Combustion can be used to power a broad range of heating systems, and even absorption-based cooling equipment. Standby and backup electric generators typically depend on combustion as well, since most models are driven by a diesel engine.
However, combustion releases flue gases that must not be allowed to accumulate in indoor spaces, and some of them are very dangerous for humans. Carbon monoxide demands special attention because it is odorless and colorless, but capable of inducing unconsciousness, coma or even death when its concentration is high enough. Given the potential consequences of indoor flue gas leaks, the NYC Department of Buildings deals with the topic in depth in several of its building codes:
Fuel Gas Code, Chapter 5 – Regulates the use of chimneys and other exhaust systems that serve gas-fired appliances.
Mechanical Code, Chapter 8 – Regulates the uses of chimneys and other exhaust systems that serve appliances running on fuels other than gas.
Building Code, Chapter 21 – Includes specific sections for masonry fireplaces (BC 2111), heaters (BC 2112) and chimneys (BC 2113). Of course, these building elements are also subject to the general requirements that apply for all masonry.
In addition, compliance with the NYC Air Pollution Control Code is required in all cases.
Some applications also require compliance with NFPA 211 (Standard for Chimneys, Fireplaces, Vents and Solid Fuel-Burning Appliances) and NFPA 37 (Standard for the Installation and Use of Stationary Combustion Engines and Gas Turbines).
NYC Fuel Gas Code Compliance: Gas-Fired Appliances
Chapter 5 of the Fuel Gas Code applies for factory-built chimneys, field-built chimneys, liners, vents and connectors, as well as masonry chimneys used for gas-fired appliances. This section will provide an overview of the general requirements. Code compliance starts with selecting the right venting system for the application, and Table 503.4 provides a useful selection guide.
They are classified into factory-built, metal and masonry chimneys, and their requirements are set forth in article 503.5. Metal and masonry chimneys must also comply with NFPA 211, and masonry chimneys must meet Chapter 21 of the NYC Building Code as well.
The height of chimneys for gas-burning appliances is determined by operating temperature and building height:
If the appliance operates under 600°F, the chimney must be at least 3 feet taller than all construction features within 10 feet of the outlet.
For appliances with operating temperatures between 600°F and 1000°F, the chimney must be at least 10 feet taller than all construction features within 20 feet.
•Finally, for appliances above 1000°F, the chimney must be at least 20 feet taller than all building features within 50 f
Keep in mind that these requirements do not apply for other chimneys nearby, but they do apply for construction features in adjacent buildings, even when the owner is different. If a chimney no longer meets height requirements due to a new construction or a major renovation in an adjacent building, the responsibility falls on whoever carried out the project, not the affected building owner.
Article 503.6 is dedicated to gas vents, and they are classified into types B, B-W and L. All vents must be UL listed and labeled, regardless of type, unless the code makes an exception explicitly. Gas vents must meet a series of spacing requirements with respect to the roof and heating equipment, as well as a minimum clearance with respect to forced air inlets. Keep in mind that different requirements may apply for B, B-W and L gas vents.
Gas vents can be shared by multiple piece of equipment if they are in the same building level; sharing vents across different floors is not allowed.
Single-Wall Metal Pipes
Their requirements are set forth in article 503.7, and the main factors to consider are the following:
They must be built from galvanized sheet steel, with a thickness of at least 0.7mm, or any approved material that is noncombustible and corrosion-resistant.
They are only allowed if they run directly from the appliance to the outdoor environment, either through the roof or a single wall, hence their name.
When multiple appliances connect to the same chimney or vent, openings must never be located at the same height and opposite to each other, unless the appliances are connected to the chimney at a 45° angle. If an opening in a chimney or vent is no longer used, it must be closed through an approved method.
Obstructions in venting systems must be prevented at all costs, and the use of manual dampers is not allowed. Only automatic dampers are allowed, and they must have control features to ensure they are always open when the respective appliance is running.
NYC Mechanical Code Compliance: Non gas-Fired Appliances
Vents are covered in Section MC 802. All vents must meet NFPA 211, and Type L vents must be tested per UL 641.
Direct-vent, integral vent and mechanical draft systems are covered in Section MC 804. With these systems, the code specifies clearances that must be met with respect to walkways, windows, air inlets, metering equipment and adjacent properties.
Factory-built chimneys are covered in Section MC 805. These can only be used if they are listed and labeled per UL standards.
Metal chimneys are covered in Section MC 806. The must meet the NFPA 211 standard, and those for exterior uses must be made from galvanized or stainless steel, with the exterior surface painted with a heat-resistant paint.
Switching from Gas to Fuel Oil
When a heating system switches from gas to fuel oil, special considerations must be taken because the Mechanical Code applies now instead of the Fuel Gas Code. The change is only allowed if the following conditions are met:
The chimney must have suitable characteristics for fuel oil exhaust gases, and requires a test run and a smoke test.
The chimney must be sized so that adequate draft is provided.
The chimney must be cleaned extensively to eliminate any flue accumulations.
This situation is common in New York City – there are many cases where fuel oil is more affordable than gas.
Internal Combustion Engines and Turbines
These appliances must comply with NFPA standards 211 and 37, and must use an exhaust pipe to get rid of flue gases. In the case of backup and emergency generators, the exhaust pipe can be connected to a chimney used by other equipment, but only if it will not affect other exhaust systems.
Combustion gases can be lethal for humans, which means adequate flue design is not only about ensuring high performance, but also about protecting human life and health. Keep in mind that the applicable code changes depending on the fuel type: the NYC Fuel Gas Code is for gas-fired appliances, the Mechanical Code applies for other fuels, and the Building Code applies whenever masonry chimneys are used, regardless of the fuel.