Combustion Fan Testing & Identification Guide
(Simple Field Guide for Service Technicians)
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