Heat-pump cascade

For pools with strongly varying heat load (outdoor heat-up in April, near-zero summer, hall pool year-round) a cascade of several modulating pool heat pumps beats one large unit. A 500 kW industrial air-water HP with inverter min 30-40 % covers a 175-500 kW window. A cascade of 4 × 125 kW pool HPs covers 37-500 kW (8 % min) via stepped switching — far closer to the actual load profile. Benefits: redundancy (loss of one HP is 25 % of capacity, not 100 %), better modulation onto the PV profile, less cycling in low summer load.

Basis: VDI 4650 / DGfdB R 65.10

VDI 4650-1 defines the seasonal coefficient of performance (SCOP) averaged across the year. Crucially it includes heat-up phases, standstill losses and auxiliary loads (pumps, controls). A realistic SCOP for an inverter pool air-water HP on a warm source is 8-12 (versus a rated-point COP of 14-18). Factors that depress SCOP: a cold source in winter (outdoor air), a high sink (pool heat-up) and frequent cycling (oversizing). For grant applications a conservative SCOP of 8 is advisable — even where a model computes 11.3, that is a best-case assumption.

Basis: VDI 4650-1 + DGfdB

CHP units were standard in pools in the 2010s but are usually economically inferior to heat-pump concepts today. CHP advantage: constant base heat load (a hall pool is ideal) and self-generated power. Disadvantages: high specific heat-generation cost (~7 ct/kWh) versus a heat pump on a warm source (~3 ct/kWh at SCOP 10); no regional grant when paired with federal CHP funding; worse CO₂ balance (natural gas) than the power mix. CHP makes sense only for hot-water hygiene needs above 60 °C or as peak-load back-up. A pool without special hot-water hygiene needs should not prioritise CHP.

Basis: DGfdB + AGFW + practice

Modern inverter pool air-to-water heat pumps reach rated-point COPs (source 26 °C, sink 27 °C) between 14 and 18 (e.g. AquaForte InverterPro 16.5, Microwell HP1500 Split 17, Pontaqua Inverter+ 14). These are data-sheet values at the thermodynamically most favourable operating point. The seasonal COP (SCOP) — accounting for heat-up phases, cooler sources and standstill losses — sits at 8-12. Inverter pool HPs typically modulate between 30 % and 100 % of rated output; some industrial models (Carrier 30RB, Stiebel WPL) down to 25 %. The source must be > 5 °C for the evaporator to work without icing.

Basis: Manufacturer data sheets (AquaForte, Microwell, Pontaqua)

Filter backwash water is the most valuable heat source for pool heat pumps. Temperature: pool set-point minus 3-5 K (26 °C for a hall pool, 19 °C outdoor). Volume: typically 1× the filter-bed volume per day, approx. 5-10 m³ per filter. Energy content: with a 60 m² backwash tank surface at 25 °C water this yields 290 kWh/d of usable heat on the HP evaporator side. Connection: a heat exchanger between backwash water and the HP brine loop keeps the evaporator clean. Investment: ~10 k€ for the exchanger + controls.

Basis: DGfdB R 65.10 + manufacturer data