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Combi Boiler vs Tankless Water Heater in Ontario (2026): One Appliance or Two in a Radiant House, Winter Flow, Venting, Cost and Service
Heating, Cooling & Ventilation
Combi Boiler or Boiler Plus Tankless: One Box or Two, and the January Arithmetic That Decides It
If the house has radiant floors and gas, the hot water question comes down to this: one combi that does both jobs, or a heating boiler with its own water heater beside it. Inside, a combi and a tankless have the same kind of burner and the same kind of heat exchanger, so the flow at the tap is the same for the same input. The real differences are elsewhere: what happens to the floors while someone showers, how many holes go through the wall, what it costs, and what you lose when one of them quits.
The Flow Is the Same Arithmetic Either Way
Ontario water is cold in winter, around 5 C coming in from the street or the well, and you want it at 50 C at the tap. That is a 45 C rise, 81 F. A 199,000 BTU/h appliance at 95 percent efficiency puts 189,050 BTU/h into the water. Divide by 500 times the rise in F and you get 189,050 / (500 x 81), about 4.7 US gallons a minute, or 17.7 litres. It does not matter whether the box says combi or tankless on the front.
| Incoming water | Rise to 50 C | 199,000 BTU/h unit | A unit at three-quarters the input |
|---|---|---|---|
| 5 C, January | 45 C (81 F) | about 4.7 US GPM, 17.7 L/min | about 3.5 US GPM |
| 15 C, late summer | 35 C (63 F) | about 6.0 US GPM, 22.7 L/min | about 4.5 US GPM |
Size for January, not for the day the salesman visited in August. Not every combi has a 199,000 BTU/h hot water side; read the spec sheet. Fixture-by-fixture sizing is on the tankless water heater page and in the sizing calculator below.
Say two showers at 2 GPM each and a kitchen tap. In January, 4.7 GPM covers the two showers and leaves the tap a little short. That is the whole sizing conversation for most houses, and it applies equally to both choices.
What the Two Choices Cost
| Setup | Installed range | Budget |
|---|---|---|
| Combi boiler, heat and on-demand hot water | $5,000 to $10,000 | $7,500 |
| Condensing boiler, space heating only | $5,500 to $11,000 | $8,000 |
| plus a condensing tankless, replacement install | $2,800 to $5,500 | $4,150 (middle) |
| or plus an indirect tank off the boiler | $1,500 to $2,500 | $2,000 allowance |
| Condensing tankless in a new house, with venting | $3,500 to $6,500 | |
| Gas line upsizing, either way, if needed | $500 to $1,500 |
Why Ontario prices are all over the map: the contractors and price guides we checked were far apart on both appliances. For a combi, one price guide says $3,500 to $10,000 and up while our own existing page says $5,000 to $8,000. For a tankless, a renovation quote site gives $2,500 to $4,000 non-condensing, $3,000 to $5,000 condensing and $4,500 to $6,000 premium with recirculation, against $3,500 to $6,500 in a new house on our page. The difference is venting, the gas line, old-unit removal and the brand. Budget $7,500 for a combi or $12,150 for boiler plus tankless, and get two or three quotes on the same written scope.
So on the installed price: combi $7,500, boiler plus indirect tank $10,000, boiler plus tankless $12,150. The combi wins by $2,500 to $4,650 before HST, and it is one appliance on the wall instead of two.
The Radiant House and DHW Priority
A combi cannot heat the floor and the shower at full fire at the same time, so it picks the shower. That is DHW priority, and in a radiant slab it costs you nothing you can feel: the concrete holds hours of heat and coasts through the shower like a woodstove after the fire dies down. Two appliances heat both at once, which matters in a house heated by low-mass panels or a fan coil, or one with a long morning rush on a minus 25 C day.
The other radiant point is turndown. The hot water side wants a big burner and a tight house wants a small one. A boiler sized only for the heat loss can be a small, quiet appliance that runs long and steady; a combi has to be big for the shower and then turn way down for the floors. Read the minimum firing rate. The combi cost page has the modulation story and the radiant design page covers loop and supply temperatures, so I will not repeat them here.
One warning. A tankless water heater is not a boiler. Do not let anyone pipe your floors off a water heater unless the manufacturer lists that exact unit for space heating and the drinking water stays separate from the heating water.
Venting, Gas and Service Day
A combi is one vent through the wall, one gas connection and one condensate drain. Boiler plus tankless is two of each, and both vents have to keep their clearances from windows, doors, the gas meter and the HRV intake, which on a small wall is like parking two trucks in a one-car driveway. Boiler plus indirect tank is one vent and one burner, which is why it is a good answer for a big family.
All of it is gas work. It is done by a TSSA-registered fuels contractor with a certified gas technician, usually a G2 for a house, and the gas line is sized for every appliance on it at full fire at once. If the house has just been air sealed, have the venting and makeup air checked, and keep a CO alarm on every floor.
On service day the combi is one appointment. When it fails, the heat and the hot water go together, usually on a Sunday. With a boiler and a separate tankless, a dead water heater means cold showers in a warm house, and a dead boiler leaves you hot water. An indirect tank goes cold with its boiler, so it buys storage, not redundancy. Some families pay the extra $4,650 for the boiler and tankless pair and are glad of it.
- Pick the combi for a normal-sized radiant house, one or two showers at once, a small mechanical room and a tight budget.
- Pick boiler plus indirect tank for a big house, a big tub or three showers at 7 a.m.
- Pick boiler plus tankless when the hot water and the heat are far apart in the house, or when you want two independent appliances.
Questions people ask
Is a combi boiler better than a tankless water heater?
For hot water alone they are the same burner with the same flow for the same input. A combi is better when the same appliance also heats a radiant house; a tankless is better when the heat comes from something else, a furnace or a heat pump.
Will the floors go cold while someone showers?
Not in a slab. The concrete holds heat for hours and the combi goes back to the floors as soon as the tap closes. In a house heated by fan coils or thin panels, a long shower on a cold night can be noticed.
How many showers can a 199,000 BTU/h combi run in winter?
About 4.7 US GPM with 5 C water coming in, so two showers at 2 GPM each with a little left for a tap. In late summer, with warmer water, about 6 GPM.
Which costs less to install?
The combi, about $7,500 against $10,000 for a boiler with an indirect tank and $12,150 for a boiler with a tankless, before HST. The gas line and venting can move all three.
Run your own January numbers
The tankless and combi sizing calculator takes your incoming water temperature and the fixtures you run at once and tells you the BTU/h and GPM you need, and what a 199,000 BTU/h unit actually delivers. If the quotes are already on the table, the $99 review reads up to three of them against these numbers.
Sources and further reading
- TSSA: hire registered and certified fuels professionals: Technical Standards and Safety Authority: why gas furnace, boiler and water heater work must be done by a registered contractor with a certified technician, and how to check.
- Gas furnaces: Canada energy efficiency regulations: Natural Resources Canada: the minimum AFUE rules for gas furnaces sold in Canada and what the efficiency ratings mean.
- Enbridge Gas Ontario rates: the current gas supply, transportation and delivery charges and the monthly customer charge.
