Hayward
Hayward Universal H-Series 400K BTU Gas Pool & Spa Heater (W3H400FDN)
Best for fast heat, cold climates & spas — The fastest way to heat — 400,000 BTU of natural-gas output that warms a pool or spa in any weather, with a cupro-nickel exchanger and 84% thermal efficiency
Specs
| Heat source | Natural gas (propane model available) |
| Output | 400,000 BTU |
| Run cost | Highest of the three |
| Heating speed | ~2°F/hr (fastest) |
| Heats on demand | Yes, in any temperature |
| Best for | Cold climates, spas & fast heat-ups |
Pros
- Heats fastest and works in any weather — the only type that can warm a pool in a cold snap or extend the season into the shoulder months
- 400,000 BTU plus a standard cupro-nickel heat exchanger suits large pools, spa duty, and frequent cold-starts
- 84% thermal efficiency, Low-NOx (California/Texas compliant); natural-gas or propane versions
- Ideal for a pool-and-spa combo where you want hot water fast on demand
Cons
- Highest running cost of the three by far — you're burning fuel, not moving free heat
- Needs a gas line (or a propane tank) sized for 400,000 BTU/hr — a real install consideration
- Overkill for a small pool in a warm climate, where a heat pump runs for a fraction of the cost
“Turned heater on and had it up to 88 degrees in 4 hours for a pool 18x38 inground — absolutely thrilled.”— verified buyer, INYO Pools · Verified owner
Size your pool first → ·All pool gear →
Data last verified: Jun 2026. Ratings are real, dated reads — never scraped or invented. Full methodology and monetization disclosure: how we test.