How to Flush a Series of Hot Water Heaters Plumbed Together

If you’ve noticed reduced hot water pressure, strange noises from your tanks, or rising energy bills, sediment buildup might be the culprit—especially if you’re running a series of hot water heaters plumbed together. Flushing these systems isn’t just about cleaning one tank; it requires understanding how they’re interconnected and which order to flush them in. In this guide, you’ll learn exactly how to flush a series of hot water heaters plumbed correctly, safely, and effectively—so your system runs smoothly for years.

Why Should You Flush Multiple Water Heaters in Series?

Water heaters connected in series (also called “daisy-chained”) are common in large homes, multi-family units, or commercial buildings where demand exceeds a single unit’s capacity. Over time, minerals like calcium and magnesium settle at the bottom of each tank as sediment. According to the U.S. Department of Energy, sediment can reduce heating efficiency by up to 30% and shorten tank life by several years.

When heaters are plumbed in series, sediment in the first tank can actually flow into the second, compounding the problem. That’s why flushing all units—not just the most accessible one—is critical.

💡 Expert Insight: “In series-plumbed systems, neglecting even one tank during maintenance can undermine the entire setup,” says Maria Lopez, licensed master plumber with over 18 years of experience in residential HVAC systems.

What Does “Plumbed in Series” Actually Mean?

Before diving into the flush process, confirm your setup. In a series configuration:

• Cold water enters the first heater.
• The output (now warm/hot) from the first heater feeds directly into the second heater as its “cold” input.
• The second heater further heats the water before sending it to fixtures.

This differs from parallel setups, where both heaters receive cold water independently and supply hot water to the same line.

🔍 How to check: Trace your pipes. If the hot outlet of Heater A connects to the cold inlet of Heater B, you have a series system.

Tools & Safety Gear You’ll Need

Gather these before starting:

• Garden hose (heat-resistant, 5–6 ft long)
• Bucket or floor drain access
• Adjustable wrench
• Screwdriver (for older drain valves)
• Towels or rags
• Safety goggles & gloves
• Optional: Wet/dry vacuum (for stubborn sediment)

⚠️ Safety First: Always turn off power/gas and shut off the cold water supply before beginning. For electric heaters, flip the circuit breaker. For gas, turn the dial to “Pilot.”

Step-by-Step: How to Flush a Series of Hot Water Heaters Plumbed Together

Follow this sequence from the first heater (closest to cold water source) to the last. Flushing out of order can push debris backward or overload downstream tanks.

Step 1: Turn Off Power & Water Supply

• Shut off the main cold water valve.
• Turn off electricity (breaker) or gas supply.
• Open a hot water faucet upstairs to relieve pressure.

Step 2: Connect Hose to First Heater’s Drain Valve

• Attach the hose securely to the drain valve at the base of Heater #1.
• Run the other end to a floor drain or outside.

Step 3: Open Drain & Relief Valves

• Open the temperature and pressure relief (TPR) valve on top of the tank.
• Open the drain valve slowly. Water will flow out—initially cloudy or gritty.

Step 4: Flush Until Water Runs Clear

• Let water run for 5–10 minutes, or until it’s visibly clear.
• If flow slows, sediment may be clogging the valve. Briefly close the valve, then reopen to dislodge chunks.
• For heavy buildup, use short bursts: open 30 sec, close 10 sec, repeat.

📊 Pro Tip: Use 2–3 gallons of water per flush cycle. Most sediment clears within 10–15 gallons total per tank.

Step 5: Repeat for Downstream Heaters

• Once Heater #1 is clean, close its drain valve, disconnect the hose, and move to Heater #2.
• Do not restore water or power yet—keep the system depressurized.
• Repeat Steps 2–4 for each subsequent heater in the series.

Step 6: Refill & Restart System

• Close all drain and TPR valves.
• Turn the cold water supply back on.
• Let tanks refill completely (listen for gurgling to stop).
• Once full, restore power or relight the pilot.
• Check for leaks at all connections.

🌐 Learn More: Understand how water hardness affects sediment buildup in this overview on hard water.

Common Mistakes to Avoid

How Often Should You Flush Series-Plumbed Heaters?

Frequency depends on your water hardness:

• Soft water (< 3.5 gpg): Every 12–18 months
• Moderate hardness (3.5–7 gpg): Every 6–12 months
• Hard water (> 7 gpg): Every 3–6 months

💡 Real-World Example: A duplex in Phoenix (known for hard water) reduced annual energy costs by $140 after implementing biannual flushing of its two series-connected 50-gallon tanks.

FAQ Section

Q1: Can I flush tankless and tank-style heaters in series?

No. Tankless (on-demand) units shouldn’t be flushed using the same method as storage tanks. They require descaling with vinegar or manufacturer-approved solutions. Mixing methods can damage components.

Q2: What if my drain valve is corroded or won’t open?

Don’t force it—this often cracks the valve. Instead, call a plumber to replace the valve with a ball-type drain valve, which is more durable and easier to operate.

Q3: Will flushing void my warranty?

Generally, no. In fact, many manufacturers (like Rheem and AO Smith) require annual maintenance to keep warranties valid. Keep records of your service dates.

Q4: Can I use a pump to speed up flushing?

Yes—submersible utility pumps can help, but ensure they’re rated for hot water (up to 140°F). Never use a pump on a pressurized tank.

Q5: Should both heaters be the same size in a series setup?

Not necessarily, but mismatched sizes can cause inefficiency. Ideally, the first tank should handle ~60% of total demand to avoid overworking the second unit.

Q6: What if water never runs clear?

Persistent cloudiness may indicate internal corrosion or a failing anode rod. Consider a professional inspection—your tank might need replacement.

Conclusion

Knowing how to flush a series of hot water heaters plumbed together is a powerful skill that saves money, boosts efficiency, and extends system life. By following this guide—from identifying your setup to executing a safe, sequential flush—you’ll keep hot water flowing reliably while avoiding premature breakdowns.

Don’t let sediment sabotage your comfort. Set a calendar reminder, grab your hose, and give your water heaters the care they deserve.

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