S.20 Low

Glow-worm S.20 Fault Code

Also written as S20 — same fault.

The Glow-worm S.20 fault code means: your boiler is temporarily heating the internal tank to ensure hot water is ready for use, which is a normal operating status rather than a fault.. It appears on 27 Glow-worm models in our database.

Monitor it

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: domestic hot water cycling mode active

What causes the Glow-worm S.20 fault?

This status occurs when the boiler detects that the temperature in the integrated hot water storage tank has dropped below the required level. The boiler temporarily stops circulating heat to your radiators to focus all its energy on reheating the internal cylinder. Once the water inside the tank reaches the correct temperature, the boiler will automatically return to its normal heating schedule.

DIY fix possible

Check your hot water cylinder thermostat settingsEnsure your heating and hot water timers are set correctlyMonitor the boiler to see if it moves to a standard heating or standby mode once the water is warm

Find a Gas Safe engineer

Search the official register for a qualified engineer in your area.

Gas Safe Register →
Boiler cover options

Avoid the next call-out bill

A cover plan would handle faults like S.20: no call-out fees, no surprise bills.

Approved partner
Domestic Appliance Guard
Unlimited call-outs, 24/7 helpline, no price-hike promise
Get a quote →

Affiliate links. We may earn a commission at no extra cost to you.

Glow-worm models that show the S.20 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Glow-worm S.20 fault code information on this page is sourced and cross-checked against the official Glow-worm service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026