540 High exhaust (flue gas) temperature

What the control is telling you

The exhaust thermistor has measured flue temperature above the allowed limit. Factory setup is for PVC venting, which limits exhaust to below 149°F (65°C).

Applies to: I-Series Plus boilers — 150K BTU and smaller and 175K BTU and higher (Combi and Solo)

Likely causes

• Parameter A2 vent material setting doesn't match installed vent
• Faulty or poorly connected exhaust thermistor
• Blocked or restricted intake/exhaust or excessive vent length
• Wrong gas type or wrong orifice (175K+)
• Scale in heat exchanger or blocked heat exchanger fins
• Damaged exhaust duct, seal or venting
• Failed PC Board

Test and repair steps

1. Warning: Only set the boiler for higher exhaust temperatures when venting is CPVC, listed polypropylene or stainless steel. Never do this with PVC — fire or carbon monoxide leak may result.
2. Remove the front panel (two screws), locate the PC Board at lower left, and hold SW1 for 5 seconds to enter parameter settings.
3. Set Parameter A2 to match the vent (A = PVC; B = CPVC, PP or other non-PVC), press select to save, then press SW1 to exit.
4. Check performance data 06 against a thermometer at the same spot; a difference over 5°F (2.8°C) means continue, and -40°F means the thermistor is unplugged or poorly connected.
5. Inspect the exhaust thermistor harness back to the PC Board for loose, broken or damaged connections.
6. Warning: Turn off electrical, gas and water supply. Remove the thermistor and clean its surface.
7. Measure the thermistor at PC Board connector CN7 (150K: pins 3 and 6; 175K: pins 3 and 14) and compare against the Appendix B table in the source manual (p.53); replace if off by more than 5°F.
8. Check vent termination clearances, look for restrictions at intake and exhaust, and confirm vent length is within the manufacturer's limit.
9. Confirm the gas supply matches the rating plate gas type.
10. 175K and higher confirm the orifice matches the gas type (black = NG, grey = LPG, white = NG above 2,000 ft).
11. In hard water areas, check the heat exchanger for lime/scale; also inspect the exhaust duct, seal and vent for damage.
12. Check the heat exchanger for hot spots; if found, remove it, separate the burner and clean blocked fins with 120 psi compressed air, or replace the heat exchanger and all gaskets. Never reuse a removed burner gasket.
13. Replace the exhaust temperature thermistor.
14. If 540 still appears after all of the above, replace the PC Board.