Tankless Water Heater Installation Mistakes Homeowners Should Avoid

Upgrading to a tankless water heater seems like a no-brainer given the unlimited hot water, lower utility bills, and more space in the utility room. But installing one isn’t as simple as replacing a tank with a sleeker unit. When the installation is done wrong, you’ll notice right away. Inconsistent hot water, long wait times, or even system failure are all signs that something was missed.

The good news? Most of the major issues are completely preventable if you know what to look out for. Here are the most common tankless installation mistakes homeowners make, and how to avoid turning your upgrade into a costly problem.

Mistake #1: Choosing the Wrong Size for Your Home

This is one of the biggest reasons people regret going tankless. If your unit is too small, it won’t keep up with simultaneous usage—like showers, laundry, and dishwashing happening at once. You’ll get temperature drops or cold water cutting in mid-shower.

Avoid it by:

• Calculating your home’s peak hot water demand
• Factoring in the number of bathrooms and appliances used at the same time
• Asking a licensed plumber to assess your actual usage instead of guessing

Oversizing can also be a problem. You’ll spend more on equipment and installation than necessary, without gaining any extra comfort or performance.

Mistake #2: Ignoring Local Code and Venting Requirements

Tankless units need proper venting to expel exhaust safely. Depending on the model (gas vs. electric), local building codes may require specific vent materials, clearances, and drainage systems for condensate.

What can go wrong:

• Improper venting leads to carbon monoxide buildup or system shutdowns
• Non-compliant installs may void your warranty or fail inspection
• Moisture from condensate can damage surrounding walls or floors

Solution: Always have your installation permitted and inspected. A good plumbing professional will plan your vent runs correctly and use the right materials for code compliance and long-term safety.

Mistake #3: Reusing Old Gas or Electrical Lines Without Upgrading

Tankless systems don’t just connect to whatever lines you had before. They require a specific amount of gas or electrical capacity to run at peak performance.

Common problems we see:

• Gas supply lines that are too small for the demand
• Breakers or wiring that can’t handle the system load
• Inconsistent heating or frequent error codes caused by supply issues

Fix it before it happens:
Have your gas or electrical lines inspected and upgraded if needed before install day. This is not the place to cut corners as supply problems lead to frustrating performance issues and can become fire or safety hazards.

Mistake #4: Skipping the Water Quality Check

If you live in an area with hard water, your tankless water heater is more vulnerable to mineral buildup than a traditional tank. Without a filter or softener, scale forms inside the heat exchanger and chokes performance.

What happens over time:

• Slower heating response
• Higher energy usage
• Early failure of the unit

What to do:
Have your plumber check your water quality. In hard water zones, install a scale filter or softener at the same time as your heater. Schedule regular flushing to keep the system clean.

Mistake #5: Poor Installation Location

Tankless units are compact, but they still need space for airflow, access, and venting. Installing them in tight closets, small cabinets, or unventilated areas can shorten the unit’s lifespan or violate manufacturer guidelines.

Better install choices include:

• Garage walls with clear side access
• Basement utility rooms with venting potential
• Outdoor-rated units for mild climates (with freeze protection)

Location affects everything from serviceability to efficiency. Ask your installer to walk you through vent clearances, drain paths, and access points before deciding.

Mistake #6: Hiring a General Contractor Instead of a Licensed Plumber

Tankless water heaters require knowledge of gas line sizing, venting, water pressure balancing, and manufacturer specs. General tradespeople often miss important steps or treat the job like a simple tank swap.

Why this matters:

• An improperly installed unit may not qualify for warranty coverage
• Missed steps often show up weeks later as inconsistent heat or leaks
• Fixing a bad install can cost more than doing it right the first time

Always hire a licensed plumber with experience in tankless installations. They’ll understand flow rates, venting materials, code compliance, and how to avoid every mistake on this list.

Tankless Systems Work, But Only If Installed the Right Way

Tankless water heaters are efficient, long-lasting, and great for families that use hot water throughout the day. But they’re also more technical to install than tank-style systems. The most common issues come from guesswork, shortcuts, or hiring the wrong person.

At Rockwater Plumbing, we don’t just hook up a unit; we plan the system for your home, confirm sizing and water conditions, and install to code with long-term performance in mind.

Thinking about going tankless? Call us for a proper assessment before buying a system that doesn’t fit. We’ll help you avoid costly mistakes and get it done right the first time.