Condensing boilers

Every gas boiler produces water vapour when it burns gas, because hydrogen in the fuel combines with oxygen to form water. A conventional boiler sends that vapour up the vent while it is still hot. A condensing boiler routes the flue gases through a larger secondary heat exchanger first, so the vapour cools down enough to condense back into liquid. The change of state releases latent heat, and that heat is transferred to the system water instead of going out the vent. Recovering it is the entire idea behind the design.

Because the flue gases leave the boiler only slightly above the temperature of the water passing through it, condensing boilers are sealed, fan-assisted appliances. They draw combustion air from outside through a dedicated pipe in the common direct-vent arrangement, rather than pulling room air through a draft hood, and the products of combustion are pushed out through certified plastic venting.

Condensing happens only while the water returning from the heating system is cool enough to chill the flue gas below its dew point, roughly below 55 °C in practice. The reason is simple: a warm radiator or floor needs the boiler to send out warm water, but what matters for condensing is the temperature of the water coming back after it has given up heat. Systems that run at lower supply temperatures, such as radiant floors or generously sized radiators, return water cool enough to condense for much of the heating season. Systems designed to run hot all winter keep the return warm, and a condensing boiler that never sees a cool return never condenses.

Modern condensing gas boilers sold in Canada are typically rated around 95% AFUE, as the buying guide on this site explains, and they modulate, meaning the burner steps down to a small fraction of full input when the weather is mild. That matters in British Columbia, where heating loads are modest and the shoulder seasons are long: a boiler that can fire low and run steadily condenses more and cycles less than one that is too large for the building. AFUE is a laboratory rating under standard test conditions; the real behaviour depends on how well the boiler is matched to the heat loss and set up for the water temperatures the home actually uses.

Condensing boilers dominate modern residential gas heating in Greater Vancouver for good reason. Most replacement projects start with an older atmospheric boiler that vents into a chimney; the new unit is a sealed condensing boiler that vents through a wall, which frees the chimney entirely and removes the draft problems that can come with a tall, cold masonry flue.

They pair well with the hydronic systems common in this region: baseboard and radiator loops that can run at moderate temperatures, in-floor heating that runs cool by design, and indirect hot-water tanks that need boiler heat only in bursts. In coastal BC's mild climate a properly sized condensing boiler spends much of the season idling at low fire, which is exactly the operating window these appliances are engineered for. Whether the home needs a heating-only boiler or a combi is a separate decision from whether to choose condensing; almost every current gas boiler documented on this site is a condensing design.

A condensing boiler changes three things about the mechanical room. First, venting: the cool, wet flue gas cannot go into a masonry chimney or standard metal vent. The installation uses plastic vent pipe certified to ULC S636, the Canadian standard for Type BH gas venting systems that CSA B149.1 requires for plastic vents, as Technical Safety BC's directive on plastic venting confirms. The directive exists because uncertified plastic venting systems have failed in Canadian homes and created carbon-monoxide hazards, so both new installations and appliance replacements must use the certified system.

Second, condensate: the boiler produces a steady trickle of mildly acidic liquid that must drain to a suitable point, often through a neutralizer before it reaches a sewer connection or floor drain. Third, the system itself: a boiler rated around 95% AFUE needs a sealed system at the right pressure, an expansion tank, and controls that keep the return water cool enough to condense. None of this is a reason to avoid condensing boilers; it is simply the difference between buying an appliance and commissioning a heating system.

In BC, replacing or installing a gas boiler requires a gas permit, and the homeowner-permit route Technical Safety BC describes is available only to owners of single-family homes; owners of strata units, duplexes and properties with a home business must use a licensed contractor, who applies for the permit. Municipalities such as Burnaby issue their own gas and heating permits for single-family and duplex dwellings, so the contractor should confirm which authority issues the permit for the property before work begins.

Ask which boiler size the heat-loss calculation produced and how the installer arrived at the number, since the building's emitters and hot-water demand, not the old boiler's rating plate, should drive it. Ask what supply and return temperatures the system is designed for and whether the existing radiators or in-floor loops can run cool enough for the boiler to condense. Ask where the certified plastic vent will terminate, how the condensate will be drained and neutralized, and who pulls the gas permit.

Ask what happens if the boiler is installed before the piping is flushed and the system water treated, since debris and sludge shorten heat-exchanger life in condensing designs. Ask about the controls: outdoor reset, which lowers water temperature as the weather warms, is one of the details that turns a condensing rating into condensing behaviour. If you are weighing repair against replacement, the instant estimator on this site gives a ballpark range, and the consultation service on this site can walk through these questions before any equipment is proposed.