Replacing My Old Tank Water Heater With a Tankless System: A Full One‑Day DIY Project

Why I Switched to Tankless

My old tank heater still worked, but it was inefficient, took up a lot of space, and delivered inconsistent hot water during heavy use. A tankless system offers:

• endless hot water
• higher efficiency
• smaller footprint
• better long‑term reliability

But replacing a tank with a tankless unit is not a simple swap. It touches multiple trades—plumbing, gas, electrical, venting, and drainage—and each has its own code requirements.

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1. Permit: Easier Than Expected

Before starting, I applied for a water‑heater replacement permit through my city’s online portal. The process was quick:

• fill out the form
• pay the fee
• receive approval by email

After installation, the city inspector only needs to verify that everything meets code. No in‑person office visit required.

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2. Understanding the Differences: Tank vs. Tankless

A traditional tank heater is simple:

• cold water in
• hot water out
• gas line
• vertical flue
• no electricity

A tankless heater, however, requires three utilities:

Electricity

Even gas tankless units need 120V power for the control board, fan, and ignition.

Gas

Tankless units require a sediment trap and often a re‑routed gas line.

Water

The inlet/outlet positions differ from a tank, so copper pipes must be cut and re‑soldered.

Venting

Tankless systems use direct venting through the wall, with separate intake and exhaust pipes.

This last part is the biggest structural change.

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3. Electrical Work: Adding a New 120V Outlet

My old tank heater didn’t use electricity, so there was no outlet nearby. The tankless unit doesn’t draw much power, but it still needs a dedicated 120V circuit.

I ran a new electrical line from the nearest junction box and installed a surface‑mounted outlet next to the heater. This part was straightforward compared to the plumbing.

⚠️ Critical Electrical Initialization Note

• The 90-Second Rule: When you first connect the unit to power, it will undergo an automatic initialization sequence lasting about 90 seconds. Do not try to operate or press buttons on the digital control panel during this window.
• Pre-Power Safety: Your installation manual strictly forbids plugging the unit into a live socket before the venting system is completely installed and sealed.

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4. Gas Line: Re‑Routing and Building a Sediment Trap

Tankless heaters require a gas sediment trap (drip leg). These are surprisingly hard to find pre‑assembled, so I built one myself using black iron pipe and fittings.

The photo below shows the custom sediment trap I assembled and connected to the yellow gas flex line:

Gas Fittings and Hose Regulation Notice

• Bushing Fittings Code Violation: In my original configuration, I used a bushing fitting prior to the sediment trap. Thanks to a heads-up from the Reddit DIY community, I learned that bushing fittings are strictly prohibited by gas codes in many jurisdictions because their thin walls crack easily under stress, creating severe leak hazards. I have since replaced them with proper standard reducing tees/couplings (the physical hardware is fixed, though the photo above remains un-updated as a code lesson).
• 1/2-Inch Gas Hose Clarification: You will note I used a standard 1/2-inch yellow flexible gas hose. While some setups require 3/4-inch lines, my run is only 36 inches long, which safely supports gas capacities well over 30,000 BTUs, satisfying the unit's demands perfectly.

After tightening everything, I performed a leak test with soapy water—no bubbles, no leaks.

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5. Water Lines: Cutting and Soldering Copper

This was the most challenging part of the entire project. Because the tankless heater’s water connections sit in different positions, I had to cut the existing copper pipes, clean/prep the ends, dry‑fit new sections, heat the joints with a torch, and solder everything with high‑temperature solder.

⚠️ Copper Soldering Caution

When welding your copper pipe adapters, never apply the torch directly near the water heater’s inlet or outlet connections. The intense heat will instantly travel down the metal and destroy the delicate internal plastic components, seals, and flow sensors. Always solder the copper pipe onto the adapters completely first, let them cool down to room temperature, and only then thread and secure them to the machine.

The Absolute Necessity of Service Valves (Isolation Kits)

Though not explicitly shown in basic plumbing schematics, you must install a dedicated Service Valve Kit (Isolation Valves) on the hot and cold lines. This isn't just an installation convenience. Tankless systems require annual flushing with food-grade citric acid or vinegar to clear out hard water scale buildup from the narrow heat exchanger. Without these isolation valves and their integrated flushing ports, future maintenance will be completely impossible.

Fortunately, all joints held pressure on the first test.

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6. Venting: Drilling Two New Holes Through the Wall

Tankless units require two PVC vent pipes: one for fresh air intake and one for exhaust. Drilling two large holes through the exterior wall took far more effort than expected—especially keeping the angles correct.

Once the pipes were through, I installed the combined intake/exhaust exterior termination (shown below). This must be sealed tightly to prevent moisture intrusion.

Venting Slope Correction (Venting Slope)

• The Correction: If you look at standard DIY conceptual diagrams, the venting pipe might appear to slope downward toward the outside wall. My physical execution was correct, but the design sketch was flawed. For Condensing Units, your vent lines must have a 1/4-inch per foot upward slope (approx. 2%) as they head toward the outside wall exit.
• The Reason: Because condensing systems extract so much heat, the exhaust cools rapidly and creates massive amounts of acidic condensate water. An upward slope forces this liquid to safely run back into the heater's built-in collector and neutralizer. If it slopes down toward the exterior wall, the acidic runoff will erode your home's outside siding and cause dangerous freezing/ice blockage at the vent outlet during winter.

Termination Spacing Clarification

• Generally, standard pipes require extending at least 12 inches out from the wall and maintaining a 12-inch gap between intake and exhaust to prevent exhaust recirculation. However, this 12-inch spacing rule does not apply if you use a proprietary termination kit or specialized concentric cap like the one shown above. The unique internal layout of these kits prevents cross-contamination automatically. Always defer to your specific manufacturer's manual.

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7. Condenser & Condensate Management: Why Tankless Heaters Produce Water

Modern high‑efficiency tankless heaters are condensing units, meaning they extract additional heat from the exhaust gases. This increases efficiency but also creates condensate—a steady flow of acidic water (typically pH 3–5).

Unlike a traditional tank heater, which vents hot exhaust straight up through a metal flue, a condensing tankless unit cools the exhaust so much that water forms inside the heat exchanger and vent pipes. This water must be drained properly to prevent corrosion inside the heater, damage to vent pipes, water pooling inside the unit, and code violations during inspection.

Strict Drainage Regulations

The instruction manual explicitly notes: Never run the heater without a functioning condensate drain line connected.

My installation location didn’t have a floor drain or a lower‑level drain nearby. The heater sits higher than any available drain line, so gravity drainage wasn’t possible. To solve this, I installed an automatic condensate pump:

• The pump collects the acidic condensate from the heater’s drain port.
• When the internal reservoir fills, the pump automatically activates.
• It pushes the water through a small line to a nearby drainage point located at a higher elevation.

(Note: If you do not have a floor drain, you must factor a dedicated condensate pump into your project budget).

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8. High-Altitude & DIP Switch Adjustments

If your home sits at an altitude above 2,000 feet (610 meters), air density changes require adjusting the internal fuel-to-air combustion ratio. You must open the front cover panel and toggle the specified DIP switches on the primary circuit board according to the manual's altitude chart. Failure to do this will cause incomplete combustion, massive soot buildup, and frequent system error faults.

(Note: My home doesn't have this high-altitude issue, but it is an essential step to note for inland or mountain installations).

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9. Advanced Consumer Advice: Micro-Flame Tech & Temperature Stability

If you are currently shopping for a tankless system, pay attention to these two real-world comfort factors:

• Micro-Flame Technology: Without this low-BTU modulation capability, standard heaters usually cannot drop below a minimum output temperature of roughly 96°F. During hot summer months when incoming groundwater is already warm, the lowest burner setting will still feel scalding hot. Look for a unit with micro-flame capabilities to keep summer showers comfortable.
• The "Cold Water Sandwich" Effect: If someone turns on a tap elsewhere in the house while you are showering, it can cause sudden water temperature fluctuations. Premium brands incorporate a small built-in buffer tank or active bypass mixing valves to smooth this out. My unit lacks this buffer feature, so we do experience occasional minor temperature jolts—something worth upgrading for if you have a larger family.

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10. Tax Credits and Utility Rebates Reminder

Don't leave free money on the table! Buying an energy-efficient condensing tankless water heater qualifies you for significant financial incentives. In the US, federal energy tax credits (like Section 25C) can cover up to 30% of your total hardware and installation costs (typically capped around $600). Furthermore, check your local gas and electric utility companies—they frequently offer direct cash rebates between $200 and $800 for upgrading to certified Energy Star tankless units.

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11. Mounting the Tankless Unit and Final Setup

With utilities ready, I mounted the tankless heater on the wall, connected the water, gas, electrical lines, vent pipes, and the condensate pump. After powering it on, the unit initialized immediately. I verified stable operation and set the temperature to 110°F.

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12. What I Did With the Old Tank

The old heater was still functional, so I listed it online for a low price. Someone picked it up the same day—better than sending it to recycling.

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13. Final Thoughts

The entire replacement took one full day. The permit process was easy, but the installation required real effort across plumbing, electrical, venting, and gas modifications. For additional structural reference details and parameters, you can check out the DIY_2.md file saved in the project repository.

If you’re highly comfortable with multi-trade plumbing, gas calibration, and electrical work, this is an incredibly rewarding, high-ROI DIY project. If not, hiring a certified professional is absolutely justified—tankless installations are significantly more complex than standard tank replacements.

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