TQ / TQR DGH – FC32 & FC45 Combustion Air Pressure Too Low at Start or During Run

⚠️ 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.

Click link below to view:

TQ / TQM Fault Codes (FC Errors) + ICS LED Flash Guide – Diagnostics & Quick Checks: Seeley International Pty Ltd

Fault Description

FC32 – Combustion Air Pressure Too Low at Start

The heater was unable to achieve the required combustion suction pressure during the startup sequence. 

FC45 – Combustion Air Pressure Too Low During Run

The heater initially started correctly but lost combustion suction pressure during operation.

These faults are commonly caused by:

• Incorrect flue installation.
• Incorrect flue terminal/cowl selection.
• Excessive flue length or bends.
• Wind affecting the flue terminal.
• Restrictions in the flue system.
• Condensation or blockages in pressure sensing hoses.
• Air leaks in the combustion pressure sensing system.

Before replacing components, the complete flue system should be checked against the installation requirements.

Step 1 – Verify Combustion Pressure Testing Method

Before carrying out any pressure checks, ensure the manometer is connected correctly.

For TQR5/TQM5 and TQR6/TQM6 modulating heaters, combustion pressure must be measured using a T-piece fitted into the black combustion pressure sensing hose, not by connecting directly to the combustion fan pressure spigot.

Why This Is Important

The heater controls combustion fan speed using feedback from the pressure sensor. If the pressure sensor is disconnected during testing, the heater will continue increasing fan speed in an attempt to achieve the target pressure.

This can result in:

• Incorrect pressure readings.
• Misdiagnosis of the fault.
• Pressure values that do not represent normal heater operation.

Correct Testing Method

✅ Leave the pressure sensing system connected.

✅ Fit a temporary T-piece into the black suction hose.

✅ Connect the manometer to the T-piece.

✅ Measure the pressure being seen by the heater's pressure sensor.

Incorrect Testing Method

❌ Connecting a manometer directly to the combustion fan pressure spigot.

❌ Disconnecting the pressure sensor hose during testing.

Recommended Parts

The following spare parts can be used to create a temporary test point:

These components can be purchased through Spare Parts.

IMPORTANT

When testing a modulating heater, the pressure sensor must remain connected while the manometer is fitted.

If the pressure sensor is removed from the circuit, the combustion fan will continue ramping up in speed while trying to achieve the target pressure. Because the heater is no longer receiving pressure feedback, it will never know it has achieved the correct pressure, resulting in misleading readings.

Using a T-piece allows both the pressure sensor and the manometer to remain connected, ensuring the pressure being measured accurately reflects normal heater operation.

Step 2 – Confirm the Correct Flue Terminal Has Been Fitted

Incorrect flue terminals can increase resistance and make the installation more susceptible to wind-related pressure faults.

TQR5 / TQM5 Modulating Non-Condensing Heaters

External / Underfloor Installations

Flue Terminal P/N 641706.

Internal Installations

100mm AGA Approved Flue System with 100mm AGA Approved Cowl.

TQR6 / TQM6 Modulating Condensing Heaters

Internal Installations

Flue Terminal P/N 641713.

External / Underfloor Installations

Flue Terminal P/N 641607.

Adaptor Flue Terminal P/N 641748.

Step 3 – Check Flue Length and Number of Bends

TQR5 / TQM5 Modulating Non-Condensing Heaters

The installation manual specifies:

Maximum Flue Lengths

• Maximum vertical flue length: 4.8m
• Maximum horizontal flue length: 4m
• Maximum combined flue length: 6m

Maximum Bends

• Maximum 2 x 90° bends
• Two 45° bends are equivalent to one 90° bend

Flue Requirements

• Heater must be flued using 100mm AGA approved metal flue pipe.
• External socket joins must face downward.
• Internal socket joins must face upward.
• Flue joins must be adequately sealed.
• Twin-wall metal flue must be used where the flue is not readily accessible.
• Twin-wall metal flue is recommended on flue runs exceeding 2m.

TQR6 / TQM6 Modulating Condensing Heaters

The installation manual specifies:

Maximum Flue Lengths

• Maximum vertical rise: 5m
• Maximum total flue length: 12m

Maximum Bends

• Maximum 4 x 90° bends
• Two 45° bends are equivalent to one 90° bend

Flue Pipe Requirements

• Must be installed using 100mm DWV plastic pipe and fittings compliant with AS1260.
• Flue system must terminate using a 100mm DWV vent cowl.
• The flue terminal on the heater is designed to accept either:
  • The male end of a 100mm DWV M-F elbow, or
  • 100mm DWV pipe.

Flue Joint Requirements

All flue and condensate drain joints must be sealed using:

• Silicone sealant, or
• PVC solvent cement and primer.

Step 4 – Inspect the Complete Flue System

Step 5 – Check Flue Terminal Location

Wind effects are one of the most common causes of intermittent FC45 faults.

Step 6 – Verify Flue Clearances

Minimum clearances from the flue terminal must be maintained.

Step 7 – Condensing Models Only (TQR6 / TQM6)

Moisture trapped in pressure sensing hoses can reduce the pressure signal seen by the ICS and generate FC32 or FC45 faults.

Step 8 – Inspect the Pressure Sensing System

Check:

• Pressure sensing hose.
• Hose joiners.
• Combustion fan pressure tapping.
• Pressure sensor connections.

Look for:

• Kinks.
• Splits.
• Cracks.
• Restrictions.
• Moisture contamination.

Any restriction between the combustion fan and the pressure sensor can affect the pressure reading seen by the ICS.

Step 9 – Inspect Heater Joins and Combustion Fan Sealing

Particularly on heaters that have been split during installation.

Check:

• Black joiner between heater halves.
• Combustion fan O-ring seal.
• Combustion fan mounting.
• Combustion fan locking tabs.
• Combustion fan sealing surfaces.

Look for:

• Cracks.
• Air leaks.
• Poor sealing.
• Damage to hose joiners.

Air leaks at these locations can reduce combustion suction pressure and generate FC32 or FC45 faults.

Before Replacing Components

Verify the following: