Comparison Between Asset Rating Design Assumptions and Measured Internal Temperatures for Homes with Heat Pumps: Observations from Ireland

Summary of a paper presented at the joint 45th AIVC conference and ASHRAE 2025 IEQ conference “IEQ 2025: “Rising to New Challenges: Connecting IEQ to a Sustainable Future” will be held on September 24-26, 2025, in Montreal, Quebec together with the 13th TightVent and the 11th venticool conferences.

Key words: heat pumps, energy performance certificates, DEAP, occupant behaviour, internal temperatures, social housing, retrofits, Ireland, energy performance gap

Across Europe, Energy Performance Certificates (EPC) are used as Asset Rating Tools to predict household energy use from standardised assumptions, but real homes often perform differently in practice. This study compares Ireland’s Dwelling Energy Assessment Procedure (DEAP) assumptions with actual indoor temperatures and Air Source Heat Pump (ASHP) operation in 15 retrofitted social homes, revealing systematic misalignment between design predictions and real-world heating practices.

EPC Energy Use vs Real Homes

Ireland’s DEAP assumes fixed indoor temperatures of 21°C in living areas and 18°C in bedrooms, with 56 heating hours per week (7-9 am and 5-11 pm) from 1^st October to 31^st May. These uniform assumptions are useful to support transparent, comparable energy ratings allowing Ireland comparatively benchmark its building stock, but real households do not heat their homes in a single standard way. Occupants typically adjust heating to their personal comfort preferences, usage patterns, and routines. Modern ASHPs allow flexible, often continuous operation rather than fixed on/off cycles. Existing EPC calculation methods, including DEAP, tend to assume conventional heating patterns and may underrepresent operational flexibility or the diversity of comfort‑seeking behaviour. This mismatch has measurable consequences. This performance gap weakens confidence in retrofit investments, complicates policy evaluation, and risks misaligned decarbonisation strategies, particularly as Ireland scales up heat pump deployment as part of a national renovation wave towards 2030 and beyond. The gap arises from multiple factors: simplified occupancy profiles, standardised climate data, variations in construction quality and operation, and, critically, occupant behaviour that asset-based ratings estimate based on total floor area only. Everyday choices such as thermostat settings, heating duration, and which rooms are kept warm can influence demand as much as the building fabric, yet static EPC assumptions treat all homes the same, with two temperature set points (21° for living area, 18° to bedrooms).

Improving EPC accuracy, and the effectiveness of retrofit and decarbonisation policy, requires evidence on how occupants use modern heating systems in retrofitted homes in practice. This study contributes such empirical data for Irish social housing retrofits with heat pumps.

Study Design and Methods

We monitored 15 single‑storey A-Rated social housing dwellings in County Waterford, each retrofitted to the same energy standards and equipped with ASHPs. Floor areas ranged from 34.8m² to 56.0m²; eight dwellings used monobloc heat pumps and seven used split systems. All were assessed using DEAP by a registered Building Energy Rating (BER) Assessor. From October 2022 to April 2024, wireless temperature sensors recorded internal conditions every 12 minutes (hourly averages computed). Manufacturer heat pump data, logged at one‑minute intervals, were downloaded monthly. Thermostat setpoints and weekly operating hours were also extracted.

Bedrooms Warmer, Living Rooms Cooler

Figure 1 shows significant discrepancies between DEAP assumptions and measured conditions. Bedrooms averaged 1.8°C warmer than assumed (p<0.001); living rooms 1.0°C cooler than assumed (p<0.001). House‑level variation ranged from +3.2°C in bedrooms to - 7.0°C in living rooms, but the pattern was consistent with occupants prioritising warmth in sleeping areas while accepting cooler daytime living spaces. Living rooms account for only 36% of floor area with excess bedroom warmth having a disproportionate impact on the whole‑dwelling heating demand. When bedrooms run warmer than predicted, actual energy use may exceed DEAP estimates. This finding challenges the uniformity assumption in standardised EPC methods.

Figure 1. Boxplots of measured internal temperatures for Bedroom and Living room during the heating season for 14 out of 15 homes indicated in Table 1. (Dashed line indicates assumed temperature in each room type in DEAP).

Setpoint Temperatures and Heat Pump Operation

Figure 2 (Top) compares recorded thermostat setpoints with DEAP’s 21°C living room assumption. In all homes, actual setpoints were lower by up to 1.3°C, confirming that occupants routinely operate heating systems below standard assumptions.

Figure 2. Top: Comparison of internal system temperatures and set-point temperatures. Bottom: Comparison of hours per week where heat pump is operational in space heating mode only. Dashed lines indicate thresholds used in DEAP for living spaces, bedrooms (Top) and a threshold of hours per week of 56 (Bottom).

Figure 3 reveals the temporal distribution of heat pump operation across a 24‑hour day. DEAP assumes heating only during 7-9 am and 5-11 pm (56 hours/week total). Measured operation showed continuous activity throughout the day in many homes, with substantial heating outside the assumed windows. Weekly operation ranged from 4 to 114 hours, a twenty-five-fold variation reflecting occupants’ actual comfort needs and occupancy patterns. Many homes maintained background heating throughout day and night, and extended operation beyond the predicted October to May heating season. This flexible, continuous operation is more efficient for heat pumps than intermittent cycling which fundamentally contradicts DEAP’s static schedule. The combination of lower living room setpoints and highly variable weekly hours demonstrates that DEAP assumptions do not represent how occupants use modern heating systems in practice.

Figure 3. Averaged number of minutes / hour for a full sample set for a 24-hour period (Shaded lighter blue rectangles indicates the hours assumed in DEAP).

Implications for Energy Policy and Retrofits

When standardised EPC assumptions diverge systematically from measured conditions, energy predictions become unreliable, leading to inaccurate retrofit savings estimates and misaligned policy decisions. Occupant behaviour, thermostat adjustments, heating duration, room by room priorities drives actual energy demand more than DEAP captures. Any retrofit strategy relying solely on envelope improvements or system efficiency ratings risks underestimating operational energy use. This is critical for Ireland’s heat pump rollout and building renovation wave. If EPCs systematically inaccurately predict performance, retrofit investment may not deliver anticipated carbon savings. Both heat pump types exhibited similar patterns of lower setpoints and variable operation, confirming these are mostly driven through occupant driven behaviours.

Toward Behaviour Informed Energy Ratings

Rather than fully individualised occupancy profiles which would be impractical for routine EPC assessments - we propose that DEAP offer assessors a limited range of selectable heating profiles reflecting common occupancy and comfort patterns (e.g., full‑time occupied vs shift workers, Elderly couple vs young couple, with bedrooms vs living spaces prioritised and so on). Further research is needed to develop and model representative occupancy profiles to better align assumed DEAP energy use with measured data, with the potential to improve accuracy while maintaining standardisation and comparability. Such profiles, informed by empirical data on real heating practices and accounting for how heat pumps enable continuous, lower‑load operation, would provide updated guidance on typical setpoint ranges and operating hours differentiated by occupancy type. This would enhance EPC accuracy without abandoning the asset‑rating framework.

Conclusion

Analysis of 15 retrofitted Irish social housing dwellings revealed systematic departures from DEAP assumptions: bedrooms 1.8°C above assumed, living rooms 1.0°C below assumed, and heat pump operation ranging from 4…114 hours/week (vs. 56 assumed). Thermostat setpoints in living spaces were consistently up to 1.3°C below standard. These discrepancies indicate that static, uniform EPC assumptions fail to capture the dynamic, occupant driven, comfort-oriented heating practices enabled by modern heat pumps. For Ireland’s retrofit and decarbonisation goals to succeed, energy performance certificates will need to evolve to a closer alignment with occupant behaviour, in a bid to reduce the performance gap between assumed EPC measurements and actual energy performance of dwellings. This evidence supports flexible, behaviour informed rating methodologies that balance standardisation with accuracy in predicting actual energy demand and thermal comfort.

Acknowledgements

This project was funded by the Sustainable Energy Authority of Ireland (SEAI) under the Research, Development and Demonstration Programme, grant numbers RDD744 and 23/RDD/1032. Data collection was approved by Munster Technological University’s Human Research Ethics Committee (approval MTU‑HREC‑MR‑23‑003‑A).