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Combi Boiler Cost in Ontario (2026): The Replacement Price, the Complete Mechanical Room, What Belongs in Each Quote, and the Houses a Combi Suits
Heating, Cooling & Ventilation
What a Combi Boiler Costs in Ontario: $7,500 to Swap One, $14,000 and Up to Build the Whole Room, and Why the Two Quotes Look Nothing Alike
A combi boiler is one wall-hung gas appliance that heats the house and makes the domestic hot water on demand, with no tank. The box itself is only part of the price. A straight replacement on an existing radiant system is one number; a new mechanical room with the manifolds, pumps, mixing and controls is a much bigger one, and homeowners get the two confused every week. This page prices both, says what belongs in each quote, and explains the two things a combi does that a furnace never has to think about: DHW priority and modulation.
The numbers
$7,500combi replacement, the budget number
$14,000+complete new combi mechanical room, low end
4.7 GPMhot water from a 199,000 BTU/h combi in January
$395CSA F280 heat loss to size it from
| Item | Range | Default or note |
|---|---|---|
| Combi boiler replacement, existing radiant system, venting and gas line adequate | $5,000 to $10,000 | $7,500 |
| Complete new combi mechanical room: combi, manifolds, pumps, mixing, controls | $14,000 to $25,000 and up | quoted per house |
| Manifolds and zone controls, one bathroom | $500 to $1,500 | |
| Manifolds and zone controls, a basement | $1,500 to $3,500 | |
| Manifolds and zone controls, whole house | $4,000 to $10,000 | |
| Gas line upsizing for a large on-demand appliance | $500 to $1,500 | only if the meter or pipe is too small |
| Municipal permit, where your municipality wants one | $90 to $350 | |
| For comparison: condensing boiler, space heating only | $5,500 to $11,000 | $8,000 |
| For comparison: indirect tank off a heating boiler | $1,500 to $2,500 | allowance |
| CSA F280 heat loss report / full HVAC design and drawings | $395 / $695 | OntarioHeatLoss.ca |
Why Ontario prices are all over the map: the price guides we checked could not agree on what a combi costs. One national price guide says $3,500 to $10,000 and up, our own tankless page says $5,000 to $8,000 for a combi feeding radiant and domestic water, and a complete new combi mechanical room runs $14,000 to $25,000 and up. Most of that spread is scope: the low numbers are a box hung where the old one was, the high ones include piping, manifolds, mixing and controls. Brand matters, and so does who is quoting and whether it is a January no-heat call. Budget $7,500 for a replacement and get two or three quotes on the same written scope.
The in-floor tubing is not on this table. Tubing in a new slab runs $6 to $12 a square foot of heated area, and the radiant floor cost page prices it.
Two Quotes, Two Different Jobs
A replacement. A 1990s house with radiant in the basement slab and an old combi that has started throwing codes. The piping, manifolds and zone valves stay. The new combi goes on the same wall, with new venting if the old vent does not match the new appliance, which it often does not. Combi $7,500, and the installer finds the gas line is too small for the new burner at full fire, $1,000 to upsize it. That is $8,500, HST $1,105, total $9,605.
A new mechanical room. A 2,000 sq ft ICF house, radiant on both floors, nothing in the basement but a gas stub and a floor drain. The quote comes in at $20,000: the combi at $7,500, whole-house manifolds and zone controls at $6,000, and $6,500 for everything quotes rarely break out, which is the circulators, the mixing valve that keeps the water down at slab temperature, the air separator, the expansion tank, the piping between them and the hour it takes to commission it properly. HST $2,600, total $22,600. The heat loss report and the design are separate, $395 and $695 at OntarioHeatLoss.ca, and the tubing in the slab is separate again.
If one quote says $7,500 and the other says $20,000 for your new house, they are not quoting the same job. The cheap one is the box. Ask what is missing before you celebrate.
DHW Priority and Modulation, in Plain Words
DHW priority means that when someone opens a hot tap, the combi stops heating the floors and puts its whole burner on the domestic water. When the tap closes, it goes back to the floors. In a slab with tonnes of warm concrete in it, nobody notices a 15 minute shower; the floor coasts. In a house heated by low-mass panels or a fan coil, a long bath on a cold night can be felt, and that house may be better off with a boiler and a separate tank.
Modulation means the burner turns itself down to match what the house is asking for instead of running flat out and shutting off. That matters more in a combi than anywhere else, because the burner is sized for hot water, which is a big load, and then spends most of its life heating a house, which in a tight house is a small one. A 2,000 sq ft 1990s frame house in Simcoe might lose 40,000 to 60,000 BTU/h at design; the same house in ICF built to our standard is about half that. A 199,000 BTU/h burner heating a 25,000 BTU/h house is a pickup truck hauling a bag of groceries. What saves it is how far down it can turn, so ask for the minimum firing rate on the spec sheet and hold it against the heat loss.
Rule of thumb: the combi is sized by the hot water you want in January and checked against the heat loss for its lowest fire. Square footage tells you neither.
Where a Combi Fits, and Where It Does Not
A combi fits a radiant house of ordinary size with gas at the road, one or two bathrooms in use at once, and a mechanical room the size of a closet. It is one appliance, one vent, one gas connection and one annual service, which is a lot of tidiness for the money. The flow numbers, and the comparison with a boiler plus a separate water heater, are on the combi vs tankless page.
It fits less well in a big house with three showers going at 7 a.m., where a heating boiler with an indirect tank stores hot water for the rush. It does not fit a house with no gas, and it is the wrong question for a house where the plan is an air-to-water heat pump. When it breaks, you lose heat and hot water on the same morning; that is the honest trade for having one box instead of two.
On rebates: the Home Renovation Savings offers we checked in October 2026 cover heat pumps, insulation, windows and a few other items; we found no gas boiler or combi offer. Check the program site before you sign anything on the strength of one.
Where Homeowners Go Wrong
- Sizing the combi from the old one. The old one was sized by someone guessing. Size it from a room-by-room CSA F280 heat loss for the heating side and from your fixtures for the hot water side.
- Comparing a box price with a room price. Ask every quote to list the combi model number, the manifolds, the pumps, the mixing, the controls, the venting and the gas line as separate lines.
- Forgetting the gas line. A large on-demand burner needs the gas to arrive. $500 to $1,500 found on install day is $500 to $1,500 that should have been on the quote.
- Letting anyone but a TSSA-registered contractor do it. The boiler, the gas piping and the venting are gas work, done by a TSSA-registered fuels contractor with a certified gas technician, usually a G2 for a house. Ask for both numbers. Nobody else opens a sealed combustion appliance, and neither do you.
- Tightening the house and leaving the venting alone. After air sealing or new windows, have the venting and makeup air for every gas appliance checked. Put a CO alarm on every floor, which you should have anyway.
Questions people ask
How much does a combi boiler cost to install in Ontario?
A replacement on an existing radiant system is $5,000 to $10,000 installed, $7,500 as a fair budget, before HST. A new mechanical room built around a combi, with manifolds, pumps, mixing and controls, is $14,000 to $25,000 and up. The tubing in the floor is extra.
Is a combi boiler cheaper than a boiler and a water heater?
Usually on the installed price. A combi at $7,500 against a condensing boiler at $8,000 plus a $1,500 to $2,500 indirect tank or a separate tankless is a few thousand dollars less, and one vent instead of two. The trade is that one failure takes out both heat and hot water.
How much hot water does a combi make in winter?
A 199,000 BTU/h combi at 95 percent puts 189,050 BTU/h into the water. With water coming in at 5 C and going out at 50 C, a 45 C (81 F) rise, that is about 4.7 US gallons a minute, 17.7 litres. A smaller unit makes proportionally less.
Does a combi boiler need a permit in Ontario?
The gas work is TSSA-regulated and done by a registered contractor whatever the municipality says. Some municipalities also want a building permit, $90 to $350, especially for new venting or a new system; a new house has the heating design in the permit package.
Can a combi heat a large house?
On heat, often yes, because tight houses need less than people think. On hot water, a big house with several showers at once may outrun it. The heat loss tells you the first answer and your morning routine tells you the second.
See the combi against the other four systems
The HVAC system cost comparison calculator prices the same house five ways, including gas combi with radiant and ductless cooling, and gives the installed cost and the 15-year cost side by side. If you already have quotes, the $99 review reads up to three of them line by line and says what is missing.
Sources and further reading
- 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.
- 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.
- OntarioHeatLoss.ca: the CSA F280 room-by-room heat loss and the HVAC design and drawings a permit needs, and the number every piece of equipment should be sized from.
