Buffer tank sizing
Free buffer tank calculator for a heat pump or a biomass boiler: volume in litres, litres per kW and which of the three requirements governs the size.
Minimum volume from calorimetry — run time, defrost and hydraulic separation, not a litres-per-kW rule.
Buffer tank sizing — method and FAQ
Three reasons to exist, the largest governs
A buffer tank exists for three independent reasons, each with a minimum volume of its own: to let the compressor reach its minimum run time before the differential shuts it down; to keep an air unit's defrost from draining the rooms; and to hold a minimum circulating volume when the zones close. The tank has to satisfy the largest of the three — sizing on one and forgetting the others is the usual way one ends up too small.
All of it is computed from plain calorimetry, V = E/(ρ·c·ΔΤ), not from an empirical litres-per-kW rule. The well-known "20 l/kW" rule is not a method, and it does not even reproduce from the numbers usually quoted beside it: an on/off unit at a 10 minute minimum run time and a 5 K differential gives 28.7 l/kW, not 20.
Biomass: a different question, different numbers
A wood boiler does not stop when the tank comes up to temperature — the load burns out to the end and all of that energy has to be stored somewhere. With a full burn time of about two hours, a 20 kW boiler leads to a tank of close to 2,000 litres — that is, 85+ litres per kW, four times the heat pumps' "rule".
Frequently asked questions
Does a heat pump need a buffer tank?
Usually yes, but not always for the reason usually cited. An inverter unit that modulates down low needs a far smaller tank than an on/off unit of the same output. Defrost matters just as much: an air unit with no stored volume takes the defrost heat out of the rooms. A typical result for a 10 kW air unit: about 150 litres.
Does the 20 litres per kW rule hold?
Not as a method, and not even as a special case. The calorimetry for an on/off unit at a 10 minute minimum run time and a 5 K differential gives 28.7 litres per kW, not 20 — 20 would correspond to about seven minutes. An inverter that turns down to a third of nominal needs around 9.6 litres per kW of nominal output, and a biomass boiler on this tool's defaults reaches 86. The spread from one end to the other is ninefold: which is why this tool computes rather than applying a rule.
Related tools
- Heat pump sizing for a dwelling — Heat loss, cooling load and required capacity — always with the operating conditions stated, so quotations become comparable.
- Radiator output at low flow temperatures — Checks existing radiators to EN 442 for a heat-pump retrofit — real output and the lowest flow temperature that suffices.