Poor ignition or loud combustion resonance - Lennox SL280UHNV

What is going on

SL280UHNV (ultra-low NOx air/gas burner) has trouble lighting or lights with a loud resonance. Hard ignition means too much combustion air (air orifice too large); resonance means too little air.

Likely causes

• Wrong or loose air orifice
• Plugged air inlet screen
• Vent/air intake restriction
• Kinked, loose or plugged signal hoses to gas valve
• Gas valve out of range
• Low supply voltage or gas pressure (-05 models)

Check in this order

1. -05 and later confirm incoming voltage is 108-132 V and gas supply pressure is 5-14 in. w.c.; fix power or gas supply first.
2. On -01 to -04 (not -040), make sure the inducer was slowed to 4300 RPM. If it wasn't, call your Lennox service rep.
3. Check the air orifice (inside the gray coupling between the clamps): correct size per the orifice table, fully seated against the elbow, and air inlet screen clear.
4. Check manifold pressure (-01 to -04 Table 18/19; -05 Table 23 0.0-0.10 in. w.c.). The gas valve is not adjustable - if manifold is out of range, replace the gas valve.
5. If manifold is OK, check signal / system operating pressure (-05 Table 24). In range and still a problem: replace the air orifice; out of range: check vent/intake restriction, inlet screen and signal hoses.
6. If pressure still wrong after fixing restrictions, replace the gas valve.
7. Ignition problem after all checks fit the 'difficulty igniting' (smaller) air orifice. -05 resonance: install Low Profile Combustion Air Intake Kit 25B23 (-01 to -04: fit the 'resonance' larger orifice).

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