Heating, cooling, domestic hot water and renewables: load calculation, distribution and generation
Loads, distribution and generation
The 10 W/m² heating load limit is what allows supply air to carry the heat: at roughly 30 m³/h per person and a supply temperature near 50 °C, air delivers about 10 W/m². Above that you need radiators or floor heating, a design warning sign. Domestic hot water frequently dominates the remaining demand in small, well-insulated dwellings, so distribution losses - pipe insulation, short runs, avoiding unnecessary circulation - matter more than nominal generator efficiency.
Renewables and primary energy
Heat pumps, solar thermal, PV and biomass are evaluated through PER factors in PHPP: the same kWh of electricity counts differently depending on the energy carrier and renewable share. Hot water storage must be sized to avoid stagnation and Legionella risk; 60 °C storage with a recirculation regime needs careful design.
Worked example: heating load
A 150 m² TFA house has total transmission UA = 36 W/K. At design ΔT = 30 K, transmission is 36 × 30 = 1,080 W. Ventilation: 0.35 ACH × 375 m³ = 131 m³/h; with 80 % heat recovery the effective rate is about 26 m³/h, giving 26 × 0.33 × 30 ≈ 260 W. Total 1,340 W = 8.9 W/m² - within 10 W/m², so air heating is feasible and no radiators are needed.
Traps
Sizing the generator from annual demand rather than design load. Omitting distribution losses and DHW circulation. Using electric-resistance backup that ruins the PER result. Ignoring that heat-pump COP falls at the low outdoor temperatures used for the load calculation.