Watts Circulating System with Tankless Water Heaters?

Watts circulating systems are popular for delivering instant hot water in traditional tank setups, but many homeowners wonder if they work with modern tankless water heaters. This post explains the compatibility requirements, installation steps, and alternatives so you can decide if a Watts system is right for your tankless unit.

Simply put, a Watts circulating system can work with some tankless water heaters, but only if the unit has a built-in recirculation pump or if you add an external pump and a dedicated return line. Compatibility depends on the specific tankless model, plumbing layout, and control strategy.

Key Takeaways

• Watts circulating systems are designed for tank water heaters, but they can be adapted for tankless units with the right pump and return line.
• Most tankless water heaters require an external recirculation pump and a dedicated return line to work with a Watts system.
• On-demand tankless models without a buffer tank may not be compatible without modifications.
• Proper installation includes a check valve, expansion tank, and temperature control to avoid short cycling.
• Alternatives like built-in recirculation pumps or dedicated recirculation valves may be simpler and more cost-effective.

How Does a Watts Circulating System Work?

A Watts circulating system uses a pump to continuously move hot water through the pipes, ensuring hot water is always available at the faucet. In a standard tank setup, the pump circulates water from the water heater through the loop and back to the tank, keeping the water in the pipes warm.

Without a circulating system, you have to wait for cold water to flush out of the pipes before hot water arrives. This wastes water and energy. A Watts system eliminates that wait by keeping the loop primed, which is why it’s a popular upgrade for tank heaters.

• Pump pushes hot water through the hot water line.
• Water returns to the water heater via the cold water line or a dedicated return line.
• A check valve prevents cold water from mixing with hot.
• Temperature sensor or timer controls when the pump runs.
• This system reduces water waste and improves comfort.
• For tank heaters, it works seamlessly because the tank maintains a large volume of hot water.
• Tankless units, which heat water on demand, present a different challenge.

Important: Watts circulating systems rely on a constant source of hot water. Tankless heaters only produce hot water when a flow sensor detects demand, which conflicts with continuous circulation.

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Compatibility of Watts Systems with Tankless Water Heaters

The core issue is that most tankless water heaters are designed to heat water on demand. They turn on only when a faucet opens enough to trigger the flow sensor. A Watts circulating system that runs continuously or on a timer can cause the tankless unit to cycle on and off frequently, leading to reduced lifespan and efficiency.

However, compatibility is not impossible. It depends on three factors: the tankless model, the type of recirculation pump, and the plumbing configuration. Some tankless heaters come with a built-in recirculation pump that works with an integrated return line.

Others require an external pump and a dedicated return line.

According to the American Society of Plumbing Engineers (ASPE), recirculation with tankless heaters can work if the flow rate is above the minimum activation threshold and the pump is controlled by a temperature sensor rather than a timer. This prevents the heater from firing unnecessarily.

Installation Requirements for a Watts System with Tankless

If your tankless water heater is compatible, installing a Watts circulating system requires some specific components. You cannot simply connect the pump to the cold water inlet and expect it to work. Here is what you need:

1. Dedicated return line – A separate pipe that returns cool water from the farthest fixture back to the heater. Without it, the system will send water back through the cold water supply, which can cause mixing and temperature swings.
2. External recirculation pump – Watts offers pumps like the Watts 500800 that can be used with tankless units. The pump must have a low-flow rating to avoid triggering the heater unnecessarily.
3. Check valve – Prevents backflow and ensures water moves only in the intended direction.
4. Expansion tank – Absorbs thermal expansion when the system is closed, protecting the heater and pipes.
5. Temperature sensor – Mounts on the return line to signal the pump only when water temperature drops below a set point (e.g., 95°F). This prevents short cycling.
6. Flow restrictor – Some installs use a restrictor to keep flow below the heater’s activation threshold, but this can reduce comfort.

Warning: Never connect a standard Watts circulation pump directly to a tankless water heater without a dedicated return line and proper controls. Doing so can void the heater warranty and cause rapid on/off cycling, leading to premature failure.

A professional plumber familiar with both Watts systems and tankless technology should handle the installation. The cost for parts and labor typically ranges from $400 to $1,200, depending on the complexity of the plumbing.

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Benefits and Drawbacks of Using a Watts System with Tankless

When done correctly, using a Watts circulating system with a tankless water heater offers several advantages. However, there are also notable downsides you should consider before committing to this setup.

Benefits

• Instant hot water at every fixture, improving convenience.
• Reduces water waste – according to the U.S. Department of Energy, recirculation can save up to 15,000 gallons of water per year for a typical family.
• Can be integrated with smart controls to run only during peak usage times.
• Compatible with condensing tankless models that support recirculation.
• Preserves the energy efficiency of tankless heating while adding comfort.

Drawbacks

• Higher installation cost due to dedicated return line and external pump.
• Potential for short cycling if pump flow rate is too high.
• Increased standby heat loss through the pipes, slightly reducing tankless efficiency.
• May not be suitable for all tankless models, especially non-condensing or outdoor units.
• Requires regular maintenance of the pump and check valve.

For many homeowners, the convenience of instant hot water outweighs the small efficiency trade-off. However, if you prioritize maximum energy savings, a Watts system may not be the best choice for a tankless heater.

Alternative Solutions for Instant Hot Water with Tankless

If a Watts circulating system seems too complex or incompatible with your tankless water heater, several alternatives can provide similar benefits. Consider these options before making a decision.

Built-in Recirculation Pumps

Many modern tankless water heaters, such as those from Rinnai, Navien, and Rheem, come with integrated recirculation pumps. These systems are designed to work seamlessly with a dedicated return line or a crossover valve at the farthest fixture. They automatically manage pump cycles to prevent short cycling.

Dedicated Recirculation Valves (Tankless Recirculation Kits)

Products like the ThermOmegaTech or Grundfos Comfort valves can be installed under the sink farthest from the heater. They use a thermal bypass to circulate water back to the heater only when the temperature drops. No dedicated return line is needed, but they are less effective for large homes.

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Demand-Controlled Systems

Systems such as the Chili Pensieri D’Mand or ACT D’Mand use a push-button at each faucet to trigger the pump on demand. This avoids continuous circulation and works well with tankless heaters because the pump runs only when you need hot water.

Tip: Check your tankless water heater’s manual for recirculation compatibility. Many manufacturers list approved pumps and installation kits. Using their recommended parts often preserves the warranty.

Common Mistakes When Adding a Watts System to Tankless

Installers and DIY enthusiasts often run into problems when adapting a Watts circulating system for tankless water heaters. Avoiding these common errors can save time, money, and frustration.

1. Using a timer-only pump without temperature control – A simple timer runs regardless of pipe temperature, forcing the tankless heater to fire even when pipes are already hot. This wastes energy and wears out the unit.
2. Skipping the dedicated return line – Connecting the pump to the cold water supply without a return line causes hot water to enter the cold pipes, leading to lukewarm water at other fixtures and potential scalding.
3. Setting pump flow too high – If the pump pushes more than 0.5 gallons per minute, the tankless heater will activate and may short cycle. Use a low-flow pump or a flow restrictor.
4. Ignoring the minimum activation flow rate – Each tankless heater has a minimum flow rate (usually 0.4 to 0.6 GPM) needed to ignite. The pump must stay below this threshold to avoid triggering the heater during circulation.
5. Not installing an expansion tank – A closed-loop system with a tankless heater and recirculation can cause pressure buildup, damaging the heat exchanger. An expansion tank absorbs thermal expansion.
6. Using a standard check valve that restricts flow – Some check valves reduce flow enough to cause the pump to work harder. Use a low-resistance spring-loaded check valve designed for recirculation.

According to a study by the Plumbing-Heating-Cooling Contractors (PHCC), over 30% of recirculation system failures are due to improper pump sizing or lack of a return line. Always consult a licensed plumber for your specific setup.

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Frequently Asked Questions

Can I use a Watts circulation pump with any tankless water heater?

Not all tankless models support external recirculation. You need a unit with a dedicated return port or the ability to connect an external pump without causing short cycling. Condensing tankless heaters from major brands are generally compatible, while non-condensing models may not be.

Does a Watts circulating system void the warranty on my tankless water heater?

It can if the installation is not approved by the manufacturer. Many tankless warranties require that any recirculation pump be installed per the manufacturer’s instructions. Always check the warranty terms before proceeding.

How much does it cost to add a Watts system to a tankless heater?

Costs vary widely based on plumbing complexity. Expect to pay between $400 and $1,200 for parts and professional installation. A dedicated return line adds the most expense, especially in a finished home.

Will a Watts system make my tankless water heater less efficient?

It can cause a slight increase in standby heat loss because the pipes stay warm even when not in use. However, the convenience and water savings often offset the minor efficiency drop. Modern systems with temperature-controlled pumps minimize this effect.

What is the best alternative to a Watts system for tankless water heaters?

Built-in recirculation pumps from the heater manufacturer are the most reliable. Demand-controlled systems like the D’Mand kit are also excellent because they only circulate when you activate them, avoiding unnecessary heater cycling.

Final Thoughts

A Watts circulating system can work with tankless water heaters, but it requires careful planning, a compatible heater, and professional installation. The key is using a temperature-controlled pump, a dedicated return line, and respecting the heater’s flow limitations. If that sounds too complex, built-in recirculation or demand-controlled valves offer simpler solutions.

Weigh the upfront cost against the water savings and daily convenience to decide what fits your home best.