Size a heat pump buffer tank at 20–50 litres per kW of heating capacity for radiator and zoned systems, 12–20 L/kW for open underfloor circuits, and 30 L/kW or more where defrost cycles are long. A 10 kW heat pump therefore pairs with a 200–500 L vessel. Always satisfy the manufacturer's minimum system volume, which the buffer tops up beyond pipework content.
What is the litres-per-kW rule for buffer tanks?
The working rule: 20–50 litres of buffer tank volume per kW of heat pump heating capacity for radiator and zoned systems, 12–20 L/kW for open underfloor circuits, and 30 L/kW or more where winters bring long defrost cycles. The span is not vagueness — it prices in emitter water content, zoning behaviour and climate:
| System configuration | Sizing band | 10 kW example |
|---|---|---|
| Open underfloor, inverter unit | 12–20 L/kW (sometimes none) | 120–200 L |
| Radiators or zoned circuits | 20–50 L/kW | 200–500 L |
| Cold climate, long defrosts | ≥ 30 L/kW | 300 L + |
| Bivalent or solar-integrated | Per hydraulic design | 500 L + |
Whether your system needs a buffer at all is covered in Do I need a buffer tank with a heat pump? — this page is the sizing maths.
How much volume does the defrost cycle demand?
Air-source units periodically reverse the refrigerant cycle to melt evaporator ice, pulling that energy from the heating water — the defrost cycle. Manufacturers therefore state a minimum system volume, typically 10–20 L per kW, that must be open to flow whenever a defrost can start. Only guaranteed water counts: pipework and permanently open loops yes, zones behind closable valves no. An 8 kW unit needs 80–160 L that no thermostat can lock away — which is precisely what a return-line buffer guarantees.
How do you calculate buffer volume from run time?
Water stores 1.16 Wh per litre and kelvin, and the sizing formula follows directly: volume in litres = heat pump output in watts × desired run time in hours ÷ (1.16 × usable temperature swing). To guarantee a 15-minute minimum run for an 8 kW unit cycling across a 10 K buffer band: 8,000 × 0.25 ÷ (1.16 × 10) ≈ 170 L. A 300 L vessel at the same swing stores about 3.5 kWh — around 26 minutes of full-load output from that 8 kW unit. Longer runs mean fewer starts, and compressor wear tracks start counts: design toward three starts per hour or fewer. An inverter compressor reduces the cycling problem by modulating down, but it does not change the defrost arithmetic.
Does SG-Ready or an off-peak tariff change the size?
Yes — upward. A buffer that must bridge utility interruptions is sized on blocked energy, not run time: German utilities may pause a heat pump up to three times two hours per day under classic EVU-lock tariffs, and an SG-Ready controller invites the same logic for price-driven operation. Bridging two hours of a 4 kW average heat load with a 15 K swing needs about 460 L. Solar thermal integration pushes the same direction at 50–100 L per m² of collector — at which point the buffer also takes on hydraulic separation duty between the generator circuits.
What do worked examples look like at 6, 10 and 16 kW?
| Heat pump size | Open underfloor | Radiators / zoned | With EVU-block or solar duty |
|---|---|---|---|
| 6 kW | 75–120 L | 120–300 L | 300 L + |
| 10 kW | 120–200 L | 200–500 L | 500 L + |
| 16 kW | 200–320 L | 320–800 L | 800 L + |
Round up to the next commercial size and check the vessel's standing loss class — a well-insulated ErP class B/C 300 L buffer costs far less energy per year than the compressor cycling it prevents. The Solimpeks Solibuffer range spans 50–1,000 L in seven sizes, from a compact 50 L minimum-volume vessel upward, so every band in the tables above maps to a standard product.
Frequently asked questions
Can a buffer tank be too big?
Yes. Oversizing adds standing losses around the clock and slows temperature response without meaningfully improving compressor life. Staying inside the sizing band for your system type and choosing a good ErP insulation class beats 'bigger is safer'.
Does pipework volume count toward the minimum system volume?
Yes — everything permanently open to flow counts: pipes, manifolds, open loops and the buffer itself. Volume behind zone valves that can close does not, which is exactly why zoned systems need buffers.
Do inverter heat pumps need smaller buffers?
Smaller, not zero. Modulation cuts cycling whenever the load stays above the unit's minimum output, but defrost volume and hydraulic separation requirements are unchanged — hence 12–20 L/kW even for open underfloor systems.
Two-pipe or four-pipe connection for the buffer?
Two-pipe series buffers preserve flow temperature and suit volume and defrost duty at the small end of the band; four-pipe separation buffers fully decouple circuits but mix temperatures and usually sit at the larger end. Follow the heat pump manufacturer's hydraulic scheme.