What makes a boiler condensing
Every gas boiler produces water vapour when it burns gas. A conventional boiler sends that vapour up the flue at high temperature. A condensing boiler passes flue gases through a larger heat exchanger so the vapour cools, condenses and gives up its latent heat to the system water. That recovered heat is where the efficiency gain comes from.
The catch is that condensing only happens when the return water is cool enough, roughly below 55 °C. A condensing boiler bolted onto a radiator system that runs at 80 °C all winter will still work, but it will spend much of the year not condensing. Emitter sizing, weather-compensation controls and lower design temperatures are what turn a 95% AFUE rating into real savings.
Reading the efficiency number
AFUE, annual fuel utilization efficiency, is the standard rating for residential boilers in Canada and the United States. It estimates seasonal efficiency under test conditions. Two boilers rated 95% AFUE can perform differently in a real home depending on how they are set up, and a brochure headline can quote the best model in a range while smaller or larger siblings are rated lower.
Boilers.ca lists the model-level figure on each model page and notes when a manufacturer's documents disagree. For the IBC SL Series G3, for example, the Canadian brochure says up to 96% while the 115, 160 and 199 models are rated 95%.
Sizing: input, output and why square footage is not enough
Boilers are specified by input (gas energy consumed, in MBH or BTU/h) and output (heat delivered to the water). A 115 MBH input boiler at 95% efficiency delivers about 109 MBH output. Marketing often quotes input; your home needs output.
The right output comes from a heat-loss calculation that considers walls, windows, insulation, air leakage, orientation and design temperature, plus any indirect tank recovery load. Rules of thumb based on floor area routinely oversize boilers, and an oversized boiler short-cycles, wears faster and condenses less. Modulating boilers with a wide turndown forgive some oversizing, but they do not fix it.
| Model | Input range | CSA output (max) | AFUE | Weight |
|---|---|---|---|---|
| SL 14-115 G3 | 14 – 115 MBH | 109 MBH | 95% | 85 lb |
| SL 30-199 G3 | 32 – 199 MBH | 181.3 MBH | 95% | 152 lb |
Heating-only with an indirect tank, or a combi boiler
A heating-only boiler paired with an indirect water tank stores hot water and recovers it using the boiler. It suits homes with several bathrooms, simultaneous draws or a large soaker tub, and it separates the two jobs so a hot-water problem does not take out the heating.
A combi boiler heats domestic water on demand through a plate heat exchanger. It saves the space and cost of a tank and works well in townhouses and smaller homes with modest simultaneous demand. Its limit is flow rate: two showers plus a dishwasher can exceed what a combi delivers in winter when incoming water is cold.
- Choose heating-only plus indirect tank for larger households and high simultaneous demand
- Choose a combi for compact homes, secondary suites and where tank space is scarce
- Either way, size the hot-water side separately from the space-heating side
Venting, condensate and gas supply
Condensing boilers vent cool, wet flue gases and cannot use an unlined masonry chimney. They use PVC, CPVC or polypropylene pipe run to an exterior terminal that meets clearance rules. Maximum equivalent vent length varies by model and pipe size, which can decide where the boiler is mounted.
Condensate is mildly acidic and must be drained to a suitable location, often through a neutralizer. A boiler with a higher input than the old one may need a larger gas line. These three items, venting, condensate and gas supply, are the most common reasons two replacement quotes for the same boiler differ.
Permits, warranty and who installs it
In BC, gas appliance installation and replacement requires a gas permit and inspection under Technical Safety BC, and municipalities such as Burnaby add a heating-system permit. Manufacturer warranties commonly require installation by a certified contractor through an authorized distributor and annual servicing; IBC's SL series warranty is one published example.
Buying through Boilers.ca is a consultation and supply-and-install process with ROMA Heating & Cooling in Greater Vancouver. The instant estimator gives a ballpark CAD range; a site assessment produces the actual quote.