Overview
S.31 on a Vaillant EcoTEC Pro / EcoTEC Plus is a status message meaning “No heat demand – boiler in summer operating mode.” It is not a fault code indicating a broken part; it simply tells you the boiler’s heating circuit has been switched off (summer mode) so the unit will not fire to drive central heating. This happens when the boiler’s heating flow temperature has been set to "Heating off" or the external controls (room thermostat, programmer, wireless receiver, zone valve) are telling the boiler there is no call for heat. Severity is low because the boiler is behaving as instructed rather than failing. If you want the heating on you can usually re-enable it yourself by changing the boiler or controller settings. However, if the boiler repeatedly switches itself off, shows S.31 when you expect heat, or you need to make changes to wiring, zone valves, or internal controls, you should contact a qualified heating engineer (Gas Safe registered) — do not attempt gas or mains-electrical repairs yourself.
Possible Cause: No heat demand boiler in summer operating mode
Troubleshooting Steps
Safety precautions:
1. Do not work on gas or live electrical components unless you are a qualified engineer. For any gas-side or internal electrical work call a Gas Safe engineer.
2. Before you touch the boiler’s terminal block or wiring, isolate the electrical supply at the mains (or the boiler isolation switch) and confirm power is off. If you are unsure, do not proceed.
3. Keep the flue and airways clear. Do not obstruct or remove safety covers while the boiler is powered and in operation.
Initial checks a homeowner can do (safe, no tools required):
1. Check the boiler display text: S.31 / “Heating off” or summer mode indicates heating flow temperature has been set to Off. Check the hot water still works — if hot water works and heating is off, S.31 is expected behavior.
2. Check your room thermostat or thermostat schedule: ensure it is set to a temperature above the current room temperature and the programmer/timer is calling for central heating right now.
3. Open at least one radiator fully and ensure any thermostatic radiator valves (TRVs) are not set to frost or very low settings.
4. If you have smart controls (wireless room stats, smart programmer), check their app or receiver to confirm they are calling for heat and have batteries and power.
How to re-enable heating on the boiler (user control method):
1. On the EcoTEC front panel press the heating selector button (the button for central heating settings). The display will show the set heating flow temperature or the text "Heating off".
2. While that setting is shown use the + button to increase the flow temperature from "Heating off" to the desired temperature (eg 50°C).
3. Confirm the new setting by pressing OK (or the right-hand selector again depending on model). The boiler should then accept the call for heat when the external controls request it.
Diagnostic steps if heating still does not come on when it should:
1. Verify the programmer/timer is set to ON or to a period that should be calling for heat. Temporarily set the programmer to permanent ON for a quick test.
2. Raise the room thermostat setpoint well above current room temperature to force a call for heat. If wireless, check receiver is showing a demand signal.
3. Check the boiler’s external control terminals: if you are competent and have isolated mains power, you can test the CH call input by placing an insulated jumper between the CH call terminals (commonly terminals 3 & 4 on some Vaillant models) to simulate a demand. If placing a link calls the boiler and it runs, the problem is upstream (thermostat, programmer, wiring, receiver). Only perform this if you understand safe isolation and wiring procedures.
4. If a link calls the boiler but the thermostat/wireless controls do not, inspect wiring and battery condition on thermostats/receivers and confirm zone valve actuators are moving. Replace batteries and check terminal connections.
5. If the boiler starts but cuts out and S.31 returns after a few minutes, note that the boiler has an anti-cycling delay; it may stop between cycles for a short period. If it repeatedly shuts down immediately, record any other messages or fault codes and the sequence of events.
6. Check for tripped external controls or interlocks: some properties have timers, immersion thermostats, frost thermostats, or external safety switches that can suppress heating demand. Verify these are not engaged.
When to call a professional:
1. Call a Gas Safe registered heating engineer if you are not comfortable isolating electrical supplies, performing the jumper test, or working with wiring.
2. Call a professional if the boiler does not respond to a direct CH call (jumper test), if S.31 appears but you are sure all controls are calling for heat, or if other fault codes appear alongside S.31.
3. Call a professional for any gas-side, internal component, fan, pump, or PCB issues, or if the boiler locks out and will not reset.
Final notes:
- S.31 itself is informational: most of the time the fix is to re-enable heating in the boiler menu or correct an external control setting. - Do not attempt gas repairs or internal PCB/electrical repairs yourself. If in doubt, stop and book a qualified heating engineer to diagnose and repair.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant EcoTEC Pro / EcoTEC Plus.