How Does a Heat Interface Unit Work?
An HIU responds to heating or hot-water demand by regulating network heat flow through the appropriate path. Exchangers separate circuits where required; controls aim to deliver the specified output while returning cooler water to the heat network.
Quote on request · Confirmed London & South East / Nottingham & NG coverage
Primary flow and regulation
Hot water from the shared network enters the unit through a primary flow connection and leaves through a return. A control valve, driven either mechanically or electronically, regulates how much primary flow is admitted according to demand from the dwelling. A strainer ahead of the controls protects narrow passages and exchanger plates from debris carried in the network water.
Because the unit only regulates what it receives, its performance is tied to primary conditions it does not control: the temperature arriving at the dwelling and the differential pressure available between flow and return. If either is below the unit's design range, heating or hot water can suffer even with the local controls working correctly.
The indirect heating path
Where heating is indirect, primary water passes through one side of a plate heat exchanger while the dwelling's own heating circuit circulates on the other side, picking up heat without the two waters mixing. A secondary pump, where fitted, moves that circuit around the radiators or underfloor loops, and an expansion vessel accommodates the water's change in volume as it heats and cools.
A direct arrangement works differently: network water itself is let into the dwelling's heating circuit under valve control, with no exchanger in between. This removes some secondary-side parts but means the heating circuit shares the water quality and pressure characteristics of the wider network.
Hot water production on demand
On a combined unit, opening a hot tap activates the model’s mechanical or electronic demand regulation, which operates the primary valve to send heat across the domestic hot-water exchanger while mains cold water passes through the other side. Many designs prioritise hot water over heating for the short period a tap is open, since heating demand is less time-sensitive.
The controller aims to hold the leaving hot-water temperature close to its setpoint across a range of flow rates, within the limits the exchanger and valve were sized for. Drawing more flow than the unit's rated duty, for example running two outlets together, can reduce the temperature achieved even when nothing is faulty.
Why delivery depends on network conditions, not just the unit
Some control valves — a differential pressure control valve (DPCV) or pressure-independent control valve (PICV) — are set to operate within a particular range for a specific model and installation; that range is not a generic figure that applies across all units or schemes. A valve operating outside its intended envelope can throttle flow even when it is mechanically sound.
Checking how an HIU is actually working under demand, rather than only in a static or no-draw state, is the only reliable way to separate a control fault from a supply-side limitation. That is why diagnosis compares measured flow, return and differential-pressure readings against the figures for that installation.
Official product references
Common Questions
Why is my heating fine but hot water not, or vice versa?
Heating and hot water usually run through different paths inside the unit — heating either directly or via one exchanger, hot water via a separate dedicated exchanger. A fault, restriction or control issue on one path does not necessarily affect the other, so it is possible for one service to work normally while the other does not.
What is differential pressure and why does it matter?
Differential pressure is the difference between the primary flow and return pressure available to the unit. The control valve needs enough of it to pass the flow the unit was designed for. Where it is too low, heat delivery can fall even though nothing inside the HIU has failed.
Does a DPCV or PICV setting apply to my HIU model?
Not generically. These valves are set for a specific model within a defined operating envelope for that installation. A setting correct for one unit or scheme is not necessarily correct for another, which is why settings should be checked against that unit's own documentation.
Can heating work normally even if the HIU itself has a fault?
Yes, in some configurations heating and hot water are sufficiently separate that a fault confined to one exchanger, valve or sensor leaves the other service largely unaffected. Diagnosis still needs to check both, since some faults, such as a restricted shared strainer, can affect both at once.
What happens to the water that returns to the network?
Water leaving the unit through the return goes back to the shared network at a lower temperature than it arrived, having given up heat to the dwelling. Network operators typically monitor return temperatures across a scheme, since consistently high returns can indicate a control or bypass issue worth investigating.
