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

**Summary:**I recently replaced my traditional tank water heater with a modern tankless unit. The project involved permitting, electrical work, gas line modifications, copper pipe soldering, drilling new vent holes through the exterior wall, and installing a condensate pump for drainage.The photos in this article show several key parts of the installation, including the new exterior intake/exhaust vent, the custom‑built gas sediment trap, the mounted tankless heater, and the vent routing layout. This article documents the entire process from start to finish.

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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.

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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:

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 and prep the ends
• dry‑fit new sections
• heat the joints with a torch
• solder everything with high‑temperature solder

Copper soldering requires patience and confidence. One bad joint means leaks, re‑heating, and re‑doing the work. 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
• one for exhaust

These must exit the wall with a specific downward slope and spacing. 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.

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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
• code violations during inspection

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 a condensate pump:

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

This setup works reliably and keeps the installation compliant with code.

The exterior vent assembly shown earlier also reflects the condensing design: the exhaust temperature is low enough that PVC venting is allowed, and the intake/exhaust separation helps maintain proper airflow while minimizing moisture buildup.

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8. 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 set the temperature to 110°F and verified stable operation.

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9. 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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10. Final Thoughts

The entire replacement took one full day. The permit process was easy, but the installation required real effort:

• electrical wiring
• gas line modifications
• copper pipe soldering
• drilling vent holes
• installing a condensate pump
• mounting and connecting everything

If you’re comfortable with plumbing, gas, and electrical work, this is a rewarding DIY project. If not, hiring a professional is absolutely justified—tankless installations are significantly more complex than tank replacements.

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