HIU Not Heating Water? Common Causes and Fixes

First, is it your flat or the whole building?

A HIU does not make heat. It takes hot water from a shared pipe running through the building and uses it to warm your radiators and taps. That changes how you diagnose a fault, because the problem can sit in three places: the network feeding the building, the unit on your wall, or the pipework and controls inside your flat.

So the first question is not "what is wrong with my HIU" — it is "is anyone else affected?" Ask a neighbour, or check the building's residents group. If two or three flats have gone cold at the same time, the fault is on the network side, and it is the heat supplier or managing agent who needs to sort it. Ring the number on your heat billing statement before you pay anybody to attend.

If every other flat has hot water and yours does not, the fault is inside your four walls.

Check your heat account has credit on it

This catches people out constantly and costs nothing to check. Many heat networks bill through a prepayment or credit meter next to the HIU, or in the riser cupboard on the landing. If the account runs out, the supply stops and the unit heats nothing — no fault code, no warning, just cold water at every tap. Check the meter display or your heat supplier's app before assuming anything has broken. We have attended flats where the entire fix was topping the account up.

Note the fault code, then be precise about what has stopped

Most HIUs have a small display or a set of indicator lights. Write down whatever it shows before you reset anything. Then work out exactly what has failed, because it narrows the job down enormously.

• No hot water, but the radiators still warm up. The network is feeding your flat perfectly well, so the fault is on the hot water side — usually the flow sensor, the hot water plate heat exchanger, or the valve that feeds it.
• Hot water fine, radiators cold. Check the room thermostat and programmer first, then the heating side of the unit. On an indirect HIU, low pressure in your flat's own heating circuit locks the heating out while hot water carries on as normal.
• Neither one works. Either the supply into the flat has stopped, the credit has run out, the unit has lost its electrical supply, or a primary strainer is choked.
• Warm but never properly hot. Almost always a restriction rather than a failure — and the most common call we get.

The lukewarm shower: the classic HIU complaint

Water that runs warm but never hot, or goes hot for half a minute and then fades, is usually a flow problem rather than a temperature problem. Somewhere between the riser and your tap, the water is not moving fast enough to pick up the heat it should.

The usual culprits are the strainers. Every HIU has at least one fine mesh filter, and most have one on the primary side and another on the secondary. They catch the debris that circulates in any large heating system — flakes off the network, jointing compound, swarf left at installation. They do their job, they gradually fill, and flow drops away. Cleaning them is a short task for an engineer with the right isolation valves and a bucket, and it fixes a large share of the weak hot water calls we attend.

The next suspect is the plate heat exchanger, the slim stainless block that passes heat from network water to your tap water without the two ever mixing. It fouls over the years and its output falls off. Birmingham's water comes down from the Elan Valley and is soft, so we see far less limescale in HIU plates here than in the hard-water south; what we do see is sludge carried over from the communal side. A fouled plate can often be chemically cleaned rather than replaced, which is a large part of what the Premium tier of our HIU service and repair work covers.

What else actually fails inside a HIU

The flow sensor

A HIU has no way of knowing you have turned a tap on unless something tells it, and that job falls to a small flow sensor in the cold inlet. When it fails you get the frustrating situation where the unit is otherwise healthy but never wakes up. The giveaway is cold water at every hot tap and complete silence from the unit when you open one.

The control valve and its actuator

Inside the case is a motorised valve metering hot network water through the heat exchanger. If the actuator fails or the valve seizes part way, the unit delivers nothing, delivers scalding water, or hunts up and down while you are stood in the shower. A valve stuck part open also shows up as a unit that feels warm all the time and a heat bill that has crept up for no obvious reason.

Temperature sensors

The unit modulates on readings from a couple of small sensors. When one drifts, the HIU is making decisions on bad information and the outlet temperature wanders. Cheap part, quick job — but it takes somebody who can interrogate the unit properly rather than guess.

Low pressure on an indirect unit

If your HIU keeps your flat's heating circuit separate from the network, that circuit has its own pressure, expansion vessel and filling arrangement, exactly like a sealed boiler system. Below roughly 1 bar the heating stops. Pressure that keeps falling means water is leaving somewhere and needs finding, not topping up week after week — repeated topping up drives fresh oxygen in and quietly corrodes the system.

The differential pressure control valve

Blocks are designed so every flat gets a sensible pressure across its unit no matter what the neighbours are drawing, and a differential pressure control valve does that balancing. When it fails your HIU behaves unpredictably — and it is often blamed on the network when it is actually sat in your cupboard.

What you can safely check yourself

• Whether neighbours are affected too.
• Whether the heat account still has credit.
• Any fault code — write it down, then try one reset. One. If it faults again straight away, stop.
• That the room thermostat has working batteries and is calling for heat, and that the programmer is in a heating period.
• If there is a pressure gauge reading below 1 bar and you have been shown how to repressurise your unit, do it once and see whether it holds.
• Underneath the unit, for water. Any leak at all — close the isolation valves and call somebody.

What we would ask you not to do is start opening strainers or cracking unions. The primary side carries water from the building's network at a temperature and pressure well beyond anything in a domestic sealed system.

What happens when we come out

An emergency HIU repair is £225, covering the callout plus up to three hours of labour to find and fix the fault, which handles the large majority of repairs in a single visit. If a part is needed we price it and tell you before we fit it, so nothing appears on an invoice you did not agree to. If a job is going to run past three hours, you hear that before we carry on rather than afterwards.

It helps if you send a photo of the unit and its data label when you book — WhatsApp is easiest. Knowing the make and model means we turn up with the right gaskets, sensors and actuator instead of making a second trip for a small part.

Stopping it happening again

Nearly every HIU breakdown we attend in January was showing warning signs the previous autumn. Strainers do not block overnight, plates do not foul in a week, and actuators get slow and noisy long before they give up. An annual service catches all three while they are still cheap to deal with, and on most units it is a condition of keeping the manufacturer's warranty alive. The numbers are set out plainly in our guide to what a HIU service should cost.

And if you are not sure what is on your wall in the first place, start with our plain-English explainer on what a heat interface unit actually is.

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