Low
S.15 on Glow-worm Ultracom 35 Store

Your boiler is continuing to run its internal components after the heating or hot water has been turned off to safely cool down the system.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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.

What the manual says

Manufacturer manual

pump / fan overrun

From this model's manual
Verified against the Glow Worm Ultracom 2 Cxi installation & service manual, page 52.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

What causes this fault?

This is a normal function where the boiler continues to run its internal pump or fan after you have turned off your heating or hot water. It happens to move residual heat away from the heat exchanger to prevent the unit from overheating internally. Once the system reaches a safe, cool temperature, the components will switch off automatically.

DIY fix possible

Check that your room thermostat or programmer is turned down or off.Wait for 5 to 10 minutes to allow the cooling process to complete naturally.Check that at least one radiator valve (usually the bypass) is open to allow water to circulate.

Find a Gas Safe engineer
Search the official register →
Boiler cover options

Avoid the next call-out bill

A cover plan would handle faults like S.15: 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.

Related guides you might find useful

No hot water but heating works

Almost always a stuck diverter valve on combis.

Read →

Banging / kettling / whistling noise

Each noise points to a different fault.

Read →

Boiler won't turn on — 9-step checklist

Work through these before the call-out.

Read →