling thermostat setting
Closed when incident solar irradiance on glazing exceeds 75 W/m ².

5. OUTPUTS

6. AREA ADJUSTMENT AND STAR RATINGS

During the development of the second generation NatHERS tools, the Ministerial Council on Energy agreed that the new NatHERS software tools should remove the naturally occurring bias that results in larger houses being advantaged over smaller houses. It was agreed that an Area Correction Factor similar to that developed by the Sustainable Energy Authority of Victoria (SEAV) for FirstRate, be developed for the second generation NatHERS software tools. A study was carried out by Mr Tony Isaacs for the then Australian Greenhouse Office in suggesting the Area Correction Factor for the use in AccuRate. The area adjustment calculation methodology for NCC 2022 can be found in a note by Dong Chen in 2022 [15]. The background and history of the area correction factors in previous versions of NatHERS software tools, please refer to Dong Chen (2011) [16].

Download:

Chen D., Area Correction Factors in AccuRate for NCC 2022, 2022.

Chen D., Area Correction Factors in AccuRate, note to DCCEE, 2011. (for NatHERS rating before NCC 2022)

7. ACKNOWLEDGEMENT

The success of AccuRate and the Chenath engine is impossible without the contributions from various institutions, associations, consultants, related departments of state and federal governments, software developers and software distribution companies collaborated with CSIRO over the years.

AccuRate and the Chenath Engine for Residential House Energy Rating

Dong Chen

Enquiries should be addressed to:

Dong Chen, CSIRO Energy

Important Disclaimer

Contents

NOMENCLATURE