Introduction
Few components divide heat pump installers like the buffer tank. Some fit one on every job as insurance. Others call it an efficiency tax and design it out. With UK installers fitting a record of more than 60,000 certified heat pumps in 2025, the question lands on more design desks every week, and the honest answer is neither camp's default. It is a calculation.
This guide covers what a buffer tank actually does, when a volumiser is the better answer, why the sizing rules of thumb disagree so widely, and how to make the decision from system data rather than habit.
What a Buffer Tank Actually Does
A buffer tank adds water volume and hydraulic separation between the heat pump and the heating circuit. Its core job is to stop short cycling: a compressor that starts and stops too often wears prematurely and runs inefficiently, and BS EN 14511 recommends a heat pump should not start more than three times an hour. The buffer also guarantees minimum flow when zone valves and TRVs start closing, and it holds an energy reserve that matters during defrost cycles on air source units.
None of that is free. A four-pipe buffer mixes flow and return water, which distorts flow temperatures and forces the heat pump to work harder than the emitters need. Kensa's design guidance is blunt about the trade-offs: an extra pump, standing heat loss, and reduced efficiency, with every degree of unnecessary temperature rise costing performance. Badly integrated buffers are one of the most common findings in underperforming heat pump systems.
Buffer Tank or Volumiser?
Most residential systems that lack water volume do not need a buffer at all. They need a volumiser: a simple vessel, usually on the return, that adds litres without creating a mixing point. Because a volumiser does not blend flow and return temperatures, it preserves system efficiency in a way a four-pipe buffer cannot. The distinction sounds academic and is worth thousands of kilowatt-hours over a system's life.
The decision method is arithmetic. Take the manufacturer's minimum system volume for the heat pump. Add up what the system already holds: pipework, radiators or underfloor circuits, cylinder coil. If the total meets the minimum, you need neither vessel. If it falls short, a volumiser sized to the gap usually closes it. A worked example: a heat pump requiring 200 litres against a system holding 170 needs a 30-litre volumiser, not a 200-litre buffer.
A genuine buffer earns its place in specific situations: heavily zoned systems where most of the circuit can close down, fixed-output or twin-compressor units, cascades, and systems needing a defrost energy reserve that the emitters cannot provide. Kensa's alternative is worth knowing too: keep roughly a quarter of the system uncontrolled, with no TRVs or actuators on those circuits, and the minimum flow problem often disappears without any vessel.
Sizing: Why the Rules of Thumb Disagree
Search for buffer sizing and you will find numbers that differ by a factor of five. Kensa suggests 10 litres per kW for its ground source units. BS EN 14511 guidance points to around 25 litres per kW for defrost purposes, while some air source manufacturers recommend 40 to 50 litres of system volume per kW. Radiator systems typically need more than underfloor heating because they hold less water per kilowatt of output.
The numbers disagree because they answer different questions: minimum flow for one manufacturer's unit, defrost energy for air source machines, total system volume for low flow temperatures. For overall system volume, current UK guidance suggests around 17 to 18 litres per kW of heat loss as a bare minimum, rising to 25 to 27 litres per kW when targeting very low flow temperatures of 30 to 35°C, and where those litres sit matters as much as the total. Volume in radiators and underfloor circuits delivers heat into rooms; volume in a buffer only keeps the compressor happy. The right answer is always the manufacturer's stated minimum volume and flow requirement for the specific unit, checked against the actual system, not a generic ratio.
Design It, Don't Default It
The pattern we see behind most buffer decisions is that they are made on habit rather than data, in both directions. The fix is to make system volume a standard output of the heat loss and system design stage: calculate the room-by-room heat loss, size the emitters, total the system volume, compare it to the unit's minimum, and let the gap dictate the vessel. When that calculation lives in the design file, the decision is defensible to the customer, the MCS assessor and the engineer who services the system in five years. Heat pump design platforms handle this alongside the heat load calculation itself; Reonic includes heat load calculation in the same planning environment as PV and battery design, so hybrid projects carry one consistent set of numbers. For the system integration side, see our guide to combining solar PV and heat pumps.
Conclusion
The buffer tank is neither essential nor evil. It is a solution to three specific problems: short cycling, minimum flow and defrost reserve, and it carries real efficiency costs when fitted where those problems do not exist. Run the volume calculation on every design. Most homes need a modest volumiser or nothing; a minority genuinely need a buffer, and those that do need it sized to the unit, not to a rule of thumb. The installers who can show the arithmetic will design better systems and defend them when the performance data comes in.
FAQ
Q1: Does every heat pump need a buffer tank?
No. Modern inverter-driven heat pumps modulate output, and systems with enough water volume and open circuit often need no vessel at all. Buffers earn their place in heavily zoned systems, fixed-output or cascade installations, and where defrost demands an energy reserve.
Q2: What is the difference between a buffer tank and a volumiser?
A buffer provides hydraulic separation and mixes flow and return water, which costs efficiency. A volumiser simply adds water volume, usually on the return, without creating a mixing point. When the problem is insufficient system volume, the volumiser is almost always the better answer.
Q3: How do I size a buffer tank for a heat pump?
Start from the manufacturer's minimum system volume and flow requirement, not a generic ratio. Rules of thumb range from 10 litres per kW for some ground source units to 25 litres per kW for defrost under BS EN 14511 guidance, with radiator systems needing more volume than underfloor heating.
Q4: Why do buffer tanks reduce heat pump efficiency?
Four-pipe buffers mix flow and return water, so the heat pump must produce a higher flow temperature than the emitters actually need, and every unnecessary degree costs performance. Add standing heat loss and an extra circulation pump, and a poorly justified buffer becomes a permanent efficiency penalty.
Q5: How much system volume does a heat pump system need?
As a sanity check, around 17 to 18 litres per kW of heat loss is a bare minimum, with 25 to 27 litres per kW more appropriate at very low flow temperatures. Count the volume in pipes, emitters and cylinder coil first, then close any remaining gap with a volumiser.
