Tankless water heaters are energy-efficient, space-saving, and deliver hot water on demand—but only if you choose the right size. Unlike tank-style heaters that store gallons of hot water, tankless systems must produce hot water instantly as you use it.
If your unit is too small, you’ll run into cold showers and low pressure. Too large, and you’ll spend more than necessary. Here’s how to get it just right.
Step 1: Know Your Peak Hot Water Demand
To size a tankless water heater correctly, you need to calculate the maximum gallons per minute (GPM) of hot water your home might need at once.
Here are average GPMs for common fixtures:
- Shower: 2.0–2.5 GPM
- Kitchen sink: 1.5 GPM
- Washing machine: 2.0 GPM
- Dishwasher: 1.5 GPM
Example:
If two people take showers at the same time (2.5 GPM each) and someone’s running the dishwasher (1.5 GPM), your peak demand is around 6.5 GPM.
Step 2: Determine Required Temperature Rise
Temperature rise is the difference between the incoming groundwater temperature and your desired hot water temperature (usually 120°F).
Example:
If your incoming water is 50°F and you want 120°F water, you need a 70° temperature rise.
Northern states may require a higher rise due to colder incoming water, while homes in warmer regions may need less.
Step 3: Match Both Numbers to the Unit Specs
Now that you know your target GPM and temperature rise, look for a tankless unit that can handle that flow rate at that temperature rise.
For example, a unit might deliver:
- 8 GPM at 40°F rise
- 6 GPM at 60°F rise
- 4 GPM at 80°F rise
Choose a model that meets your needs without stretching it to its limit.
Step 4: Consider Whole-House vs. Point-of-Use
If your home has high usage or multiple bathrooms, a whole-house unit is likely the best fit. In smaller homes or for single fixtures (like a guest bath), a point-of-use tankless unit might be more efficient.
Step 5: Consult a Pro Before You Buy
Sizing depends on more than just math. Gas line capacity, electrical availability, venting needs, and elevation can all impact performance. An experienced plumber can help you choose a properly sized unit and install it for maximum efficiency.
Get It Right the First Time
At Poe’s Plumbing & Backflow, we offer expert sizing, installation, and maintenance for modern, efficient tankless water heater services tailored to your home’s needs.
Not sure what size you need? Contact us today for a free consultation.
