When cooktops are not vented and exhausted, the ceiling, or cabinets above the cooktop, will become soiled and stained by grease and steam. Although it is a desirable option, a regular kitchen cooktop does not need to be vented and exhausted to the outside. A downdraft cooktop and grill must be vented from the kitchen to the outside, following manufacturer's specifications.
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Most kitchen cooktops, when installed under a space saver microwave, will be vented because the microwave has a built-in vent and exhaust system. The vent and exhaust system on a microwave is recirculating and self-cleaning. Filters are intended to be replaced on a regular basis. Microwave vent and exhaust systems are designed to handle normal kitchen cooking steam, smoke and grease. The vent and exhaust system of a microwave can be vented or exhausted to the outside to accommodate heavy kitchen use.
Oven exhaust hoods serve the same purpose. Island cooktops do not need to vented and exhausted unless required by local building codes. The location and design of kitchens in high rise residential buildings often make it impossible to vent and exhaust kitchen cook tops. Downdraft cooktops and inside grills must be vented and exhausted to the outside in order to avoid buildup of dangerous fumes. A downdraft cooktop is designed with a grate to suck smoke and grease down, and to the outside through a crawl space, instead of out of the air. A built-in cooktop grill is a natural-gas-fueled grill that is incorporated into a kitchen range, usually between the burners. A vent system, which removes dangerous fumes, grease and smoke to outside the house, should be included when these type of cooking appliances are installed.
Never install a bathroom vent fan over a cooktop. Do not tap into a clothes dryer vent to exhaust a cooktop. These vents are not intended to handle steam, grease and other kitchen exhaust. Both of these scenarios create serious fire hazards and are in violation of National Building Code Regulations, as well as manufacturer's specifications for vents and cook tops. When adding any exhaust features to your kitchen, always consult with a local building code official and a qualified HVAC contractor for advice and regulations.