Quick answer
Size a tankless water heater by flow at a GTA winter temperature rise. Incoming water is about 5°C in January, so a 199,000 BTU condensing unit — the largest residential class — delivers roughly 5 gallons per minute of hot water, enough for two showers at once. A family of four with two bathrooms needs that 199,000 BTU class; a one-bathroom household manages with 140,000–160,000 BTU. Undersizing is the usual cause of tankless disappointment.
What size tankless water heater you need is where Ontario differs sharply from the American advice that dominates search results. Our incoming water in a GTA January runs 4–7°C, far colder than Texas or California, which means the same tankless unit delivers roughly half the hot water here that its brochure headline suggests. Sizing for the winter inlet — not the best case — is what separates a great tankless experience from a lukewarm one.
Step 1: Add up your peak demand
- Shower: 2.0–2.5 GPM each (gallons per minute)
- Bathroom or kitchen tap: 1.0–1.5 GPM
- Dishwasher: 1.0–1.5 GPM
- Washing machine: 1.5–2.0 GPM
- Soaker or rain shower head, or a tub filler: 3.0–4.0 GPM
Use your real worst case, not a theoretical one: two showers plus a kitchen tap on a Saturday morning is roughly 5.5–6.5 GPM, while one shower and a dishwasher is about 3.5 GPM. If a fixture never runs during the morning rush, leave it out of the number.
Step 2: Apply the GTA temperature rise
Tankless capacity is quoted at a temperature rise — the difference between incoming water and the set point, typically 49°C (120°F). With a GTA winter inlet around 5°C you need a 44°C rise, about 78°F. Flow drops in direct proportion: a unit advertised at 9.8 GPM makes that number at a 35°F rise, and at our winter rise it delivers roughly half — around 5 GPM for a 199,000 BTU condensing model. Read the manufacturer's flow chart at a 75–80°F rise, and remember the same unit nearly doubles its output in summer when the inlet is in the high teens.
What that means by household
- 1–2 person condo or household with one bathroom: 140,000–160,000 BTU (about 3.5–4 GPM at the winter rise — one shower plus a tap).
- Family of 3–4 with two bathrooms: a 199,000 BTU condensing unit (about 5 GPM in January — two showers at once, or a shower plus the dishwasher and a tap).
- Larger homes with three-plus bathrooms, a big soaker tub or a body-spray shower: a 199,000 BTU unit plus staggered use, a small buffer tank, or two units cascaded.
That middle line is the most common GTA install — a mid-tier 199,000 BTU gas unit from Rinnai or Navien replacing an existing tank — and it typically lands at $4,200–$5,000 installed. Whether the conversion itself makes sense is a separate question; is a tankless worth it in Ontario weighs it.
Don't forget the gas line and venting
A 199,000 BTU tankless draws four to five times the gas of the tank it replaces. Many GTA homes need the gas line upsized from 1/2-inch to 3/4-inch between the meter and the unit, and the honest caveat is that upsizing the run can add $400–$1,200. Condensing models also need PVC intake and exhaust venting to an outside wall plus a condensate drain. All of it belongs in a professional quote up front — the tankless cost guide breaks down full installed pricing.
Condensing or non-condensing?
Condensing units — the Navien NPE and Rinnai RU families are the common GTA picks — pull extra heat out of the exhaust, vent in inexpensive PVC and need a condensate drain; non-condensing units vent in stainless steel and run hotter exhaust. In a basement install the condensing unit usually wins: the venting costs less to run, and the higher efficiency is what makes the $80–$200 a year saving over a standing tank possible.
When one unit is not enough
A household with a two-person body-spray shower, a jetted tub and a teenager schedule can beat 5 GPM on a January morning, and the fixes are a second unit cascaded, a small buffer tank, or staggering the heavy draws. A recirculation pump does not add capacity — it shortens the wait at the far taps, and left running unscheduled it adds gas use. Maintenance matters more than brand here: an annual descale flush ($150–$300) keeps the heat exchanger efficient — especially on GTA hard water.
ZK Mechanical sizes every tankless installation from your actual fixture count and the winter inlet temperature, not the brochure headline — and will say plainly when a quality tank fits your house better. Book a free assessment or call (647) 801-1252.