BoilerFaultCodesYour boiler is simply letting you know that someone has turned on a hot tap or a shower and it is currently working to heat that water.
This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.
Technical description: DHW demand. The domestic hot water demand is activated.
What causes this fault?
This code is triggered when your boiler detects water flowing through your hot water pipes. It is a signal that the internal flow sensor has spotted a demand for hot water, prompted by someone opening a tap or starting a shower. The boiler is not broken; it is simply communicating that it is busy heating your water.
This status is slightly more common in winter when incoming mains water is much colder, meaning the boiler has to work at full power for longer to reach your desired temperature.
DIY fix possible
Turn off all hot water tapsCheck that the code changes to a different status (like S.0 or S.4)If the code stays on despite all taps being closed, check for a dripping tap or a leaking pipe
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Protect yourself from future repair bills
A boiler cover plan would cover faults like S.20 — no call-out fees, no surprises.
Affiliate links — we may earn a commission at no extra cost to you.