When it comes to pool heating, homeowners often face a variety of options, from solar heaters to heat pumps and gas-fired systems. One question that has grown in popularity in recent years is: Can a tankless water heater be used to heat a swimming pool? This article dives deep into the feasibility, efficiency, and practicality of using a tankless water heater for pool heating, offering detailed insights and helping you make an informed decision for your home or project.
Understanding How Tankless Water Heaters Work
Traditional water heaters rely on a storage tank where water is continuously heated and stored for use when needed. In contrast, tankless water heaters, also called “on-demand” water heaters, heat water directly without the use of a storage tank. When a hot water tap is turned on, cold water flows through a pipe into the unit. Depending on the model—either gas-powered or electric—the heating system (generally a powerful burner or heating element) activates and warms the water as it passes through in real time.
This design offers several benefits:
- Energy efficiency (no standby heat loss)
- Longer lifespan compared to traditional models
- Compact size and space-saving footprint
These advantages have made tankless heaters a popular choice for residential homes, especially for everyday hot water use like showers, dishwashers, and laundries.
Why You Might Consider a Tankless Water Heater for Pool Heating
Given that tankless heaters are designed for continuous hot water flow, some pool owners—and even small commercial pool operators—consider using them to heat their pools, especially as an alternative to traditional pool heaters. Let’s look at the main reasons behind this growing interest.
1. Potential Cost-Savings
For an existing tankless water heater, especially a gas-fired unit, the upfront cost has already been incurred for home use. Repurposing an oversized model or installing a second, dedicated tankless unit can appear economical compared to buying a fully dedicated pool heater, which can cost several thousand dollars.
2. Faster Heating Cycles for Smaller Volumes
If the pool is small—like a spa or plunge pool—a tankless water heater might provide a faster and adequate way to heat the water compared to passive options like solar blankets or solar heaters.
3. Energy Efficiency (Potentially)
In some configurations, tankless water heaters—specifically high-efficiency gas models—can offer better thermal efficiency than some traditional pool heaters, especially those that are older or less advanced.
Comparing Pool Heater Systems
Before diving into whether a tankless water heater is the right choice, it’s important to understand your options. Here’s a quick side-by-side:
| Type | Heating Method | Efficiency | Average Cost | Best For: |
|---|---|---|---|---|
| Gas Pool Heater | Burns natural gas or propane | Modest (typically 80–95% efficiency) | $2,000–$5,000 | Quick heating, outdoor pools, variable climates |
| Heat Pump | Transfers heat from the air to the water | High (300–500% efficiency) | $2,500–$5,500 | Warm climates, long-term use |
| Solar Heater | Uses solar thermal collectors | Very High (free fuel) | $3,000–$7,000 | Sunny climates, budget-conscious owners |
| Tankless Water Heater (repurposed) | Instant water heating using gas or electric | High (up to 98%) | $1,000–$3,000 (excluding system design costs) | Temporary setups, spas, small pools |
Why Tankless Water Heaters Are Not Ideal for Standard Pool Heating
While they have pros, tankless water heaters face several limitations that often make them unsuitable for standard full-size pool heating. Here’s a breakdown.
1. Insufficient Flow Rate for Large Volumes
Pools require significant water volumes to be heated at once. For example, a standard 15,000-gallon pool requires a heat pump or dedicated pool heater capable of handling high circulation rates and raising the temperature over several hours consistently.
In contrast, most tankless water heaters are engineered to supply domestic hot water—typically 2–5 gallons per minute (GPM) at best—far below what a pool system requires.
2. Lack of Suitable Temperature Range
Pool water needs to be raised and maintained within a specific range (commonly 78°F–84°F). While tankless units can heat water to over 120°F quickly, they are not designed for continuous operation or for heat dissipation through pool flow mechanics. They simply aren’t built for extended, high-volume thermal maintenance.
3. System Design Challenges
A tankless heater integrated with a pool must be hooked into the pool pump system, filtration unit, and often heat exchangers or bypasses. This creates a complex plumbing demand. The system must also ensure that the hot water is mixed appropriately to avoid damaging pipes, pool surfaces, or filtration components.
4. Risk of Overworking and Shortened Lifespan
Tankless heaters, while robust, are not built to be on cycle for hours at a time, particularly when heating large volumes of water. Continuous operation to heat a 10,000+ gallon pool would put extreme strain on the unit, leading to component failures and early replacement costs, which can negate the cost savings.
5. Safety and Code Compliance Concerns
For residential use, a tankless unit repurposed as a pool heater may violate building, HVAC, and pool safety codes. Most tankless water heaters are not certified or approved for continuous outdoor or pool-related operation. As such, using one for pool heating could void warranties and even lead to insurance complications in the event of property damage or injury.
When Might a Tankless Water Heater Be a Practical Option?
Despite the challenges, there are scenarios where using a tankless water heater to heat a pool or spa may be viable or even optimal.
1. For Small Pools or Hot Tubs
Smaller spas or hot tubs with limited water volume may be compatible with tankless technology, especially when paired with a dedicated system design that balances flow, temperature, and pump capacity.
2. Short-Term or Seasonal Pool Use
If you plan to heat your pool only occasionally, such as for a summer swim season or for special family gatherings, a tankless water heater may serve as a cost-effective temporary solution, especially if you already own one with sufficient output capacity.
3. Supplemental Heating or Hybrid Systems
Tankless heaters can also be used in hybrid setups. For example, pairing a solar heater with a tankless booster system allows for faster heating when sunshine is insufficient. This method can reduce overall energy usage and increase flexibility in maintaining the desired pool temperature.
4. Commercial Applications with Proper Setup
In some smaller commercial installations—such as guesthouses, rental cabins, or boutique wellness centers with multiple small outdoor plunge pools—tankless water heaters can be part of the thermal loop system when installed correctly with proper control mechanisms.
Important Considerations for Using Tankless Water Heaters to Heat Pools
If you’re considering using a tankless water heater for pool heating, here are some essential factors to carefully evaluate:
1. Water Flow and GPM Requirements
To effectively heat a pool, your heating system must handle the GPM throughput required by your pool’s pump system. Most pool pumps circulate 20–80 GPM depending on pool size and filter configurations. Tankless heaters on the market today usually max out at 5–10 GPM, meaning a direct swap or adaptation is not usually viable unless the pool pump is downgraded or the heater is used in a bypass mixing system.
2. Output Limitations of Tankless Heaters
Even high-output tankless water heaters (such as 10+ gpm units) still pose issues when applied to pool water heating. A high-flow pool system cannot rely on a tankless unit to raise the bulk temperature of a large pool volume quickly or cost-effectively.
3. Operating Costs – Electric vs. Gas
Choosing between electric or gas heaters also significantly affects performance and cost. Gas-powered tankless units offer higher heating capacity and are usually faster to raise water temperatures, while electric models may overheat or draw on a disproportionately large amount of power over time—leading to steep electricity bills.
4. Integration with the Pool System
To integrate successfully, you must consider:
- Plumbing setup with the pool pump and filter
- Placement of the heater (especially avoiding water splash in outdoor setups)
- Automatic temperature control and regulation systems
- Proper bypass valves for pressure and flow
5. Cost of Installation
If you’re starting from scratch, factoring in custom plumbing, thermostatic control units, and possibly upgraded electrical or gas infrastructure can make tankless pool heating more costly than simply getting a dedicated pool heater.
Alternatives to Tankless Water Heaters for Pool Heating
So what works better for your pool? Here’s a quick comparison of the most used alternatives:
Gas Pool Heaters: Reliable and Fast
Gas pool heaters (using natural gas or propane) provide rapid heating and maintain pool temperatures even in cold weather. They’re often the top pick for cold climate areas and work well for commercial or large residential pools.
Heat Pumps: Energy-Efficient and Eco-Friendly
Heat pumps pull heat from the ambient air and transfer it to the water. Highly energy-efficient and long-lasting (10+ years with maintenance), heat pumps are ideal in warm to moderate climates.
Solar Pool Heaters: Sustainable but Seasonal
For owners aiming for the most cost-effective and eco-friendly solution, solar heaters use solar thermal panels to heat water. While effective, they are weather-dependent and best for regions with high sun exposure.
Combination Units and Hybrid Solutions
In smart installations, pool owners can use a tankless heater as a backup for a solar heater or as a topping mechanism with a heat pump. This hybrid system can offer both sustainability and flexibility.
Expert Insights and Industry Recommendations
According to the Department of Energy, heating a pool generally accounts for about 70% of the total operating cost of a pool, especially for outdoor setups. This makes choosing the right heating system crucial not just for efficiency but long-term cost-effectiveness.
Most HVAC professionals and pool contractors discourage using tankless heaters for direct pool heating due to thermal limitations, safety risks, and long-term inefficiency.
They do suggest that:
- High-capacity gas tankless heaters might be **useful in hybrid pool heating applications**
- Homeowners with existing or oversized tankless units might get **limited supplemental use** for temporary pool or spa heating
- DIYers should be cautious of overtaxing a small unit to avoid early failure
Conclusion: Is a Tankless Water Heater Right for Your Pool?
To sum up: Yes, a tankless water heater can technically heat parts of a pool, particularly small ones like spas or hot tubs, as long as the system is correctly designed and managed. However, for a full-sized residential or commercial swimming pool, a dedicated pool heater remains the clear winner in terms of efficiency, reliability, cost, and suitability for the task.
Investing in the right heater from the beginning can save you time, money, and hassle. If you’re curious about alternative methods such as hybrid heat solutions, solar boosters, or gas-electric combinations that include tankless components, you can consult with a licensed HVAC and pool contractor to explore the full potential of integrating diverse systems safely and efficiently.
Ultimately, the decision should be based on your climate, pool size, usage frequency, and energy goals. Whatever route you choose, prioritizing safety, compliance, and long-term performance is key to enjoying your pool year-round without expensive and unexpected complications.
Key Takeaways
- Tankless water heaters offer high energy efficiency but are primarily built for domestic hot water use.
- While they may work in limited configurations for small pools or spas, they are not designed for high-volume pool heating.
- Full-sized pools may suffer from inconsistent heating, high energy costs, and shortened equipment lifespan if a tankless heater is used improperly.
- Considered use of tankless heaters in complementary or hybrid heating systems may offer some advantages for sustainability-focused builds.
- Always consult with a professional before modifying water heating systems to ensure code compliance and safety.
If your goal is consistent, long-term, and powerful pool heating, go with pool-specific systems like heat pumps or modern gas-fired heaters that are engineered for the task.
Can a tankless water heater be used to heat a pool?
Yes, a tankless water heater can be used to heat a pool, but its effectiveness depends on several factors such as the size of the pool, the desired water temperature, and the capacity of the heater. Tankless units are typically designed for residential hot water demands like showers and sinks, not the large volumes required for pool heating. However, for smaller pools or spas, and in situations where the ambient temperature doesn’t drop too low, a high-output tankless water heater might provide a cost-effective and efficient alternative to traditional pool heaters.
That said, tankless water heaters generally lack the BTU (British Thermal Unit) output required to maintain the temperatures needed for larger in-ground pools. Pool heaters, especially gas or heat pump models, are built for continuous operation and are optimized for heating large volumes of water over extended periods. Using a tankless water heater for a large pool may result in frequent cycling, reduced lifespan of the unit, and inconsistent water temperatures. Therefore, while possible, its use should be limited to appropriate applications.
What are the differences between tankless water heaters and traditional pool heaters?
Tankless water heaters and traditional pool heaters operate differently and are built for different purposes. Tankless systems heat water on demand without storing it, making them highly efficient for household use. They work well for intermittent hot water needs but are not specifically engineered to handle the large and continuous thermal demand of pools. In contrast, traditional pool heaters—such as gas-fired or heat pump models—are designed for sustained heating over large water volumes and can maintain a consistent water temperature over long periods.
While tankless water heaters may provide a temporary or supplemental heating solution for small pools or spa applications, their flow rate and energy output often fall short when compared to dedicated pool heaters. Pool heaters are also equipped to handle corrosion and chemical exposure better than residential tankless models. Considering installation, maintenance, and operating costs, dedicated pool heaters are typically more suitable for long-term pool use.
What size tankless water heater would be needed to heat a pool?
The size of a tankless water heater needed to heat a pool depends on the pool’s volume, the desired temperature rise, and the flow rate required. For example, to heat a 10,000-gallon pool from 60°F to 80°F in a reasonable time, a heater must deliver a substantial BTU output—often in the range of 100,000 to 400,000 BTUs. Standard residential tankless units typically range between 150,000 to 200,000 BTUs, which may fall short for large pools. Therefore, a high-capacity tankless heater or even multiple units in parallel might be required to meet the thermal demand effectively.
Moreover, even if a large enough tankless water heater is installed, it’s important to consider its intended use and design limitations. Most tankless heaters are optimized for household point-of-use applications and are not built to run continuously, as would be required for steady pool heating. Overworking the unit can lead to early failure and increased maintenance costs. Determining the appropriate size and type of heater should involve consulting a professional to ensure the best balance between performance and durability.
Are there any advantages to using a tankless water heater for pool heating?
One key advantage of using a tankless water heater for pool heating is energy efficiency, particularly for small pools or when occasional heating is required. Because tankless systems heat water only when it is flowing through the unit, they can reduce standby energy losses, unlike traditional pool heaters that may draw power even when not in active use. Additionally, if a tankless unit is already installed for household use, it may offer a dual-purpose setup that can reduce initial equipment costs, especially if the pool does not require intensive heating.
Another benefit is the reduced space required for installation—tankless heaters are typically compact and wall-mounted. This makes them a good option for homeowners with limited room for additional systems. If the pool is small and used only during certain times, a tankless heater could fulfill the demand without the need for a separate pool heater. However, these advantages are best realized under specific conditions, and a tankless system should not be viewed as a universal replacement for dedicated pool heating technology.
What are the disadvantages of using a tankless water heater for a pool?
One of the major disadvantages is that tankless water heaters may struggle to provide sufficient heating for standard-sized pools due to their relatively low BTU output. They are designed to heat water for short-term domestic uses, not for the continuous operation needed to maintain a large pool’s temperature. This can lead to extended runtime, overheating, and eventually, premature failure of the unit. Additionally, the flow rate required for pool heating is often higher than what a standard tankless model is capable of handling efficiently.
Another drawback is the potential for inconsistent water temperatures, especially in unpredictable weather conditions. Tankless systems may not have the same built-in features as pool heaters, such as anti-corrosion elements and automated controls for fluctuating temperatures. Furthermore, warranties on tankless water heaters may be voided if the unit is used outside its intended purpose, including for pool heating. For these reasons, despite the potential cost savings in some scenarios, using a tankless heater for pool heating comes with notable limitations.
Can a tankless water heater damage the pool water chemistry?
Using a tankless water heater for a pool does not directly alter the water chemistry, but certain external factors may indirectly impact it. If the heater is not designed for prolonged contact with chemically treated pool water, such as chlorine or bromine, corrosion and metal degradation inside the heat exchanger could occur. This breakdown of materials might introduce trace metals or impurities into the pool water, affecting chemical balance and leading to staining or discoloration of the pool surface.
Additionally, improper water circulation and uneven heating can cause chemical volatility in localized areas of the pool. If the tankless heater struggles to maintain a consistent temperature or water flow, it may create thermal stress points that affect sanitizer efficiency and encourage algae growth. It’s crucial to ensure that the system includes corrosion-resistant components and that proper water treatment and circulation are maintained to prevent such issues.
How can I safely use a tankless water heater to heat a pool?
To safely use a tankless water heater for pool heating, it’s important to start with a system that is rated for high-output applications and has the capacity to handle the desired water flow and temperature increase. A professional plumber or HVAC technician should assess the system’s compatibility, including evaluating gas lines, venting requirements, and electrical connections to ensure the heater can function without issues. Installing a dedicated circulation system that filters and circulates the pool water through the heater can help maintain efficiency and reduce strain on the unit.
Additionally, installing a buffer tank or tempering valve can help regulate water temperature and minimize thermal stress on the heater. Regular maintenance is also key—cleaning the heat exchanger, checking for corrosion, and ensuring no scale buildup will help maintain the heater’s performance. Monitoring the water chemistry closely is important to prevent chemical imbalances that could damage both the heater and the pool. If the tankless system is not sufficient for long-term heating, consider using it as a supplemental system in combination with or as a backup for a traditional pool heater.