Architecture is defined as the art or science of building. This term commonly references the individual designing the envelope and specifying the overall appearance, but architecture requires much more than that.

It brings together a team from multiple fields to document a design for a singular goal. That goal will enable the client to bring his or her vision to life. Each team member takes responsibility for their field to meet the client’s vision while balancing energy efficiency, aesthetics and other ideas in harmony with all disciplines. 

Two pieces of equipment that typically require additional architectural consideration for integration are natural gas boilers and water heaters. The coordination of the venting and exiting requirements for natural gas equipment, as well as building openings and intakes, is one of the critical conversations needed in the design phase between the architect and the engineer.

Boilers and water heaters can use a variety of fuel types, including electric, natural gas and other fuels such as propane (not under consideration in this article). While the primary difference is the fuel source, there are several pros and cons for each type. 

While electric equipment can take advantage of renewable energy sources, it can also lead to increases in the electrical service size, which can result in increased sizes of generators, transformers, uninterrupted power supply systems and quantities of panels for a building. Additionally, this makes it difficult for equipment to work in a building on an existing power service. While natural gas is a less expensive fuel source than electricity, it is often seen as more stable since it is not impacted by power cuts. 

Also, electric water heaters and boilers have limited capacities as compared to natural gas and require more time to get to temperature due to their increased mass. Often natural gas is specified for hospitals, universities and laboratories due to increased demand and versatility of a natural gas fuel. Natural gas equipment, however, has a larger footprint and requires integration on the design side to meet building codes. 

For flue vents, the sizing and routing are driven by the International Fuel Gas Code Chapter 5, National Fire Protection Association (NFPA) Code 54, also known as ANSI Z223.1, and by recommendations of the equipment manufacturer. The International Mechanical Code (IMC) also provides guidance relative to the design and placement of flue venting relative to other building components.

Venting Classifications

NPFA 54 breaks down the venting classification into four categories. The main differentiating factor for the categories is the potential for condensation at the dew point. The dew point is the temperature below which moisture in the air may condense and, with it, absorb combustion gases. 

This condensed liquid is often acidic and can damage the venting materials. Often manufacturers label their equipment as condensing or noncondensing to simplify information for specifying engineers. The other dividing factor for a category is whether the vent operates under negative or positive pressure.

• Category I: The most common category whereby the flue vent system operates under negative pressure and the flue gas is hot enough to stay above the dew point. Under Category I, Type B venting materials are used and typically consist of a double-wall construction comprised of an aluminum inner shell and a galvanized-steel outer shell. 

Note this category also represents a reduction in energy efficiency due to the release of hot flue gases (limited amount of heat recovery within the appliance).

• Category II: The flue vent system operates under negative pressure like Category I but has flue temperatures that may drop below the dew point, resulting in condensation of combustion gases within the flue vent. Venting materials are as specified or furnished by the manufacturers. 

Category II flue vent systems are not common in new construction but can still show up on retrofit projects with existing appliances. Condensate neutralization kits are recommended to protect plumbing systems and drains.

• Category III: The flue vent system includes a power exhauster, operates with positive pressure, and has flue temperatures that remain above the dew point. Venting materials are as specified or furnished by the manufacturers and address potential leakage with positive pressure. A best practice does not route venting through occupied areas or return plenums as there is a risk of flue gases spilling during a leak. 

• Category IV: The flue vent system includes a power exhauster and operates with a positive pressure system but has flue temperatures that may drop below the dew point, resulting in condensation of combustion gases within the flue vent. Venting materials are as specified or furnished by the manufacturers and address potential leakage with positive pressure. 

Under this category, vent systems commonly use corrosion-resistant steel, polyvinyl chloride or polypropylene due to the acidic condensation. Condensate neutralization kits are recommended to protect plumbing systems and drains.

Combustion Air Intake

While flue vents lead directly to the outdoors and have minimum code-driven distances from building intakes, openings or walkways, the combustion intake can be either directly ducted or ventilated from the room, depending on which category the system falls under. 

Combustion air intake to the equipment room, if placed on an exterior wall, will need a minimum of two permanent outside air openings sized according to the total heating load of the equipment: one within a foot of the ceiling and another within a foot of the floor. Note that these louvered openings can get very large and often become a point of discussion with the architect relative to placement and appearance. 

Combustion air for natural gas or other fuel-fired appliances can be provided at the room level based on the total capacity of equipment, as defined by the IMC and NFPA 54. This can be accommodated by a set of high and low louvers in the wall or a ducted system in which one duct is extended to within one foot of the floor. 

Other venting options are available for appliances not categorized directly under NFPA 54. These systems are developed by manufacturers and are commonly known as two-pipe systems. Under two-pipe systems, there is a direct-ducted connection to the equipment for both combustion intake and the flue venting. These systems are performance-tested by the manufacturers. 

If available, this approach is more flexible in that it removes the need for louvered combustion air intakes but also must address issues of routing that must be integrated into the design. 

If the equipment room was not planned for natural gas-fueled equipment and does not have direct access to the roof, vents must exit the building on an exterior wall or traverse to a shaft, where they can then be vented to the exterior. Wall exits outside of loading areas are almost always rejected as they take away from the building design. 

Ideally, the vent requirements are discussed early in the design process to situate the equipment room with an exterior wall that is not forward-facing or elsewhere in the building, either below the roof or adjacent to the shaft.

Mechanical Room Issues

Another conversation that ideally occurs early in design regards mechanical equipment spaces and the potential need for a second egress exit. Per the International Building Code (IBC), any mechanical room housing more than 400,000 BTU/hour of fuel-fired equipment that is
500 square feet or more must be provided with an emergency shut-off button and two means of exiting. 

The IBC requires the two exits to be separated by at least half the diagonal distance of the opposite corners of the room. The two means of exiting can be via two sets of doors, or the code allows one means of egress to be a fixed ladder or alternating tread device. The second exit could have exit-only hardware as well to improve security. 

With the push for decarbonization across the globe, the demand for natural gas-fired equipment is decreasing. Even with the decreased demand, natural gas-fired equipment still has its place for certain applications. 

It remains critical to understand the nuances inherent in natural gas equipment and be able to communicate these requirements to the architect and design team so the vision of the client can be fully realized while still meeting necessary code and building system requirements. 

Dylan Hendrix is a mechanical designer at SmithGroup’s Phoenix office. He is a member of the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and a 1st Lt. in the U.S. Army Reserves. With 5-plus years of experience in the construction industry, Hendrix focuses on designing mechanical systems for various building types. 

Megan Kittredge is a mechanical designer at SmithGroup’s Phoenix office. She is a member of ASHRAE and the Society of Women Engineers with 4-plus years of experience in designing mechanical systems for various building types.