All Topics
Draft & Ventilation
How combustion gases flow through the boiler. Natural, forced, and induced draft systems.
In this topic:
What Is Draft?
Draft is the difference in pressure between the furnace and the outside atmosphere that causes combustion gases to flow through the boiler and up the stack.
- Measured in inches of water column (in. W.C.)
- Negative furnace pressure = normal (gases flow toward stack)
- Positive furnace pressure = dangerous (gases/flame push outward)
- Draft moves combustion gases across heating surfaces and out the stack
Positive furnace pressure can cause flame rollout — an immediate shutdown condition.
Types of Draft
Different methods are used to create and control draft through the boiler.
- Natural draft: created by the temperature difference between hot stack gases and cooler outside air
- Forced draft (FD): fan pushes air into the furnace (positive pressure air supply)
- Induced draft (ID): fan pulls gases out of the boiler (negative pressure in furnace)
- Balanced draft: combination of FD and ID fans — furnace pressure held near neutral
Draft Problems
Draft problems affect combustion efficiency and safety.
- Too much draft: pulls excess air through, reducing efficiency and cooling the furnace
- Too little draft: poor combustion gas flow, potential positive pressure
- Causes of poor draft: blocked stack, failed fan, stuck damper, dirty tubes
- Rising stack temperature: may indicate soot buildup reducing draft and heat transfer