When Heating Demand Passes but PER and Summer Comfort Fail
A 1,200 m² TFA, 14-dwelling apartment building is modelled in PHPP 10 by a designer who reports 'Passive House achieved' because the heating demand is 14.2 kWh/(m²a) and the heating load is 9.5 W/m². The airtightness test result is 0.55 h⁻¹. However, the same PHPP shows PER demand of 74 kWh/(m²a), summertime overheating of 11% of hours above 25°C, and domestic hot water produced by a 6 kW electric immersion heater in each 300 L tank. The client has roof area for a 12 kWp PV array, which is not yet in the model. Mechanical ventilation has specific fan power of 0.35 Wh/m³. The designer proposes three changes: add the PV, replace electric DHW with an air-to-water heat pump (COP 2.6), and add external venetian blinds to south windows. The budget allows only two measures before certification.
1. Which Passive House criteria are met and which are not, and what is the consequence for a Classic certificate?
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The project meets the heating demand criterion (14.2 ≤ 15 kWh/(m²a)), the heating load criterion (9.5 ≤ 10 W/m²) and the airtightness criterion (0.55 ≤ 0.6 h⁻¹). It fails two other mandatory Classic criteria: PER demand is 74 kWh/(m²a) against the 60 kWh/(m²a) limit, and summer overheating is 11% of hours above 25°C against the 10% limit. Certification is a whole-building verification, so passing heating demand alone does not justify the claim. The electric immersion DHW is the obvious PER driver: resistance heating turns almost 1 kWh of electricity into 1 kWh of heat, and electricity carries a high PER factor. The certifier should reject the current model and require a revised PHPP with demonstrated compliance on all criteria, not issue a certificate with a note that heating demand is acceptable. The result also matters for the certificate class: Classic has the PER limit, while Plus and Premium add renewable-generation requirements.
2. Rank the three measures and recommend two within the budget. Support the ranking with approximate numbers.
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Assume a DHW heat demand of 25 kWh/(m²a). Electric resistance at COP 0.95 uses about 26.3 kWh/(m²a) of electricity; with an illustrative PER factor of 1.8 this is about 47 PER kWh/(m²a). A heat pump at COP 2.6 uses about 9.6 kWh/(m²a), or about 17 PER kWh/(m²a), saving roughly 30 PER kWh/(m²a) and bringing 74 down to about 44 before other changes. This is the largest permanent reduction and should be first. External blinds do not improve PER, but without them the summer criterion still fails, so they are also mandatory for certification. A 12 kWp PV array might yield around 10,500 kWh/a, or 8.75 kWh/(m²a); even at full PER credit that is about 16 PER kWh/(m²a), and self-consumption may be much lower. PV also does nothing for the 11% overheating. The defensible choice is therefore heat pump DHW plus external blinds; PV can be added later or used to pursue a higher certificate class. Replacing glazing with low-g glass instead of blinds would reduce useful winter gains and is not the best summer fix.
3. What must the certifier verify in the revised PHPP and supporting evidence before accepting the project?
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The certifier should require an as-built PHPP, not a design-phase model. For DHW, verify the heat pump type, COP at the actual operating conditions, storage and distribution losses, and whether an electric backup or immersion is retained; the backup must be modelled honestly because it affects PER. For summer, verify the shading device, its control strategy, the shading factor used, and the resulting percentage of hours above 25°C; a manually operated blind cannot be credited as if it were always closed. For PV, verify array size, yield, metering arrangement and the PER treatment of export versus self-consumption, and ensure it is not double-counted as both a demand reduction and an export credit. The certifier should also confirm that the ventilation specific fan power of 0.35 Wh/m³ is at design airflow and that the airtightness result is from a valid test. Finally, the certificate class must be checked against the PER and renewable-generation rules, not just the heating demand.