How Does a HIU Work?

How a HIU Works

An HIU works by taking heat from a shared heating system and using it to provide hot water and, in many homes, room heating. HIU stands for Heat Interface Unit. It connects your home to a communal or district heat network.

The unit does not burn gas to make heat inside your flat. Instead, it controls how heat from the network is used. When you turn on a hot tap or ask for heating, the HIU responds to that demand.

How Does an HIU Provide Hot Water?

Many HIUs heat water when you need it. This is known as instant hot water production.

Here is what happens when you turn on a hot tap:

1. Cold mains water starts to flow. The HIU detects the demand for hot water.
2. A valve controls the network water flow. Hot water from the shared system passes through one side of a heat exchanger.
3. Heat passes to the cold water. The mains water flows through separate channels on the other side.
4. Warm water reaches your tap. The HIU controls the heat transfer to help maintain the set temperature.

The two water supplies do not mix. Only heat passes between them.

When you close the tap, the demand ends and the controls adjust the flow. Some models also have a keep-warm function to reduce the wait for hot water.

What Does the Heat Exchanger Do?

A heat exchanger lets heat pass from one water supply to another without mixing them.

Inside a plate heat exchanger are thin metal plates. Hot network water flows on one side of the plates. Cooler water flows on the other side and picks up heat through the metal.

For hot taps and showers, this keeps the mains water separate from the water in the heat network. In an indirect HIU, another heat exchanger does the same job for your room heating.

How Does an HIU Heat Your Home?

When your thermostat calls for heat, the HIU controls the heat supplied to your radiators or underfloor heating.

There are two main arrangements:

Indirect Heating

An indirect HIU uses a heat exchanger to warm the water in your home’s heating circuit. This water stays separate from the network water.

A pump circulates the heated water through your radiators or underfloor pipes. It then returns to the HIU to be warmed again.

Direct Heating

A direct HIU allows water from the shared network to flow through your home’s heating circuit. Valves control the supply, but there is no separate heat exchanger for room heating.

Both types can supply hot taps through a separate heat exchanger. The words “direct” and “indirect” describe how the room heating connects to the network.

How Is Your Heat Use Measured?

Many homes on a heat network have a heat meter fitted within or near the HIU.

The meter uses the water flow and the difference between the supply and return temperatures to calculate how much heat your home uses. This is usually recorded in kilowatt-hours, or kWh.

Your heat supplier can explain how these readings relate to your bill and any fixed charges.

Why Does an HIU Need Servicing?

An HIU relies on clean filters, working valves and correct settings. Blocked filters, worn parts or poor settings can affect its performance.

You may notice:

• Hot water that changes temperature.
• Radiators that do not warm up properly.
• Leaks around the unit.
• Unusual noises.
• Higher heat use than expected.

These signs do not always mean the HIU is at fault. The shared heat supply or another part of your home’s system may also need checking.

At Add Detail, we recommend an annual HIU service. Our engineers check for leaks, clean strainer filters, inspect controls and assess how the unit is working.

If you already have a problem, our HIU repair service in London can help identify the cause.

Common Questions About How HIUs Work

Is an HIU the Same as a Boiler?

No. A boiler produces heat, while an HIU uses heat supplied by a shared system. It does not burn gas inside your home.

Can an HIU Supply Underfloor Heating?

Yes. A suitable HIU can serve underfloor heating or radiators. Its design and settings must suit the heating system it supplies.

Does an HIU Need a Hot Water Cylinder?

Many HIUs heat mains water on demand, so they do not need a hot water cylinder. Other designs work with stored hot water, so it depends on your system.

Get Help With Your HIU

Understanding how your HIU works can make it easier to spot changes in its performance. If your heating or hot water is not working as expected, Add Detail can help.

Call 020 8822 4451 to discuss a service or repair in London.