If you live in a flat with a communal or district heating system, you may have a Heat Interface Unit (HIU) instead of a boiler.
An HIU is the box (often in a cupboard) that transfers heat from the building's central plant into your home's hot water and heating.
This guide covers the most common HIU problems, what you can safely check, and when it's time to call an engineer.
Quick safety note
- Don't remove covers or touch internal components.
- If you smell gas (rare with HIUs), follow gas safety advice immediately.
- If there's water leaking, protect electrics and turn off water where safe.
First: is it just your flat, or the whole building?
Before you assume the HIU has failed, check:
- Neighbours: are they also without heating/hot water?
- Building notices / concierge / managing agent: any known outage or maintenance?
If the whole building is affected, it's usually a communal plant/heat network issue, not your HIU.
Problem 1: No hot water at all
What you can check
- Other taps: does any tap run hot, even briefly?
- HIU display/indicators: is it powered on? Any fault lights or error codes?
- Isolation valves: if you've recently had work done, a valve may have been left closed.
- Water pressure (your heating circuit): some HIU setups still have a local heating pressure gauge. If it's very low, heating may lock out (hot water may still work depending on design).
Common causes
- Heat network not supplying heat (building-side issue)
- Faulty sensor/actuator inside the HIU
- Blocked strainer/filter (common after works or in older systems)
- Plate heat exchanger scaled or blocked (reduced transfer)
Problem 2: Hot water is warm, but not hot (or runs hot then goes cold)
What you can check
- Run one hot tap only for a minute (avoid mixing multiple outlets while testing)
- If you have a thermostatic shower, test a hot tap at the basin/kitchen too
Common causes
- HIU temperature control issue (sensor/valve)
- Heat network temperature/flow fluctuating
- Partially blocked strainer reducing flow through the HIU
- Limescale/scale in the plate heat exchanger (more likely in hard water areas)
Problem 3: Poor hot water flow (but it's hot)
What you can check
- Compare hot vs cold flow at the same tap
- Check if the issue is at one outlet (tap/shower cartridge) or throughout the flat
Common causes
- Blocked tap aerator or shower head (local issue)
- Partially blocked HIU strainer/filter
- Pressure reducing valve (PRV) issue (where fitted)
Problem 4: No heating, but hot water works
What you can check
- Room thermostat: is it calling for heat? Try turning it up
- Programmer/timer: is heating scheduled on?
- TRVs: are radiator valves open?
- Heating pressure (if your system has a gauge): low pressure can stop heating
Common causes
- Heating control/actuator fault inside the HIU
- Low pressure on the heating circuit (where applicable)
- Air in radiators / circulation issues on the flat side
Problem 5: Heating is on, but radiators are cold or patchy
What you can check
- Are all radiators affected, or just one?
- Do radiators feel cold at the top (often air) or cold at the bottom (often sludge/flow issues)?
Common causes
- Air in radiators (may need bleeding)
- System balancing issues
- Sludge/debris in the heating circuit (flat side)
Problem 6: HIU is noisy (buzzing, whistling, banging)
Common causes
- Control valve hunting (trying to regulate temperature/flow)
- Debris in a valve or strainer
- High differential pressure in the network (building-side)
If the noise is new or getting worse, it's worth getting it checked before it causes a bigger failure.
Problem 7: Leaking HIU or damp cupboard
What to do now
- If safe, place a container/towel to limit damage
- Avoid electrics in the cupboard
- Contact your managing agent/engineer promptly
Common causes
- Pressure relief discharge (where fitted)
- Failed seal/connection
- Corrosion or pinhole leak
When to call an engineer (or your managing agent)
Call for help if:
- You have no hot water and no heating and neighbours are fine
- Hot water temperature is unstable or unsafe
- There's leaking, repeated pressure loss, or visible corrosion
- You see a fault code you don't recognise
- The HIU has been noisy for more than a day or two
What to tell the engineer (saves time)
- Your HIU make/model (photo of the label is ideal)
- Any fault code or flashing lights
- Whether the issue affects hot water, heating, or both
- Whether neighbours are affected (possible building-side issue)
- How long it's been happening and anything that changed recently (works in the building, water shut-off, etc.)
Need HIU help in South London?
If you're in South London or North Surrey and you're having HIU issues, we can help diagnose what's happening and advise on the best next step.
If it's a building-side heat network issue, we'll tell you plainly and if it's within your flat's HIU, we'll help get you back up and running.