Overview
S.28 on a Vaillant EcoTEC Plus Regular is the boiler’s hot-water (DHW) anti-cycling status. Anti-cycling is a built-in protection that prevents the burner from re-igniting too quickly when the control thinks conditions will cause short cycling (for example when flow/return temperatures are still high or there is insufficient system flow). In practical terms the boiler will show the egg-timer icon and refuse to fire until the anti-cycling condition clears. This error is usually not a catastrophic fault but it will stop the boiler supplying heat or hot water until the condition ends or the unit is reset. Common triggers are high return temperatures, reduced circulation (pump speed, closed TRVs, partially closed valves), incorrect 3-port valve behaviour or a faulty temperature sensor or wiring. Occasionally the control may appear to remain locked in S.28 even when the anti-cycle timer (d.67) reads zero; that can indicate a sensor, wiring or control logic issue rather than a simple timing delay. Some basic checks a homeowner can do are useful, but diagnosing and changing boiler parameters, wiring or internal components should be done by a Gas Safe registered engineer.
Possible Cause: Anti-cycling mode
Troubleshooting Steps
Safety precautions:
- If you smell gas, stop, leave the building immediately and call the gas emergency number. Do not attempt any checks.
- Before opening the boiler or touching internal components switch the boiler off at the power isolate and turn off the gas supply. Do not attempt internal repairs unless you are a Gas Safe registered engineer.
Initial checks a homeowner can do (no tools or safe to perform):
1. Check the display and note S.28 and any other symbols. Try a normal reset using the boiler reset/power button and observe whether it returns to S.28 or clears.
2. Check boiler pressure on the gauge. If pressure is low (below about 1 bar) top up the system per the manual and then re-test. Low pressure can cause odd behaviour.
3. Inspect the condensate pipe for blockages or freezing (common in cold weather) and clear/defrost if needed.
4. Confirm the 3-port valve is moving correctly: call for DHW and watch the valve move to mid position; listen for the pump and valve motor. If the valve is not reaching mid or its end switches are not changing, note that.
5. Open one or two radiator TRVs slightly (especially those closest to the boiler) to ensure there is adequate flow through the system and reduce return temperature.
Specific diagnostic steps and possible fixes (do not open the boiler casing unless qualified):
1. Replicate the fault: with heating running and a CH demand active, call for DHW and watch behaviour. If S.28 appears when the valve goes to mid position, this confirms the sequence described.
2. Check pump operation: feel pipes (with care) or listen to confirm the pump is running when a demand exists. A dead or slow pump can give high return temps and trigger anti-cycling.
3. Check thermostat/room stat and TRVs: ensure they are not shutting down most radiators so the system has low flow. Temporarily open a radiator to see if that allows the boiler to come out of S.28.
4. Check wiring and end-switch logic for the 3-port valve (Y-plan or S-plan): incorrect wiring or a failed end-switch can leave the boiler confused about DHW/CH priority and trigger anti-cycling. If you are not competent with wiring, get an engineer to inspect.
5. Sensor checks: the boiler uses flow/return and DHW NTC sensors to judge temperatures. If a sensor or its wiring is faulty (open circuit, shorted, or intermittent), the controller can misread temperatures and lock into anti-cycling. A Gas Safe engineer can measure NTC resistor values and continuity and replace sensors if needed.
6. d.67 and software behaviour: you noticed d.67 reading 0 while S.28 remains. That can indicate a control or firmware logic issue. Try a full power-down: isolate the boiler for several minutes (switch off at the mains and isolate power), then restore power to see if the control clears the status. If a simple reset or power-cycle temporarily clears it but the problem recurs, suspect sensor, valve or control board fault.
7. If you have a service history or recent work done, mention it to the engineer; incorrect parameter changes or component swaps can cause repeat anti-cycling.
When to call a professional and why:
- If the checks above don’t clear the S.28 condition reliably, or if you find faulty wiring, sensors, pump or valve issues, call a Gas Safe registered heating engineer. They have the test equipment and authority to safely isolate gas, open the boiler, test NTC sensors, check PCB diagnostics and, if required, update parameters or replace parts.
- Do not attempt to change boiler parameters (d.## menus) or replace gas-carrying components yourself. Incorrect adjustments can create unsafe conditions and will likely void warranties.
Severity and expected outcome:
- Anti-cycling itself is a protective and efficiency feature; occasional activation is normal. Persistent or repeat S.28 that prevents DHW or CH is a fault that needs investigation. A competent engineer will check circulation, valve wiring/feedback, sensor integrity and the control board; fixing those typically resolves the lock-in. If you need immediate hot water and the boiler is unusable, call a professional for urgent attendance.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant EcoTEC Plus Regular Gas Boiler.