Heat Interface Unit No Heating
No heating from a heat interface unit can originate in the user controls, heating valve, a secondary heating circuit or the shared heat supply. Hot-water performance and the unit hydraulic arrangement help determine which checks are relevant.
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Narrowing down where the fault sits
No heating from a heat interface unit can originate in the user controls, heating valve, a secondary heating circuit or the shared heat supply. Hot-water performance and the unit hydraulic arrangement help determine which checks are relevant.
If hot water works normally but heating does not, the fault is more likely confined to the heating-specific components: the heating valve, actuator, room thermostat/programmer interface, or, on an indirect unit, the secondary circuit pump or expansion arrangement. If both services are affected, the primary supply is a more likely shared cause.
Heating valve and actuator faults
The heating circuit on most HIUs is controlled by a valve driven by an actuator responding to a room thermostat, programmer or internal controller signal. A valve stuck closed, or an actuator that has failed to open, gives a complete loss of heating on that circuit even though hot water and the rest of the unit function normally. A valve or actuator that only partially opens gives weak or intermittent heating rather than a total loss.
Secondary circuit issues on indirect units
Where the unit uses an indirect arrangement with its own secondary heating circuit, a failed circulating pump, air trapped in the circuit, or a secondary-side pressure loss can prevent heat reaching the radiators even though the primary side of the unit is functioning. These components are not present on every HIU, so their relevance depends on confirming the installed arrangement first.
Zone valves and distribution issues
In some installations, zone valves further along the heating circuit, beyond the HIU itself, control distribution to different parts of a dwelling or shared pipework. A zone valve fault can present as no heating in part of a property while the HIU itself is operating correctly, which is distinguished by checking the unit's own flow output before investigating further downstream.
Pressure regulation, heat transfer and reliable restoration
A heating-only fault still needs measurement of the incoming heat supply. Hot water functioning during a short draw does not establish that the network can sustain heating at the specified flow, and an individual branch can have a pressure restriction while adjacent flats remain comfortable. Record primary temperatures and available differential pressure during a genuine heating call before altering controls or ordering parts.
A restricted strainer or unsuitable DPCV/PICV setting can limit primary flow. On an indirect arrangement, fouling of the heating exchanger adds thermal resistance; adequate primary flow with poor secondary temperature response needs different treatment from a pump which is not circulating. Sensor drift can persuade an electronic controller that sufficient heat has already been supplied. Compare measured temperatures with displayed values and confirm the controller output and actuator travel against the model instructions.
On direct heating, investigate the dwelling distribution without assuming a secondary pressure gauge or filling loop exists. On an indirect circuit, check its documented pump, air, expansion and safety arrangements where fitted. A persistent pressure loss needs its cause established rather than repeated filling. Repairs may include compatible actuator, valve or sensor replacement, approved exchanger cleaning and secondary-circuit restoration after safe isolation.
Mechanical Danfoss or SAV control arrangements and electronic Caleffi SATK, Baxi AquaHeat or Evinox models use different interfaces. Match control signal, valve connection, pump duty and seal material rather than cabinet appearance. Verify heating reaches the relevant emitters, record flow/return temperatures under demand, test shutdown and complete approved joint-tightness checks. For combined units, confirm hot water remains stable at a measured flow rate after the heating repair. Use ServiceM8 to request a quote with symptoms and access details.
Diagnosis, repair and verification
Diagnosis includes confirming whether hot water is unaffected, checking the heating valve and actuator response, reviewing secondary circuit components where fitted, and measuring primary flow and return temperature at the unit to rule out a supply-side cause. Repair may involve actuator or valve replacement matched to the model, pump replacement, or bleeding and re-pressurising a secondary circuit.
After repair, a heating demand test confirms the circuit responds and sustains an acceptable flow and return temperature differential, and pressure is checked against the relevant circuit's stated operating range.
Cost factors
Cost depends on whether the fault is an accessible valve or actuator, a secondary circuit component requiring drain-down and re-pressurisation, or further investigation needed into zone valves or network supply.
Official product references
Common Questions
If hot water works but heating does not, is it the HIU or the network?
This usually points toward the heating-specific components of the unit, such as the heating valve, actuator or secondary circuit, rather than the shared network supply.
Does every HIU have a separate secondary heating circuit?
No. Only indirect units with a plate heat exchanger on the heating side have a secondary circuit with its own pump and pressure; direct units do not, and this is confirmed before diagnosing a secondary-circuit fault.
Can a zone valve away from the HIU cause no heating in one room?
Yes, a zone valve controlling part of the heating distribution can fail independently of the HIU, which is checked by confirming the unit's own flow output first.
What does a heating demand test check?
That the heating circuit responds to a call for heat and sustains an acceptable flow and return temperature differential under that demand, rather than just reaching a set display temperature.
What is checked for pressure after a heating repair?
Pressure on the relevant circuit is compared against that unit's stated operating range, which differs between direct and indirect arrangements.
How is a no-heating fault booked for a visit?
Through the central ServiceM8 booking form with the symptoms, model and postcode, or by calling 020 8935 5572.
