Universal H-Series Low NOx Heater 400K BTU - NG - H400FDN



Part # H400FDN

By Hayward Pool Products

  4.89 Average  



Part # H400FDN

Warranty 1 Year

Product Description

Click here to save BIG $ on an Identical Low Nox Cupro Nickel Heater by TRANE


Universal H-Series Low NOx Heaters
For In-Ground Pools and Spas - Electronic Ignition Dual Thermostat

Now With Cupro Nickel Heat Exchanger!

NEW Hayward Universal H-Series Low NOx heater combines industry-leading hydraulic performance; environmentally responsible low NOx emissions and a standard cupro nickel heat exchanger with a low profile appearance, lighter weight design, independent left or right side electric, water and gas connections plus 2" x 2 1/2" CPVC union connections to provide the ultimate installation flexibility for new or existing equipment pads.

It also includes a forced draft system that constantly moves air through the combustion chamber at a precise flow rate. As a result, all outside weather variables that can affect the heating performance, such as wind conditions, are eliminated - without having to install a high-wind stack.

NOTE: The Hayward Universal H-Series heaters meet the NOx Emission standards set by the California South Coast Quality Air Management Commission for 2001 and the Texas Natural Resource Conservation Commission Code.


  • Forced draft system constantly moves air through the combustion chamber at a precise flow rate, eliminating all outside weather variables that can effect the heating performance, such as high wind conditions -- without a high wind stack.
  • Our patented single header "V" baffle design heat exchangers allow for long life, fast heating and virtually no condensation. All Hayward heat exchangers are now constructed with highly resilient Cupro Nickel for greater durability and longevity even in salt-based, high flow or aggressive water chemistry set ups.
  • Easy-to-read L.E.D. digital Control Panel provides digital temperature readout and diagnostics for quick identification of components that might need service.
  • New polymer header provides superior hydraulic performance to save energy and protect against the damaging effects of erosion from high flow conditions for improved reliability.
  • Designed for today's larger diameter plumbing systems with 2" x 2 1/2" CPVC unions.
  • 1 Year limited warranty
  • Now with Cupro Nickel Heat Exchanger

Overall Dimensions

Product Specifications

400,000 BTU
197 lbs.
36-1/2 in.
0-10,000 ft.
Heat Exchanger:
Cupro Nickel
Special Feature:
Low NOx Emissions

?     Asked on 4/21/2019 by Pete Kamper

I'm looking at the Hayward N G Heaters. The 400,000 BTU and the 500,000 BTU The 400,000 has a shield over the control panel.Does the 500,000 also have this shield? Thank You

A  Answered on 4/25/2019 by InyoPools Product Specialist Lennox H.

Hi, Pete. All of the Universal H-Series Low NOx Heaters will include a Keypad Cover Kit - FDXLKPC1930

?     Asked on 11/6/2018 by Gershon A Goldman

For a three wire remote connection to the Hayward Pro Logic board, where does the third wire, (spa) attach?

A  Answered on 11/9/2018 by InyoPools Product Specialist Lennox H.

Hi, Gershon. The 3-Wire is for COM, POOL, and SPA. 

?     Asked on 6/17/2018 by Roy

I have a pentair 400,000 btu heater now however the water lines come in from the left side of the heater. Can this heater be retro fit so i don't have to re route existing plumbing?

A  Answered on 6/20/2018 by InyoPools Product Specialist Joe S.

Yes, this unit will allow you to rotate the unit. There will be a How To: Rotate UHS Heat Exchanger in the manual. 

?     Asked on 6/13/2018 by

How do you turn the heater off?

A  Answered on 6/13/2018 by InyoPools Product Specialist Lennox H.

Hi. You can turn off the heater from the power source, the breaker. 

?     Asked on 5/21/2018 by Bill

I currently have a pentair heater that is at least 20+ years old that I am replacing. (1) will this heater work with my Easy Touch Automation Remote? (2) there is a 3/4 inch gas line running about 85 feet from my meter to the current heater. Will that work with this?

A  Answered on 5/24/2018 by InyoPools Product Specialist Lennox H.

Hi, Bill. This heater will work with an EasyTouch Automation Remote 2-Wire. The gas connection at the heater is 3/4", this unit will only work with 3/4" gas line if you have a High-Pressure, 2-Stage application. 

?     Asked on 5/6/2018 by Jack

I'm opening the pool and starting up the Hayward heater, there is a crossover pipe between the inlet and outlet on the heater, is the crossover pipe in the open or closed position during normal operating function? I have it in the closed position but the heater keeps turning on and off. What is wrong? Thanks

A  Answered on 5/6/2018 by InyoPools Product Specialist Lennox H.

Hi, Jack. The valve should be in the opening position. The water flow might be too much for the unit. 

?     Asked on 5/5/2018 by Ronnie

What size pool is this heater good for???

A  Answered on 5/5/2018 by InyoPools Product Specialist Lennox H.

Hi, Ronnie. This heater can be used to heat a pool/spa that is 10K-40K gallons. If your pool is larger you might need another unit depending on how fast you want the water heated.

?     Asked on 4/16/2018 by CURT


A  Answered on 4/18/2018 by InyoPools Product Specialist Megan C.

The Universal H-Series Low NOx Heaters are equipped with Keypad Bezel Cover to protect the keypad. 

?     Asked on 1/4/2018 by Mike

I have a 15 year old Teledyne Laars 400000 BTU heater supplied by three-quarter inch natural gas line. Is the 3/4 inch gas line existing going to be enough to supply this heater they are both 400000 btus. Thx. Mike

A  Answered on 1/5/2018 by InyoPools Product Specialist Lennox H.

Hi, Mike. A 3/4 gas line is sufficient for specific applications, refer to page 22-24 of the Hayward H-series LoNox Installation Guide

?     Asked on 11/22/2017 by Patrick S.

Can turn this pool heater on and off whenever I want? or once I turn it on for the season (say April), I have to keep it on running 24/7 until I winterize it in September? Is there a "On/Off" button? I live in PA, 45'X25' pool

A  Answered on 11/22/2017 by InyoPools Product Specialist Lennox H.

Hi, Patrick S. You can put the unit in STANDBY mode which will turn the Heater off. 

?     Asked on 9/20/2017 by Steve

What is the best way to determine the BTU size of heater required for my pool? I currently have a 406,000 BTU unit and my pool is 27,000 gallons.

A  Answered on 9/20/2017 by InyoPools Product Specialist Lennox H.

Hi, Steve. Our Sizing A Pool Heater Blog has all the steps you'll need. 

?     Asked on 8/24/2017 by Elisha

I ordered this heater, Is there a service installed?

A  Answered on 8/25/2017 by InyoPools Product Specialist Joe S.

Good afternoon, for a list of installers you will need to review Hayward's Dealer locator. All you need to do is Enter a postal code to find the nearest Hayward authorized pool professional.

?     Asked on 6/6/2017 by Jeff

What size propane line is required?

A  Answered on 6/12/2017 by InyoPools Product Specialist Lennox H.

Hi, Jeff. The Hayward H-series LoNox Installation Guide provides the gas line info on page 22 under Gas Supply and Piping.

?     Asked on 5/25/2017 by Arnold Zaffos

With regards to the Hayward H400FDN, is the cover a new design for 2017?

A  Answered on 5/25/2017 by InyoPools Product Specialist Scott W.

Yes, the new design has a cover on the key pad.

?     Asked on 2/21/2017 by

Is the keypad cover on this unit or does it need to be purchased separately? Does price include shipping?

A  Answered on 2/21/2017 by InyoPools Product Specialist Scott W.

Yes, the keypad cover comes with the unit and the price includes shipping.

?     Asked on 12/9/2016 by Bill

how do i properly winterize my hayward h 250 ? as far as i can see they tell you to open the drain on the header. this doesn't seem to be enough.

A  Answered on 12/9/2016 by InyoPools Product Specialist Lennox H.

Hi, Bill. Draining all of the water from the unit is all it takes to winterize the unit. You can take it an extra step and get a heater cover to protect the unit from the cold.

?     Asked on 6/7/2016 by Matt

I have an existing 1-1/4" gas line run at around 140'. Will that be sufficient for this heater?

A  Answered on 6/11/2016 by InyoPools Product Specialist Lennox H.

Hi Matt, According to the Hayward H-series LoNox Installation Guide you will need 1-1/2" pipe.

?     Asked on 5/25/2016 by Chuck

Can I control a Hayward H400FDN heater with my Jandy AquaLink RS control system?

A  Answered on 5/29/2016 by InyoPools Product Specialist Lennox H.

Hi Chuck, The H400FDN can be wired and or controlled by a Jandy Aqualink RS controller.

?     Asked on 5/21/2016 by

when is the date of delivery

A  Answered on 5/22/2016 by InyoPools Product Specialist Alex V.

Hello, Freight deliveries can take 5-10 business days.

?     Asked on 3/27/2016 by Rich

Can this heater be installed outside and used in all weather conditions?

A  Answered on 3/28/2016 by InyoPools Product Specialist Lennox H.

Hi Rich, This unit can be installed outside and can be used in all weather conditions. Please refer to the Hayward Universal H-Series Owner's Manual for winterizing instructions.

?     Asked on 2/19/2016 by

do all the heaters have a right side connection or do you have to order it that way?

A  Answered on 2/19/2016 by InyoPools Product Specialist Megan C.

Normally heaters are factory set to the rights side for connection. The H400FDN is factory set to the right, it may be reversed by a Certified Pool Technician before installation.

?     Asked on 9/6/2015 by tammy

is there any specific winterizing instructions the manual does not have any.

A  Answered on 9/10/2015 by InyoPools Product Specialist Lennox H.

Hi Tammy, You can find the Winterizing Instructions in the Hayward H-series LoNox Installation Guide.

?     Asked on 5/21/2015 by Jack Green

When wiring this heater is it 110 or 220?

A  Answered on 5/21/2015 by InyoPools Product Specialist Patrick P.

Hi Jack, This heater can be wired either 110V or 220V, it comes set up for 220V but can be easily converted to 120V by removing the 220V jumper.

?     Asked on 4/21/2015 by George

I want to install the Hayward Lownox heater on my pool but I live in the mountains at 3,000 foot elevation; do I need to buy a special high elevation heater?

A  Answered on 4/21/2015 by InyoPools Product Specialist Matt S.

Hello George, The Hayward Universal Low Nox are self adjusting up to 10,000 feet of elevation. Any installation above 2,000 feet will require a special restriction plate to be installed. Luckily, that plate comes standard with every heater so no need to purchase it separately.

?     Asked on 4/19/2015 by

I would like to replace an older Model H400 IDL serial n°30831409 dated 2002 with this Low Nox 400K NG. Is this the right choice? Will the new unit be compatible with the existing external electronic panel control?

A  Answered on 4/20/2015 by InyoPools Product Specialist Patrick P.

Yes the H400FDN would be the correct replacement for the H400IDL heater. Yes this heater would be compatible with any external controller you were using on the H400IDL.

?     Asked on 10/2/2014 by Guest

Heater model # H400FDN, can be used on pools of all different sizes? The larger the BTU's the quicker the pool will get warm

A  Answered on 10/2/2014 by InyoPools Product Specialist Patrick P.

The required size of a heater depends on the size of your pool or spa and how fast you want to heat up your pool. In the northern states you will probably want the larger heaters to compensate for the colder temperatures. Above ground pools use heaters from 55K to 130K BTUs. In ground pools and spa general run from 100K to 400K BTU. For an average 15' x 30' pool (15,000 gallons), a 300K BTU heater would raise your pool temperature 2 degrees per hour; a 400K BTU heater, 2.7 degrees per hour. For a 20' x 30' pool (20,000 gallons) the temperature rise for 300K and 400K heaters would be 1.5 and 2 degrees respectively. By contrast these same heaters would raise the temperature of an average size spa 30 degrees in 35 and 20 minutes respectively.

?     Asked on 10/8/2013 by Guest

Where can my heater be installed?

A  Answered on 10/8/2013 by InyoPools Product Specialist Patrick P.

There are specific venting requirements and clearance distances that must be met to ensure proper operation. Consult the owner’s manual for exact specifications for your model of heater.NOTE: Gas fired heaters should be diagnosed and repaired by trained personnel.

?     Asked on 10/7/2013 by Guest

Is it possible to convert this heater from natural gas to propane?

A  Answered on 10/7/2013 by InyoPools Product Specialist Patrick P.

Yes, it is possible to convert this heater from natural to propane gas as there is a conversion kit for it. To view and purchase this kit you you will need to click here.

?     Asked on 4/28/2013 by Guest

How can I tell the difference between a Propane or Natural Gas heater listed on your website?

A  Answered on 4/28/2013 by InyoPools Product Specialist Patrick P.

A heater's model number will usually end with an 'N' or a ''P' to denote the gas type. if the model number is only numbers we also have the gas type in the description of the product. The abbreviations "NG" is for natural Gas and "LP" is form Liquid Petroleum, also called Propane.

?     Asked on 1/17/2013 by Guest

Is a vent required for typical outside installation for the Hayward 150k Ng Lo Nox heater?

A  Answered on 1/17/2013 by InyoPools Product Specialist Patrick P.

For the Hayward  Lo Nox heater, you would not need to purchase an additional vent if you are using it outside. If you are installing the heater outside, we recommend you purchasing a heater cover. This will protect your heater from debris, rain, snow, heat- or pretty much anything that could get caught in your heater. The Weather-Out Pool Heater Cover  can be viewed on our website.

?     Asked on 10/18/2012 by Guest

What is the elevation rating for the Hayward H-Series Low Nox Heaters

A  Answered on 10/18/2012 by InyoPools Product Specialist Patrick P.

Per Hayward, the Hayward H-Series Low Nox Heaters have an elevation rating up to 7,700'

?     Asked on 10/10/2012 by Guest

What is the difference between an Electronic and Millivolt ignition?

A  Answered on 10/10/2012 by InyoPools Product Specialist Patrick P.

The basic answer is that Millivolt is the old style of ignition, using a standing pilot light. While the electronic ignition uses a spark start system like a gas grill. Electronic ignitions are more efficient in gas usage and also are better suited for high wind areas where a pilot light might get extinguished.

?     Asked on 1/16/2012 by Guest

Will a gas heater operate at the low speed of a dual speed motor?

A  Answered on 1/16/2012 by InyoPools Product Specialist Patrick P.

Probably not - Most gas heater will shut off at low GPM levels (20-40) to keep the heater cores from overheating - 20 for 200 BTUs, 40 for 400 BTUs. Depending on your pool system you might get these GPM levels at the low speed but generally these heaters must be run at the motor's high speed.

?     Asked on 12/29/2011 by Guest

How do you know if a pool heater is a propane heater or a natural gas heater?

A  Answered on 12/29/2011 by InyoPools Product Specialist Patrick P.

Typically in the product name we use the abbreviation of "LP" for propane or "NG" for natural gas. Click here for a full listing of our >Pool Heaters.


Est. Repair Time

2 to 4 hrs

Tools Needed

Screwdriver, Pliers, Wrench Set, Adjustable Wrench, Multimeter

Old heater was rusted out and not working

Written on 4/12/2018 by Guest

Asked INYOPOOLS.COM forum which heaters were recommended. Got a reply almost immediately and proceeded to look through internet for best price on that brand/model. INYOPOOLS.COM not only had an extremely competive price but also provided the knowledge on which product would work best for my needs and info on how to install it.

Got heater from delivery service - they placed right in front of garage. Heater was heavy but not too much for two adults to move into place. Old heater was already removed from exisiting pad. Connected gas line with minimal adjustments to old setup. Replumed pool PVC lines as it was slightly different setup than old Jandy heater. Tested elecrircal volts to confirm it was 240v. Connected electric and tested heater - it kicked right on after about 30 seconds.

Was in hot tub that night enjoying!

Were these instructions helpful? 


Est. Repair Time

Less than 1 hr

Tools Needed

Screwdriver, Adjustable Wrench

I needed a new pool heater.

Written on 10/3/2017 by Guest

I had this heater installed by a plumber and electrician in about 30 minutes! It is so quiet that the only noise is from the flames on the burner, and it heats my pool water at about 2 - 3 degrees per hour. Plus, because I live in the Northeast I have to winterize my filter, etc. This heater is very easy to winterize as the instructions are precise and very easy to follow. A perfect pool heater!

Were these instructions helpful? 


Review Summary

 (Average 4.89  out of 5 stars)
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