A.O. Smith Signature 300 40-Gallon Tall 40000-BTU Natural Gas Water Heater - Item #839885 | Model #G12-CADT4040NV
A. O. Smith's Signature 300 Series 40-gallon gas water heater provides reliable hot water for households with 3 to 4 people when sized appropriately. With an environmentally friendly 40,000 Btu/h Low-NOx gas burner, this flue damper model delivers 80 gallons of hot water in the first hour. This water heater does not require external power and is as simple as a standard atmospheric model. The automatic damper prevents cool air from traveling through the flue in standby mode to maintain overall heat and efficiency. The natural gas hot water heater is built with safety in mind. It has a safety system that reduces the risk of accidental flammable vapor ignitions. A factory-installed temperature and pressure relief valve and tamper-proof drain valve also ensure safe operation. This model is built to last. It features Tank Guard® which is designed to provide ultimate tank protection and increase lifespan of the water heater. This added protection comes with a for additional peace of mind.
Features
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Delivers the ideal amount of hot water for households with 3 to 4 people when sized appropriately
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Fitted with an automatic flue damper that opens and closes automatically to increase efficiency and reduce heat loss in standby mode
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Simple installation that uses standard 3-in or 4-in vent ducting and requires no power
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Features an environmentally friendly 40,000 BTU low NOx gas burner that delivers 82 gallons in the first hour
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Tank Guardprovides advanced tank protection that extends the life of your gas water heater
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Tall, slim 62.5-in high x 20-in diameter profile enables easier installation in narrow spaces
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High-altitude certified from 0 ft to 10,100 ft
Specifications
Diameter (Inches) |
20.0 |
Rated Storage Capacity |
38-Gallon |
Family Size |
4.0 |
Vent Diameter(s) |
3-in; 4-in |
Height (Inches) |
62.5 |
Water Connection Size |
3/4-in |
Insulation Thickness (Inches) |
2.0 |
Weight (lbs.) |
135.0 |
Nominal Capacity (Gallons) |
40 |
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What our Experts Think
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Strong everyday performance: This 40‑gallon tall natural gas water heater delivers a robust first‑hour hot water output (~80+ gal) with a 40,000 BTU Low‑NOx burner, ideal for homes with ~3–4 people doing showers, laundry, and dishes.
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ENERGY STAR‑certified efficiency: The automatic flue damper increases efficiency by reducing standby heat loss, helping lower annual energy costs compared with basic atmospheric units.
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Durable tank protection: Tank Guard® and fused ceramic tank shield help resist corrosion and extend service life, backed by a 12‑year limited residential warranty for lasting peace of mind.
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Precise temperature & diagnostics: Electronic gas control with LED status indicator provides accurate temperature regulation and self‑diagnostics that make troubleshooting easier.
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Space‑friendly tall design: Slim tall profile fits better in closets and utility rooms with limited floor space while still offering full residential capacity.
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High‑altitude capable: Certified for installation from 0 ft to ~10,100 ft elevation, making it versatile for varied geographic locations.
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Safety built in: Factory‑installed temperature & pressure relief valve and tamper‑proof drain valve support reliable, code‑compliant operation.
Q&A
Q: What household size is this water heater best for?
A: Ideal for households of 2–3 people, providing consistent hot water for showers, laundry, and everyday use.
Q: What type of fuel does it use?
A: Operates on natural gas, delivering reliable heating and energy-efficient performance.
Q: How powerful is the heater?
A: Features a 40,000-BTU burner for fast recovery and consistent hot water supply.
Q: What are the dimensions of this tall water heater?
A: Designed as a tall, narrow profile, ideal for closets or small utility spaces.
Q: Does it include safety features?
A: Yes. Includes a temperature and pressure relief valve and other standard safety protections for reliable operation.
Q: Is it energy efficient?
A: Yes. The Signature 300 Series is designed for high thermal efficiency and reduced gas usage.
Q: How is the tank protected?
A: Equipped with a protective anode rod and premium tank materials to resist corrosion and extend lifespan.
Q: What are the installation requirements?
A: Requires natural gas connection, proper venting, and indoor residential installation.
Maintenance Tips
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Inspect and flush the tank annually: Drain and flush the tank every 6–12 months to remove sediment and maintain heating efficiency.
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Check the anode rod: Inspect the sacrificial anode rod yearly and replace if heavily corroded to prevent internal tank rust.
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Examine gas and vent connections: Ensure gas lines, fittings, and venting are secure, unobstructed, and free from corrosion.
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Test the T&P relief valve: Operate the temperature & pressure relief valve yearly to ensure it opens properly and safely.
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Monitor burner operation: Watch for irregular ignition or flame color changes, which may indicate venting or gas issues.
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Inspect for leaks: Regularly check water connections, fittings, and tank base for moisture or corrosion.
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Maintain proper space and airflow: Keep the area around the water heater clean, dry, and well-ventilated for safe operation and easy maintenance.
Damage 
Loss 
Theft 
