Common problems guide

The most common Heatline C24 problems

The faults most likely to send a Heatline C24 into lockout — with plain-English causes, what to check first, estimated repair costs and whether you need a Gas Safe engineer.

This model is discontinued. Parts can be harder to source and expensive. If repair costs are mounting, a new A-rated boiler may be the smarter choice.

The 3 most common faults

01

Overheat Thermostat High Engineer

Your boiler has detected that it is running too hot and has automatically shut itself down to prevent damage.

Est. cost: £120-250
Parts: Overheat thermostat, Central heating pump, Diverter valve

Full guide for Overheat Thermostat

02

Fault on DHW sensor Medium Engineer

Your boiler is struggling to detect the temperature of your hot water supply, so it has turned off the tap water heating as a precaution.

Est. cost: £120-200
Parts: DHW NTC Sensor, Wiring harness

Full guide for Fault on DHW sensor

03

1% Low Engineer

Your boiler is struggling to detect the outside temperature, so it may not be adjusting its heating levels as efficiently as usual.

Est. cost: £120-180
Parts: Outdoor temperature sensor, Wiring loom

Full guide for 1%

Parts most often replaced

on the Heatline C24

Overheat thermostatCentral heating pumpDiverter valvePCBDHW NTC SensorWiring harness

Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.

Call a Gas Safe engineer if…

You can smell gas or see signs of a leak
The Heatline shows an Emergency or High severity code
The boiler keeps locking out after repeated resets
You've tried the DIY checks and the fault hasn't cleared
There's visible water leaking from the boiler
The flame is yellow or orange instead of blue

See the full 3-code list for the Heatline C24