Buffer tank
A hydronic vessel sitting between a heat pump and a zoned distribution system, holding enough water volume to keep the heat pump running at or above its manufacturer-stated minimum flow even when zone valves close and reduce the volume actively circulating through the rest of the system.
Also called low-loss header, hydraulic buffer
What does a buffer tank do on a heat pump system?
A buffer tank is a water vessel between a heat pump and its zoned distribution, sized so the heat pump keeps circulating at its own required minimum flow regardless of how many zone valves are calling at a given moment.
Buffer tank: how it works
A heat pump has a minimum run time or flow rate below which the compressor risks short cycling. In a zoned system, closing valves during low demand can drop circulated volume below that minimum. A buffer tank lets the heat pump circulate through its own loop at its required minimum regardless of zone state, while the zone circuits draw from the tank's stored volume independently.
Where you meet the term
- In a zoned hydronic heat pump installation, piped between the heat pump's primary loop and the zone distribution.
- In a manufacturer's installation manual, where minimum system volume or minimum flow requirements are stated.
- In a diagnosis of a heat pump that short cycles despite correct sizing, where the actual cause is circulated volume falling below the equipment's minimum during low zone demand.
How to check it
- Calculate volume from minimum capacity and minimum run time. Buffer volume follows from the heat pump's minimum output, its manufacturer-stated minimum run time, and the design temperature rise the buffer is allowed to reach, not from a fixed litres-per-kilowatt rule. Technician
- Check whether the system already guarantees minimum flow. A primary loop with a dedicated bypass or a minimum number of zones left permanently open may already satisfy the heat pump's minimum flow requirement without any added buffer volume. Technician
Buffer tank: what it is not
- Buffer tank size is not set by a fixed litres-per-kilowatt rule. It follows from the heat pump's minimum capacity and minimum run time, which differ between models with different turndown ranges.
- A buffer tank does not add efficiency by itself. Its role is preventing short cycling; where the flow mismatch it addresses does not exist, it adds cost and thermal lag without a benefit.
Buffer tank questions
Does every zoned heat pump system need a buffer tank?
No. A system where the primary loop always maintains adequate flow regardless of zone valve position may never expose the heat pump to a flow condition below its minimum, removing the need for one.
Can an existing hot water cylinder serve as a buffer tank?
Some designs share one vessel for both domestic hot water and buffering, but this requires careful control sequencing so the two functions do not conflict, and the sizing calculation for each still has to be satisfied.
Is a larger buffer tank ever a problem?
Going oversized trades away installed footprint and a slower warm-up from a cold start, and adds a standing heat-loss surface, none of which is the kind of hard failure an undersized tank actually produces.
Where this definition comes from
Written from standards body publication material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- ASHRAE Handbook, FundamentalsASHRAE reference volume carrying the psychrometric relationships, the climatic design condition tables and their annual percentile basis, duct and pipe fitting loss coefficients, and the bin and degree-day methods used for seasonal energy estimation.
- CIBSE Knowledge PortalCIBSE guidance library covering United Kingdom design practice for heating and cooling load calculation, external design conditions, pipe and duct sizing, water treatment, and heat pump system design, used here where United Kingdom convention differs from North American procedure.