Energy Manager

Hot water heating renewal series: The importance of boiler venting

 Flue details are regulated by the Technical Standard and Safety Authority (TSSA).  Improper flue design can be rejected by a TSSA inspector, leaving an expensive fix.  At the same time, the rules are confusing and seemingly ever-changing so this is an area that requires attention from a knowledgeable and trained professional.

July 6, 2009  By Mike Zongor

In the last article, entitled Building Code and Safety Issues, we left off with a brief discussion on flues, chimney liners, and boiler operating conditions. The American National Standards Institute (ANSI) has defined 4 categories of boiler depending on operating characteristics with respect to pressure and flue gas condensation in the vent system.  The categories are as follows:

Category I – Non-positive vent static pressure and high temperature that avoids excessive condensate in the vent.
Category II – Non-positive vent static pressure and a vent temperature that may cause excessive condensation in the vent.
Category III – Positive vent static pressure and high temperature that avoids excessive condensate in the vent.
Category IV – Positive vent static pressure and a vent temperature that may cause excessive condensation in the vent.

Categories 2, 3, and 4 by code all require “BH” venting, which is a high alloy stainless steel that is watertight and can withstand the corrosion caused by the very acidic flue gas condensate.  Category 1 boilers are approved for use with “B” venting; however, when they operate with high and low firing rates the venting manufacturers acknowledge this as a category 2 appliance and will require “BH” venting to be installed.  This means that for high efficiency boilers of 85% or greater, with only some rare exceptions of single stage boilers, “BH” venting is required.  

While positive pressure venting will be smaller since there is a fan assisting the exhaust, boilers of this type are required to be independently vented.  This is in order to avoid flue gas from back-feeding an idle boiler, except in a few rare cases where boiler manufacturers have achieved approval with integral back draft dampers.  Alternatively, non-positive vent static pressure appliances can be co-vented, where multiple boilers may be vented through a single combined vent or chimney.  


There are many additional venting requirements and venting termination requirements as laid out by the Natural Gas and Propane Installation Code that increase the complexity of boiler venting.  These include the vent termination distance from windows or doors, above sidewalks and driveways, above the height of a roof, to mechanical air inlets, above gas meters or regulator assemblies, above expected snow accumulation levels, and at a distance away from the building.

In retrofit projects these venting requirements can present a significant technical challenge and become an economic constraint that may limit your boiler selection options.  In a penthouse boiler room the technical challenges are much less pronounced.  Regardless of venting pressure, the flues are short, there is no need for co-venting, sidewall venting can be considered, and the cost premium of venting materials is not great as compared to the overall cost of the project.

The limitations can become a concern where basement boiler rooms exist.  As positive pressure appliances must be vented individually, it might not be feasible to find a path for numerous vents to the roof prompting the designer to look for a sidewall venting location if available or at alternative solutions.  On the other hand with non-positive pressure boilers if they are to be combined into a single chimney it may not be feasible to run a liner within the existing location. A couple of factors affect the chimney size in this case.  As buoyancy lifts the flue gas up the chimney air enters the flue through a barometric damper, which adjusts the opening size depending on the pressure difference between the flue gas and the outdoors.  In tall buildings the chimney draft varies significantly with the outside air temperature such that the barometric damper may swing wide open when it is very cold outside. This will create a very strong draft and actually pull more heat out of each boiler that significantly reduces the overall boiler efficiency.

As is illustrated, there are many factors that must be considered with respect to venting when selecting retrofit boilers. Some of the venting constraints can limit your choice of boilers and significantly increase the cost of the project. Building owners and managers should consult an experienced heating plant designer to help them optimize your objectives for code compliance, energy efficiency, and project cost.

Relevant Hot Water Heating Plant Codes
•    National Building Code
•    Ontario Building Code
•    Ontario Fire Code
•    National Fuel Gas Code
•    Canadian Electrical Code
•    Ontario Hydro Electrical Safety Code
•    CSA B.149 Natural Gas and Propane Installation code
•    Canada Plumbing Code
•    CSA B51 –[M1991] Boiler, Pressure Vessel, and Pressure Piping Code
•    CSA-B214 Installation Code for Hydronic Heating Systems
•    SMACNA Guidelines

Mike Zongor, EIT, Senior Project Manager with Efficiency Engineering Inc. in Cambridge, specializes in energy efficiency and control of central plants. He can be reached at

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