Many newer blocks of flats in London and Essex don’t have individual boilers. Instead, heat comes from a central plant room through a communal or district heat network, and each flat has a heat interface unit (HIU), usually in a cupboard. When the heating or hot water stops working, the HIU is the first place to look.
What an HIU does
An HIU is the link between the building’s heat network and your flat. Inside the unit (like the one above, with its cover removed) you’ll typically find:
- plate heat exchangers, which transfer heat from the network to your radiators or underfloor heating and to your hot water, without the two water supplies mixing
- control valves and actuators, which open and close to deliver heat when you need it
- a pump to circulate water around the flat’s heating system
- strainers (filters), which catch debris before it reaches the heat exchangers
- a heat meter, which measures the heat your flat uses for billing
- a pressure gauge, safety valves and isolation valves
Common HIU faults
No heating, but hot water works: often a faulty heating control valve or actuator, a failed pump, low pressure on the flat’s heating circuit, or a room thermostat or programmer problem.
No hot water, or lukewarm hot water: commonly a blocked strainer, a failed hot water control valve, or a scaled-up hot water heat exchanger. Limescale is a big problem in hard-water areas.
Hot water temperature swings or takes a long time to arrive: control valve or sensor problems, or a partly blocked strainer.
Low pressure on the gauge: a leak somewhere on the flat’s heating system (radiators, valves, pipework), or a pressure that needs topping up.
Leaks from the unit: failed valve seals or fittings, which need attention before they damage the cupboard or the flat below.
What to check first
- Is it just your flat? If neighbours have no heating or hot water, the problem is likely the building’s plant room. Contact your managing agent or heat network operator.
- Is your account in credit? Many heat networks use prepayment, and heat may be cut off if the balance runs out.
- Check the pressure gauge on the HIU against the level in the unit’s instructions.
- Check thermostats and timers are set correctly, and that batteries in wireless controls aren’t flat.
A recent example
On one recent job, a flat’s underfloor heating wasn’t warming up. The primary strainer in the HIU was blocked, and the plate heat exchanger and circulation pump had failed. Charlie cleared the strainer and replaced the plate heat exchanger and circulation pump, which restored the heat. Read the full story in our underfloor heating investigation.
Who is responsible for HIU repairs?
This varies. In many buildings, the heat network operator or managing agent maintains the HIUs as part of the service charge. In others, the HIU belongs to the flat and repairs are the owner’s or landlord’s responsibility. Check your lease or heat supply agreement before booking a repair, so you don’t pay for something that’s covered.
For landlords and letting agents, it’s worth knowing which applies in each block before a tenant reports a problem.
How we repair HIUs
Our plumbing and heating team works on HIUs and flat heating systems, including:
- cleaning strainers and restoring flow
- replacing control valves, actuators and sensors
- replacing pumps
- descaling or replacing hot water plate heat exchangers
- fixing leaks and repressurising the heating circuit
- balancing radiators on the flat’s heating system
Where a fault is on the building side, or the unit is maintained under a contract, we’ll tell you who you need to contact.
Book an HIU repair
We look after flats for owners, landlords and letting agents across London and Essex. See our plumbing and heating service and landlords and agents page.
We cover London and Essex, with our own engineers working every day across Chigwell, Loughton, Buckhurst Hill, Woodford, Ilford, Romford, Brentwood and Epping. Call 020 8227 0035 or request a quote.