Product Overview

The Hayward Universal H-Series Natural Gas Heater W3H400FDN is a powerful 400,000 BTU pool and spa heater designed to deliver fast, dependable, and efficient heating for residential in-ground pool and spa applications. Built on Hayward’s Universal H-Series platform, this heater combines strong natural gas performance with Low NOx emissions, flexible installation features, and durable construction for long-lasting pool comfort.

Featuring a cupro nickel heat exchanger, electronic ignition, digital controls, and a forced-draft combustion system, the W3H400FDN is engineered to provide reliable heat-up performance while helping protect against corrosion caused by challenging water chemistry conditions.

Powerful Natural Gas Heating for Pools and Spas

The Universal H-Series 400K BTU natural gas heater helps raise and maintain comfortable water temperatures for pools and spas. Whether used to extend the swimming season, warm cooler water, or provide fast heat recovery for an attached spa, this heater delivers strong on-demand heating when properly sized and professionally installed.

Its 400,000 BTU output makes it a strong choice for larger residential pools, pool/spa combinations, and applications requiring faster heat-up times.

Built for Low NOx Performance

The Hayward Universal H-Series Low NOx heater is designed to help reduce nitrogen oxide emissions while delivering dependable heating performance. This makes it suitable for many areas with stricter clean-air requirements.

The W3H400FDN uses natural gas and should only be installed with the proper natural gas supply, gas line sizing, ventilation, electrical setup, and code-compliant installation.

Flexible Universal Installation Design

The Universal H-Series platform is designed for adaptable installation and easier replacement. Its compact cabinet, connection flexibility, and service-friendly layout make it a strong option for both new equipment pads and heater replacement projects.

This heater is commonly selected for larger residential in-ground pools, spas, and pool/spa combinations where natural gas service is available and high BTU heating performance is needed.

Ideal For

Larger residential in-ground pools

Attached spas and pool/spa combinations

Applications requiring faster heat-up performance

Homes using natural gas

Saltwater pools with properly balanced water chemistry

Replacing an older natural gas pool heater

Pool owners wanting stronger heating performance than lower-BTU models

New equipment pad installations

Installations requiring 120/240 V compatibility

Areas where Low NOx heater performance is required

Key Features

Genuine Hayward Universal H-Series natural gas heater

400,000 BTU heating capacity

Natural gas fuel configuration

Low NOx emissions design

120/240 V electrical compatibility

60 Hz, single-phase operation

Designed for residential in-ground pools and spas

Cupro nickel heat exchanger for enhanced corrosion resistance

Excellent choice for saltwater pools when water chemistry is properly maintained

Forced-draft combustion system for consistent performance

Electronic ignition system with no standing pilot

Digital LED control panel with temperature display

Built-in diagnostics for easier troubleshooting

Universal-style design for flexible installation

Compact, low-profile cabinet

Independent left- or right-side water, gas, and electrical connections

2" x 2.5" CPVC union plumbing connections, where supplied

Designed for new installations and heater replacement applications

Benefits

Fast Heating for Larger Pools: The 400K BTU output delivers strong heating performance for larger residential pools, spas, and pool/spa combinations, helping customers enjoy comfortable water temperatures faster than lower-BTU models.

Natural Gas Performance: Natural gas provides consistent heat output, making this heater a reliable choice for homes with natural gas service and larger heating demands.

Corrosion-Resistant Heat Exchanger: The cupro nickel heat exchanger helps resist corrosion and premature wear caused by unbalanced water chemistry, saltwater systems, and demanding pool environments.

Cleaner Low NOx Operation: The Low NOx design helps reduce emissions and supports use in many areas with stricter air-quality requirements.

Easy Operation and Diagnostics: The digital LED control panel makes it simple to adjust water temperature, monitor heater operation, and support easier troubleshooting.

Flexible Replacement Option: The Universal H-Series design helps make the W3H400FDN a practical replacement choice for many existing pool heater installations.

Specifications

What’s Included

• One Hayward Universal H-Series Natural Gas Heater, 400K BTU
• Digital control system
• Cupro nickel heat exchanger
• Electronic ignition system
• Factory-installed heater components
• Manufacturer documentation, where supplied by packaging

Gas line components, venting materials, electrical whip, plumbing fittings, equipment pad materials, and installation labor are not included unless specified separately.

Warranty & Compatibility

The Hayward W3H400FDN is designed for natural gas pool and spa heating applications. It is commonly used for residential in-ground pools and spas when properly sized for the pool volume, desired temperature rise, plumbing flow, and local climate conditions.

Before ordering, confirm:

• Fuel type: Natural gas
• Heating capacity: 400,000 BTU
• Electrical requirements: 120/240 V, 60 Hz, single phase
• Natural gas line sizing and supply pressure
• Pool size and desired heat-up time
• Plumbing flow rate requirements
• Ventilation and installation clearance requirements
• Local gas, plumbing, electrical, and building codes
• Professional installation requirements

This heater should not be used with propane unless the heater model is specifically configured for propane gas.

Customer-Facing Q&A

Q: What type of fuel does the Hayward W3H400FDN use?
A: The W3H400FDN uses natural gas.

Q: How many BTUs is this heater?
A: This heater is rated at 400,000 BTU.

Q: What voltage does this heater require?
A: The W3H400FDN is designed for 120/240 V, 60 Hz, single-phase electrical operation.

Q: Is this heater for pools or spas?
A: This heater is designed for residential in-ground pools and spas when properly sized and installed.

Q: Does this heater have a cupro nickel heat exchanger?
A: Yes. The W3H400FDN features a cupro nickel heat exchanger for enhanced corrosion resistance.

Q: Is this heater compatible with saltwater pools?
A: Yes. It can be used with saltwater pools when the pool water is properly balanced and maintained.

Q: Is this a propane heater?
A: No. This model is configured for natural gas. Do not use it with propane unless the heater model specifically supports propane.

Q: What does Low NOx mean?
A: Low NOx means the heater is designed to produce lower nitrogen oxide emissions, helping meet clean-air requirements in many areas.

Q: Does this heater require professional installation?
A: Yes. Gas pool heaters should be installed by qualified professionals according to local gas, electrical, plumbing, and building codes.

Q: What is the barcode for this product?
A: The UPC / Barcode is 0610377387255.