A heat interface unit (HIU) is neither a gas appliance nor an electric heater. It involves gas only indirectly, if the energy centre serving your building runs gas boilers, because the burning happens there and not in your flat. It uses electricity directly, because the unit needs a permanent 230 V supply for its controls, valves and pump. The heat itself arrives as hot water from the heat network.
HIU gas or electric: what each supply does in your flat
Neither supply makes the heat inside your flat on its own. Gas, where it is used at all, is burned in the building's energy centre, while the electricity at the HIU runs the controls and the pump.
| Criterion | Gas | Electricity |
|---|---|---|
| Connection at the unit | No gas connection is required at the HIU itself. | A permanent electrical supply is required, or the unit will not run at all. |
| Where it is used | Only in the central plant room or energy centre, if that plant is gas fired. | At the HIU on your wall, for the electronics, the motorised valves and the pump. |
| What it provides | The fuel that may heat the network water upstream of your home. | Power for the unit only. It does not generate the heat for your radiators or taps. |
| Combustion and flue | Nothing is burned inside the HIU, so the unit needs no flue. | No combustion either, so no flue gas checks at the unit. |
| Who may work on it | Gas fittings in England are work for a Gas Safe registered engineer under GSIUR 1998. | The HIU and its wiring are for competent persons, as the manufacturer's instructions require. |
| Metering | No gas meter is involved in what you pay for heat. | Separate from your heat charge, which comes from the heat meter at or in the HIU. |
An HIU contains no burner, no gas valve and no flue. Any gas in the chain is burned in the building's energy centre, and the electricity at the unit powers controls and a pump, not the heating itself.
What a heat interface unit is and where its heat comes from
A heat interface unit is a wall-mounted unit inside a flat that takes heat from a shared heat network. Inside it sit plate heat exchangers, stacks of thin plates that pass heat from the network water across to your own heating water and your hot water. The network water never mixes with the water in your radiators or taps. The unit also keeps the two systems hydraulically separate, so the pressure in the network stays out of your flat.
Some units serve heating only, others handle heating and hot water, and some are supplied together with an indirect cylinder. Baxi states that an HIU has an annual maintenance need for safe and efficient operation, with the interval set by the manufacturer and the network operator rather than by the resident. If the flat has underfloor heating running off the HIU, the floor circuits raise their own version of this question, and underfloor heating: electric, wet or gas explains which type you are dealing with.
When gas is involved with an HIU: in the energy centre, not your flat
Gas enters the picture only where the heat is generated, which is normally a plant room or energy centre serving the whole block or estate. If that plant runs gas boilers, the burners, the gas pipework and the flues are all there, behind a locked door. Work on those gas fittings in England falls under the Gas Safety (Installation and Use) Regulations 1998 as amended. Regulation 3 requires anyone carrying out such work to be competent, which in practice means Gas Safe registered.
Your HIU has no burner, no gas valve and no gas supply. The one case where gas is still inside the flat is a separate appliance such as a gas cooker or a gas fire. Those are gas appliances in their own right, and the gas rules follow that appliance, not the HIU on the wall.
When electricity matters: the HIU runs on a 230 V supply
Electricity is what the heat interface unit itself runs on. Baxi states that an HIU requires a direct electrical supply, because the controls, the motorised valves and the pump all need power. Worcester Bosch specifies 230 V AC at 50 Hz for its Greenstar heat interface unit, single phase rather than three phase, with the unit earthed and wired as a fixed appliance to the current IET Wiring Regulations.
That power does not make your heat. It moves water and opens valves so that heat already in the network can cross into your flat. If the supply to the unit is off, the HIU can sit dead while the network outside is still hot. The electrical connection is work for a competent electrician, not a householder task, and it is outside what a plumbing and heating firm takes on.
Do you need a gas safety certificate for a flat with an HIU?
In England, a let flat whose only heat source is an HIU normally needs no annual gas safety check, because there is no gas appliance in it to check. The duty under the Gas Safety (Installation and Use) Regulations 1998 as amended attaches to relevant gas appliances and flues in the property. HSE is clear that where none are present the annual check does not arise. Gas pipework left in the property still has to be kept in a safe condition, so the duty does not disappear entirely.
As soon as a gas cooker, hob or fire is in the flat, the annual check applies to that appliance and a record follows. The position for a property with pipework but no appliances is set out in gas safety certificate with no gas appliances. In a block with a communal plant room, the boilers there are a separate duty on whoever controls them, which is explained in gas safety in a leasehold flat.
No heating or hot water from your HIU: what you can check yourself
When a heat interface unit gives no heating or hot water, four things are safe for a householder to look at. Check the room thermostat and the programmer, including the clock and the hot water schedule. Read the pressure gauge on your own heating side and note what it shows. Press the reset button if the unit has one and the user instructions describe it, and bleed a radiator that is cold at the top.
Everything else belongs to someone else. The inside of the casing, the plate heat exchangers, the valves on the network side and the electrical supply are for a competent engineer or the network operator, as the manufacturer's instructions state. What a falling gauge usually means is covered in boiler low pressure: what the gauge is telling you. GOV.UK advises heat network customers to contact their heat supplier first, and to establish before moving in who to call, what the response arrangements are and what happens during an unplanned outage.
Can you replace an HIU with a gas boiler?
Swapping an HIU for a gas boiler is rarely a decision a resident can take alone. Government material on heat networks notes that customers on a network generally cannot switch heat supplier or install their own heat source, and that this is often written into the supply contract. A lease usually adds its own restrictions on altering building services. A new boiler would also need a flue route out through the building, which in a flat is often the hardest part.
Where such a change is permitted, the gas side remains regulated work. In England, installing a gas boiler is work in relation to gas fittings under the Gas Safety (Installation and Use) Regulations 1998 as amended, so it is for a Gas Safe registered engineer. The consent question in a leasehold flat is set out in freeholder permission to replace a boiler.
Gas or electric: which answer fits your flat with an HIU
Say gas, if you are asking where the heat ultimately comes from and your building's energy centre runs gas boilers. The burning happens there, under gas regulation, and never inside the unit on your wall. Say electric, if you are asking what the HIU itself needs to run, because it takes a 230 V supply for its controls, valves and pump and nothing more.
Say neither, if the question is really about safety paperwork. An HIU is not a gas appliance, so it does not appear on a gas safety record, and it has no flue to inspect. If a gas cooker, gas fire or gas boiler is also in the home, that appliance decides what applies, not the HIU. If your property has a gas boiler as well, or you are moving from a heat network back to one, our page on boiler service and repairs sets out what that work covers.
Sources
- Baxi – Does a Heat Interface Unit (HIU) use gas or electricity
- Baxi – What is a Heat Interface Unit?
- Baxi – What is a Hydraulic Interface Unit
- Worcester Bosch – Does the Heat Interface Unit require a flue to be fitted?
- Worcester Bosch – Installation and maintenance instructions, Heat Interface Unit
- Gas Safety (Installation and Use) Regulations 1998, regulation 3
- HSE – Gas safety checks: what needs them?
- GOV.UK – Heat Networks: advice for customers
- Ofgem – Heat networks regulation is now live
- The Heat Network (Metering and Billing) Regulations 2014