Heat Interface Unit No Hot Water
No hot water from a heat interface unit can mean missing network heat, a restricted exchanger or strainer, or a hot-water demand/control fault. Check whether heating works and whether other homes are affected before deciding the HIU needs a replacement.
Quote on request · Confirmed London & South East / Nottingham & NG coverage
First distinctions to make
No hot water from a heat interface unit can mean missing network heat, a restricted exchanger or strainer, or a hot-water demand/control fault. Check whether heating works and whether other homes are affected before deciding the HIU needs a replacement.
If heating works but hot water does not, investigate the hot-water path while still measuring the incoming supply under draw-off. The relevant demand device and valve depend on the model. If both heating and hot water are affected, and especially if neighbouring flats report the same thing, the primary network supply is a more likely cause.
Exchanger and strainer fouling
The plate heat exchanger used for instantaneous hot water has fine waterways that are more sensitive to fouling than heating circuit pipework. A fouled exchanger reduces heat transfer even when primary flow is present, giving lukewarm rather than fully cold water at the tap. A blocked primary strainer upstream of the exchanger has a similar effect by reducing the flow reaching it.
Flow switch, diverter valve and sensor faults
Instantaneous hot-water regulation responds to draw-off using the mechanical or electronic demand arrangement documented for that model. A flow switch that fails to trigger, or a diverter valve that sticks, can produce no hot-water response even though the heating side functions normally.
A hot-water temperature sensor reading incorrectly can cause the controller to under-supply flow to the exchanger, giving weak or inconsistent hot water rather than a complete absence of it.
Checking network supply conditions
Where multiple flats on the same riser report no hot water, measuring the primary flow temperature at an affected unit indicates whether adequate heat is reaching the dwelling at all. A low primary temperature arriving at the HIU limits every downstream HIU's hot-water output regardless of how well each individual unit is maintained.
Separate demand recognition from inadequate heat transfer
Working heating does not prove the network can meet hot-water duty. Radiators can deliver useful warmth with primary temperatures too low for the required hot-water temperature at a high draw-off rate. Likewise, the available differential pressure may collapse during a tap demand even if static pressure looks normal. Compare primary supply under demand with the unit design before concluding that a hot-water-specific part has failed.
Self-acting units can sense potable flow and temperature mechanically; electronic models may use a flow sensor and modulating valve. Neither a diverter valve nor a separate flow switch exists on every unit. Test the sequence in the relevant manual: does the primary valve respond to draw-off, does it travel sufficiently and does the control reading agree with an independent temperature measurement? An actuator or sensor fault can then be distinguished from lack of incoming heat.
Inspect common restrictions including primary strainers, cold-water-side fittings and a fouled plate exchanger. A DPCV regulates pressure across its controlled branch; a PICV limits or modulates flow within its operating envelope. Neither can compensate for insufficient incoming differential pressure. Cleaning, valve service, actuator replacement or approved exchanger treatment should follow the confirmed cause. Seals, sensor curves, valve travel and actuator voltage must match the installed revision.
Danfoss and SAV mechanical controls differ from electronically regulated Caleffi SATK, Baxi AquaHeat and Evinox ModuSat equipment. After work, record sustained draw-off flow and temperature at the HIU and representative outlet, inspect disturbed joints and apply any manufacturer-specified tightness procedure. Check heating still functions and the hot-water valve shuts down appropriately. Residents should note symptoms and use ServiceM8 for an authorised enquiry, without opening covers or pressurised strainers.
Diagnosis, repair and verification
Diagnosis typically includes isolating whether the fault is on the hot-water side specifically, checking the strainer and exchanger for restriction, testing the flow switch/diverter response, and comparing sensor readings against an independent measurement. Repair may involve cleaning, part replacement matched to the model, or escalation to the network operator where supply conditions are the limiting factor.
After repair, a sustained draw-off test at a representative outlet confirms flow rate and delivered temperature over time, and pressure on the hot-water side is checked against the model's stated operating range.
Cost factors
Cost depends on whether the fault is confined to accessible components such as a strainer or sensor, or requires exchanger or diverter valve replacement, along with access and whether network-side investigation is also needed.
Official product references
Common Questions
If heating works but there's no hot water, is it the network?
It suggests checking the hot-water-specific controls and exchanger, but does not rule out primary temperature or differential-pressure limitations during draw-off.
Can a blocked strainer cause no hot water at all?
A badly restricted strainer can reduce flow enough to give only lukewarm or intermittent hot water, and in some cases contribute to an apparent loss of hot water, alongside other possible causes.
What triggers hot water to start flowing in an HIU?
Demand can be recognised by a self-acting mechanical assembly or an electronic sensor/controller arrangement. The primary valve then regulates heat transfer according to the model design; a diverter valve is not universal.
How do you know if several flats share the same no-hot-water cause?
By comparing complaints across the riser and measuring primary flow temperature at an affected unit, since a low network temperature affects every HIU downstream similarly.
What is checked after a hot-water repair?
A sustained draw-off test confirming flow rate and delivered temperature, and a pressure check on the hot-water side against the model's stated range.
How quickly can a no-hot-water fault be looked at?
Availability depends on the postcode and current schedule; book through the central ServiceM8 form or call 020 8935 5572 to confirm timing for your site.
