Building regulations - new domestic buildings: identification and assessment of energy standard improvements

Appendix C Summary of model differences

Occupancy

Each model has different occupancy assumptions, resulting in different number of occupants in each archetype. This impacts ventilation requirements, hot water usage and gains (except for PHPP, which uses a default W/m² gains assumption for certification).

Set points and programmes

SAP 10.2 assumes 21°C in zone 1 and 18°C in zone 2 is maintained for a specific intermittent heating programme. The heating hours consist of short period on a weekday morning and a longer period in the afternoon and evening. Weekends are assumed to have all day heating. No heating is assumed overnight and is assumed to be off in the summer months. For low responsive heating systems, such as ASHPs, longer heating hours are predicted to capture the longer timeframes required to get the dwelling up to temperature.

PHPP assumes a 20°C set point for the whole dwelling and that the dwelling is continuously heated. All year round.

HEM assumes 21°C in zone 1 and 20°C in zone 2 is maintained for a specific intermittent heating programme. The heating hours consist of a short period on a weekday morning and a longer period in the afternoon and evening. Weekends are assumed to have all day heating. Outside of the periods where the set points are targeted, the HEM has been run with an 18°C set back temperature and with a 1-hour advance start function to assist ASHP function. Without this setback and advanced start, the model predicts underheating will occur due to the time taken for wet heating system emitter circuits to get up to temperature. The DE does not require an advanced start as the assumed capacity is sufficient to meet the demand when required. The setback for the DHN option was set to 12°C due to the higher flow temperature creating a more responsive system.

Near shading impact on openings

SAP 10.2 applies an annual default 23% factor for shading reduction and 10% reduction for incident reflection.

PHPP applies a static 10% incident reflection factor and 5% dirt factor to account for the fact that windows are not maintained with the stock glazing characteristics. The near shading impacts of balconies and window reveals are calculated based on the monthly average solar angle.

HEM applies a static 10% incident reflection factor¹ and calculates the near shading impacts using the hourly calculated solar angle.

Emitter circuit characteristics

Both SAP 10.2 and PHPP assume that the emitter circuit is sized to deliver the design demand meaning that the heating generator characteristics represent the efficiency of the entire system. SAP 10.2 assumes intermittent heating so wet heating system are assumed to have some small amount of non-useful energy at the end of the heating period (which represents the energy retained in the thermal mass of the emitter circuit). As PHPP assumes constant heating, all output is considered useful.

HEM requires specific details to be input for the heating system characteristics. This includes the power output and thermal mass of the circuit, flow temperature ranges and flow rates. AECOM have sized the emitter circuits to best represent reality (and the components used for the costing exercise) and used the HEM to confirm if the heating system is sufficiently sized. Some unmet demand has been reported but this represents only a handful of hours per year where the zone temperature is only 0-1°C below the set point. This typically occurs on the colder days of the year during the startup periods or after a hot water heating event.

Window thermal performance

SAP 10.2 and HEM require the average window characteristics to be entered. This consists of a U_window value made up of all heat loss through the glazing, frame and glazing spacer thermal bridge. Thermal bridging of the window installation is accounted for separately in the model.

PHPP requires all individual window components with heat loss to be input separately. To obtain an equivalent value between the models a standard window size (1.48m x 1.23m) calculation for an actual uPVC DGU window was used to break down the component performance. For the BAU window specification this approach for a 1.4W/m² results in a U_frame, U_glass and ψ_spacer of 1.3W/m².K, 1.2W/m².K and 0.05W/m.K respectively.

PHPP and SAP assumptions for ventilation losses

SAP 10.2 assumes a default 0.5ACH being required for ventilation purposes. For intermittent MEV systems, assumed present in the notional dwelling, it is assumed that the ventilators are manually controlled to achieve the 0.5ACH rate (i.e. mechanical fans are not used and ventilator shut if the infiltration rates exceed 0.5ACH). For the dMEV option modelled in the BAU case, the ventilation system is assumed to achieve a constant rate of ACH to meet the requirements. Uncontrolled ventilation^[7] through the ventilators is not accounted (i.e. all air drawn if through the ventilators is assumed to be due the negative pressure created by the MEV fans).

PHPP ventilation is based on a set extract rate per wet room type, therefore the ventilation ACH varies per dwelling (typically lower than 0.5ACH for larger houses but higher for small flats). PHPP BAU have been run with the assumption that the ventilation system is “Extract air only”. The ventilation requirements are assumed to be achieved by the MEV fans, and no uncontrolled ventilation is assumed through the ventilators.

HEM ventilation requirements are based on an input value. The input is based on the ventilation rates outlined in Section 3.14 of the technical handbook. Ventilator equivalent areas have been inputted based 3.14 requirements. HEM assumes that occupants will open and close the ventilators in response to ventilation rates outside of the required range.

Solar energy on external surface of building elements

SAP 10.2 does not account for solar absorption on the external surfaces of building elements.

PHPP accounts for the solar absorption on building elements based on the declared absorption and emissivity values for each element. PHPP applies a default 0.7 shading reduction factor for the environmental solar energy hitting the dwelling external surfaces. Opaque solar gains are calculated and subject to a usefulness calculation before being discounted from the space heating demand.

HEM accounts for the solar absorption on building elements. Absorption and emissivity values are used to calculate the outer element surface temperatures, if the outer surface temperature is greater than the inner wall temperature then losses are not recorded. As such, it is expected that fabric energy balance in the HEM is much lower or even positive when compared to the SAP or PHPP energy balance.