Draft, Spillage & Chimney Systems
Draft hoods, draft measurement, spillage testing, chimney liners, condensation in chimneys, and common venting rules.
- Explain draft principles and measurement techniques
- Perform spillage testing and interpret results
- Apply chimney liner and common venting requirements
Lesson 1
Draft Principles & Measurement
What Is Draft
Draft is the negative pressure in the vent system that draws combustion products up and out of the building. It is created by the buoyancy of hot flue gases - hot gas is lighter than cool air and rises naturally through the vent.
Factors Affecting Draft
- Chimney height - taller chimneys produce more draft
- Flue gas temperature - hotter gases produce more draft
- Outside temperature - cold outside air increases draft (greater temperature difference)
- Wind - can help or hinder draft depending on direction
- Chimney condition - blockages, leaks, and deterioration reduce draft
Draft Measurement
Measure draft using a manometer or digital draft gauge at the draft hood or vent connector. A negative reading (below atmospheric) indicates proper upward flow. A positive or zero reading indicates inadequate draft or downdraft.
Draft Hood Function
The draft hood (also called a draft diverter) serves two purposes: it prevents downdraft from extinguishing the burner, and it admits dilution air to moderate the draft. Never block, modify, or remove a draft hood.
Draft is the negative pressure that draws flue gases upward. Minimum draft is typically -0.02" WC. Measure with a manometer at the draft hood. Draft depends on chimney height, flue gas temperature, and outdoor conditions.