A low loss header is a short, oversized vertical vessel with four connections that hydraulically separates the heat generator circuit from the heating circuits while storing almost no water — typically under 20 litres, against 100 litres or more for a buffer tank. Because the pressure drop across it is near zero, the generator pump and each circuit pump operate independently without interfering with one another.
How does a low loss header work?
A low loss header joins the flow and return of the heat generator to the flow and return of the heating circuits inside one short, wide vertical pipe. Because its cross-section is many times larger than the connecting pipework, water velocity inside collapses to a few centimetres per second and the pressure difference between the four ports becomes practically zero — which is exactly what "low loss" means. The result is hydraulic separation: the primary (generator) pump and the secondary (heating) pumps each see only the resistance of their own circuit, so neither can starve or overpower the other. When primary and secondary flow rates match, water passes straight across the header; any imbalance simply recirculates vertically inside it instead of disturbing the pumps.
When do you need a low loss header?
You need one whenever two or more pumps would otherwise fight each other on shared pipework. The classic cases are a heat pump or boiler feeding several independently pumped zones, a cascade of two or more generators on one distribution system, and retrofits where the existing pipe network has an unknown or highly variable resistance. Many heat pump manufacturers also demand a guaranteed minimum flow through the unit at all times; a header provides that even when every zone valve downstream is closed. A modern inverter heat pump feeding a single open circuit usually does not need one — separation only earns its place once multiple pumped circuits or generators meet at one point.
Low loss header vs buffer tank: what is the difference?
| Feature | Low loss header | Buffer tank |
|---|---|---|
| Stored volume | Typically 5–20 L | Typically 100–1,000 L |
| Hydraulic separation | Yes | Yes (4-port piping) |
| Prevents short-cycling | No — too little volume | Yes |
| Defrost energy reserve | No | Yes |
| Standing heat loss | Negligible | Roughly 1.5–2.5 kWh/day for a 300 L class B/C vessel |
| Space and cost | Small, low | Larger, higher |
A header solves a pump problem; a buffer tank solves a volume problem — and a 4-port buffer happens to solve both at once. If a system only lacks water volume and has no pump conflict, an in-line volumiser is the third option. The decision logic is worked through in Low loss header vs buffer tank: which do I need?
What is the downside of a low loss header?
Mixing. If the secondary pumps move more water than the primary pump, cool return water is drawn up the header and blends into the flow, lowering the temperature delivered to the circuits; if the primary side dominates, hot flow water short-circuits into the generator return. Both distortions cost efficiency: as a widely used design rule, each kelvin of unnecessary flow-temperature increase costs a heat pump roughly 2 % of its COP, and elevated return temperatures reduce condensing boiler efficiency as well. Good practice is to balance primary flow slightly above secondary flow, place the control sensor downstream of the header, and size the header generously so internal velocity stays low — around 0.2 m/s or less is a common target — preserving internal temperature layering.
Can a low loss header be replaced by anything simpler?
Sometimes. Closely spaced tees achieve the same near-zero pressure difference in compact systems, and a plate heat exchanger separates circuits fully where different fluids or pressures are involved (for example glycol on one side). A 4-port buffer tank replaces the header entirely in heat pump systems that need added volume anyway, which is why many installers skip the header and fit the buffer.
Frequently asked questions
Does a low loss header store hot water?
No. It typically holds under 20 litres and is not intended as thermal storage. It cannot prevent compressor short-cycling or supply defrost energy — those jobs need a buffer tank or volumiser with real water volume.
Does every heat pump need a low loss header?
No. A single inverter-driven heat pump on one open, correctly sized circuit runs best fully open with no separation. Headers pay off when several pumped zones, a cascade of generators, or an unknown retrofit pipe network must be decoupled.
Why is my flow temperature lower after the header than before it?
Secondary flow is exceeding primary flow, so cool return water is being drawn up the header and mixed into the supply. Rebalance the pumps so the primary side delivers at least as much flow as all secondary circuits combined.
What size should a low loss header be?
Size it so internal water velocity stays very low — a vessel diameter of roughly three times the connecting pipe diameter is a common rule of thumb. Manufacturers publish maximum flow ratings per header model; select against the larger of primary or secondary design flow.