Troubleshooting · 9 min read
Common HIU Faults Explained
Heat Interface Units fail in a fairly predictable set of ways. This guide covers the faults we attend most often across London, what actually causes each one, and how a specialist engineer fixes it.
By Ilyas Safi, Founder & Director, Collide Group
Quick answer
The most common HIU faults are a blocked strainer and a scaled plate heat exchanger — both restrict heat transfer and show up as weak or lukewarm hot water. Other frequent faults include failed diverter valves and actuators, drifting temperature sensors, PCB faults and leaking seals. The large majority are repairable, and most are preventable with an annual service.
HIU Symptom-to-Cause Table
Use this to narrow down what is likely happening before an engineer attends. Note that several different faults can produce the same symptom, so this is a starting point rather than a diagnosis.
| Symptom | Likely cause | Typical fix |
|---|---|---|
| No hot water at all | Failed DHW/diverter valve, failed actuator, PCB fault, or loss of primary flow from the network | Diagnose supply and controls, then replace the failed component |
| Hot water is lukewarm | Scaled or fouled plate heat exchanger, partially blocked strainer, valve not fully opening | Descale or replace the plate heat exchanger; clean strainer; service valve |
| Weak flow / slow to run hot | Blocked strainer or filter, debris in the secondary side, low network differential pressure | Clean or replace strainer, flush the unit, check network pressure |
| Heating works, hot water does not | DHW valve, actuator or flow switch fault on the domestic side only | Test and replace the DHW-side component |
| Hot water works, heating does not | Heating circuit valve, actuator or thermostat/controls fault | Test heating circuit, actuator and room controls |
| Water leaking from the unit | Failed seal or gasket, corroded fitting, cracked plate heat exchanger, valve gland | Isolate, identify the leak point, replace the failed seal or component |
| Banging or knocking pipes | Air in the system, water hammer from rapid valve closure, network pump noise | Vent the system, check valve operation and network pressures |
| Whistling or singing noise | Excessive differential pressure across a PICV or partially blocked valve | Adjust or clean the PICV, verify differential pressure settings |
| Pressure dropping repeatedly | Leak on the secondary circuit, failed expansion vessel, faulty pressure relief valve | Trace the leak, test expansion vessel charge, replace PRV if failed |
| Fluctuating temperatures | Drifting or failed temperature sensor, sticking control valve, unstable network supply | Recalibrate or replace sensor, service the control valve |
The Faults in Detail
1. Blocked strainers and filters
Every HIU has a strainer on the primary side, and usually one on the secondary side too, designed to catch debris before it reaches the plate heat exchanger. In newer buildings these clog with construction debris in the first years after handover. In older buildings they fill with corrosion products and sludge from the wider system.
A blocked strainer restricts flow, and restricted flow means less heat transfer. Because it happens gradually, residents often do not notice until the hot water is noticeably weak. Cleaning the strainer is a routine part of an annual service and is one of the simplest faults to prevent entirely.
2. Scaled or fouled plate heat exchangers
The plate heat exchanger is the core of the HIU — the component that actually transfers heat from the network to your water. London water is hard in most areas, and limescale gradually coats the plates, insulating them and reducing heat transfer.
The classic symptom is hot water that has slowly become lukewarm over months or years. Depending on severity, this is resolved by chemically descaling the exchanger or replacing it. Regular servicing catches scaling early, when a descale is still sufficient.
3. Failed valves and actuators
HIUs rely on motorised valves and actuators to direct flow between the heating and domestic hot water circuits. Actuators fail, valve stems seize, and pressure-independent control valves (PICVs) can block or drift out of adjustment.
When the domestic hot water valve or its actuator fails, you typically get heating but no hot water, or hot water that never reaches temperature. These are individually replaceable components — a failed actuator does not mean a failed HIU, despite what residents are sometimes told.
4. Temperature sensor drift and PCB faults
Modern HIUs use temperature sensors feeding a control board that modulates the valves. Sensors drift out of calibration over time and will eventually report temperatures that are simply wrong, causing the unit to behave erratically — fluctuating temperatures, short cycling, or shutting down as a protective measure.
Control board faults are less common but do occur, particularly in units that have suffered water ingress from a leak elsewhere in the unit.
5. Leaks
Leaks come from failed seals and gaskets, corroded fittings, valve glands, or occasionally a cracked plate heat exchanger. A leaking HIU should be dealt with urgently — not only because of the water damage risk to your own flat and those below, but because a slow leak on the secondary circuit causes repeated pressure loss.
6. Noise
Whistling or singing generally indicates excessive differential pressure across a valve — often a PICV that needs adjusting, or one that is partially blocked. Banging and knocking is more commonly air in the system or water hammer caused by a valve closing too quickly.
Noise is easy to dismiss as a quirk of the building, but it usually means the system is not operating at its designed pressures, which shortens component life.
7. Network-side problems that are not your HIU
Not every fault is inside the flat. If the communal network is running at the wrong flow temperature, or differential pressure across the building is too low, every HIU on the riser will underperform. The quickest diagnostic a resident can do is ask neighbours whether they have the same problem. If they do, it is a building issue for the managing agent or heat network operator, not an individual HIU repair.
Most of These Are Preventable
Looking at the list above, the majority of faults fall into two groups: things that build up gradually (scale, sludge, debris) and things that wear out (seals, actuators, sensors). Both are exactly what an annual HIU service is designed to catch.
An unserviced HIU does not usually fail suddenly. It degrades — slowly enough that residents adjust to weaker showers and longer waits without registering that anything is wrong, until the unit stops entirely on the coldest week of the year.
Frequently Asked Questions
What is the most common HIU fault?
Why has my HIU stopped producing hot water completely?
Why is my HIU hot water lukewarm but not hot?
Why is my HIU making banging or whistling noises?
Can a faulty HIU increase my heating bills?
Is it worth repairing an HIU or should it be replaced?
Recognise Any of These?
We diagnose and repair HIU faults across all 26 London boroughs. £120 call-out covers attendance and full diagnosis.