Vaillant S.31 status codewhat it means
Not a fault. S.31 is summer mode: heating has been switched off at the boiler while hot water carries on. If you want the radiators back, turn the heating flow temperature up from "Heating off".
Before you start
What S.31 means
S.31 is the status "No heat demand: Summer mode — summer mode is activated; there is no heat demand". It is the boiler telling you that heating has been switched off on its own controls, not by a thermostat elsewhere in the house, and Vaillant's operating instructions describe exactly how that happens: "to switch off heating mode without switching off hot water generation … use the button to set the heating flow temperature to Heating off … the symbol appears on the display".
Turning the heating flow setting down past its minimum is how you enter summer mode on an ecoTEC plus, pro, exclusive or ecoFIT, and it is easy to do by accident while adjusting the temperature. The ecoFIT pure, ecoTEC sustain and open-vent plus widen the row to "No heating demand: summer mode, eBUS control, waiting period", because on those boilers an eBUS control such as a sensoCOMFORT can put the boiler into summer mode itself when the weather is warm enough.
On the 2005 to 2012 ecoTEC the row is "central heating thermostat knob turned off or no heat demand by the eBUS control unit", and the turboMAX plus prints "summer mode (heating control knob in off position)", which is the same message from the days of a dial. The 2022 ecoTEC plus shows S.031, "heating mode has been deactivated and there is no domestic hot water demand". The boiler is not faulty, has not locked out and will still fire for a tap or a cylinder.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Heating turned off at the boiler | The heating flow temperature has been set to "Heating off", usually by turning it down past its lowest value while trying to lower the radiator temperature. | Common |
| eBUS control has chosen summer mode | A weather-compensating control with an outdoor sensor stops asking for heating above its summer threshold. Normal in warm weather, and the control's own setting decides when it ends. | Common |
| Left in summer mode from last year | The setting is remembered through power cuts. A boiler switched to summer in May will still show S.31 on the first cold night in October. | Occasional |
What to try yourself
Look at the display. The summer mode symbol the operating instructions describe, a sun on most ecoTEC displays, sits beside the temperature, and the heating flow temperature reads "Heating off" or "--" when you press the heating button.
Press the heating flow temperature button, use the plus button to raise the value from "Heating off" to the temperature you want, and confirm. The symbol disappears and, if the room thermostat is calling, the boiler runs on to S.03 and S.04.
If you have a sensoCOMFORT, VRC or other eBUS control, the boiler may show S.31 because the control is in summer mode. Change the operating mode or the summer threshold on the control rather than at the boiler.
Get a Gas Safe registered engineer to look at your Vaillant S.31
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What it costs to fix
Typical repair cost
Depends on the cause
Time on site
5 minutes
Nothing. Raising the flow temperature from "Heating off" is a button press. A call-out to do it for you is £80 to £120.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Vaillant fault codes
- S.08Usually simple to fixNot a fault. S.08 is the burner anti-cycling time: the boiler has just finished a heating burn and is deliberately waiting, up to about twenty minutes, before it is allowed to relight.Related: status codes
- S.30Usually simple to fixNot a fault. S.30 means the room thermostat or programmer is not asking for heat, so the boiler is idle on purpose. If the house is cold, look at the thermostat, not the boiler.Related: status codes
- S.34Usually simple to fixNot a fault. S.34 is frost protection: the boiler has fired by itself because its flow temperature dropped below 5 °C, and it will warm the water to about 30 °C and stop.Related: status codes
- S.40Get it checkedNot a lockout, but not normal either: comfort protection mode. The boiler has detected a fault and is running on with reduced comfort. Its fault memory will say what it found.Related: status codes
- S.88Usually simple to fixNot a fault. S.88 means the purging programme is running: the pump is cycling to push air out of the boiler, 7.5 minutes per circuit, and the burner stays off until it finishes.Related: status codes
- S.96Usually simple to fixNot a fault. S.96, S.97 and S.98 are the boiler testing its own sensors — return, water pressure, flow and return — with heating demands blocked for the few seconds it takes.Related: status codes
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant S.31
Hot water still works, so is the boiler broken?
No, that is what summer mode is for. Vaillant designed it to leave hot water running while heating is off. The boiler will fire for a tap and show S.10 to S.14 while it does, and it will still protect itself against frost.
Why did it switch itself off?
It did not, unless an eBUS control did. On the boiler itself, S.31 follows a setting someone made — the flow temperature turned down to "off". With a weather-compensating control, the control decides the season from the outdoor sensor and puts the boiler in summer mode itself.
Is frost protection still active in summer mode?
Yes. The operating instructions say the boiler runs if the flow temperature falls below 5 °C while it is switched on, whatever mode it is in, and shows S.34 while it does.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.