The room must have a combustion air fan properly sized for the requirements of the gas fired appliances in the room. The combustion air fan must have a CFM capacity at least 10% larger than the installed exhaust fan in an equipment room. The equipment room for gas fired appliances must always be at a zero to a slight positive pressure.
Any covering over an opening reduces the free area. A ½” mesh screen has a 90% free area, a ¼” mesh screen has an 80% free area and an insect screen only has a 50% net free area. Screens on a combustion air inlet can cause a major reduction in net free area.
These are not reliable or acceptable combustion air openings per code requirements. Only the designated combustion and ventilation air openings are considered by the codes and the code inspectors when sizing the available combustion and ventilation air openings.
It is only acceptable if there is a combustion air fan operating to draw air into the room that has at least 10% more capacity than the exhaust fan.
Almost any type of sealed duct material can be used to route combustion air to a Lochinvar appliance designed for a Direct Vent or DirectAire vent system. The material should be sealed to ensure that combustion air is taken from the desired location. When a unit is installed in a cold climate it is recommended that the air inlet duct be a Type “B” vent or and insulated stack to prevent the condensing of airborne moisture in the combustion air supply.
Multiple combustion air intakes can be combined as long as the common pipe has an area equal to or larger than the total cross sectional or all of the combined air inlets. It is also important to ensure that the common air inlet is adequately sized to prevent a restriction at the point of termination. Total installed length of the combined air inlet system can not exceed the maximum specified for the product.
A draft hood equipped appliance requires a combustion air fan sized on the total Btu/hr input of all atmospheric gas fired appliances ÷ 2000 = Required CFM of combustion air. A fan assisted combustion appliance requires a combustion air fan sized on the total Btu/hr input of all gas fired fan assisted appliances ÷ 3000 = Required CFM of combustion air. Local codes may have additional air requirements for ventilation of the equipment room. These requirements would have to be added to the CFM required for the combustion process.
The National Fuel Gas Code and local codes make no difference in the combustion air requirements for atmospheric; fan assisted or forced draft combustion appliances. There is only one standard for all types of combustion appliances. In reality, different types of combustion systems will require varying amounts of air to operate the combustion and venting systems but the code requirements do not address this when sizing the required openings to the equipment room.
Typically a metal louver assembly is used for combustion and ventilation air into an equipment room. Most of these purpose built louvers are stamped with the free area. If the free area is not known, assume that metal louvers are 75% free area and wood louvers are 50% free area.
Multiple air inlet pipes are combined into a common single air inlet pipe based on the cross sectional area of the multiple pipes. The area of a single common air pipe must have an area equal to or greater than the total of all the multiple pipes it will supply. It is also important to ensure that the point where the common air inlet connects to the outdoors is also properly sized to provide an opening with free area equal to or greater than the common air inlet pipe.
The basic area is the length times the width of the opening. The free area is the portion of this total area that will allow unrestricted flow of air to the gas fired appliances in an equipment room. Free area is typically a percentage of the basic area, such as 75%. If the length in inches times the width in inches is multiplied by 0.75 you can determine the free area of the louvered opening in this theoretical example. The exact percentage of free area may vary based on the manufacturer of the louvered opening.
Direct venting allows the combustion air to drawn from a specific area instead of from the general atmosphere in the equipment room. A direct vent system can prevent airborne contaminants in an equipment room from being drawn into the combustion process. This reduces the need for louvers into the equipment room and prevents freezing of pipes in cold winter months. An equipment room with a direct vent high efficiency boiler or water heater will usually require supplemental heat from a space heater to warm the mechanical room.
When an equipment room is under a negative pressure it means that more air is being drawn out of the room than is being drawn into the room. This starves the gas combustion process for the air need for complete combustion and the air movement needed to vent flue products to the atmosphere. Negative pressure in the equipment room can result in incomplete combustion, high combustion chamber temperatures, thermal fatigue of the finned copper tubes, nuisance operational problems and possible dangerous spillage of flue products into an occupied living space.
A typical metal combustion air louver will have approximately 75% of its area free to draw in combustion and/or ventilation air.
Per the National Fuel Gas Code, most equipment rooms with gas fired appliances require two openings. The combustion air opening should be installed within 12” of the floor and the ventilation air opening should be installed within 12” of the ceiling. This provides convective air movement in the equipment room.
Combustion and ventilation air openings must be provided to the equipment room for gas fired appliances per the requirements of the National Fuel Gas Code and local codes. An old leaky building is no guarantee of adequate combustion and ventilation air to the equipment room. The first time any renovation or repair is made to the building, any combustion or ventilation coming from infiltration in a old leaky building could be eliminated and result in operational problems.
My inspector tells me I need ventilation air for the workers in this space. The use of a direct vent system provides the a separate air inlet pipe to provide the necessary combustion air to operate the Lochinvar equipment. Local codes may have additional requirements for ventilation of the equipment room. These are typically based on a specific number of air changes per hour. The local code requirement must be satisfied.
The national Fuel Gas Code allows the installation of a single opening into the equipment room for gas fired appliances. The opening must be installed within 12” of the ceiling and prove a free area based on one square inch per each 3000 Btu/hr of gas appliance input in the equipment room. Not all local jurisdictions have adopted this requirement in the national code.
The national Fuel Gas Code allows the installation of a single opening into the equipment room for gas fired appliances. The opening must be installed within 12” of the ceiling and prove a free area based on one square inch per each 3000 Btu/hr of gas appliance input in the equipment room. Not all local jurisdictions have adopted this requirement in the national code.
Lochinvar does not recommend openings from the roof top to supply combustion air to a mechanical room. In almost all cases, the vertical stacks from the roof top tend to have either no air movement or have a negative draft and actually draw air our of the equipment room. The only way we have seen a vertical roof top opening function properly is when it is equipped with a fan sized to draw in the required volume of air to operate the combustion and ventilation process.
The basic area is the length times the width of the opening. The free area is the portion of this total area that will allow unrestricted flow of air to the gas fired appliances in an equipment room. Free area is typically a percentage of the area, such as 75%. If the length in inches times the width in inches is multiplied by 0.75 you can determine the free area of the louvered opening in out theoretical example. The exact percentage of free area may vary based on the manufacturer of the louvered opening.
The operation of a barometric damper in the full open position may add as much as 30% to the volume of the flue products. The air drawn into the open barometric damper must be supplied from the equipment room. This is less air than would be used for dilution air in the draft hood of an atmospheric combustion appliance so the standard sizing for the louvers free area per the National Fuel Gas Code will be sufficient.
The area of the combustion and ventilation air openings must be sized per the requirements of the National Fuel Gas Code and local codes. See the unit’s installation and operating instructions for detailed instructions on how to size the opens based on where the combustion and ventilation air are drawn from. Combustion and ventilation air louvers with direct contact to the out doors must be sized with one square inch of free area for each 4000 Btu/h of gas appliance input. Combustion and ventilation air drawn through horizontal gravity ducts will require each duct and free area of the openings to be sized with one square inch of free area for each 2000 Btu/h of gas appliance input. If combustion and ventilation air is drawn form the interior of the building, the free area of the openings must be sized with one square inch of free area for each 1000 Btu/h of gas appliance input. If the local code will allow the installation of a single opening to the equipment room, the opening with direct contact to the outdoors must be sized with one square inch of free area for each 3000 Btu/h of gas appliance input.
A dirty combustion air filter will reduce the available volume of combustion air supplied to the burners. A dirty filter will effectively derate the burner input until the low air switch shuts down operation. All filters in the combustion air inlet must be placed on a regular maintenance schedule and changed with a new filter every thirty days or more often if the unit is installed in a dirty environment.
A unit installed in a cold winter climate area should have either a Type “B” air inlet pipe or an insulted air inlet pipe. This helps to control the condensing of airborne moisture in the combustion air as it enters the relatively warm equipment room.
A sealed stack must be used to draw in combustion air from the outdoors. The air inlet can typically be Type “B” vent, single wall metal vent pipe, PVC pipe, CPVC pipe of flexible vent pipe. The installation instructions for each specific product details multiple acceptable materials available for installation as an air inlet pipe for each type of boiler or water heater product.
The combustion air inlet pipe is sized based on both the volume of air required to operate the combustion process and the diameter required to minimize the frictional losses in the pipe as defined by the specified maximum acceptable installed length. In conventional venting and direct vent systems the internal combustion air fan is responsible for drawing in adequate combustion air. The performance level of the fan is also influenced by the vent system employed on a specific model.
A unit that is starved for air operates with a fuel rich flame in the combustion chamber. This is typically seen as a yellow lazy flame. This will typically yield incomplete or smelly combustion by-products. If this condition is allowed to persist the combustion may be incomplete to the point of forming soot on the heat transfer surface. Sustained operation of a unit starved for air may result in a hazardous operating condition. This problem must be corrected.
The typical equipment room should have two openings. There should be one for combustion air and one for ventilation air. The National Fuel Gas Code also makes provisions to allow operation with a single properly sized opening to the equipment room. See the installation and operating instruction furnished with each unit and local code requirements for detailed instructions on sizing combustion and ventilation air openings.
An equipment room for gas fired appliances may be provided with combustion and ventilation air openings sized based on drawing all air from the interior of the building. In this case, each opening must have one square inch of free area for each 1000 Btu/hr of gas appliance input in the room. Equipment rooms that draw air form the interior of the building are generally reserved for small, low Btu/hr appliances such as a small residential type water heater. Large commercial gas fired boilers and water heaters will typically have combustion and ventilation air openings with direct contact to the outdoors to ensure proper operation.
All gas fired appliances require combustion and ventilation air for proper operation. Most code requirements are based on operation of an atmospheric gas fired appliance with a draft hood because this type of appliance generally requires the largest amount of air for combustion and venting. Fan assisted and power burners generally require less air because there is less secondary air in the combustion process and less air required for dilution air in the venting process. Regardless of the differences, it does not change the code requirements. Combustion air is required for any combustion process.