Error S.28

Safety first: if you are not Gas Safe registered do not attempt gas-side or internal boiler repairs. Turn off electrical power to the boiler before accessing internal components. If you smell gas, evacuate and call your gas emergency number immediately. Do not repeatedly reset the boiler as a long-term solution—this is only a temporary workaround and can mask an underlying fault.

Initial homeowner checks you can safely do:

- Check system water pressure on the front display. It should typically be around 1–1.5 bar. If pressure is low, top up the system using the filling loop per the manual (only if you know how). Low pressure can cause unusual behaviour. Do not over-pressurise.

- Observe the display and symbols. S.28 / egg-timer means anti-cycling. Note whether it appears only when DHW is requested while CH is already firing.

- Try a controlled reset: press the reset/fault-clear button once and watch the start sequence. If the unit immediately returns to S.28, note what preceded it (e.g., CH on and then tap turned on).

- Check for visible obstructions: ensure the condensate pipe/tundish is clear and not frozen or blocked. Clear any obvious ice or debris safely.

- Check room thermostat and programmer settings to ensure no conflicting commands. If fitted, temporarily set room stat higher to ensure CH is genuinely demanded during tests.

Diagnostic steps and possible fixes:

1) Reproduce the fault safely: with the heating running and still calling, open a hot tap and watch the valve movement. Confirm the 3-port/diverter valve moves to mid-position (both HW and CH symbols as expected). If the valve moves but the boiler goes into S.28 and does not fire, continue.

2) Check pump and flow indications: listen for the boiler pump running. Insufficient flow or a seized/slow pump can raise return temperatures or trigger anti-cycle. If the pump is silent when it should be running, call an engineer.

3) Bleed radiators and purge the system: air trapped in the system reduces flow and can cause high return temps. Purge with central heating off and then restart. Ensure radiator TRVs are open on the circuit being tested.

4) Check for blocked filters/strainers and the plate heat exchanger: if your system has an inline filter/strainer on the DHW or pump circuit, isolate and clean it (only if you know how). A heavily scaled or blocked plate heat exchanger (common on hard water areas) will reduce flow and increase return temperature and may cause persistent anti-cycling.

5) Observe diagnostic parameters: if you can access the boiler's service menu, note d.67 (remaining anti-cycling time) and d.2 (anti-cycling setting). If d.67 reads 0 but the boiler remains in S.28, that suggests a control or sensor issue rather than a simple timer. Do not change service menu parameters unless competent and confident—the wrong change can disable safety features.

6) Temperature sensors and flow switch: a faulty flow sensor or NTC sensor(s) that read unexpectedly high flow/return temperature (or a false no-flow) can lock the controller in anti-cycle. These faults require an engineer to test sensors and wiring and replace components as necessary.

7) Diverter/3-port valve and wiring: if the valve is mechanically or electrically faulty (sticking, incorrect feedback to the boiler), the boiler may interpret the situation as unsafe and hold anti-cycling. Check that the valve motor gives correct end-stop feedback; if the valve returns incorrect signals the actuator may need replacing. This is an installer/engineer task.

8) System configuration and flow temperature: sometimes high CH flow setpoints or incorrect weather compensation/heat curve settings produce high flow/return temps when switching modes. Lowering CH flow temperature or DHW setpoint can reduce return temperature spikes—do this only through normal front-panel controls, not service menu changes, unless you are qualified.

When to call a professional: If the fault repeats, the boiler remains locked in S.28 despite resets, d.67 shows zero but S.28 persists, the pump is not running, the diverter valve appears to operate incorrectly, or you suspect sensor or PCB faults, call a Gas Safe registered engineer experienced with Vaillant boilers. They can check flow/return temperatures under load, test the NTC sensors and flow switch, examine wiring and the diverter actuator, inspect the plate heat exchanger, and if needed update firmware or replace faulty components.

Notes and cautions: Repeatedly resetting the boiler is only a temporary measure and will not fix underlying causes. Do not enter the boiler’s gas or burner area, or attempt internal repairs unless you are a qualified engineer. Keep a record of when S.28 appears and what actions preceded it (e.g., CH on, tap opened) to give the engineer useful diagnostics. Persistent S.28 behaviour can be caused by flow/return temperature spikes, insufficient flow, blocked heat exchanger or strainer, faulty sensors, or wiring/control faults—these often require professional diagnosis and repair.