Draft measurement quantifies the pressure difference between a furnace or boiler combustion chamber and the surrounding environment, expressed in inches of water column (in. w.c.). Technicians use a manometer connected to the flue vent to verify adequate draft that safely removes combustion gases. Proper draft ensures complete combustion and prevents dangerous back-drafting into living spaces.
Draft Requirements and Testing
Natural draft furnaces typically require 0.01 to 0.05 in. w.c. of negative pressure at the draft hood. Induced draft furnaces need 0.05 to 0.15 in. w.c. at the inducer inlet. Insufficient draft below 0.01 in. w.c. causes incomplete combustion and CO production. Excessive draft exceeding 0.15 in. w.c. increases heat loss through the chimney. Testing involves reading the manometer while the appliance operates at steady state for 5 minutes.
Operational Importance
Draft measurement verifies chimney adequacy, identifies blockages or restrictions, and detects combustion spillage into living areas. Back-drafting caused by negative house pressure or blocked vents creates serious carbon monoxide hazards. Draft problems signal the need for chimney cleaning, vent clearing, makeup air installation, or appliance replacement. Regular draft testing during maintenance prevents dangerous conditions and optimizes equipment performance.