Why Has My HIU Lost Hot Water?

Check These First (Before Calling an Engineer)

Before assuming your HIU has a fault, there are a few quick checks that take less than five minutes and may save you a call-out fee:

1. Check with your neighbours. If the building's communal heating system has gone down — due to a plant room fault, pump failure or scheduled maintenance — all flats in the building will lose hot water at the same time. Knock on a neighbour's door or check a building management notice board. If it is a communal outage, you need to contact your building manager or landlord, not an HIU engineer.
2. Check the HIU display for error codes. Most modern HIUs have a small digital display or LED indicator. If there is an error code showing, note it down — this will help the engineer identify the fault faster. Common codes are explained in our HIU fault codes guide.
3. Check that isolation valves are open. HIUs typically have isolation valves on the primary inlet and outlet and on the domestic hot water circuit. These are occasionally closed accidentally during maintenance work or after a repair. If a valve has been turned to the closed position (usually 90 degrees to the pipe), this could be blocking flow.
4. Check the system pressure. Some HIUs display secondary circuit pressure. If the pressure has dropped below 1 bar, the system may need repressurisng — check your HIU manual or contact your engineer.

If none of these quick checks identify an obvious cause, the fault is internal to the HIU and you will need a qualified engineer.

Most Common Causes of HIU Hot Water Failure

1. Blocked Plate Heat Exchanger

This is by far the most common cause of HIU hot water problems in London. The plate heat exchanger (PHX) is a stack of thin corrugated metal plates through which hot primary water and cold secondary water flow in alternating channels. In London's hard water areas, limescale deposits build up on the plate surfaces over time, progressively restricting water flow and reducing heat transfer efficiency.

A partially blocked PHX will cause the hot water temperature to drop gradually — you may notice it taking longer to run hot, or the maximum temperature being lower than it used to be. A heavily scaled PHX can cause complete hot water loss.

The fix is either a chemical descale (circulating a descalant solution through the PHX to dissolve the scale) or, in severe cases, replacement of the heat exchanger assembly.

2. Failed DHW Valve or Actuator

The domestic hot water (DHW) valve opens when you turn on a tap, allowing primary hot water to flow through the heat exchanger. The actuator is the electromechanical device that opens and closes this valve in response to a signal from the control board.

If the actuator fails in the closed position, hot water flow to the heat exchanger will be cut off, and you will get cold water from the tap. If it fails in the open position, you may experience the opposite problem — heat continuously running even when no hot water is being demanded, causing high heat charges.

An engineer will test the actuator electrically and mechanically and replace it if faulty. Actuators are a common wear item and are generally inexpensive to replace.

3. Faulty Thermostatic Cartridge

Some HIU designs use a thermostatic cartridge (also called a thermostatic mixing valve or TMV) to blend hot water to a safe outlet temperature. If the cartridge fails or jams, it can restrict or completely block the hot water flow, or cause the outlet temperature to fluctuate erratically.

4. PICV Valve Fault

The Pressure Independent Control Valve (PICV) regulates the flow of primary hot water into the HIU. If the PICV becomes stuck, corroded or its cartridge fails, it can prevent adequate primary water flow from reaching the heat exchanger — causing reduced hot water performance or complete failure.

PICV faults can be subtle: the valve may pass some flow but not enough to heat water to temperature under high demand. Engineers test flow rates to diagnose PICV issues.

5. Low Pressure from the Communal System

Even if the communal system is “working” (i.e., other flats have heat), the primary pressure available to your specific flat's HIU may be insufficient. This can happen if there is a fault in the riser serving your floor, or if the building's balancing has drifted over time. An engineer can measure the differential pressure across your HIU to diagnose this.

6. PCB / Control Board Fault

The printed circuit board (PCB) is the electronic controller that manages all functions of the HIU. A faulty PCB may fail to signal the actuator to open, misread sensor inputs and shut the unit down unnecessarily, or display fault codes that do not correspond to any obvious mechanical fault.

PCB replacement is one of the more expensive repairs on an HIU, and on older units it can sometimes be more cost-effective to replace the entire unit rather than the board — your engineer will advise.

What an HIU Engineer Will Do

When a Collide Group engineer attends an HIU no hot water call, they follow a structured diagnostic process:

1. Check primary flow and temperature — confirm that the communal system is delivering adequate heat to the HIU.
2. Read error codes — review the HIU display and control board logs for stored fault history.
3. Test the DHW actuator — electrically test the actuator and observe valve operation.
4. Measure heat exchanger performance — compare primary and secondary temperatures to identify heat transfer losses consistent with scaling.
5. Test the PICV — measure differential pressure and flow rate.
6. Inspect sensors and wiring — check for damaged cables, failed temperature sensors or loose connections.

Once the fault is identified, the engineer will carry out the repair — either on the same visit using parts carried on the van, or by ordering specific components for a follow-up.

How Quickly Can It Be Fixed?

The majority of HIU hot water faults are resolved in a single visit. Collide Group engineers carry the most common spare parts for Danfoss, SAV and Switch2 HIUs on the van as standard — including actuators, thermostatic cartridges, sensors and strainer filters.

A plate heat exchanger descale typically takes 45–90 minutes on site. A PHX replacement, if required, adds additional time and may require parts to be ordered if the specific unit is not in stock.

We offer same-day emergency callouts across Greater London — call us on 020 4548 4226 and our team will arrange the earliest available appointment.