Heat Interface Units (HIU) in London: Communal Heating Faults, Who Pays, and Your New 2026 Ofgem Rights

There is a call we take more of every winter, and it almost always starts the same way: "I've got no hot water, but I haven't got a boiler." The caller is in a flat built in the last fifteen years — Stratford, the Royal Docks, City Road, Elephant and Castle, Greenwich Peninsula, Wembley — and somewhere in a hallway cupboard there is a white steel cabinet with a couple of pipes, a small display and no flue. They have been ringing round for a Gas Safe engineer and nobody wants the job.

That cabinet is a Heat Interface Unit, and the reason nobody wants the job is that half the trade still treats it as somebody else's problem. It is not a boiler. It needs no gas ticket. And the fault that has left you cold is very often not in your flat at all.

What an HIU Actually Is

Start with what it is not. A combi boiler is a heat generator: it burns gas inside your home, and that is why it needs a flue, a gas supply and an annual safety check by a Gas Safe registered engineer.

An HIU generates nothing. On a communal or district heat network, the heat is made somewhere else — a central energy centre with large boilers, CHP engines, heat pumps or an energy-from-waste connection — and pumped around the building or the district as hot water. That circuit is the primary side. Your flat's radiators and hot taps are the secondary side. The HIU is the box where the two meet: a set of stainless steel plate heat exchangers that pass heat from the primary water into your secondary water without the two ever mixing, plus the valves, sensors and controls that decide how much heat crosses and when.

There are two broad types. An indirect HIU uses a plate heat exchanger for the heating circuit as well as one for hot water, keeping your flat hydraulically separate from the network. A direct HIU sends network water straight into your radiators and uses a plate exchanger only for the hot water. Indirect is more common in newer London blocks because it isolates each flat from the network's pressure and water quality.

The consequences of that design are worth stating plainly, because they are what people get wrong:

• No gas in your flat. No combustion, no flue, no carbon monoxide risk from the unit, and no CP12 needed for it.
• No Gas Safe requirement. A Gas Safe ticket is not the qualification the job calls for. HIU-specific competence is.
• You cannot "top up the pressure". There is no filling loop in the way a combi has one. If an indirect HIU's secondary side has lost pressure, something has leaked and needs finding.
• You are a captive customer. You cannot switch heat supplier the way you can switch your electricity. That single fact is why the sector needed a regulator, and why it now has one.

Why This Matters So Much in London

Heat networks are not a niche. Government figures covering January 2019 to December 2022 recorded over 10,000 registered heat networks in England, and 3,503 of them are in London — more than any other English region. About three quarters of London's registered networks are communal schemes serving a single building rather than district networks spanning a neighbourhood. Between them they serve more than 15,000 buildings in the capital.

The customer number is the striking one: roughly half of all GB domestic heat network consumers — over 240,000 of them — are in London. If you live in a new-build block in Zone 1 or 2, the odds that your heat arrives through an HIU rather than a boiler are high, and rising, because London Plan policy has pushed connection to heat networks for large developments for years.

So this is not an obscure appliance. It is how a substantial and growing share of Londoners get their hot water — and it is concentrated in exactly the places our engineers work: the Olympic-legacy blocks in Stratford and East Village and the Royal Docks towers in Newham, and the City Road, Old Street and canal-basin developments in Islington.

The First Check: Is It You or the Block?

This is the most valuable thing in this article, and it costs nothing.

Before you call anybody out, find out whether your neighbours have heat. Knock on a door, or put it in the building's WhatsApp group. The answer splits the problem in two, and the two halves have completely different owners:

• The whole block is cold. The fault is on the network — the energy centre, the primary pumps, a plant failure, or a shutdown for works. This is the operator's responsibility. It is not your HIU, and you should not be paying an engineer to come and tell you that. Phone the heat supplier's emergency number, which since 27 January 2026 must be printed on your bill.
• Only your flat is cold. Now it is your HIU or your secondary side, and an engineer visit makes sense.

We say this because we get called to flats where the answer was upstairs all along, and the resident pays a call-out to be told the energy centre is down. Two minutes of asking saves that.

The Faults That Cause No Hot Water

Assuming it is your unit, the fault set is small and repeats. In rough order of how often we find them:

1. A blocked strainer or filter

Every HIU has strainers on the primary inlet — a fine mesh catching debris circulating in the network. Networks are big, old-ish, and full of magnetite and construction debris; the strainers do their job and clog. Symptom: heat output falls off gradually, or dies. This is the cheapest fix in the list and the most commonly missed, and it is frequently the entire problem.

2. A limescaled plate heat exchanger

The London classic. The hot water plate heat exchanger has narrow passages, and it is heating hard mains water with a high-temperature primary. Calcium carbonate drops out and bonds to the plates, insulating them and narrowing the channels. Symptom is distinctive and gradual: hot water that is only ever warm, or that runs hot for a minute then fades, and a longer and longer wait at the tap. A chemical descale can recover it; a badly scaled exchanger gets replaced. The permanent answer is a scale reducer on the incoming main.

3. A stuck or failed actuator

Actuators are the small motorised heads that drive the control valves — they decide how much primary flow crosses the exchanger when you open a tap or the thermostat calls. They seize, they burn out, and they fail in whatever position they were in. If they fail closed you get nothing; if they fail open you can get scalding water and a network efficiency complaint. Because hot water and heating usually have separate actuators, a single failure explains the very common "one works, the other doesn't".

4. A failed sensor or flow switch

The unit only fires the hot water path when it detects you have opened a tap, via a flow sensor or differential pressure switch, and it modulates on temperature sensors (thermistors). A dead sensor means the HIU never learns it should be doing anything. Symptom: nothing happens at all when you open the tap, but the unit otherwise looks alive.

5. Differential pressure control valve (DPCV) problems

A DPCV keeps the pressure across your unit stable regardless of what the rest of the block is doing at 7am. When it fails you get erratic performance that changes with the time of day — fine at midnight, useless during the morning peak.

6. Leaks and "network bypass"

Leaks show up on unions, on the plate exchanger gasket, or on the valves. The other issue is subtler and mostly the operator's concern rather than yours: a poorly performing HIU returns primary water to the energy centre too hot — network bypass — which wrecks the efficiency of the whole scheme and pushes up everybody's standing charge. It is one of the reasons a badly maintained HIU is not just your problem.

Who Is Responsible — and Who Pays

This is where heat networks get genuinely messy, and where residents lose money they did not need to spend. There is no single answer, because it turns on your lease and your heat supply agreement. But the shape is usually this:

• The energy centre, the primary network and the risers — the operator's or freeholder's, funded through the standing charge or service charge. Never yours.
• The HIU itself — it depends. On many schemes the HIU sits inside your demise and is therefore the leaseholder's to maintain. On many others — increasingly, the newer ones — the operator retains ownership of the HIU and maintains and replaces it as part of the heat supply agreement you are already paying for. Check before you pay.
• Everything after the HIU — your radiators, your thermostat, your taps and your pipework. Yours.

So the practical sequence is: establish whether the block is affected; if it is just you, read the heat supply agreement and the lease to see who owns the HIU; and only then book an engineer. We would rather tell you that on the phone than take a call-out fee for a unit somebody else was contractually obliged to fix.

Your New Rights: Ofgem, 27 January 2026

For years the honest complaint about heat networks was that customers had all the obligations of a utility and none of the protections. You could not switch supplier, you could be billed on estimates indefinitely, and when it went wrong there was no ombudsman to escalate to. Gas and electricity customers had Ofgem; heat customers had, in most cases, nothing but their lease and the voluntary Heat Trust scheme.

That changed. The Energy Act 2023 designated Ofgem as the statutory heat networks regulator, and on 27 January 2026 the first package of consumer protection rules took effect. The intent, in Ofgem's own framing, is to give heat network consumers protection comparable to gas and electricity customers. What is live now:

• Standards of Conduct. Suppliers and operators must treat you fairly in every interaction — act honestly and professionally, give complete and intelligible information, keep customer service reachable and effective, and cooperate with each other where responsibility is shared across parties (which on heat networks it very often is).
• Accurate billing. Bills must be accurate and, on metered networks, based on an actual reading at least once a year. Each bill must show your current charges and consumption, the terms, how to contact the supplier and the emergency service, how to complain and reach redress, and where to get energy-saving and consumer advice.
• Complaints and redress. There must be a clear complaints process, you must be told about it at least annually, and the supplier is normally your single point of contact even where an operator or contractor is really at fault. Once the internal process is exhausted, unresolved complaints can be escalated to an approved redress body — Ofgem signposts the Energy Ombudsman.
• Vulnerable customers. Suppliers must run a Priority Services Register, proactively identify vulnerable households and provide free support, with specific constraints on disconnection for non-payment — particularly in winter and where there are very young children, older residents or people with medical needs.

Two caveats worth knowing. First, this is phased: some authorisation conditions started on 27 January 2026, operators and suppliers must go through Ofgem's registration process over the following year, and the majority of the regime is expected to be in place and enforced by January 2027. Second, none of it makes anybody fix your HIU faster on a Sunday night — but it does mean that a supplier who bills you on estimates for three years, or ignores your complaints, is now answerable to somebody.

If your bill does not carry an emergency contact number and a complaints route, that is now a compliance failure, not just bad service. Say so.

If You Rent: the Landlord's Duty

Nothing about a heat network dilutes your landlord's repairing obligation. Section 11 of the Landlord and Tenant Act 1985 puts the duty to keep installations for space heating and heating water in repair and proper working order on the landlord, and the Homes (Fitness for Human Habitation) Act 2018 reinforces that the property must be fit to live in throughout the tenancy. An HIU is the installation for heating water; a landlord cannot point at the freeholder and call it your problem.

The practical difference from a boiler is only this: because there is no gas appliance in the flat, there is no annual CP12 for the HIU itself. If the flat has no other gas appliance — and most heat-network flats have none at all — there is no gas safety certificate to issue. That surprises landlords, and it occasionally means nobody has looked at the unit in a decade, because the annual gas check was the only thing that ever prompted a visit.

What You Can Safely Check Yourself

Before booking anyone, in this order:

1. Ask a neighbour. Whole block cold? Stop. Call the heat supplier. It is theirs.
2. Read the display. Most HIUs show a fault or error code. Photograph it. It saves diagnostic time and tells us what to bring.
3. Check the obvious controls. Room thermostat calling? Batteries alive? Programmer on the right schedule? A dead thermostat battery presents as a dead heating system.
4. Check your electricity supply. The HIU needs power for its controls and its actuators. Check the fused spur or breaker serving the cupboard — an HIU with no power looks exactly like a broken HIU.
5. Check for prepayment credit. A good number of London schemes are prepay. If the credit runs out, the heat stops, and the unit is faultless.
6. Look for water. Any wet in the cupboard, isolate if you safely can and call — a leak inside an HIU cupboard in a block is somebody's ceiling downstairs.

What not to do: do not attempt to strip the plate heat exchanger or the strainers yourself. The primary side runs hot and pressurised, and opening the wrong union in a hallway cupboard produces a scald and a flood in the same second.

HIU Costs in London (2026)

Job	2026 London cost	Notes
Fixed call-out (weekday daytime)	£60	Covers travel + first 45 minutes — often enough to diagnose and clear a strainer
Fixed call-out (evenings, weekends, bank holidays)	£90	Same 45-minute window
Strainer clean + filter change	£90–£160	The most common fix, and the most commonly missed
Chemical descale of DHW plate heat exchanger	£180–£320	Recovers a scaled unit where the plates are not too far gone
Plate heat exchanger replacement	£250–£500	Where descaling will not recover it; part cost varies by model
Actuator / thermistor / flow sensor replacement	£150–£350	Depends on part availability for the make
DPCV replacement	£200–£400	For pressure-related faults that track the block's peak demand
Full like-for-like HIU replacement	£1,200–£2,500	Make, model and cupboard access drive the range
Annual HIU service	£90–£150	Strainers, scale check, actuator function, return-temperature check

Figures exclude VAT. And the point we keep making because it keeps saving people money: check the heat supply agreement first. On a meaningful share of London schemes, the operator already maintains and replaces the HIU under the standing charge you pay every month, and the correct call is to them, not to us.

Where the HIU genuinely is yours, the annual service is worth it in a way it is not always worth it on a modern combi — because the two things that kill London HIUs, debris in the strainers and scale on the plates, are both cumulative, both invisible until they bite, and both cheap to deal with early. A £120 service that catches a furring plate exchanger is the difference between a descale and a £400 part.

If your flat is on a heat network and you have lost hot water or heating, call us on 0207 046 1363. We will ask you first whether your neighbours are affected — and if the answer is yes, we will tell you to ring your heat supplier instead of sending you a bill.