A carbon film heater not heating can mean three different things: the system may not heat at all, some strips may stay cold, or the system may work but the room does not warm up enough. In this article we list the most common causes and checks for each situation.
Warning: Electrical measurements and repairs must be carried out by a qualified electrician.
1. The System Does Not Heat at All
- No power: The circuit breaker or RCD may have tripped. If it keeps tripping, see tripping the breaker.
- Thermostat setting: The set temperature may be below room temperature, the unit may be off, or it may be outside its time schedule.
- Wrong mode: The thermostat may be in F (floor) mode but the floor sensor is missing or faulty.
- Thermostat fault: The relay does not switch, or there is an error code on the display.
- Main connection broken: The supply cable or clamps may have come loose.
2. Some Strips Do Not Heat
- Loose or faulty clamp: The clamp may not be in full contact with the copper busbar.
- Broken copper busbar: The film may have been cut lengthways or folded, or the busbar damaged.
- Incorrect cut: If the film was cut through a carbon strip, that section will not work.
- Broken cable: The supply cable to that strip may have come away.
For correct cutting and connection, see our technical installation guide.
3. It Works but the Room Does Not Warm Up Enough
- Insufficient installed power: The film area or power per m² is too low for the room’s heat loss. See sizing per square metre.
- No underlay insulation: Heat escapes into the floor. See should you lay felt underneath?
- Thick carpet: Heat struggles to pass into the room.
- Floor limit too low: The thermostat’s floor limit cuts in early and switches the film off.
- Building insulation: Open windows, uninsulated walls, large glazed areas.
- Short running time: The system is switched on too late. See how long it takes to warm up.
Step-by-Step Checklist
- Are the circuit breaker and RCD switched on?
- Is the thermostat on, with the correct set temperature and mode?
- Is there voltage at the thermostat output?
- Is each strip’s measured resistance at the expected value?
- Check by hand or with a thermal camera which strips are heating.
- Check the clamps and cables of any strip that is not heating.
- If everything works, assess power, insulation and running time.
Frequently Asked Questions
Does carbon film lose power over time?
With quality films, power loss is very limited. Overheating, moisture and poor-quality carbon ink can accelerate it. See carbon film heater lifespan.
If one strip fails, does the whole film need replacing?
No. Because strips are connected in parallel, only the faulty strip or its connection is repaired or replaced.
Conclusion
A carbon film heater that won’t heat is usually down to power supply, thermostat settings, connections or insufficient power/insulation. A step-by-step check is needed for an accurate diagnosis. Contact us for service and fault finding.
