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Tankless Water Heater Sizing Calculator Ontario (2026): How Many Showers at Once, in January, With Lake or Well Water at 5 C, and What a 199,000 BTU Tankless or Combi Really Delivers
Heating, Cooling & VentilationFree Calculator
How Big a Tankless Do You Need? Size It for a January Morning, Not the Box.
Incoming water temperature, the fixtures running at once, the flow and the BTU/h it takes, and what 120,000 to 199,000 BTU tankless and combi units really deliver at that rise. No email address required.
The number on the box is the maximum gas input. The number in your shower is how many litres a minute it can heat from the temperature your water comes in at, and in an Ontario January that is 5 C off a lake or a well, not the 15 C the brochure assumed. A 199,000 BTU unit that runs three showers in July runs about two in February. This calculator does the arithmetic for your house.
Step-by-step
Tankless and Combi Sizing (Ontario)
1
The water
Size for the coldest month. A tankless that is right in July is undersized in February.
Use a mixing valve at the fixtures to prevent scalding; 49 C at the tap is the common target.
2
What runs at the same time on a bad morning
Mixed to about 40 C at the head.
About 4 US gpm each, filled to 40 C. A soaker tub is its own problem.
About 1.5 US gpm, mostly hot.
About 0.5 US gpm each, half hot.
About 1 US gpm hot while filling.
About 2 US gpm, half hot.
3
The appliance you are looking at
4
Where is the house?
Sets the installed-price adjustment, the heating degree-days and the winter design temperature.
Used only to tell you whether the house is in our direct service area. Not stored.
Enter a postal code to see service coverage.
BuildersOntario.com
What the House Needs, and What the Unit Gives
Illustrative sizing
| Line | Value |
|---|
Hot water needed at the peak0
Example only. Hot water flow is each fixture’s flow times the share of it that has to be hot to reach its use temperature (40 C for showers and tubs) from your incoming temperature. Output needed is 500 x US gpm x temperature rise in F, in BTU/h; the gas input is that divided by the efficiency. Real units derate with altitude, gas pressure, venting length and scale; use the manufacturer’s flow chart for the exact model. Not a quote and not an equipment selection.
Request a Real Quotation
Your fixtures, your water temperature and the peak flow become a written scope a plumbing or HVAC contractor can quote without guessing, including the gas line, the venting and whether a combi has to heat the floors too. It goes straight to our inbox, and a copy comes back to you; reply to that copy with a photo of the existing water heater label, the gas meter and the spot where it would go.
What this calculator does, and what it will not do!
It adds up the hot water your house asks for at the worst moment of the day, works out the temperature rise from your incoming water, and turns both into the BTU/h a tankless or combi has to burn. Then it runs the appliance you picked backwards and shows what it can actually deliver at that rise. If the second number is smaller than the first, somebody’s shower goes cold, and it is usually the last person up.
It does not pick a model. Every maker publishes a flow chart, and that chart, for the exact unit, at your gas pressure and vent length, is the final word. It also does not size a combi for heating; that comes from the heat loss, and the two numbers are usually far apart.
A rule of thumb: a 199,000 BTU/h condensing unit at 95% puts out about 189,000 BTU/h. Heating 5 C water to 49 C is a 44 C (79 F) rise, so it delivers about 4.8 US gpm, or about 18 L/min. Two showers at 2 gpm mixed to 40 C need about 3.2 gpm of that. Add a kitchen sink and you are at 4.4. Run three showers instead and you are right at the limit, with nothing left for the sink.
What a 199,000 BTU/h unit really gives you
| Incoming water | Rise to 49 C | US gpm at 95% | L/min | Showers at 2 gpm, alone |
|---|---|---|---|---|
| 4 C (lake, shallow well, February) | 45 C (81 F) | 4.7 | 17.7 | 2.9 |
| 5 C (most Ontario winters) | 44 C (79 F) | 4.8 | 18.1 | 3.0 |
| 8 C (mild southern winter) | 41 C (74 F) | 5.1 | 19.4 | 3.3 |
| 15 C (summer) | 34 C (61 F) | 6.2 | 23.4 | 4.2 |
Output 199,000 x 0.95 = 189,050 BTU/h; flow = output / (500 x rise in F). Showers counted at 2 gpm mixed to 40 C, so the share that has to be hot rises as the incoming water gets colder. These are theoretical; real units derate a little. Installed prices are on the tankless water heater and combi boiler cost pages.
Why Ontario prices are all over the map: the contractors and published price guides we checked put a condensing gas tankless at $3,000 to $5,000 installed (a renovation quote site), $4,500 to $6,000 with recirculation, and $3,500 to $6,500 in a new house on our own tankless page. The gap is the gas line, the venting run, the condensate drain, scale protection and whether the old rental tank comes out cleanly. Budget about $4,500 for a straightforward swap, and get two or three quotes on the same written scope.
Where homeowners go wrong
- Sizing from the summer. The same unit gives about 30% more in July than in February. Size for February.
- Reading the gpm on the box. Many makers quote flow at a small rise. Find the flow at your rise, about 79 F for most of Ontario in winter.
- Forgetting the gas line. A 199,000 BTU/h unit needs a gas line that can carry it, often bigger than the one that fed the old tank. That is $500 to $1,500 that did not make the first quote.
- Buying a combi for the floors and a family of six. The combi sized for the showers is far too big for the slab in an ICF house. It works, but its turndown has to be good, or an indirect tank on a smaller boiler is the better design. Heat sources for radiant.
- Skipping the softener on well water. Scale is the number one tankless failure. It is cheaper to treat the water than to replace a heat exchanger.
Questions people ask
How many showers can a tankless water heater run in an Ontario winter?
A 199,000 BTU/h condensing unit runs about three 2 gpm showers at once with 5 C incoming water and nothing else on, or two showers and a kitchen sink. A 150,000 BTU/h unit runs about two. Low-flow heads at 1.8 gpm buy you a little more.
What size tankless do I need for a family of four?
For two showers at once plus a sink in winter, about 4.5 US gpm at a 79 F rise, which is about 190,000 BTU/h of input at 95%: in practice a 199,000 BTU/h unit. With one shower at a time, a 150,000 is plenty.
Does a combi boiler give the same hot water as a tankless?
At the same input, about the same flow, because the arithmetic is the same: output divided by the rise. The difference is that a combi also heats the house and pauses the heating while it makes hot water. Combi vs tankless.
Why does my tankless go cold when two showers run?
Either the unit is at its limit at that incoming temperature, the gas line is too small to feed full fire, or the heat exchanger is scaled. A technician can check the gas pressure at full fire; scale shows up as falling output over the years.
Is a tank better than a tankless?
For big simultaneous demand, a soaker tub or hard water, often yes. A tank stores 150 to 200 L ready to go; a tankless makes it as you use it. A heat pump water heater uses the least energy of all, and needs no gas. Rent or buy.
Quotes in hand?
The $99 quote review checks up to three water heater, tankless or combi quotes against your peak flow, the gas line, the venting and the manufacturer’s flow chart, and lists what each one left out.
