A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater 79 Error Fix: Insufficient Water Flow

Error code 79 on the A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater means: Insufficient Water Flow. The water flow rate is below the minimum activation threshold. The unit requires a minimum flow rate (typically 0.4-0.5 GPM) to activate the burner. Check the inlet water filter screen for debris. Fully open the hot water fixture to increase flow. Check for partially closed isolation valves. Descale the heat exchanger if mineral buildup is restricting flow. Verify the water supply pressure is adequate. Install a recirculation pump if the fixture is far from the unit.

The most successful fix is "Descale Heat Exchanger and Inspect Water Supply Pressure" (hard difficulty, ~90 minutes). Estimated DIY cost: $15-$30. HomeMD has 2 community-ranked fixes for this error.

This error is rated low severity. It is generally not urgent and can often be resolved with a simple DIY fix.

Yes — the top-ranked community fix ("Descale Heat Exchanger and Inspect Water Supply Pressure") is rated hard difficulty and takes about 90 minutes. Tools needed: Adjustable wrench, Descaling pump kit, Pressure gauge, Bucket.

DIY cost: $15-$30. Professional repair typically costs $250-$400. Many homeowners save money by trying the top-ranked community fix first.

The A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater has 8 other known error codes, including: 11 (Ignition Failure), 12 (Flame Failure), 13 (Combustion Abnormality), 14 (Overheat Protection Activated), 16 (Over-Temperature Warning), and 3 more.

Error code 79 also appears on 1 other A.O. Smith model: ATI-540H-N 0.54 GPM 199,900 BTU High Efficiency Tankless Gas Water Heater. The fix may vary by model, so check each model's specific page for tailored repair steps.