Cleaver-Brooks - Complete Boiler Room Solutions

Properly Designing and Installing a Boiler Exhaust System

Tip Sheet: December 2012

Key Facts

  • A poorly designed stack system can result in boiler inefficiency, burner shutdowns and flame ignition failure upon start-up
  • Incorrect sizing causes poor or excess draft
  • A sophisticated building ventilation system brings in less air than it vents out causing outside air to enter through the stack

A properly configured stack system can avoid boiler inefficiency, burner shutdowns and flame ignition failure upon start-up.


Properly designed stack systems can alleviate many costly and aggravating problems associated with boilers. Boiler inefficiency, burner shutdowns, and flame ignition failure upon start-up can result from a poorly designed stack system. Following are the factors that lead to most problems:   

Poor or Excess Draft: The combustion flue gases inside the chimney or stack can be much hotter than the ambient outside air, and therefore, less dense than the ambient air. This variance causes the bottom of the vertical column of hot flue gas to have a lower pressure than the pressure at the bottom of a corresponding column of outside air. The higher pressure outside the chimney is the driving force that moves the required combustion air into the combustion zone and also moves the flue gas up and out of the chimney. That movement or flow of combustion air and flue gas is called “natural draft.” The taller the stack, the more draft is created. There can be cases of diminishing returns; if a stack is disproportionately tall relative to the heat exiting the stack, the flue gases may cool down before reaching the top of the chimney. This condition can result in poor drafting. A number of variables must be evaluated in order for a chimney or stack to be designed with the right amount of natural draft.    

Breeching and Stack Sizing: Incorrect sizing leads to poor or excess draft. A good rule of thumb dictates that the vertical stack be at least as tall as the length of the breeching. If you combine multiple boilers, the air flow through the common breeching system should be the sum of the flow of all the boilers at the minimum. The design of the stack and breeching must provide the required draft at each boiler’s flue gas outlet. Proper draft is critical to burner performance. Although constant pressure at the flue gas outlet is not always required, it is necessary to size the stack/breeching to limit flue gas pressure variation. Check the manufacturer’s recommendations for the allowable pressure range.  

Poorly Designed Ventilation System: Buildings today are constructed to save energy and money in the long run. As a result, the ventilation system often is very sophisticated and brings in less air than it vents out. This causes negative pressure in the building. To obtain a balance, the outside air comes in the path of least resistance, which many times is the stack, especially if it is not a sealed circuit.  

Proper design and installation of an exhaust system helps to ensure that a boilersystem operates at peak efficiency. Cleaver-Brooks manufactures installation-ready exhaust systems and engineered freestanding stacks. For installation-ready systems, Cleaver-Brooks uses an exclusive male/female jointing system, which accelerates installation time by up to 40%.  


Installation of an exhaust system with the male/female jointing system involves just a few simple steps. There are arrows and labels indicating where to connect the ends, and two sealant applications within the system ensure an airtight fit. To learn more about boiler exhasut systems, watch the Properly Designing and Installing a Boiler Exhaust System webinar or contact your local Cleaver-Brooks representative.