What size tankless water heater do I need?
Sizing a tankless water heater correctly is the single most important factor in being happy with it. Too small and you’ll get lukewarm water when demand is high; oversized and you’ve paid for capacity you don’t use. Unlike a tank, a tankless unit is rated by how much hot water it can deliver per minute, not by storage volume.
Proper sizing comes down to two numbers: peak flow rate (how much hot water you might use at once) and temperature rise (how much the unit must heat the incoming water).
Here’s how to work through it for a Metro Vancouver or Fraser Valley home, where cooler incoming water makes correct sizing especially important. HeatLand sizes and installs tankless units across the Lower Mainland to match your real demand.
Step 1: Calculate Your Peak Flow Rate
Start by figuring out how much hot water you might use simultaneously during your busiest moment. Each fixture has an approximate flow rate: a modern showerhead runs roughly 1.5 to 2.5 gallons per minute (about 6 to 9.5 litres), a kitchen or bathroom faucet around 1 to 2 gpm, and a dishwasher or washing machine adds more.
List the fixtures you realistically expect to run at the same time, for example, two showers plus a kitchen sink, then add their flow rates together. That total is your peak flow demand, the amount of hot water the unit must supply at once without running cold.
Be honest about your household’s habits. A busy family home with multiple bathrooms used in the morning has a much higher peak than a couple who stagger their usage. Sizing to your realistic peak, not your average, is what prevents the lukewarm-water complaint. When in doubt, size up slightly rather than risk falling short.
Step 2: Determine the Temperature Rise
The second number is temperature rise, the difference between your incoming (groundwater) temperature and your desired output temperature. Most homes target around 120F (49C) at the tap for showers and general use.
Incoming water temperature is where climate matters. In much of the Lower Mainland, winter groundwater can be cool, sometimes in the 40s F (single-digit C). If your incoming water is 50F and you want 120F, you need a 70F temperature rise. In summer the incoming water is warmer, so the rise, and the demand on the unit, is smaller.
This is critical because a tankless unit’s flow rating drops as the required temperature rise increases: the same unit that delivers plenty of hot water in summer can fall short in winter if it was sized for mild conditions. Always size for the coldest, most demanding scenario, your peak flow at your coldest winter temperature rise, so the unit performs year-round, not just in warm months.
Step 3: Match a Unit and Get It Right
With your peak flow and required winter temperature rise in hand, you select a unit rated to deliver that flow at that rise. Manufacturers publish flow charts showing gallons or litres per minute at various temperature rises, so you match your numbers against real specs rather than a single headline figure.
For example, a household needing about 5 gpm at a 70F winter rise requires a fairly high-capacity gas unit; an electric unit may struggle to reach that in cold weather, which often steers larger Lower Mainland homes toward gas. Gas supply size and venting must also support the chosen unit.
Because an undersized unit disappoints and an oversized one wastes money, and because the calculation depends on your fixtures, habits, water temperature, gas capacity, and climate, professional sizing pays for itself. Installation involves gas and venting and must be done by a licensed, insured, TSBC-certified technician. HeatLand assesses your real peak demand and local water temperatures to size and install the right unit, with financing available.
Quick Checklist
- List fixtures you’ll run simultaneously
- Add their flow rates for peak demand
- Find your incoming winter water temperature
- Subtract from your target temp for temperature rise
- Match a unit rated for that flow at that rise
- Confirm gas supply and venting support the unit
- Get professional sizing to avoid over/undersizing
| Household Scenario | Approx. Peak Flow | Typical Fit |
|---|---|---|
| Small home / condo, staggered use | 2-3 gpm | Smaller gas or high-capacity electric |
| Family, 1-2 fixtures at once | 4-5 gpm | Mid-to-large gas unit |
| Large home, multiple simultaneous | 6+ gpm | High-capacity gas unit |
| Single distant fixture | 1-2 gpm | Point-of-use unit |
Frequently Asked Questions
Why does winter matter for tankless sizing?
Because a tankless unit’s hot-water output drops as the temperature rise increases. In winter, Lower Mainland groundwater is cooler, so the unit must heat it more, reducing how much it can deliver per minute. Sizing for warm-weather conditions leaves you short in winter, so always size for your coldest, highest-demand scenario.
Can one tankless water heater serve a whole house?
Yes, if it’s sized for your peak simultaneous demand at your winter temperature rise. A properly sized high-capacity gas unit can run multiple fixtures at once in a typical home. Larger homes with high simultaneous demand sometimes use two units. Accurate sizing based on your real habits is what makes whole-home service reliable.
What happens if my tankless unit is too small?
An undersized unit can’t maintain the target temperature when demand exceeds its capacity, so hot water turns lukewarm during peak use, such as two showers at once on a cold morning. This is the most common complaint from poor sizing. Sizing to your realistic winter peak, ideally with professional help, prevents it.
Should I just buy the biggest unit to be safe?
Not necessarily. An oversized unit costs more up front, may need a larger gas line or venting, and delivers no benefit beyond your actual peak demand. The goal is right-sizing: enough capacity for your busiest winter moment without paying for unused output. A professional assessment finds that balance for your home.
Reviewed by the HeatLand Heating & Cooling team — licensed HVAC contractor serving Metro Vancouver & the Fraser Valley since 1999. This article is for general guidance only. If you smell gas, suspect a carbon monoxide leak, or are unsure about any electrical or gas component, stop troubleshooting immediately and call a licensed technician or 911.