How efficient is a condensing boiler in cold weather?
A condensing boiler recovers heat from the water vapour in its exhaust, so it can reach an AFUE of roughly 90 percent or more. It only condenses, and only earns that rating, when the water returning from the radiators or floors is cool enough, which is why system design matters as much as the label.
What makes a boiler condensing?
Burning natural gas produces water vapour along with carbon dioxide. In an ordinary boiler that vapour leaves up the chimney, carrying a lot of heat with it. A condensing boiler adds a second heat exchanger that cools the exhaust until the vapour turns back to liquid water, releasing heat into the system water. The condensate drains away through a plastic pipe, and the cooler exhaust is vented through plastic pipe as well.
The fuel-use rating for boilers is AFUE, the share of fuel energy that becomes useful heat over a typical season. Condensing models are normally in the 90s, while older non-condensing boilers are often rated in the 80s or lower. The label on your model gives the exact figure. See boiler versus furnace in Canada for wider context.
Why does return water temperature matter?
Exhaust only condenses if the surface it touches is cooler than its dew point, which is roughly 55 degrees Celsius for natural gas. If the water coming back to the boiler is hotter than that, there is no condensing and the unit behaves like a conventional boiler, with an efficiency a good deal lower than its label.
Old cast-iron radiators designed for hot water may need supply temperatures well above that, so the boiler rarely condenses on the coldest days. Radiant floors and large low-temperature emitters work with cooler water and let the boiler condense most of the time. Our guide to low-temperature hydronic heating explains the design side, and the radiant floor heating guide shows a system that suits condensing equipment.
How do controls change real-world efficiency?
Four features help a condensing boiler reach its potential.
- Outdoor reset. A sensor lowers the supply temperature in mild weather, which lowers the return temperature too.
- Modulating burner. The flame adjusts to demand instead of cycling on and off, so the boiler runs longer at a lower output.
- Proper sizing. An oversized boiler short cycles. A load calculation matches the unit to the house. Heat load calculations work the same way for any system.
- Balanced flow. Well-balanced piping avoids returning hot water because a pump runs too fast.
A tune-up that checks combustion and cleans the condensate trap keeps the unit performing. Furnace tune-ups follow a similar logic.
Is a condensing boiler worth the extra cost?
For many gas-heated homes with hydronic distribution, yes, because lower fuel use repays the difference over time. The size of that saving depends on the boiler it replaces, the temperatures the system runs at and the price of gas, which differs by province. Installation also costs more, since it needs a condensate drain and plastic venting. Get written quotes that include any piping or emitter changes.
If you are weighing a bigger change, a heat pump can supply hydronic heat too. The comparison in high-efficiency furnace versus heat pump replacement sets out the trade-offs. Drake Landing Solar Community used the same low-temperature logic, with a district loop typically at 40 to 50 degrees Celsius, so see the district heating page for how cool water enables efficient supply.
Quick answers
What AFUE does a condensing boiler have?
Condensing boilers are typically rated in the 90s, while non-condensing boilers are often lower. The rating on the unit's label is the number to use.
Why is my condensing boiler not condensing?
Usually the return water is too hot, because of high supply temperatures, undersized emitters or the lack of outdoor reset. A heating contractor can check settings and flow.
Do condensing boilers need special venting?
Yes. Their exhaust is cooler and acidic, so they use plastic or other approved venting and a drain for condensate.