Heating · Glossary
Low-Loss Header
A vertical hydraulic separation device that decouples primary and secondary circuit flows in a heating system, often used with system boilers and heat pumps.
A low-loss header (LLH) is a hydraulic separator: a vertical pipe (typically 4-6 inch diameter) with primary flow/return on one side and secondary flow/return on the other, sized so flow velocity drops below 0.1 m/s. This decouples the two circuits hydraulically — primary and secondary pumps no longer interact, eliminating overpump and pump-fight problems.
LLHs are common with system boilers driving multiple heating zones plus a hot water cylinder. With heat pumps they are less commonly necessary because the integrated controller and primary pump are designed to manage flow without an LLH — but they can be useful in retrofits where existing pipework is undersized.
Caveat: an LLH is a thermal mixing point, so primary and secondary temperatures equalise. That is fine for mid-temperature radiator circuits but defeats the purpose of weather compensation if you mix hot DHW load with low-temp space heating. CCS specifies LLHs sparingly and only where the design demands. See our heat pump page.