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Question DetailsAsked on 6/5/2013

How much to add exhaust fan to kitchen and bath?

House built in 1977. There are no exhaust fans in the kitchen or bathroom. The bathroom is always moist. It takes bath towels over 24 hours to dry. How much and what has to be done to add these?

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I have listed in Source, below, an article that is a primer on exhaust fans. You can also google "kitchen exhaust fan size) for more articles.

It requires cutting a hole in the ceiling over the stove (or in center of room in case of bathroom ceiling), installing the fan, running duct work (usually 3x10 rectangular or 6 to 12 inch round metal for kitchen, 4 or 6 inch round metal or plastic for bathroom, depending on local code) from there to the outside (through an outside wall if there is a floor above, or through attic to roof if on top floor), and installing a rain hood with backdraft damper on the exit point (on outside wall or in roofing). Also requires pulling electrical power from a circuit that has adequate capacity to run the fan and associated lights - usually using the light circuit in the same room, and changing the light switch box out to a double box or triple box and switch plate - for the existing lights, for the fan, and for separate light if bathroom fan has a separately turned on/off light in it. (Personally, I do not like the idea of having a light in the bathroom fan unit - too much moisture present, with resultant chance of a short and fire in your ceiling, especially since almost all bathroom fans come in plastic cases).

You need to check if you have space in the kitchen for the fan unit - cabinets are usually installed with a standard spacing for the fan equal to the space left for the range, though fans can come in 20 to 60" widths. If space was not left for a vent unit, you may hae to modify cabinets. Also, a reduced-height cabinet is usually installed over the stove - you are going to lose part of the suew of this cabinet, as the vent duct has to go up through it to the ceiling. The kitchen fan unit (intake hood) should be at least as wide as the range or cooktop it is serving.

Cost, assuming direct shot into ceiling, through attic, to roof - about $300-600 for kitchen depending on difficulty, difficulty of pulling power, and cost of fan unit. Likewise, $200-400 for bathroom fan.

If you have to go sideways out the side of the house because of another floor above (not recommended if you can avoid it), can run from above costs if short run and floor joists run favborable direction, to double cost if difficult run, going through brick or stone outside surface, etc.

Three very important factors -

1) the moist exhaust air has to vent OUTSIDE the house (NEVER in the attic, floor joist space, or under the eaves) to prevent the moisture from causing rot in the house structure, and not near (3-6 feet, depending on code) to any air intakes or windows. Also, if your house is energy efficient (i.e. upgrade from original 1977 condition) you need to have makeup air flow evaluated, otherwise in a "tight" house you can cause a partial vacuum in the house (called negative air pressure) that xxxx in outside air through any available opening, and if openings around doors and windows are not enough, can cause backdrafting of your furnace and hot water heater, sucking combustion gases and CO2 and CO (carbon monoxide) into the house. The units should be properly sized by an HVAC (Heating, Ventilating, and Air Conditioning) contractor, who is also who should install them.

2) make a special point with the contractor that ceiling penetrations are to have the fan unit and ductwork sealed to the vapor barrier AND airtight sealed/caulked at the ceiling, so moist air does not leak up around the fan box or duct into the floor joists above or attic.

3) the outside of the penetrations through the roof or siding should have the ductwork and adjacent area sealed with ice and water shield (a bitumastic stick-on waterproofing membrane) bonded to the duct, UNDER the vent hood. This will help prevent condensation in the vent hood from dripping back down along the vent pipe into the wall or attic. I require on jobs I control that ice and water shield be sealed to and around the duct, extending to the outside edge of the vent hood skirt on the uphill and two adjacent sides, but terminating about 1/2 inch short of the hood skirt on the downhill side, to provide an exit point for condensate. For some reason, manufacturers NEVER account for condensation in the hood (yet it always happens when the moist, warm air hits the cold metal), so the hoods have no trap shield around the ductwork or drainage holes provided for this condensate to be trapped and discharged outside the hood.

Source: http://www.cmhc-schl.gc.ca/en/co/maho...

Answered 7 years ago by LCD




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