Boiler Replacement in Ontario: Condensing Boilers, Radiant Heat and Heat Pumps

Boiler replacement in Ontario usually means choosing a high-efficiency condensing boiler, often a modulating "mod-con" unit, sized from a proper heat loss calculation and paired with outdoor reset controls. Done well, that combination can cut gas use noticeably compared with an old cast-iron boiler; done poorly, a new condensing boiler may never actually condense and will perform little better than the old one.

Boilers heat water rather than air, and deliver heat through radiators, baseboard convectors or in-floor tubing. They are common in older Ontario homes, especially in central Toronto, Ottawa, Kingston and Hamilton, and in many newer homes with radiant floors.

How does a hydronic heating system work?

A boiler heats water, a circulator pump moves it through pipes to emitters in each room, and the cooled water returns to the boiler. The main types of emitter are:

• Cast-iron radiators, typical of homes built before about 1950. They hold a lot of heat and give steady, comfortable warmth.
• Baseboard convectors (fin-tube), common from the 1960s onward. They need hotter water to deliver the same heat.
• In-floor radiant tubing in concrete slabs, basements or under wood floors. It uses low water temperatures and is very comfortable.
• Panel radiators, slim modern steel units that can work at lower temperatures.

Some older homes have steam systems rather than hot water. Steam is a different technology with its own maintenance needs; replacing a steam boiler is specialist work.

What makes a boiler efficient?

New gas boilers are rated by AFUE (annual fuel utilization efficiency). Older non-condensing boilers are typically 80–85% or lower in practice; condensing boilers are rated around 90–95% or more.

Condensing boilers extract extra heat by cooling exhaust gases until water vapour condenses. With natural gas, that only happens when the water returning to the boiler is below roughly 55 °C. If the system runs at 70–80 °C all winter, as many older systems are set to do, the boiler rarely condenses.

Emitter type	Typical water temperature needed on a cold day	Condensing boiler fit	Air-to-water heat pump fit
In-floor radiant (slab)	~30–45 °C	Excellent	Excellent
Oversized cast-iron radiators	~50–65 °C (often lower than originally designed)	Good with outdoor reset	Possible, check heat loss room by room
Modern panel radiators	~45–60 °C	Good	Possible with larger panels
Fin-tube baseboard	~65–80 °C	Fair; condenses mainly in mild weather	Poor without adding emitters

Outdoor reset is the key control. It lowers the boiler water temperature automatically as outdoor temperatures rise, so the system runs cooler most of the season. Many homes were sized for −20 °C or colder but spend most of the winter much warmer, so the boiler can condense for much of the year.

Modulation lets the boiler adjust its firing rate, for example from 20% to 100% of capacity, instead of turning on and off. That reduces short-cycling, especially in homes with multiple small zones.

How much does boiler replacement cost in Ontario?

Costs vary widely with the size of the home, the condition of the piping, chimney or venting work, and whether domestic hot water is included. These are rough 2026 estimates, not quotes:

Option	Typical installed cost (estimate)	Notes
Condensing mod-con boiler, heating only	~$8,000–$15,000	Includes venting, controls and basic piping changes
Combi boiler (heating plus on-demand hot water)	~$8,000–$14,000	Replaces a separate water heater; check hot water flow needs
Boiler plus indirect hot water tank	~$10,000–$18,000	Good for larger households with high hot water use
Non-condensing replacement	Lower upfront	Usually only when venting or system constraints rule out condensing

Ask quotes to include a written heat loss calculation, the proposed boiler's modulation range, outdoor reset setup, and any chimney liner or side-wall venting work. Condensing boilers vent through plastic pipe and need a condensate drain. Our HVAC sizing guide explains why sizing matters.

Tip: Many older Ontario boilers are two or three times larger than the home needs, especially after windows, insulation or air sealing upgrades. Oversized boilers short-cycle and waste fuel. Do not let a contractor size the new boiler by copying the old one's nameplate.

Should you combine a boiler with hot water?

A combi boiler heats household water on demand and can free up space. It works well in smaller homes or where one shower runs at a time. Larger households often do better with an indirect tank, which stores hot water heated by the boiler and can handle several fixtures at once. If you currently rent a water heater, this is a natural time to review that contract. Compare options in our water heater guide.

Can a heat pump work with radiators or radiant floors?

There are two main paths.

Air-to-water heat pumps heat water for the hydronic system. They work best with low-temperature emitters such as in-floor radiant heat, and their efficiency falls as the required water temperature rises. Availability, installer experience and rebate eligibility for these systems in Ontario are more limited than for air-to-air heat pumps, so ask contractors about their experience and check program rules on saveonenergy.ca.

Ductless mini-splits alongside the boiler are more common. They provide air conditioning, which many boiler homes lack, and efficient heating in fall and spring, while the boiler handles the coldest weather. See our ductless mini-split guide.

Homes heated by oil boilers have the strongest financial case for a heat pump. Income-qualified households switching from oil may be eligible for the Oil to Heat Pump Affordability program delivered by Save on Energy; confirm current intake status before planning around it.

Boiler maintenance checklist

1. Annual service by a licensed gas technician (G2 or higher, regulated under TSSA), including combustion testing and inspection of the venting.
2. Check system pressure. A typical residential system runs roughly 12–15 psi cold; follow the boiler manufacturer's range.
3. Bleed air from radiators at the start of the season if they are cold at the top.
4. Check the expansion tank and relief valve during service.
5. Clear condensate drains on condensing boilers; a blocked drain shuts the boiler down.
6. Keep side-wall vents clear of snow, which can cause a lockout.
7. Test the carbon monoxide alarms monthly.

Frequently asked questions

Is a boiler more efficient than a furnace?

Not inherently. Both are available at around 95% efficiency. Hydronic systems avoid duct losses and are often more comfortable, but they cost more to install and do not provide cooling on their own.

Should I replace my old cast-iron boiler if it still works?

If it is reliable and serviced, there is no urgent need. If it is leaking, very oversized or more than 30 years old, plan ahead so you are not choosing a replacement during a January breakdown.

Can I add air conditioning to a home with radiators?

Yes. Ductless mini-split heat pumps are the most common solution, and they also heat efficiently in milder weather. High-velocity small-duct systems are another option where wall units are not wanted.

Do I need a new chimney liner?

If you replace a non-condensing boiler but a gas water heater still uses the chimney, the chimney may need a liner sized for the water heater alone. Condensing boilers vent through plastic pipe instead. Your contractor should assess this.