⚠️ Safety Notice – Gas Heating Products

This appliance must only be installed, commissioned, serviced and repaired by suitably qualified and licensed personnel.

Before commencing work, complete a task-specific risk assessment and ensure gas and electrical safety requirements, ventilation provisions, safe access, adequate service clearances and appropriate PPE have been considered. All work must comply with applicable regulations, standards and manufacturer requirements.

Particular care must be taken when working around gas systems, electrical components, combustion products, hot surfaces, moving parts, roof spaces, ceiling spaces and confined spaces.

Do not proceed if safe access, required clearances, ventilation requirements, confined space risks or appropriate safety controls cannot be achieved and maintained. 

Safety takes priority over diagnosis, repairs, installation schedules and customer expectations. If the task cannot be completed safely, it must not proceed until the risks have been adequately controlled.

FC4, FC10 & FC19 – High Temperature / Low Airflow Faults

Applies To

• Braemar TG / TH Series Gas Ducted Heaters
• FC4 – Thermistor >70°C During Run
• FC10 – Heat Exchanger Over-Temperature Switch 1 (HX OT1) Open
• FC19 – Heat Exchanger Over-Temperature Switch 2 (HX OT2) Open

What This Means

These faults are usually caused by the heater overheating because there is not enough airflow moving through the system.

When airflow is restricted, heat builds up inside the heater, and one or more safety devices operate to protect the appliance.

The fault may be caused by:

• Maximum fan speed set too low during commissioning
• Too few outlets installed for the heater size
• Restricted or undersized ductwork
• Blocked or dirty return air filter
• Incorrect thermistor location
• Faulty thermistor or over-temperature switch
• Damaged wiring to thermistor or HX switches

Quick Checks

1. Check Maximum Fan Speed

A common cause of FC4 and FC10 faults is the maximum system fan speed being set too low.

Verify that the maximum fan speed matches the number of installed outlets and heater size.

Important:

Do not assume you should measure 240 Vac at the fan motor.

The voltage supplied to the motor depends on:

• Maximum system fan speed setting
• Current fan speed being commanded by the controller
• Duct temperature

Refer to the fan speed voltage table at the end of this article.

If the measured voltage is significantly lower than expected, increase the maximum system fan speed setting and retest.

2. Check Number of Outlets

Confirm the system has sufficient supply outlets installed for the heater capacity.

Too few outlets can cause excessive duct temperatures and repeated overheating faults.

Refer to the installation manual outlet sizing chart.

3. Check Return Air System

Inspect:

• Return air grille sizing
• Return air openings
• Return air filter cleanliness

A blocked filter or undersized return air system can significantly reduce airflow.

4. Inspect Ductwork

Check for:

• Crushed or kinked duct
• Excessive duct lengths
• Tight bends
• Poorly balanced airflow
• Disconnected ducting

Long runs of small diameter duct can restrict airflow and contribute to overheating.

5. Check Thermistor Location

For FC4 and FC10 faults:

The outlet duct thermistor should be installed approximately:

3 metres downstream from the heater outlet

If fitted too close to the heater, temperatures may be higher than normal and nuisance faults can occur.

6. Check Thermistor Operation

Applicable Parts:

Using the wall controller:

• Check the displayed duct temperature.
• Confirm readings are realistic.
• Compare against actual duct temperature if required.

If the displayed temperature is excessively high when the duct is not hot, replace the thermistor.

7. Check HX Over-Temperature Switches

Applicable to FC10 and FC19.

Inspect:

• Wiring connections
• Switch continuity
• Signs of overheating

If an HX switch remains open when cool, replace the faulty switch.

Fault Specific Information

FC4 – Thermistor Above 70°C

The outlet duct temperature has exceeded 70°C during operation.

Most commonly caused by:

• Low airflow
• Fan speed set too low
• Incorrect thermistor location

FC10 – HX OT1 Open

The primary heat exchanger over-temperature switch has operated.

Most commonly caused by:

• Airflow restrictions
• Maximum system fan speed set too low
• Faulty HX OT1 switch

FC19 – HX OT2 Open At Start-Up

The secondary over-temperature switch is open when the heater starts.

Check:

• Wiring to HX OT2
• HX OT1 operation
• HX OT2 operation

If either switch is faulty, replace both switches as a pair.

After Repairs

After correcting the fault:

1. Verify airflow is correct.
2. Check maximum fan speed settings.
3. Confirm duct temperature is normal.
4. Clear fault history using the SCC controller if required.
5. Test heater operation through a complete heating cycle.

Helpful Videos

• Check Duct Air Temperature Using Spectrolink SCC
• View and Clear Fault History Using Spectrolink SCC

Helpful Articles

• FC19 Fault Code – HX OT2 Opened Spectrolink Reset Procedure, FC10 History Check & Overheating Diagnosis (TG, TH, THM, THM5 & THM6): Seeley International Pty Ltd
• TG TH Spectrolink System Set Up including zone installation instructions: Seeley International Pty Ltd

Technician Tip

If FC4, FC10 or FC19 occur together, investigate airflow first. In most cases the root cause is insufficient airflow caused by incorrect fan speed settings, too few outlets, return air restrictions or ducting issues rather than a failed component.