Combustion Fan Testing & Identification Guide
(Simple Field Guide for Service Technicians)
⚠️ Safety Notice – Gas Ducted Heaters (Servicing)
This appliance must only be serviced by suitably qualified and licensed personnel.
Before commencing servicing or fault diagnosis, ensure electrical and gas supplies are safely isolated where required, a task-specific risk assessment has been completed, and appropriate PPE is worn. Particular care must be taken when working around live electrical components, gas systems, hot surfaces and moving parts.
If the work cannot be completed safely, stop work and rectify the hazard before proceeding.
Safety takes priority over diagnosis, installation timeframes and customer expectations. If the task cannot be completed safely, it must not proceed until the risks have been adequately controlled.
Purpose
- Test if a combustion fan is faulty
- Identify the correct replacement fan
- Avoid confusion with multiple fan versions
✅ 1. Identify the Fan Type First
Before testing or ordering parts, confirm what type of fan you have:
230V AC Fan
- Used in Single Stage and Two‑Stage units
- Standard wiring (no control pins)
24V DC Fan
- Used in Modulating units
- Has multi‑pin plug (low voltage control)
Quick rule:
- Big plug with pins → 24V DC (Modulating)
- Standard 240V wiring → 230V AC
2. Combustion Fan Testing
A. 230V AC Fan (Single & Two‑Stage)
Test Steps
Start a call for heat
Wait approx. 10 seconds
Measure voltage at fan terminals
- ✅ Expected: 230 Vac
Isolate power and check resistance
- ✅ Expected: 40–44 Ω
Results
| Result | Action |
|---|---|
| No voltage | Check PCB / wiring |
| Voltage OK + fan NOT running | ✅ Replace fan |
| Resistance outside 40–44 Ω | ✅ Replace fan |
B. 24V DC Fan (Modulating)
Test Steps
Check supply voltage
- ✅ Pins 1–5 = 24V DC
Check control signal during startup
- ✅ Pins 4–5 > 10V DC
Results
| Result | Action |
|---|---|
| No 24V supply | Check wiring / PCB |
| Supply OK but no control voltage | Check PCB / controls |
| Voltages correct + fan NOT running | ✅ Replace fan |
⚠️ 3. Two‑Stage Only – Resistor Check
Two‑stage systems use a combustion fan resistor.
Test
- Isolate and check resistance
Correct Values
| Part Number | Resistance |
|---|---|
| 621791 | 720 Ω |
| 611570 | 500 Ω |
Result
| Condition | Action |
|---|---|
| Open circuit | ❌ Replace resistor |
4. Quick Fan Identification
Step 1 – Confirm System Type
| System Type | Fan Type |
|---|---|
| Single Stage | 230V fan |
| Two Stage | 230V fan (+ resistor) |
| Modulating | 24V DC fan |
Step 2 – Use This Rule
If replacing:
- Same type (230V or 24V) MUST match
- Same size/style (small vs large housing)
- Cross-check part number where possible
Step 3 – If Unsure
Use ONE of these:
- Match physically (best field method)
- Check unit model / serial
Pt# 620510
Pt# 638379
Pt# 642963
Pt# 621517
Pt# 638416
Pt# 642970
Pt# 641621
Pt# 642987
Pt# 641638
Combustion Fan Identification
Non‑Condensing (230V – Single & Two‑Stage)
| Part Number | Standard Description |
|---|---|
| 620510 | Combustion Fan Assembly – 230V – Complete (incl. suction fitting) – suits 3 & 4 star heaters – Pre Serial No. T10050225 |
| 638379 | Combustion Fan Assembly – Small – 230V – Galvanised lid – non‑condensing single & two‑stage heaters – From Serial No. T10050225(also used on some large units between T10071675 and T13031830) |
| 642963 | Combustion Fan Assembly – Large – 230V – Galvanised lid – non‑condensing single & two‑stage heaters – From Serial No. T13031831 onward |
Condensing (230V – Two‑Stage Only)
| Part Number | Standard Description |
|---|---|
| 638416 | Combustion Fan Assembly – Small – 230V – Stainless steel lid – condensing two‑stage heaters – From Serial No. T10070316 (to T13031830 changeover) |
| 642970 | Combustion Fan Assembly – Large – 230V – Stainless steel lid – condensing two‑stage heaters – From Serial No. T13031831 onward |
5 Star / Legacy Transition Fans
| Part Number | Standard Description |
|---|---|
| 621517 | Combustion Fan Assembly – 230V – power coated housing – suits 5 star heaters – legacy replacement – Pre Serial No. T10070316 |
Modulating (24V DC Fans)
| Part Number | Standard Description |
|---|---|
| 641638 | Combustion Fan Assembly – Small – 24V DC – Stainless steel lid – modulating heaters |
| 641621 | ⚠️ Combustion Fan Assembly – 24V DC – modulating heaters |
| 642987 | ⚠️ Combustion Fan Assembly – 24V DC – modulating heaters |
5. Field Tips
- Always test before replacing
- Most failures = fan not starting with correct voltage present
- Don’t overcomplicate:
- Voltage + no run = bad fan
- Take a photo of the fan before leaving site (incase needed for identification)
✅ Technician Takeaway
- Identify fan type first (230V or 24V)
- Confirm voltage is correct
- If fan doesn’t run → Replace it
- Two‑stage only → check resistor