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Q.In a house I am building, I need to provide adequate makeup air for a fireplace and a 600-cfm cooktop exhaust fan. How do I size a passive duct to introduce exterior makeup air into the house?

A.Pat Huelman, cold climate housing coordinator at the University of Minnesota, responds: Whether or not the fireplace or exhaust fan will cause problems depends on several factors. The most important factors are the leakage characteristics of the house, the type of combustion devices used for space and water heating, the location and type of fireplace and chimney, and the presence of other exhaust devices such as a clothes dryer, central vacuum, or bath fans.

The quantity of combustion and dilution air needed by a fireplace varies, depending on the type of fireplace, the type of chimney, and the size and burning phase of the fire. The amount might be as little as 100 or 200 cubic feet per minute (cfm) at the startup or burn-down of a small fire in a fireplace with glass doors, or as much as 800 cfm or more for a large fire in an open fireplace.

If the house is very leaky, drawing that much air may cause only a small negative pressure. But if the house is tighter, pulling that much air may cause a very large negative pressure. Although negative pressure can interfere with the combustion of the fireplace, in many cases it simply limits the magnitude of the fire, somewhat like a wood-burning stove with good air control.

Negative pressure can certainly interfere with atmospherically vented furnaces, boilers, or water heaters, potentially causing spillage of flue gases. That’s why you would be well advised to choose sealed-combustion appliances. Also, excessive soil gas and/or garage gases could be drawn into the home when the fireplace is in use.

Let’s assume for a minute that you want to limit the negative pressure caused by the fireplace to 5 pascals. The first table shows the size of combustion air opening needed, assuming three levels of combustion and dilution air requirements (see Chart A).

A. Makeup Air Required for a Fireplace

House TightnessBlower Door (cfm @ 50 Pa)Combustion Air Opening for: 200 cfm 500 cfm 800 cfm
Very Tight 600 28 sq. in. 154 sq. in. 280 sq. in.
Pretty Tight 1,200 None 98 sq. in. 224 sq. in.
Kind of Tight 1,800 None 41 sq. in. 168 sq. in.
Typical 2,400 None None 111 sq. in.
Loose /td> 3,000 td> None None 54 sq. in.
Very Loose 3,600 None None None

If your house is very leaky, existing leaks may provide adequate combustion and dilution air for your fireplace. If your house is very tight, you will need a very large passive air opening.

Now let’s focus on the 600-cfm cooktop exhaust fan. If we know the cfm @ 50 pascals of the home using a blower door, we can easily predict the negative pressure due to the operation of this fan (Chart B).

B. Negative Pressure Caused by a 600-cfm Exhaust Fan

House TightnessBlower Door (cfm @ 50 Pa)Predicted Negative Pressure Due to 600-cfm Exhaust Fan
Very Tight 600 50 pascals
Pretty Tight 1,200 17 pascals
Kind of Tight 1,800 9 pascals
Typical 2,400 6 pascals
Loose 3,000 4 pascals
Very Loose 3,600 3 pascals

A negative pressure of 3 or 4 pascals — which this exhaust fan can cause, even in a very leaky house — can pull the products of combustion down a fireplace chimney. This is especially likely during the start-up or burn-down phases of a fire. The simplest solution to this problem is never to use the cooktop exhaust fan when the fireplace is being used.

However, remember that this same negative pressure could cause spillage of combustion gases from an atmospherically vented space or from water heating equipment. With this type of exhaust device, sealed combustion equipment would be advised for space and water heating.

If you decide that these approaches are impractical, you can try to size a makeup air inlet for the cooktop exhaust fan. Let’s assume a 20-foot smooth duct with three 90-degree elbows and a screened hood. Let’s also assume that the fireplace (or water heater) can tolerate 3 pascals. Assuming no house leakage, you would need an opening of approximately 325 square inches, or the equivalent of a 20-inch diameter duct. If we know the house leakage as measured by a blower door, we could use Chart C to size the makeup air opening.

C. Makeup Air Required for a 600-cfm Exhaust Fan

House TightnessBlower Door (cfm @ 50 Pa)Predicted Negative Pressure Due to 600-cfm Exhaust Fan
Very Tight 600 50 pascals
Pretty Tight 1,200 17 pascals
Kind of Tight 1,800 9 pascals
Typical 2,400 6 pascals
Loose 3,000 4 pascals
Very Loose 3,600 3 pascals

If the cooktop exhaust fan will be used while the fireplace is in operation or with atmospherically vented space and water heating, you should install a makeup air fan.

Now, if that isn’t complicated enough, there is one last nagging issue. When the cooktop exhaust fan is used when the fireplace is not, the makeup air often comes down the fireplace chimney, commonly leading to homeowner complaints about a sooty odor in the house. Unfortunately, it is very difficult to seal the chimney completely.

In conclusion, a fireplace and a large cooktop exhaust fan just don’t belong together in the modern house. The fireplace by itself can be handled by selecting sealed combustion equipment for space and water heating and introducing the necessary combustion and dilution air, especially for tighter homes and during the critical start-up and burn-down phases. The cooktop exhaust fan can likewise be handled by itself with proper selection of combustion equipment and some provision for makeup air. It is very difficult or impossible, though, to handle combustion and makeup air requirements when a house has both a fireplace and a cooktop exhaust fan. If both are an inextricable part of your plans, I would recommend a different type of hearth product and a kitchen cooktop or exhaust fan with much smaller cfm requirements.