HIU vs Combi Boiler: What's the Difference?

The difference in one paragraph

A combi boiler makes heat. It burns gas in your home, and everything else — the flue, the gas supply, the annual safety check, the carbon monoxide risk — follows from that. A heat interface unit makes no heat at all. It takes hot water from a shared network running through the building and transfers that heat into your radiators and your taps. From the sofa the two look almost identical: a white box in a cupboard giving you heating and hot water on demand. Underneath, they are completely different animals.

Where the heat comes from

With a combi, the heat is produced in your flat. Gas comes in through your own meter, burns in the boiler, and the products of combustion go out through a flue in the wall. You buy gas from a supplier you chose and you can switch supplier whenever you like.

With a HIU, the heat is produced somewhere else in the development, in an energy centre — large boilers, a combined heat and power engine, or increasingly a heat pump. Hot water is pumped from there around a loop passing every flat. Your HIU is the handover point. You buy heat by the kilowatt hour from whoever operates that network, and you cannot switch to a different supplier, because there is only one pipe.

What is inside each one

A combi contains a burner, a fan, a flue, a gas valve, ignition electrodes, a heat exchanger fired directly by the flame, a pump, an expansion vessel and a set of safety controls whose whole purpose is to make burning gas indoors safe.

A HIU contains none of that. Instead there are plate heat exchangers, a motorised control valve, a flow sensor, temperature sensors, strainers, a differential pressure control valve and a heat meter. Fewer moving parts, no combustion, and a much simpler safety picture — but a set of failure modes of its own, mostly to do with flow being restricted rather than with anything failing to light.

Hot water performance

This is where HIUs tend to win, and it surprises people. A combi is limited by how much gas it can burn, which is why flow rate is the headline figure people argue about when buying one. A HIU is limited by the network instead, and units are commonly specified at hot water outputs well above what a domestic combi manages. In practice that often means a stronger shower and less of a drop when somebody runs a tap elsewhere in the flat.

When a HIU underperforms, it is almost never a design limit — it is a blocked strainer or a fouled plate heat exchanger choking the flow. If your shower has faded over a couple of years, that is a maintenance problem with a straightforward fix, not something you have to live with. We have written up the diagnosis in HIU not heating water.

Bills, and why the comparison is not straightforward

A combi's running cost is easy to reason about: gas price times how much you burn, and you can shop around for a better rate.

A heat network bill is usually a unit rate per kWh plus a daily standing charge covering the energy centre and the shared pipework. Those pipes lose heat continuously whether anyone is drawing any or not, and the losses are shared across the connected flats. It is why a small flat on a heat network can show a heating cost that looks nothing like a similar flat with its own combi — and it is a legitimate thing to query if it looks out of proportion.

Heat networks are covered by the Heat Network (Metering and Billing) Regulations, and under the Energy Act 2023 Ofgem has been appointed as their regulator, bringing consumer protections closer to those gas and electricity customers already have. If your bill looks wrong, take it up with the heat supplier named on the statement, then the managing agent.

Safety and legal duties

A combi burns gas, so it carries a carbon monoxide risk, needs a flue that is intact and correctly terminated, and must be worked on by a Gas Safe registered engineer. If you let the property out, an annual gas safety check on it is a legal duty and you need a certificate.

A HIU burns nothing. No combustion, no flue, no carbon monoxide from the unit, and no Gas Safe requirement arising from the HIU itself.

Landlords should read that carefully, because there is a trap in it. The gas safety duty attaches to gas appliances, not to boilers specifically. A flat with a HIU and a gas hob still has a gas appliance, and still needs a landlord gas safety certificate covering it. Only a flat with no gas supply and no gas appliances at all falls outside the requirement. Check before assuming you are exempt.

Servicing

Both need looking at once a year, for different reasons. A combi service is partly a safety check — combustion analysis, flue integrity, seals — and partly maintenance. A HIU service is purely maintenance and performance: strainers cleaned, valves exercised, temperatures and pressures recorded, heat exchanger cleaned where needed.

Neither one is optional in practice. Manufacturers require documented annual servicing to keep a warranty valid on both, and with a HIU there is often a lease or heat supply agreement saying the same thing. Our HIU servicing starts at £95 for Essential, £115 for Complete and £170 for Premium, and an emergency HIU repair is £225 including up to three hours of labour, with parts quoted before fitting. The costs and what sits behind each tier are broken down in our guide to HIU service costs.

Space, noise and siting

A HIU is generally quieter, because there is no fan and no ignition. It is often physically smaller, and it does not need a flue route to an outside wall, which is precisely why developers use them in blocks where running a flue from every flat would be impossible.

A combi needs an outside wall within reach for the flue, a condensate drain, a gas supply and clearances around it for servicing. In a flat several rooms deep into a building, all of that is a genuine constraint.

Can I take the HIU out and fit a combi instead?

Almost always, no — and it is worth being blunt about why, because we get asked regularly by people frustrated with their heat bills.

• There is usually no gas supply into the flat, and getting one run into an apartment block needs freeholder consent and is often simply not available.
• There is frequently no viable flue route from where the unit sits to an outside wall.
• Your lease will almost certainly prohibit disconnecting from the communal system, and the heat supply agreement may commit the property to the network regardless.
• Even if you disconnected, the network standing charge often continues to apply to the property.

If the bills are the real problem, the productive route is checking that your unit is working efficiently, that the heat meter readings match your usage, and that the rates match your agreement. A HIU with a valve stuck part open draws heat around the clock and shows up as a bill nobody can explain.

So which is better?

It is not really a choice most people get to make, which is the honest answer. If you live in a flat on a heat network you have a HIU; if you live in a house you almost certainly have a boiler. What matters is knowing what you have got and what it needs.

What we would say is this: a well-maintained HIU is a very good bit of kit. Quiet, compact, strong hot water, nothing burning in your home, and a long service life when it is looked after. The problems we attend are nearly always neglect rather than design — units nobody has serviced since the building went up, strainers full of debris, plates that have not been cleaned in a decade. Treat it the way you would treat a boiler and it will treat you well.

If you are still not certain what is on your wall, our plain-English guide to what a heat interface unit is runs through how to tell.

Written by the engineering team at Synergy Gas & Plumbing, Gas Safe registered 594385. Last reviewed August 2026.