A Simple Framework: Temperature Rise + Simultaneous Flow
Sizing is where most tankless disappointments are born. The correct approach is not “pick a big one,” and it’s not “match my old tank size.” It’s a two-part calculation:
- Temperature rise required (how cold your incoming water is vs how hot you want it)
- Peak simultaneous flow (how many fixtures you want to run at once)
Step 1: Estimate Your Temperature Rise
Canadian groundwater is cold enough that it changes the whole sizing conversation. Mr. Rooter Canada notes that average groundwater temperature in Canada can be roughly 1.67–5.55 °C while typical hot-water faucet temperatures are about 43–49 °C, as explained in its guide to what size tankless water heater you need. That implies temperature rises commonly in the 40 °C range, which reduces the effective flow a unit can deliver.
Use this worksheet:
Step 2: Estimate Peak Simultaneous Flow (What You Actually Do)
Tankless units are commonly described by flow rate (often in gallons per minute). For rough planning, The Spruce lists typical fixture flows—about 2–2.5 GPM for a shower and 1–1.5 GPM for a bathroom faucet—in its tankless water heater buying guide. You don’t need perfect numbers; you need an honest picture of how your household overlaps hot-water use.
Build a “peak moment” scenario:
Now apply the cold-climate reality check: the same guide provides an example where a unit rated 11.1 GPM in ideal conditions can drop to roughly 4.6 GPM with 37 °F (≈3 °C) groundwater in colder northern regions, which includes Canada. The takeaway is not the specific brand or model—it’s that cold inlet water can cut “advertised” flow dramatically, so you size with temperature rise in mind.
Step 3: Translate Your Result Into A Purchase Conversation
Most homeowners don’t need to pick the exact model from first principles; they need to walk into a quote conversation with the correct constraints:
- “I need X GPM at roughly Y °C temperature rise.”
- “My peak is two showers at once (or not).”
- “My priority is stable temperature under overlap, not infinite duration.”
Then, let the installer confirm with manufacturer spec tables for your region and fuel type.
Under-sizing doesn’t just mean “slightly cooler water.” It often creates temperature swings when another fixture opens, which many homeowners interpret as “the unit is defective” when it’s actually a capacity mismatch.
