Why is my hot water working but my radiators are cold?

An HIU handles heating and hot water through separate circuits inside the same unit. Working hot water proves the building is delivering hot primary water to your flat and that the hot water side of the unit is functioning. A cold radiator circuit therefore points to a fault on the heating side specifically: the heating zone valve or actuator, the room thermostat or programmer, trapped air, a closed valve, or sludge in the radiator circuit.

Is it my HIU or the building system?

If your hot water is normal, the building primary supply is almost certainly fine, because both circuits draw from it. That points the fault at your own flat. A quick cross-check is to ask a neighbour whether their heating is working; if several flats are affected at once, it is more likely to be a communal system or plant room issue for the managing agent to resolve.

Should I bleed my radiators?

Yes, if the radiators are cold at the top and warm at the bottom, which is the classic sign of trapped air. Use a radiator key, start with the radiator furthest from the HIU, and have a cloth ready. If radiators are cold all over, or cold at the bottom and warm at the top, bleeding will not help and the cause is different.

Why is only one radiator cold?

A single cold radiator is usually a local problem rather than an HIU fault: a stuck thermostatic radiator valve pin, a closed lockshield valve, trapped air in that radiator, or sludge settled in the bottom of it. If every radiator is cold, the cause is more likely to be at the HIU, the controls or the heating circuit as a whole.

How much does it cost to fix a heating fault on an HIU?

Collide Group charges a £140 call-out which covers attendance and full fault diagnosis. If a repair can be completed on the first visit, the repair fee is agreed with you before any work starts. Common heating-side repairs such as a replacement zone valve actuator are usually completed in a single visit.

Radiators Cold But Hot Water Fine? HIU Heating Faults Explained

Why This Combination Is Good News

Your heat interface unit does two separate jobs inside one box. Hot water from the building's communal system — the primary circuit — arrives at your flat. The HIU then uses that heat twice over: once through a plate heat exchanger to make your domestic hot water on demand, and once to feed your radiator circuit.

So if your hot water is working normally, you have already proved two things: the building is delivering hot primary water to your flat, and a good part of your HIU is working. That rules out a plant room failure, a closed riser valve and a dead unit all at once.

The fault is on the heating side specifically — which is a much shorter list of possibilities, and usually a cheaper fix.

Match Your Symptom to the Cause

All radiators cold, hot water fine

Likely cause: Heating zone valve or actuator failure inside the HIU, or no demand signal reaching the unit from the thermostat or programmer

Try first: Check the programmer is calling for heat and the thermostat is set above room temperature

Radiators cold at top, warm at bottom

Likely cause: Trapped air in the radiator

Try first: Bleed the radiators, starting furthest from the HIU

Radiators cold at bottom, warm at top

Likely cause: Sludge and magnetite settled in the bottom of the radiator

Try first: Needs a powerflush — not a DIY fix

One radiator cold, the rest fine

Likely cause: Stuck TRV pin, closed lockshield valve, or air in that radiator

Try first: Check both valves are open; a stuck TRV pin can sometimes be freed

Radiators lukewarm, never reach full heat

Likely cause: Partly open zone valve, low system pressure, or a scaled heat exchanger restricting the circuit

Try first: Check the pressure gauge if your unit has one

Heating works briefly then stops

Likely cause: Control cycling on a faulty temperature sensor, or an overheat thermostat tripping

Try first: Note how long it runs before cutting out — useful for the engineer

Check These Before You Call Anyone

A surprising number of “no heating” callouts turn out to be controls rather than a fault. Run through these first — they cost nothing and take five minutes.

Is the programmer calling for heat? Check it is in a heating period and not on a summer or hot-water-only setting. Timers lose their schedule after a power cut.
Is the room thermostat set above the current room temperature? If the room is 20°C and the stat is set to 19°C, nothing will fire. Turn it well up as a test.
Does the thermostat need batteries? Wireless thermostats are a very common cause. A blank or faint display is the giveaway.
Are the radiator valves open? Check both ends of each radiator — the TRV and the lockshield.
Is there any system pressure? If your unit has a pressure gauge and it reads near zero, the heating circuit has lost water and needs attention.
Are your neighbours affected? If several flats lost heating at the same time, it is a communal issue for your managing agent, not your HIU.

The Most Common Actual Fault

When the controls check out and every radiator is stone cold, the single most frequent cause we find is a failed heating zone valve actuator inside the HIU.

The actuator is a small motorised head that opens the valve letting primary heat into your radiator circuit. When it fails — the motor burns out, or the valve beneath it seizes — the heating circuit simply never opens. Hot water carries on working perfectly, because it runs through a different valve.

It is a routine repair. The part is inexpensive, it is one of the components we carry on the van, and it is normally done in a single visit.

The other recurring cause is sludge. Magnetite builds up in radiator circuits over years and settles in the bottom of radiators and in narrow sections of the HIU. The classic sign is radiators warm at the top and cold at the bottom, often with the circuit never quite reaching temperature. That needs a powerflush rather than a part swap.

When It Is Not Your HIU at All

Two situations point away from your unit entirely:

Several flats affected simultaneously. This is a communal heating or plant room problem. Report it to your managing agent or housing provider rather than arranging your own engineer — the cost should sit with the building, not you.
It started immediately after works in the building. If the system has been drained and refilled, air in the risers is common and the building's contractor should return to vent it properly.

Who Pays for the Repair?

In most London leasehold blocks, the communal system up to your flat is the freeholder's responsibility, while the HIU inside your flat may be yours — but leases vary considerably and some blocks include in-flat HIUs in the service charge. Check your lease, or ask your managing agent, before you commission anything.

If you rent, it is your landlord's responsibility. Under the Landlord and Tenant Act 1985, landlords must keep installations for space heating and hot water in repair and proper working order.

No Heating? Same-Day Callouts

£140 call-out covering attendance and full fault diagnosis · All HIU brands · Any repair fee agreed before work begins

Book a Callout 020 3951 5133

Radiators Cold But Hot Water Fine?