What’s Cooking? – A Look at Domestic Kitchen Exhaust Design.
With people spending more time in their kitchens, it is important that they have the correct ventilation provisions to provide a comfortable and safe space with good indoor air quality by facilitating the elimination of the gases, particles, and odors that may be caused by cooking. The first-place homeowners can look to improve the air quality in their home is their kitchen exhaust system.
When cooking occurs over a range, depending on what is being cooked, the process can release grease, oil, and vapor into the air that are inhaled by the occupants of the home and settle upon the appliances and walls of the surrounding space. Cooking also generates heat and moisture that is transferred into the air, raising the interior temperature and humidity of the kitchen and increasing air’s ability to hold moisture adding both sensible and latent load to the existing HVAC system. As the temperature gradually falls back down to the normal indoor air temperature, condensation forms on surfaces and can lead to damage of the surface finishes and mold growth. A visible example of this is when condensation forms on windows and even walls in a separate part of the home when cooking in the kitchen. To properly protect from these negative aspects of cooking, a kitchen exhaust system should be installed to remove the harmful byproducts and excess heat created from combustion to the outside.
While it is typically recommended to have a dedicated kitchen exhaust hood over your cooking appliance within your home, the current New York City Mechanical Code (NYC MC), based off the International Mechanical Code (IMC), does not specifically require it in all instances. The primary ventilation requirement for a kitchen is found on table 403.3 in the NYC MC requiring that a residential kitchen be exhausted at a rate of at least 25 CFM/FT2 when exhausted continuously, or 100 CFM/FT2 when exhausted intermittently. This requirement is easily solved by the installation of a range hood that is ducted from above the cooking range to the outdoors. The NYC MC however does allow for recirculating type “ductless” range hoods to not discharge directly to the outdoors provided that the equipment is installed per the manufacturer’s instructions and that the ventilation requirements of table 403.3 are met or if the natural ventilation requirements from the New York City Building Codes are met for the kitchen/kitchenette. Additional it is worth noting that that the NYC fuel gas code, section 501.8, specifically states that gas-fired ranges do not need to be vented and that international building code section 505.4 states that if a domestic range is installed in a non-residential space (office, church, etc…), a hood with ducted exhaust must also be installed however this provision is absent from the NYC MC which implies that non-residential spaces with domestic equipment must refer to the commercial kitchen hood requirement.
The capacity of a range hood is measured in cubic heat per minute (CFM) and are sized to meet the total heat output generated by the cooking range in British Thermal Units (BTU). Typically, the manufacturer will provide an optimal exhaust rate for the selected hood for the engineer to design the exhaust system.
Once a range hood requiring exhaust has been selected and sized, the engineer must route the ductwork to a code compliant location outside. Section 501.2.1.3 of the 2014 NYCMC requires that environmental exhaust, under which domestic kitchen exhaust is classified, be located a minimum of 3’ property lines and operable opening into the building, and 10’ from mechanical air intakes. The ductwork itself is required to be constructed of stainless steel, aluminum, or copper, and have smooth inner walls. The ducts must be airtight, equipped with a backdraft damper to prevent air infiltration as required by both the NYC MC and the New York City Energy Conservation Code (NYC ECC). The kitchen exhaust duct must also be independent of all other exhaust systems, as stated in section NYC MC 505.1.
Nowadays, as residences have envelopes that are becoming more and more well sealed, kitchen exhaust systems encounter issues stemming from the negative pressure caused by the rate at which they exhaust air from the space. For that reason, the NYCMC includes a section, 505.2, requiring make-up air to be provided to any exhaust hood systems capable of exhausting more than 400 CFM, at approximately an equal rate to that of the exhaust. The make-up air system is usually comprised of a standalone make up air unit or a series of components that duct fresh air into the space and balance out the negative pressure caused by the kitchen exhaust system. This air would be filtered and potentially conditioned or at the very least tempered to provide better indoor air quality as opposed to infiltration based make up air.
A compact solution to bring make-up air while exhausting the kitchen is by utilizing an ERV (energy recovery ventilator). The ERV unit consists of two fans, exhaust and make up, and a core where the airflows mix. The fans operate simultaneously, bringing outside air and return air to the core, where the air is then tempered and discharged as make up air and exhaust. These units are compact and effective when wired for operation with the HVAC serving the space, however they are unideal for continuous use as the unit does not lower the humidity of the outside air and can be cold in the winter months without supplemental conditioning.
Kitchen exhaust systems come in a wide array of options ranging from simple recirculating hoods to fully ducted range hoods. The recirculating hood consists of a fan with a filter that sucks up grease particles, odors, etc…, filters it, and recirculates it back into the kitchen. While these hoods are ductless and inexpensive, they do not eliminate all the harmful combustion gasses, nor the moisture and heat created when cooking that would be eliminated if using a ducted system. Fully ducted systems consist of a hood sized for the specific cooking range that sits about 2’-3’ above the face range. The hood is ducted a fan that then exhausts the air to the outside when it operates. If the hood is large enough (≥400 CFM), make up air must be provided as noted above. Typically, the make-up air system is interlocked with the exhaust system to turn on when the exhaust system is operating. If the space is served by natural ventilation, it is recommended that make up air still be provided to equalize the negative pressure cause from the ducted kitchen exhaust. If make up air is not provided, the exhaust will draw air from the cracks in the doors, walls, and windows, which can create comfort and indoor air quality.
Every project challenges us engineers to provide innovative solutions when faced with different design and field requirements. It is our obligation to enact best practice when during the design process and the design of kitchen exhausts systems is no different. It is for that reason that ducted kitchen exhaust systems are recommended on most residential projects that come across our plate. When designing with best practice in mind, it is important to think about a variety of factors such as: What is the optimal exhaust rate of the range hood? Will the room be negatively pressurized by the rate of the exhaust, and what, if any, issues could that cause? Does the exhaust rate of the hood require make-up air be supplied? Is energy recovery appropriate for the application? etc… If presented with a typical kitchen, it would be recommended to provide the kitchen with a ducted exhaust system and to deliver fresh air regardless of if it requires make-up air, so that all the byproducts of cooking and combustion are safely exhausted out of the residence, and there is little to no pressure differential.