The promise of a tankless water heater — endless hot water and lower energy bills — only comes true if the unit is sized correctly. An undersized tankless heater will run out of temperature when you need it most; an oversized one wastes money. Sizing is where the real expertise lives, so here is exactly how it works.
Two Numbers Drive Everything: Flow Rate and Temperature Rise
Tankless heaters are not rated by tank capacity (there is no tank) but by how much water they can heat at once. That depends on two figures:
- Flow rate (gallons per minute, gpm): How much hot water your household demands simultaneously.
- Temperature rise (degrees F): The difference between your incoming cold water temperature and your desired output temperature.
A tankless unit's rated gpm only applies at a given temperature rise. Demand a bigger rise and the achievable flow drops. Get both numbers right and the unit performs flawlessly.
Step 1: Add Up Your Peak Flow Rate
List the fixtures you might run at the same time during your busiest moment — say, a morning when two showers and the kitchen sink are all going. Rough flow rates:
- Shower: about 1.5 to 2.5 gpm
- Bathroom faucet: about 0.5 to 1.5 gpm
- Kitchen faucet: about 1.0 to 2.2 gpm
- Dishwasher: about 1.0 to 2.5 gpm
Total the fixtures you realistically run together. A small household might peak around 4 gpm; a busy four-bath home can need 8 gpm or more.
Step 2: Calculate Your Temperature Rise
In much of the Bay Area, groundwater entering your home sits in the 50s Fahrenheit, while most homes want hot water delivered around 120 degrees. That gives a temperature rise of roughly 60 to 70 degrees. Colder winter inlet temperatures push the required rise higher, which is exactly when demand also peaks — so size for the worst case, not the average.
Step 3: Match a Unit to Both Numbers
Now find a tankless model that delivers your peak gpm at your required temperature rise. This is the step homeowners most often get wrong, because they look at the headline gpm rating without checking the temperature rise it assumes. The right model comfortably meets your peak demand on a cold morning with margin to spare.
Gas vs. Electric
- Gas tankless: The standard for whole-home use in the Bay Area. Higher output handles multiple fixtures easily, but may require upsizing the gas line and adding proper venting.
- Electric tankless: Great for point-of-use applications or smaller homes, but whole-home electric units can demand a substantial electrical service upgrade.
When to Consider Two Units
Large homes with many simultaneous demands sometimes do best with two tankless units, or a single high-capacity unit with recirculation. Some Navien models include a built-in recirculation feature that delivers near-instant hot water at the tap and reduces the wait — a real comfort upgrade.
Do Not Forget Bay Area Hard Water
Our region's moderately hard water leaves scale inside a tankless heat exchanger over time, which hurts efficiency and shortens lifespan. Annual descaling is essential, and pairing the unit with a water softener protects your investment. We build maintenance planning into every tankless recommendation.
Brands We Install and Trust
We carry tankless water heaters from Navien, Noritz, and Rheem — three of the most respected names in on-demand heating, each with strong efficiency and reliable performance. You can browse current models on our products page, and our team can run the sizing calculation for your specific home so you buy exactly what you need.
Get the Sizing Right the First Time
Tankless done right means endless hot water and years of savings; done wrong it means cold surprises. That is why we bring expert care to every fixture, starting with proper sizing. To get a tailored recommendation, contact us or call (408) 657-3325. We serve Campbell, San Jose, and the greater Silicon Valley area.