It’s an understandable question — a tankless water heater and a pool heater both promise on-demand, no-tank heat, and a Rinnai RU199iN’s 199,000 BTU output sounds like more than enough to warm a pool. But the two products are built for different jobs, and running pool water through a domestic tankless unit isn’t a gray area — it’s a specific, named warranty exclusion.
Why the standard answer is no
Rinnai’s Limited Warranty does not cover any product used in an application with chemically treated water, such as a pool or spa heater — language that shows up consistently across the brand’s warranty documentation, per Rinnai America Corporation. Domestic tankless units are engineered and certified to run potable water only; chlorine, bromine, or salt-cell byproducts in pool water attack the heat exchanger and internal components in ways the unit was never tested against. This isn’t unique to Rinnai — it’s standard language across the category, the same way a standard tankless heater is excluded from radiant floor heating for a related reason: certification for one specific water chemistry and use case, not a general license to heat any water that flows through it.
BTU headroom doesn’t change the answer. Even our top overall pick on the best tankless water heater list, rated at up to 199,000 BTU and 11 GPM, is sized around domestic hot water — simultaneous showers, a dishwasher, a washing machine — at a fairly small total flow. Pool heating is a different math problem: thousands of gallons of standing water that needs a slow, sustained temperature rise, which is why dedicated gas pool heaters commonly run 250,000–400,000+ BTU. A domestic unit’s BTU rating and a pool heater’s BTU rating aren’t solving for the same thing.
What actually works: purpose-built tankless pool heaters
| Unit | Output | Circuit | Best for | Price |
|---|---|---|---|---|
| EcoSmart Smart POOL 18 | 18 kW | 240V / 75A | Spas, hot tubs, very small above-ground pools | ~$849 |
| EcoSmart Smart POOL 27 | 27 kW, 5.27 GPM | 240V | Small-to-mid above-ground pools | ~$549 |
| EcoSmart Smart POOL 36 | 36 kW | 240V | Larger above-ground pools, supplemental spa heat | ~$699 |
| Standard domestic tankless (Rinnai/Rheem/Navien) | Up to 199,000 BTU | Gas line or 240V | Domestic hot water only — warranty-excluded from pool/spa | $500–$1,900 |
EcoSmart Smart POOL 27
- Rated 1°F–1.5°F temperature rise per hour of continuous run, per EcoSmart's own product specifications — built for small above-ground pools and spa sections, not a full in-ground pool.
- Self-modulating technology adjusts power draw to hold a set temperature instead of cycling full-power on and off, the same approach EcoSmart uses across its domestic ECO-series tankless line covered in our EcoSmart roundup.
- Digital thermostat control lets you dial in an exact target temperature rather than relying on a fixed setpoint.
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EcoSmart Smart POOL 18
- Right-sized for spas, hot tubs, and the smallest above-ground pools rather than a general-purpose pool heater, per EcoSmart's stated scope for the model.
- Uses flow-sensor activation instead of a traditional pressure switch, which EcoSmart states reduces the risk of burned heating elements versus older pressure-switch designs.
- Requires a dedicated 75-amp, 240V circuit — confirm panel capacity before buying, the same electrical-planning step covered in our electrical requirements guide for domestic units.
For a full in-ground pool, look elsewhere
Even EcoSmart’s largest Smart POOL model is scoped to above-ground pools and spa loops — at a 1°F–1.5°F-per-hour rise rate, heating a 15,000–20,000 gallon in-ground pool would take an impractically long time and a sustained 240V load. That’s the gap dedicated gas and heat-pump pool heaters, rated in the hundreds of thousands of BTU or tens of thousands of watts, exist to fill. If your pool is a standard in-ground install, a purpose-built pool heater — not a tankless unit of any kind, domestic or pool-rated — is the right category to shop.
The bottom line
A standard tankless water heater is a hard no for pool heating — Rinnai’s own warranty language excludes any application with chemically treated water, and the exclusion is standard across the category regardless of BTU output. If you want tankless-style on-demand heat for a small above-ground pool or spa, EcoSmart’s purpose-built Smart POOL series (18kW, 27kW, or 36kW) is engineered and warrantied for exactly that job. For a full in-ground pool, skip tankless entirely and shop a dedicated gas or heat-pump pool heater sized to your pool’s actual gallon count. For sizing a standard tankless unit once pool heating is off the table, start with our tankless water heater sizing guide, or compare whole-home options on our best tankless water heater list.