Explore Tankless Water Heater Sizing Guide for Ontario Homeowners

Switching to a tankless water heater is a smart move for many Ontario homes. These systems provide endless hot water, take up less space, and are more energy-efficient than traditional tanks. But to experience those benefits, one key factor matters most: choosing the right size unit. 

If your tankless water heater is undersized, it won’t keep up with demand. Oversized, and you're paying more than necessary. Let’s walk through how to size your tankless water heater correctly based on your home, lifestyle, and hot water habits. 

Why Sizing Matters for Tankless Water Heaters? 

Unlike traditional tanks that store and heat a large volume of water, tankless systems heat water on demand. That means the unit must instantly deliver enough hot water for everything that’s running whether that’s one shower or multiple taps and appliances at the same time. 

An improperly sized unit can lead to: 

• Cold water interruptions 
• System overloads 
• Reduced efficiency 
• Premature wear 

The goal is to select a unit that matches your household’s peak demand without overdoing it. 

Key Factors to Consider When Sizing a Tankless Water Heater 

1. Flow Rate (GPM – Gallons Per Minute) 

This is the total hot water demand in your home at the same time. Each appliance or fixture has its own flow rate. Here's a quick breakdown of common usage: 

Example: If someone is showering (2.0 GPM), while the dishwasher (1.5 GPM) and a bathroom faucet (1.0 GPM) are running at the same time, you’d need a unit that can handle at least 4.5 GPM. 

2. Temperature Rise Required 

Ontario has cold groundwater temperatures, especially during the winter. The colder the incoming water, the harder your system must work to reach your desired hot water temperature (typically 120°F or 49°C). 

In Ontario: 

Groundwater in winter: ~5°C (41°F) 

Target output temp: ~49°C (120°F) 

Required rise: 44°C (79°F) 

Your tankless unit must be able to raise the temperature by this amount at your required flow rate. 

Some systems can provide: 

• 5 GPM with a 40°C rise 
• 3 GPM with a 60°C rise 

Always look at manufacturer specs showing flow rate at a given temperature rise. 

3. Number of Bathrooms and Simultaneous Users 

A general guideline based on bathrooms: 

• 1–2 bathrooms: 6–8 GPM unit 
• 3+ bathrooms: 9–11 GPM unit or dual system 

If you have multiple showers or a soaker tub, or run laundry and dishes during the same window, consider a higher capacity model or a recirculation pump for consistent delivery. 

For Ontario homes with large families or multiple floors, a mid- to large-capacity unit is usually recommended. Learn more about hot water demand in our guide: Can a Tankless Water Heater Run Out of Hot Water? 

Gas vs Electric Tankless: How Sizing Differs 

Gas tankless water heaters offer higher GPM ratings and are better suited for whole-home use in colder regions like Ontario. 

Electric models work best in smaller homes, condos, or cottages with lower hot water needs. 

Important: Most Ontario homeowners go with natural gas due to the higher output and reliability in winter. 

What Size Tankless Water Heater Do I Need?  

Choosing the right tankless water heater involves two steps: 

Step 1: Calculate Your Peak Hot Water Demand (GPM) 

Add the flow rates of fixtures you expect to use at the same time. Use this chart to estimate typical flow rates: 

Formula: Total Flow Rate (GPM) = Sum of all fixtures running simultaneously 

Example: Shower (2.0) + Kitchen Faucet (1.5) + Dishwasher (1.5) = 5.0 GPM total demand 

Step 2: Determine Required Temperature Rise 

Temperature rise = Desired Output Temperature – Incoming Groundwater Temperature 

For Ontario, groundwater can be as low as 5°C in winter. Most households want water heated to 49°C. 

Formula: Temperature Rise (°C) = 49°C – 5°C = 44°C 

Reference Sizing Table 

Here’s a simplified table that combines both GPM demand and temperature rise to help you choose the right unit: 

Tip: Always size for your peak usage, not your average. 

Still Not Sure? Let Go Lime Handle It 

If you're unsure how many gallons per minute your home really uses, or how cold your incoming water gets, we’ll handle the calculations during your consultation. We factor in family size, number of bathrooms, appliance use, and seasonal conditions in Ontario to recommend the ideal system. 

And we don’t stop at installation; we offer Protection Plans that cover maintenance and repairs, so your system stays reliable year-round. 

Need help getting started? Learn more in our Tank vs. Tankless Water Heater comparison guide. 

Final Thoughts 

Getting the right size tankless water heater is the difference between limitless comfort and frustrating cold surprises. For Ontario homeowners, it's especially important to account for low groundwater temperatures and simultaneous hot water use. 

Not sure what size is right for you? We can help. Our licensed experts will guide you through the process, recommend the best fit, and handle professional installation across Ontario. 

Book your free consultation and explore smarter water heating today.