What Happens When a Tankless Water Heater Isn’t Sized Correctly for Your Home?

Tankless water heaters are sized by the amount of hot water they can deliver at once, based on flow rate and temperature rise. An undersized tankless water heater can’t keep up with your household’s hot water demand, and an oversized one wastes energy cycling on and off unnecessarily. Getting the sizing wrong doesn’t just cause inconvenience; it shortens the unit’s lifespan and drives up your utility bills. If you rely on a tankless water heater in Chandler, AZ, here’s what you need to know before a bad sizing decision costs you more than it should.

The Number That Determines Everything

Tankless water heaters are rated by flow rate, measured in gallons per minute (GPM), and temperature rise. According to the U.S. Department of Energy, a unit must be sized based on the maximum flow rate you need and the temperature of the incoming groundwater in your area. In Arizona, groundwater tends to run warmer than northern states, which works in your favor, but simultaneous demand still adds up fast.

Running a shower, dishwasher, and washing machine at the same time can require 5 to 7 GPM or more. A unit rated for 3.5 GPM won’t come close.

What You’ll Notice When the Unit Is Too Small

The symptoms of an undersized unit show up quickly. Homeowners often report:

• Lukewarm water during peak usage hours
• Temperature fluctuations mid-shower
• The unit shutting off when multiple fixtures run simultaneously
• Longer wait times for hot water to reach faucets

These aren’t random quirks. They’re the unit telling you it’s stretched beyond its capacity.

The Hidden Damage From Getting It Wrong

Beyond comfort issues, improper sizing accelerates wear on the heat exchanger, which is one of the most expensive components to replace. A unit working harder than it was built for burns through its parts faster. Scheduling routine tankless water heater maintenance can help catch early wear signs, but no amount of upkeep will fully compensate for a unit that was never matched to your home in the first place.

How Sizing Works

A qualified plumber calculates sizing based on:

• Number of fixtures running simultaneously
• Incoming water temperature (typically 60 to 70 degrees Fahrenheit in Chandler)
• Desired output temperature (commonly set to 120°F for safety)
• Fuel type, gas or electric, and available capacity

Professional tankless water heater installation takes all of these variables into account before a unit ever gets mounted to your wall.

Frequently Asked Questions About Tankless Water Heaters

Can I add a second unit if mine can’t keep up?

Yes. Parallel units installed in tandem are a practical solution for larger households.

Does brand affect sizing requirements?

No. GPM ratings and temperature rise calculations apply regardless of manufacturer.

How often does sizing need to be reassessed?

When you add bathrooms, appliances, or more people to the household.

Get the Right Fit From a Team That Knows the Work

ServWorx Plumbing is a family-owned, locally operated company with over 15 years of experience handling tankless water heater work across the Valley. We offer free estimates, transparent pricing with no hidden fees, and 24/7 emergency service when you can’t wait. Senior citizens and military and first responders receive 10% off. We’re licensed, insured, and here when it counts.

Call us today for a free estimate.