Why is my HIU banging when I run the hot tap?

A bang or thud when a tap is opened or closed is usually water hammer, caused by the sudden stopping of water in the pipework or by a valve closing too quickly. In an HIU it commonly points to a fast-acting or worn actuator, incorrect system pressure, or a missing or failed water hammer arrestor. It is not normally dangerous but repeated hammering can loosen joints over time and should be investigated.

Is a humming or buzzing HIU dangerous?

A low hum is often normal, as the HIU contains valves and actuators that move under power. A loud or constant buzz usually indicates an actuator being driven against a stuck valve, a failing circulating pump where one is fitted, or excessive flow velocity through a partly closed valve. It is not an immediate safety risk but it will usually get worse, so it should be looked at.

Why does my HIU whistle or hiss?

Whistling or hissing is typically water being forced through a restriction at high velocity, most often a partly blocked strainer, a scaled plate heat exchanger, or a differential pressure control valve that is not set correctly for the flat. It frequently appears alongside reduced hot water flow rate and is one of the clearest early signs that an HIU is overdue a service.

Should I turn my HIU off if it is making a noise?

In most cases no, and turning it off will leave you without heating and hot water. The exceptions are if you can see or hear water escaping, if the unit is visibly leaking, or if there is a burning smell or scorching around the controls. In those cases isolate the unit using the valves on the pipework if you can do so safely, and report it immediately.

Who do I report a noisy HIU to?

If you rent, report it to your landlord or housing provider. If you own a leasehold flat, check your lease: in many London blocks the HIU inside the flat is the leaseholder’s responsibility while the communal system is the freeholder’s, but this varies. Your managing agent can confirm which applies to your building.

Why Is My HIU Making a Banging or Humming Noise?

Match the Noise to the Cause

Start here. Identify the noise your unit is making, and the likely cause is usually one of a small number of things.

Banging or thudding

Not urgent, but worth fixing

When: When a hot tap is turned on or off, or when heating starts

Usual cause: Water hammer — a fast-closing or worn actuator, incorrect system pressure, or no hammer arrestor fitted

Whistling or hissing

Book a service — flow will keep dropping

When: While hot water is running, often with weak flow

Usual cause: Restricted flow — a blocked strainer, a scaled plate heat exchanger, or a mis-set differential pressure control valve

Humming or buzzing

Book a repair before it seizes

When: Constant, or whenever the unit calls for heat

Usual cause: An actuator driving against a stuck valve, a failing pump where fitted, or high velocity through a partly closed valve

Clicking or ticking

Usually normal unless rapid

When: Intermittent, usually as heating or hot water starts and stops

Usual cause: Normal actuator and valve movement in most cases. Rapid repeated clicking can mean a control cycling on a faulty sensor

Gurgling or trickling

Often resolved by bleeding radiators

When: After a refill, or following work on the building system

Usual cause: Air in the secondary circuit or in the radiators

Kettling — a rumbling boil

Needs descaling — efficiency is already lost

When: During hot water demand, building in volume

Usual cause: Heavy limescale on the plate heat exchanger causing localised overheating. Common in London hard water areas

What Counts as Normal

An HIU contains motorised valves, actuators and — in some designs — a small pump. A soft click as the unit starts and stops, and a faint hum while it is running, are both normal and nothing to worry about. Many residents only notice them because the unit sits in a hallway cupboard rather than a plant room.

What is not normal is a noise that is new, getting louder, audible from another room, or accompanied by any change in performance — weaker hot water flow, longer waits for hot water, or radiators that are not getting as warm as they used to.

Noises You Should Report Straight Away

Most HIU noises are a maintenance issue rather than an emergency. Report it urgently if you notice any of the following:

Any sound of running or dripping water when no tap is open — this may be a leak inside the cupboard
A burning or hot plastic smell, or scorch marks around the controls or wiring
Loud, sustained kettling — a heavy rolling boil sound during hot water demand
Violent banging strong enough to move the pipework or shake the cupboard

In those cases, if you can safely reach the isolation valves on the pipework into the unit, you can close them to stop flow — then report it immediately. Do not attempt to open the HIU casing.

The Two You Can Sometimes Fix Yourself

Gurgling with cool radiators. This is usually trapped air. Bleeding your radiators with a radiator key, starting with the one furthest from the HIU, will often clear it. If the gurgling returns within days, air is getting back in and that needs investigating.

Banging only when a specific tap closes. Closing that tap more slowly is a reasonable test. If the bang disappears, it is water hammer in the pipework rather than a fault inside the unit — still worth reporting, but less pressing.

Everything else on the list above needs someone with the unit open, a set of readings and the right parts on the van.

What an Engineer Will Check

On a noise callout we work from the sound backwards. In practice that means:

Measuring primary and secondary flow and return temperatures, and the differential across the unit
Removing and inspecting the strainer, which is the most common single cause of restriction noise
Checking the plate heat exchanger for scale, and testing the hot water flow rate at the outlet
Testing actuator travel and listening for a motor driving against a seized valve
Reading the control board fault history, which often records what has been happening for months
Checking system pressure and the expansion vessel pre-charge

Most noise faults turn out to be a dirty strainer, a scaled heat exchanger or a failing actuator — all of which are routine parts of an annual HIU service. A noisy unit is very often simply an unserviced one.

Why London Units Scale Up Faster

Most of London sits in a hard water area. Limescale forms on the hot side of the plate heat exchanger, narrowing the channels water passes through. That produces the whistling and kettling noises in the table above, and it costs you twice: the unit transfers heat less efficiently, so it draws more energy to deliver the same hot water, which shows up on the heat charges for your flat.

Descaling at the annual service keeps it in check. Left for several years, the exchanger may need replacing instead.

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