Cleaver-Brooks - Complete Boiler Room Solutions

Criteria to Consider for Boiler Venting System Selection

Tip Sheet: February 2016

Key Facts

  • Improper venting can lead to fuel inefficiency, downtime and increased maintenance costs
  • Venting categories are classified by vent operating pressure and condensing vs. non-condensing
  • Air, mineral fiber and ceramic are the main stack insulating materials used today

A properly engineered and installed venting system is required for safe, reliable boiler operation. Improper venting can lead to fuel inefficiency, downtime and increased maintenance costs. These drawbacks can be avoided by attaining and maintaining adequate and consistent draft.

There are several draft classifications for packaged boilers, including atmospheric or natural draft, similar to a fireplace; forced draft using a power burner; balanced draft using both a power burner and an induced draft fan; and the induced draft fan, employed when the resistance in the venting system exceeds the draft potential thereby requiring a fan to either suck and/or push the products of combustion into the atmosphere.  

In addition, there are four venting categories classified by vent operating pressure and whether the boilers are condensing or non-condensing. Category I is negative pressure, non-condensing. Category II is negative pressure, condensing. Category III is positive pressure, non-condensing, and Category IV is positive pressure, condensing. Category classification determines which UL and NFPA requirements for venting must be followed. 

Venting systems can be complex both in their design and layout. Here is a list of factors that should be evaluated when designing and selecting the correct boiler venting system to ensure proper and safe operation.

Mass flow. The total amount of flue gas that forms in the exhaust or venting system. As this mass increases, the amount of friction also increases, which reduces draft.  

Exhaust temperature. As the mass temperature increases, its density lessens, causing the flue gas to rise, increasing draft.

Horizontal run. Similar to water flowing through a pipe or current running through a wire, resistance builds with increased linear length. This condition causes draft loss.  

Vertical height. Increases draft due to larger quantities of hot combustion gas versus ambient air, which is cooler and denser. One column replaces the other due to the temperature differential between them.  

Fittings. Transition pieces or elbows essentially add linear length to the breeching, and therefore increase the resistance and reduce the draft.  

Diameter. If it is too small for the mass running through it, resistance increases, reducing the draft.  

Heat loss. Insulating materials in the stack have an effect. The higher the temperature, the greater the density difference and the greater the draft. If the temperature difference lessens, the draft decreases.

Site elevation. Affects the density of the air, impacting the diameter of materials used and the cost variable for the exhaust system. The higher the elevation above sea level, the greater the diameter required because of the volumetric increase.  

Site temperature. Cold average temperature results in more draft, and hot average temperature conditions reduce the draft in the system due to the narrowing of the temperature differential.

Insulating materials commonly used in prefabricated stack offerings adhere to code compliance and greatly reduce the heat loss from the stack. They can significantly affect the draft in the stack, which if not used can cause not only burner issues, but condensate issues as well. This can lead to corrosion of the stack materials and in some cases the boiler too.

The main insulating materials used today include: air, mineral fiber and ceramic. These substances are placed between the stack or breeching’s double walls to produce the insulating effect. Operating conditions, type of fuel and the respective dew points of the resulting flue gas determine which non-corrosive stack/breeching materials are recommended for a particular application.

For more information about boiler venting system selection, including summaries of two real-life chimney projects, watch the Forced Draft Boiler Flue Gas Venting: Solutions to Typical Problems webinar or contact your local Cleaver-Brooks representative