Surge tank (outdoor)

Uninsulated surge tanks (steel or concrete, in the ground) lose heat continuously. Uninsulated U-value: 2.0 W/(m²·K) against ground (12 °C). Insulating with 80 mm PUR rigid foam reduces U to 0.32 W/(m²·K) — a factor of 6. Investment: 250-350 €/m² of tank wall including water-tightness and mould protection. For a 55 m² surge tank: 124 kWh/d uninsulated → 20 kWh/d insulated = 38 MWh/a saved, ~3,400 €/a in gas. Plus avoidance of condensation corrosion on the outside.

Basis: Practice (KSB, FLL)

DGfdB guideline R 65.10 fixes the order for refurbishments: 1) reduce losses (envelope, pool cover, insulate the surge tank, ventilation heat recovery), 2) recover heat (heat recovery, waste-water heat, filter backwash as a heat-pump source), 3) generate efficiently (heat pump > CHP > gas). This hierarchy is decisive for public grant applications: funders check whether losses were reduced before the generation investment. Concretely: insulate the surge tank, add an outdoor-pool night cover and service the ventilation heat recovery before or together with the heat-pump installation.

Basis: DGfdB R 65.10

Evaporation from the water surface is the dominant heat loss in an indoor pool (60-80 %). Smith/Löf variant of the Carrier formula: m_evap [kg/h] = β × A [m²] × (p_w_pool − p_w_hall) [kPa]. β-coefficient per VDI 2089 Sheet 2 Tab. 4: 0.013 unused pool (covered), 0.040 normal use, 0.080 wave/diving pool. Evaporation enthalpy at 30 °C: 0.694 kWh/kg. Pool saturation vapour pressure via the Magnus formula: 611.2 × exp(17.62 × t / (243.12 + t)) Pa.

Basis: VDI 2089 Sheet 2

Outdoor pools have higher convection and radiation losses. Convection: h_conv = 4 + 11 × v_wind [W/(m²·K)] with v in m/s. At 1 m/s wind: h ≈ 15 W/(m²·K). Night radiation (clear sky): ~60 W/m² over a 10 h night. Solar gain during the day (global radiation × absorptivity 0.82 × 12 h) is a credit — in high summer it can drop the heat demand close to zero. A night cover reduces both evaporation (−85 %) and radiation loss (−90 %).

Basis: VDI 2089 Sheet 2

DGfdB guideline R 60.03 on heating public outdoor pools: heat-up phase in April over 30 days, raising the set-point from 10 to 22 °C; slow heat-up (45 days) or solar absorbers as a pre-heat stage save energy. For 2,508 m³ of water × 12 K × 1.16 kWh/(m³·K) = 34,928 kWh — spread over 30 days that is 1,164 kWh/d of extra load. Recommendation: heat up via a pool heat pump on a warm source (backwash water already at 15 °C before season start) instead of a gas peak boiler. A pool cover shortens the time (solar gain by day, loss reduction by night).

Basis: DGfdB R 60.03