How to Plumb 1 Recirc Line to Two Water Heaters

Tired of waiting forever for hot water in a large home or multi-unit building? You’re not alone. Many homeowners install two water heaters to meet high demand—but then struggle with inconsistent hot water delivery. The solution? A smart recirculation (recirc) system that serves both units efficiently. In this guide, you’ll learn exactly how to plumb 1 recirc line to two water heaters—safely, legally, and effectively—so you get hot water fast without wasting gallons down the drain.

Why Use One Recirc Line for Two Water Heaters?

Before diving into installation, it’s crucial to understand why this setup makes sense:

• Energy efficiency: Reduces water waste by up to 12,000 gallons per year per household (EPA WaterSense).
• Convenience: Delivers hot water within seconds, even in distant bathrooms.
• Cost savings: Less water usage = lower utility bills.
• Space optimization: Avoids installing two separate recirc pumps and return lines.

However, improper plumbing can cause cross-contamination, pressure imbalances, or short-cycling of heaters. That’s why correct design matters.

💡 Pro Tip: Always check local plumbing codes. Some jurisdictions require thermal disinfection loops or backflow preventers in shared systems.

Can You Actually Connect One Recirc Line to Two Water Heaters?

Yes—but only under specific conditions. According to the International Plumbing Code (IPC) and industry best practices from the American Society of Sanitary Engineering (ASSE), a single recirc line can serve multiple heaters if:

1. Both water heaters are of the same type (e.g., two tank-style gas units or two electric tanks).
2. They’re plumbed in parallel, not series.
3. A common header system is used with isolation valves.
4. The recirc pump is sized correctly for total flow demand.

⚠️ Never connect tankless and tank-style heaters on the same recirc loop—their operating pressures and flow requirements differ drastically, risking damage or inefficiency.

For authoritative background on recirculation principles, see Wikipedia’s overview on domestic hot water recirculation.

Step-by-Step: How to Plumb 1 Recirc Line to Two Water Heaters

Follow these steps carefully. This assumes you’re using two identical tank-type water heaters (gas or electric) in a residential setting.

Tools & Materials Needed:

• Copper or PEX piping (¾-inch recommended)
• Two isolation valves (¼-turn ball valves)
• One thermostatic bypass valve or timer-controlled recirc pump
• Pipe cutter, flux, solder (for copper) or crimping tool (for PEX)
• T-fittings, unions, dielectric nipples (if mixing metal types)
• Pipe insulation (to reduce heat loss)

Step 1: Install Heaters in Parallel Configuration

• Connect cold water supply to a T-fitting that splits evenly to both heaters.
• Similarly, connect hot water outlets from both heaters to a common hot header.
• This ensures balanced draw and prevents one heater from doing all the work.

🔧 Critical Detail: Use dielectric unions if connecting copper to steel pipes to prevent galvanic corrosion.

Step 2: Create a Common Return Line

• From the farthest fixture (e.g., master bathroom), run a dedicated return line back toward the mechanical room.
• Instead of splitting the return to each heater, merge it into a single line that feeds into a central manifold just before the heaters.

Step 3: Install the Recirc Pump on the Common Return

• Mount the recirc pump on the merged return line, ideally 12–18 inches above the floor for easy maintenance.
• Choose a variable-speed or timer-based pump (e.g., Grundfos Comfort System or Taco SmartPlus) to avoid constant operation.
• Set the pump timer to run during peak usage hours (e.g., 6–9 AM and 5–8 PM).

Step 4: Add a Thermostatic Bypass Valve (Optional but Recommended)

• Install a thermostatic mixing valve (set to ~105°F) between the hot header and cold supply.
• This valve opens when return water drops below temperature, allowing cooler water to re-enter the system—keeping temps stable without overheating.

Step 5: Insulate All Hot & Return Lines

• Wrap all hot water and recirc lines with ½-inch closed-cell foam insulation.
• This reduces standby heat loss by up to 45% (U.S. DOE data) and improves system responsiveness.

Common Mistakes to Avoid

Advantages vs. Disadvantages of This Setup

✅ Pros:

• Lower installation cost (one pump, one return line)
• Simpler maintenance
• Consistent hot water across zones
• Reduced water waste

❌ Cons:

• Requires precise balancing
• Not suitable for mixed heater types
• May need professional commissioning

📊 Real-World Example: A 3,200 sq ft home in Phoenix retrofitted two 50-gallon electric heaters with a single Grundfos UP 15-29SU pump. Result: Hot water at faucet in <8 seconds, with $18/month savings on water and energy.

FAQ Section

Q1: Can I use this setup with one gas and one electric water heater?

No. Different recovery rates, pressure tolerances, and flow dynamics make this unsafe and inefficient. Always match heater types.

Q2: Do I need a dedicated return line, or can I use the cold line?

You must use a dedicated return line. Using the cold line as a return (called “crossover”) causes lukewarm water at taps and violates most plumbing codes.

Q3: What size recirc pump do I need for two 50-gallon heaters?

A ½ HP pump with 2–4 GPM flow rate is sufficient for most homes under 3,500 sq ft. Larger homes may need a ¾ HP unit. Always calculate total fixture units first.

Q4: Will this increase my energy bill?

Slightly—but water savings usually offset it. Use a timer or demand-based pump to minimize runtime. Insulated pipes further reduce energy loss.

Q5: Can I DIY this, or should I hire a plumber?

If you’re experienced with soldering, pressure testing, and code compliance—yes. But most jurisdictions require permits and inspections for multi-heater recirc systems. When in doubt, hire a licensed pro.

Q6: How do I balance flow between the two heaters?

Use isolation valves on each heater’s outlet. Open both fully, then slightly throttle the valve on the closer heater until flow equalizes (test with infrared thermometer on outlet pipes).

Conclusion

Plumbing one recirc line to two water heaters is not only possible—it’s a smart strategy for large homes seeking comfort, efficiency, and sustainability. By following a parallel configuration, using proper valves, and insulating your lines, you’ll enjoy near-instant hot water while cutting waste and costs.

✅ Key Takeaways:

• Match heater types.
• Use a common hot header and single return.
• Size your pump correctly.
• Insulate everything.

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Stay warm, save water, and plumb smart. 💧🔥