What is combustion spillage?
Combustion spillage is the escape of unvented exhaust gases from a gas appliance into living spaces instead of through the flue or vent system.
When a gas furnace, water heater, or fireplace burns fuel, it produces exhaust gases that must exit your home safely through a vent pipe or chimney. Combustion spillage happens when these gases leak back into your home instead. This typically occurs because the vent system is blocked, damaged, or undersized, or because pressure inside the home pushes exhaust gases downward and inward rather than out.
The main hazard is carbon monoxide (CO), an odorless, colorless gas produced during combustion. Even small amounts entering living spaces can cause headaches, dizziness, and nausea. Higher concentrations lead to serious injury or death. Spillage can also release other combustion byproducts like nitrogen dioxide and water vapor.
Ontario plumbers and gas fitters test for combustion spillage as part of routine safety inspections and when servicing appliances. A gas-fitting professional uses draft testing equipment to measure air flow in the vent system and observe whether exhaust gases spill into the home during appliance operation. Common causes include loose vent connections, cracked heat exchangers, blocked chimney flues, or undersized duct work.
Finding spillage during an inspection means the appliance must be repaired or replaced before use. This is not a cosmetic or performance issue: it is a life-safety problem that requires immediate attention.