Running out of hot water mid-shower is frustrating—especially in large households or commercial spaces. If you’re dealing with long wait times or inconsistent supply, how to plumb in a two water heater with return might be the solution you need. This setup not only boosts hot water availability but also improves efficiency and comfort. In this guide, we’ll walk you through everything you need to know—safely, clearly, and step by step.
Why Use Two Water Heaters with a Return Line?
Before diving into installation, it’s important to understand why this system is worth the effort.
A dual water heater system with a dedicated return line (also called a recirculation loop) ensures hot water is always available at every fixture—without wasting gallons down the drain while you wait. According to the U.S. Department of Energy, the average household wastes 20–30 gallons of water per day waiting for hot water. A properly plumbed return system can cut that waste dramatically.
This setup is ideal for:
Large homes (3,000+ sq ft)
Multi-story buildings
Homes with distant bathrooms or kitchens
Commercial applications like hotels or offices
💡 Pro Tip: The “return” line circulates cooled hot water back to the heater instead of letting it sit in pipes—keeping your system ready 24/7.
What Is a Return Line in a Water Heater System?
A return line is a dedicated pipe that connects the farthest fixture back to the water heater inlet. It works with a recirculation pump (often timer- or thermostat-controlled) to keep hot water moving in a loop.
Without a return line, you’d rely on “demand-based” systems or inefficient gravity-fed loops—both less reliable and more wasteful.
To successfully plumb in two water heaters with a return, gather these essentials:
Two compatible water heaters (same type: gas, electric, or tankless; ideally same model)
Recirculation pump (e.g., Grundfos, Taco, or Watts)
Return line piping (typically ¾” copper or PEX)
Check valves (to prevent backflow between units)
Isolation valves (for maintenance)
Expansion tanks (required by code in closed systems)
Pipe insulation (to reduce heat loss)
⚠️ Safety First: Always turn off power/gas and water supply before starting. Local plumbing codes may require permits—check with your municipality.
Step-by-Step: How to Plumb In a Two Water Heater With Return
Follow this professional-grade sequence for a safe, code-compliant installation.
Step 1: Choose Your Configuration
There are two main ways to connect dual water heaters:
Configuration
Best For
Pros
Cons
Parallel
High-demand homes
Equal load sharing, redundancy
Requires balancing valves
Series (Cascade)
Very large demand
Simpler piping
Second heater works harder
Recommendation: Use parallel for most residential setups—it extends equipment life and balances wear.
Step 2: Install the Water Heaters
Place both units close together (within 3–5 feet).
Ensure proper clearance per manufacturer specs (usually 6″ on sides, 12″ above).
Mount on drip pans with drain lines.
Step 3: Connect Cold Water Inlet
Run a single cold water line to a T-fitting.
Split equally to both heaters using ½” or ¾” pipe.
Install isolation valves on each branch for future servicing.
Step 4: Connect Hot Water Outlets
Join both hot outlets via a common manifold.
Use a header pipe (¾” minimum) to combine flow.
Install a check valve on each hot outlet to prevent cross-flow.
Step 5: Install the Return Line
Locate the farthest fixture (usually a bathroom sink or shower).
Run a dedicated return line from that point back to the cold inlet side of the system.
Critical: Install the recirculation pump on the return line, near the water heater, pumping toward the tank.
Add a check valve just after the pump to stop reverse flow.
🔧 Detail Matters: Slope return lines slightly (¼” per foot) toward the heater to aid drainage during maintenance.
Step 6: Add Controls & Insulate
Wire the pump to a timer (e.g., run 6 AM–10 PM) or thermostat (activates when return temp < 95°F).
Wrap all hot and return pipes with foam insulation (R-3 or higher).
Install an expansion tank on the cold inlet if you have a closed system (check local code).
Step 7: Test & Balance
Open all valves.
Fill the system slowly to avoid air locks.
Purge air from highest points.
Turn on heaters and check for leaks.
Measure flow at multiple fixtures—adjust balancing valves if needed.
✅ Success Metric: Hot water should arrive within 5 seconds at all fixtures.
Common Mistakes to Avoid
Even experienced DIYers slip up here:
❌ Skipping the check valves → causes one heater to backfeed the other.
❌ Oversizing the pump → leads to noise, erosion, and wasted energy.
❌ Ignoring insulation → up to 20% heat loss in un-insulated pipes.
❌ Using series configuration unnecessarily → shortens second heater’s lifespan.
According to a 2023 study by the Plumbing-Heating-Cooling Contractors Association (PHCC), 73% of dual-heater failures were due to improper return line design or missing check valves.
Maintenance Tips for Longevity
Flush both tanks annually to remove sediment.
Inspect the recirculation pump every 6 months.
Replace expansion tank every 5–7 years.
Monitor energy bills—if usage spikes, check for pump overuse or leaks.
FAQ Section
Q1: Can I use two different types of water heaters (e.g., gas + electric)?
A: Technically yes, but not recommended. Mismatched recovery rates, flow capacities, and control systems can cause imbalance, inefficiency, or damage. Stick to identical models for reliability.
Q2: Do I need a recirculation pump if I have a gravity return system?
A: Gravity systems (thermosiphon) only work in specific layouts (heater below fixtures, vertical rise). They’re slow and unreliable. A pump-assisted return is far more effective and now standard in modern builds.
Q3: How much does it cost to install a dual water heater with return?
A: Materials typically range from $1,200–$2,500 (including two 50-gal tanks, pump, piping, valves). Labor adds $800–$1,500 depending on complexity. But you’ll save 10–15% on water heating costs long-term.
Q4: Will this setup increase my energy bill?
A: Slightly—due to the pump and standby heat loss. But insulating pipes and using a timer (not running 24/7) minimizes this. Net savings come from reduced water waste and shorter heater runtime per use.
Q5: Can I retrofit a return line into an existing home?
A: Yes! Use PEX tubing, which is flexible and easier to route through walls/floors. Some systems even use the cold water line as a return (with a special valve), though a dedicated return is superior.
Q6: Are there code requirements I must follow?
A: Absolutely. Most U.S. jurisdictions follow the IPC (International Plumbing Code) or UPC (Uniform Plumbing Code). Key rules include:
Expansion tank for closed systems
Temperature/pressure relief valves
Proper venting (for gas units)
Pump must not create excessive pressure
Always consult your local building department.
Conclusion
Mastering how to plumb in a two water heater with return transforms your home’s comfort and efficiency. You’ll enjoy instant hot water, lower water waste, and a more resilient system—especially valuable in growing families or aging-in-place scenarios.
If this guide helped you, share it with a friend or on social media! Whether you’re a weekend warrior or a new homeowner, smart plumbing choices pay off for years. Got questions? Drop them in the comments—we’re here to help you succeed.
🛠️ Remember: When in doubt, hire a licensed plumber. Safety and code compliance are non-negotiable.
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