What Size Hayward H-Series Gas Pool Heater Do You Need? (W3H135FDN vs W3H150FDN vs H400FDN)
Sizing a gas heater is the one decision people get wrong most often. Go too small and the pool never really gets warm on a cool week. Go too big and you may be paying for a gas line upgrade you did not plan on. The Hayward Universal H-Series comes in a wide spread of sizes, from the W3H135FDN up to the H400FDN and beyond, so it helps to know which one actually matches your pool.
What size Hayward H-Series heater do I need?
Take your pool's surface area in square feet, multiply it by how many degrees you want to raise the water, then multiply that by 12. That number is roughly the minimum BTU rating you need to heat the pool in about 24 hours.
So a 15 by 30 foot pool is 450 square feet. If you want a 20 degree rise, that is 450 x 20 x 12, which lands around 108,000 BTU. A W3H135FDN would cover it. Most installers round up from there, because wind, no cover, and cold nights all eat into your real world numbers.
A few things push you up a size:
- An attached spa you want hot in under an hour
- No solar cover on the pool
- An open, windy yard with little shade
- A long season where you are heating in spring and fall
- Any water feature that keeps dumping heat into the air
Which Hayward H-Series model fits which pool?
Here is the short version. The smaller models are for spas and compact pools, and the big ones are for full size in-ground pools or pool and spa combos.
- W3H135FDN (135,000 BTU) is the compact end of the line. Good for spas, above ground pools, and small in-ground pools under about 350 square feet.
- W3H150FDN (150,000 BTU) suits smaller in-ground pools and spa setups where you want a little more punch than the 135.
- H400FDN (400,000 BTU) is the size most in-ground pools end up with. It is the standard pick for a 15 by 30 or larger pool, and it is the one you want if you have a spa you actually use.
- H500FDN (500,000 BTU) is for oversized pools, cold climates, or anyone who wants very fast heat up times.
If you are on the fence between two sizes, the bigger one is usually the safer buy. A heater that is too big just runs for less time. A heater that is too small runs constantly and still leaves you disappointed.
Does a bigger heater cost more to run?
Not really, and this surprises people. Heating a pool takes a fixed amount of energy no matter how you get there. A 400,000 BTU heater gets to temperature faster and shuts off sooner. A 135,000 BTU heater burns less per hour but runs far longer to do the same job.
Where a bigger heater does cost more is up front, and in the gas line. Which brings us to the part people skip.
Do you have enough gas supply for an H400FDN?
Check this before you buy. A 400,000 BTU heater needs a gas line sized to deliver that much fuel, and a lot of existing lines were run for something smaller. The right pipe size depends on the run length, the pipe material, and whatever else is already pulling gas at the house.
Ask a licensed gas fitter to look at it. If the line is undersized, the heater will struggle to fire, cycle off, or never hit its rated output. That is not a heater problem, but it gets blamed on the heater all the time. The same check applies to propane models like the W3H135FDP and W3H150FDP, where tank size and regulator capacity matter too.
Natural gas or propane?
Buy the one that matches the fuel you already have. Hayward sells the H-Series in both, and the model number tells you which is which. An N at the end means natural gas, so W3H150FDN. A P means liquid propane, so W3H150FDP. They are not interchangeable on a whim, and converting one in the field is not something to improvise.
What about venting?
If the heater sits outdoors in the open, you are fine as it ships. If it is going indoors or inside an enclosed equipment room, you need a proper vent kit, and Hayward sells them by BTU size and by vertical or horizontal orientation. Do not mix sizes here. A 400K heater needs the vent adapter built for 400K.
So which one should you buy?
For most in-ground pools in the 400 to 800 square foot range, the H400FDN is the right call. It heats fast, it handles a spa, and you are not going to outgrow it. Step down to the W3H150FDN or W3H135FDN if you have a small pool, a standalone spa, or an above ground setup where the 400 would be overkill.
Still not sure? Send us your pool dimensions and whether you have a spa, and we will tell you which H-Series size to get.
Hayward | H400FDN | Universal H-Series Natural Gas 400,000 BTU Low NOx Gas Heater
$3999.00
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