Jandy LXi Unstable Combustion: Flame Problems and Troubleshooting

Understanding Premix Combustion in the LXi

The LXi uses a premix forced-draft combustion system where the combustion blower pushes air through the burners. Air and gas are mixed in the burner venturis before entering the combustion chamber. This design produces very low NOx emissions when burning natural gas but requires precise balance between gas pressure and air pressure.

The combustion chamber operates at slightly positive pressure (0.08–0.18″ WC). If combustion products leak from the chamber or flue collector inside the cabinet, they get pulled back into the combustion air fan, creating a rich condition because the oxygen has already been consumed.

Types of Combustion Problems

Lazy flame with yellow tip

Caused by wrong gas type. Verify the gas supply matches the gas type on the rating plate. LP supplied to a natural gas heater, or vice versa, will produce abnormal flame characteristics.

Flame lifts and goes out

Caused by a lean fuel/air mixture. Either gas supply pressure is too low or combustion air volume is too high (excessive burner throat pressure). Correct supply pressure to assure 2.5″ WC (Natural) or 9.0″ WC (Propane) manifold pressure. Check burner throat pressure and adjust to 1.0″ WC.

Flashback on start-up

Caused by wrong gas type or damaged burner. Flashback occurs when the flame propagates backwards into the burner venturi. Replace the burner if damaged, and verify the gas supply matches the rating plate.

Short yellow-streaked flame

Caused by a rich fuel/air mixture. Either manifold pressure is too high or combustion air is too low. Correct manifold pressure and check burner throat pressure. Also check combustion system sealing beneath burners.

Step-by-Step Troubleshooting

Verify gas type and pressures

1. Confirm the gas type on the rating plate matches the actual gas supply.
2. Measure inlet supply pressure: NG 5.0–10.5″ WC, LP 11.0–14.0″ WC.
3. Measure manifold pressure with heater firing: NG 2.5″ WC, LP 9.0″ WC.
4. If manifold pressure is off, the gas valve regulator may need adjustment or replacement.

Measure and adjust burner throat pressure

1. Locate the burner throat pressure tap. Remove the plug and connect a manometer.
2. With the heater firing, throat pressure should read 1.0″ WC.
3. Adjust using the air orifice slider on the blower intake. Sliding to cover more of the orifice decreases air pressure; exposing more increases it.

Inspect combustion chamber sealing

1. Check that the combustion chamber and flue collector are properly sealed.
2. If service has been done involving burner or heat exchanger removal, verify all seals are intact.
3. Combustion product leaks inside the cabinet get recirculated through the blower, depleting oxygen.

Common Parts That Fix This Problem

• Gas orifices (if gas type changed or local gas has unusual heat content)
• Gas valve/regulator
• Burner assembly (if damaged from flashback)
• Combustion chamber gaskets and seals

When to Contact Zodiac Technical Support

If problems with lifting or short yellow/orange flames cannot be corrected with slight changes to manifold pressure and burner throat pressure, the local fuel supply may differ greatly from average heat content. Contact Zodiac Pool Systems Technical Support at 800.822.7933 for assistance with orifice sizing.