HIU Fault Codes Explained

Should You Reset an HIU Error Code Yourself?

The temptation when an error code appears is to reset the unit and hope the fault goes away. This is almost always the wrong approach.

HIU fault codes are safety features — the control board has detected a condition it considers outside of normal operating parameters and has taken action (usually shutting down or limiting operation) to prevent damage. Resetting without understanding the cause means:

• The underlying problem continues or worsens between resets
• You may be masking a fault that could damage expensive components
• In the case of overtemperature codes, you may be overriding a genuine safety protection
• Repeated resets on some units can lock out the controller entirely, requiring specialist intervention to restore

The correct approach is to note the code, check whether the unit is still operating (partially or fully), and contact a qualified HIU engineer — particularly for any codes involving no flow, overtemperature, or communication failure.

That said, if the code is clearly related to a communal system outage (i.e., the whole building has lost heating), waiting for the communal system to be restored and then resetting is reasonable.

Common Danfoss HIU Error Codes

Danfoss is one of the most widely installed HIU brands in London. Their HIUs display alphanumeric error codes on a small LED display. Note that exact codes vary between Danfoss product ranges (AKVA, ECL, etc.) — always cross-reference with the unit's specific documentation.

Common SAV HIU Fault Indicators

SAV Systems HIUs — widely installed in London social housing and private residential developments — typically use LED indicator lights rather than numeric codes. The LEDs are usually located on the front panel of the unit, and their meaning depends on colour, pattern and combination.

• Flashing amber LED: Often indicates a pressure alarm. This may mean the primary circuit differential pressure has fallen below the HIU's minimum operating threshold. Check with neighbours — if the whole building is affected, it is a communal system issue. If isolated to your flat, call an engineer.
• Flashing red LED: Typically a more serious fault — often a no-flow condition on the DHW circuit, an overtemperature condition, or a safety shutdown. Do not attempt to reset. Call an engineer.
• No-flow indicator / steady amber: The DHW circuit is not detecting flow when demand has been requested. This is often a failed actuator, blocked strainer, or PICV fault.
• All LEDs off / no power: Check whether the HIU has power (check the fuse or circuit breaker for the HIU circuit in your consumer unit). If the unit has power but no display, the PCB may have failed.

SAV HIUs also often have a manual override or test button — do not use this unless instructed by a qualified engineer, as it can bypass safety controls.

Common Switch2 HIU Errors

Switch2 HIUs are distinctive because they integrate smart heat metering and remote monitoring alongside the standard HIU functions. This means they have a broader range of fault types — some of which affect metering and billing rather than heating performance directly.

• Metering faults: Switch2 units monitor heat consumption for billing purposes. If the heat meter sensor or flow meter develops a fault, the unit will log a metering error. This may not affect heating or hot water immediately, but it means heat consumption is not being accurately recorded — which can lead to billing disputes. Report metering faults to your building manager promptly.
• Communication errors: Switch2 systems communicate with a central billing and monitoring platform. If the unit loses network connectivity (due to a router fault, M-Bus wiring issue, or network outage), it will log a communication error. Heating and hot water typically continue to function, but usage data will not be transmitted.
• Heating circuit faults: Like other HIU brands, Switch2 units will display faults for overtemperature, no flow, sensor failures and actuator issues. These are handled in the same way as equivalent faults on other brands.

When to Call an Engineer

You should call a qualified HIU engineer immediately in the following circumstances:

• Any fault code involving no flow — the unit has shut down due to no circulation, which can indicate a serious internal fault or a communal system problem
• Any overtemperature code — this is a safety protection that must be investigated before the unit is returned to service
• The unit has no power or is completely unresponsive, and the circuit breaker is not tripped
• There is a visible leak from the unit or associated pipework
• There are unusual noises from the unit — loud banging, hissing or grinding
• You have lost both heating and hot water simultaneously and neighbours are unaffected

For fault codes that are displaying but the unit appears to be working (e.g., a metering communication error on a Switch2 that is still providing heat), report the fault to your building manager and arrange a non-urgent inspection at the next available opportunity.