Kitchen Hood Fans and Air Quality in Your Home

I would like to write about a topic that keeps coming up at our sales meetings and in discussion with our Design Centre team:  hood fans, in particular the larger canopy-style hood fans in the kitchen and how they can affect the air quality in a home.

From time to time a new product is introduced to the residential construction community and it becomes very popular in a short period of time.  Usually this is a good thing.  However, there are instances when a new product affects the overall performance in a home, in ways that were not initially considered.  I believe the canopy-style hood fans fall into this category.

A canopy hood fan is a style of fan you are likely familiar with.  It is a hood fan style commonly seen in commercial kitchens. They are usually wider at the bottom and narrower at the top (similar to a triangle) with a chrome finish.  This style of hood has become popular in residential kitchens the past five years or so.   Why?  There are two reasons.  One is the appearance of the hood fan and how it can be an integral part of the look and style of a kitchen design.  The second reason is the improved suction or removal of cooking aromas that this appliance offers.

The removal of air from the kitchen to the outside is measured in CFM’s, or cubic feet per minute.    Cubic feet per minute is a measurement of the velocity at which air flows in and out of a space.  CFM measurement is commonly used in the HVAC (heating, ventilation, air conditioning) sector of the construction industry.

A typical kitchen hood fan in a new home operates at approximately 200 CFM.  At this level a kitchen hood fan can effectively ventilate a kitchen designed in a typical new home.

Along with the kitchen hood fan, there are other fans in a new home such as bathroom fans and possibly a fan with a power-vented water heater.  These fans remove air from the home.  In addition to these fans, most new homes include a Heat Recovery Ventilator (HRV) unit.  The HRV is integrated with the furnace system and provides fresh air to the home while removing stale air at the same time.  The HRV is air pressure neutral as it brings in the same amount of air as it removes from the home.  All of these systems are components of the overall HVAC system.

New construction standards today have evolved to a level where homes are built to be VERY air tight –   so that is there is minimal leakage of air into or out of the home.  Improved insulation methods, house wrap (commonly referred to as Tyvek) and windows are the key components to improved air tightness in new homes today.   Obviously older homes are less air tight and therefore more air leaks occur at windows and doors as well as gaps in the wall insulation.

In a new home environment, if the amount of air being blown out of the home by various fans does not equal the amount of air that gets pulled back in, a “negative air pressure” situation  is created.   Even in an air tight home, air will find its way into the home through the smallest cracks, via the fireplace or the furnace.  This is what is referred to as “back drafting.”  Back drafting through a furnace or fireplace can be very dangerous as this may introduce unhealthy levels of carbon monoxide into a home.

A kitchen hood fan in excess of 300 CFM can create a similar situation where too much air is expelled from the home without the introduction of new air into to the home.  The standard used by the HVAC industry is that that a kitchen hood fan should never operate at a level greater than 300 CFM without the inclusion of a “make up air unit.”  A “make up air unit” brings additional fresh air into the home, thereby reducing the potential for back drafting.  The make up air unit does not remove air from the home like a bathroom fan or a canopy hood fan.  It is a square box that is incorporated into the HVAC system designed to balance the air-in to air-out ratio by bringing fresh air back into the home.  It also preheats fresh air providing comfortable ventilation throughout the house.

At Tartan, we sell an upgraded canopy-style hood fan that is 300 CFM.    As it is not over 300 CFM, this fan does not require a make up air unit.  However, on occasion we are asked to supply and install a canopy hood fan that is in excess of 300 CFM.  While we are pleased to do so, we must also supply and install a “make up air unit” at the same time.  This requirement can sometimes create confusion and frustration as our customers may not understand the need to install this and do not understand why it is required.  A “make up air unit” costs in excess of $1,000 to supply and install which is significant.

Unfortunately in our experience, the appliance industry is not always as clear as it could be when it comes to providing consumers information about canopy-style hood fans and the additional requirement for “a make up air unit.”  For instance, we have received requests for 700 CFM hood fans and we have explained that a fan that size provides too much power in a residential home and that it would adversely affect the air quality in the home by expelling too much.   Because the purchaser may not be made aware of the need for a “make up air unit” by the appliance retailer, there is confusion when we explain the need for one because the unit is an additional cost they did not anticipate.

Another area of confusion relates to the rough-in pipe that is installed in the wall above the stove which is required for any hood fan.  Our standard spec calls for a 6” round pipe which is satisfactory for the majority of hood fans styles.  Unfortunately, there are instances where a purchaser elects to install their own hood fan after possession, only to find that the required rough in for their hood fan requires a larger pipe or a square shape pipe.  This makes hood fan installation difficult.  While one can purchase a “reducer” (a pipe that allows a different size or different shaped pipe to fit together) this lowers the effectiveness of a hood fan causing it to underperform and operate less efficiently because the airflow is imbalanced and the large amount of air the hood fan is producing is being forced out a mismatched pipe shape or a pipe that is too small for its design.

At Tartan, we believe a maximum 300 CFM hood fan is more than adequate for everyday kitchen use and this is why we have sourced the aforementioned canopy-style fan for this application.  We are happy to consider supplying and installing larger canopy hood fans but for the safety and comfort of our buyers, we are obligated to install the “make up air unit” at the same time.

I hope this blog clarifies the issue of hood fans and CFM’s in new homes.