Overview
S.30 on a Vaillant EcoTEC Plus means "no heating demand from external controls." In plain terms the boiler is working but it is not being told to run for central heating. That call for heat normally comes from your room thermostat and/or programmer, or from an internal microswitch in a motorised valve on systems with zone valves. The boiler sees no closed circuit or command on the CH input (commonly across terminals 3 and 4), so it will display S.30 and will not fire the burner or run the pump for heating (hot water circuits can still work if their demand input is satisfied). Why it happens: common causes are a thermostat or programmer that is off or not calling, dead batteries in a wireless thermostat/receiver, a failed thermostat receiver, a faulty or un-seated link/wire between controls and the boiler, a faulty motorised/three-port or two-port valve (its internal microswitch not making), or damaged wiring/ blown fuse in the control circuit. Intermittent faults after power cuts are often down to wireless receivers, stuck motorised valve heads, or failed microswitches. Severity and DIY suitability: the boiler itself is usually fine and this is generally not an emergency (you’ll be without heating but often still have hot water). Some checks are simple and suitable for a confident homeowner (check settings, replace batteries, try manual valve override). Anything involving mains electrical wiring, gas, or disassembly of boiler/electrical terminals should be left to a Gas Safe-registered engineer or qualified electrician.
Possible Cause: No heating demand from external controls
Troubleshooting Steps
Safety first
- If you are not confident working around electricity or heating controls stop and call a qualified engineer. Never work on gas parts of the boiler unless you are Gas Safe registered.
- Before touching any wiring, isolate the mains supply to the boiler and the programmer if you will be opening covers or changing connections. If you are only changing thermostat batteries or operating valves by hand you can do those with the boiler powered but still be cautious.
Initial homeowner checks (easy, safe)
1) Check programmer/timer: ensure the heating program is turned on for the current time period and the CH is not set to OFF or to frost mode. Set the programmer to a confirmed ON period.
2) Check room thermostat: set the room stat several degrees above the current room temperature to force a call for heat. If it’s wireless, check and replace batteries in the thermostat and in the receiver.
3) Check TRVs: on thermostatic radiator valves (TRVs) make sure at least one radiator valve is open so the system can call for heat.
4) Boiler reset: try a single reset of the boiler according to the manual (don’t repeatedly reset). Observe whether the S.30 clears and whether the boiler fires for heating.
Basic diagnostic steps (safe but requires observation)
5) Watch boiler display when you force a heating call: with programmer set to heating ON and stat set higher than room temp, look at the boiler. If S.30 remains it shows the boiler isn’t seeing the call.
6) Manual valve override: if you have a motorised zone valve (2- or 3-port), move the valve to its manual heating position or press the actuator lever to move it to heating. If the boiler then starts, the valve actuator or its internal microswitch is suspect.
7) Receiver/controls: if you have a wireless thermostat with a wall-mounted receiver, check the receiver’s indicator lights and ensure it is powered. Remove and replace batteries in both thermostat and receiver if present. Re-pair per manufacturer instructions if needed.
Multimeter/continuity checks (only if competent and with power isolated where required)
8) Check the CH control circuit: many Vaillant EcoTEC models use a call input on terminals often labelled 3 and 4. With the heating demand active from the programmer/stat, you should either see a closed circuit or an expected control voltage depending on your system wiring (some are volt-free, some supply 230V, some use low-voltage bus). If you are experienced with electrical measurement: with appropriate caution and the correct meter settings, measure across the control terminals while the programmer/stat is calling. If the terminals don’t change state when a demand is present, the fault is in the controls or wiring.
9) Bridging test (temporary diagnostic only): bridging terminals 3 and 4 at the boiler will simulate a direct call for heat. Only attempt this if you fully understand your system and local electrical safety rules. If bridging 3-4 makes the boiler fire and pump run, the boiler is fine and the fault is in the external controller/thermostat/wiring/valve. If bridging does nothing, stop and call an engineer — it suggests a boiler control fault.
What to inspect after these checks
10) Wiring and links: look for loose or corroded wires at the boiler and at the programmer/receiver. If someone previously fitted a link where external controls were removed, verify it is intact. Do not attempt rearranging live mains wiring unless qualified.
11) Valve microswitch/actuator: on systems with mid-position motorised valves a small microswitch in the valve head tells the boiler to run for CH. If the valve moves but the microswitch is not making, replacement of the actuator or valve head is required.
12) Intermittent problems after power cuts: reset the boiler and check wireless receivers for fault LED indications. Replace receiver batteries or consider replacing an unreliable wireless thermostat/receiver.
When to call a professional
- If the simple checks above don’t find or fix the problem, or if you need to open the boiler or touch mains wiring, call a Gas Safe-registered heating engineer or a qualified electrician. Also call a pro if you find a suspected gas or boiler internal fault, if bridging didn’t make the boiler run, or if you see damaged wiring or smells of burning.
- Tell the engineer the boiler shows S.30 and what diagnostic steps you’ve already done (batteries replaced, programmer set to ON, bridging test result, valve manual test). That will help them diagnose faster.
Final notes
- Do not work on gas-tight covers, gas components or sealed parts of the boiler unless you are certified to do so. Repeated resets are not a fix and can mask intermittent safety trips. The S.30 code itself is a control/communication issue rather than an immediate safety shutdown, but a qualified engineer should repair electrical/control faults and replace failed valves, actuators or control modules.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant EcoTEC Plus Regular Gas Boiler.