• By Champion Inspect

Range & Microwave Hood Venting, How to Calculate & is it Right for Your Home?

The over-the-range microwave has not changed much since I first saw it back in 1986. The typical specs are 16.2", 16.375" deep, and 30" wide to match the range.

The CFM (how many cubes of air is exhausted in a minute) is 300-400 on the average side. Most range hoods are 300-695 CFM's for a standard gas range. It was an adequate ventilator for a typical stove designed in the 1980’s and 1990’s.

However, it is not a product capable to exhaust grills, woks, or professional ranges (sorry Viking, it will NOT work), because there is little depth or capture area. Smoke is never immediately filtered. It is captured and then channeled through the duct to the outside.

The average hood is 24" in depth whereas the OTR is only 16". That could be a venting issue on a more powerful range.

Gas Ranges 

At the same time, gas ranges have changed quite a bit. The average BTU burner on a gas stove was about 10,000 BTU's. Most ranges now have at least one or two burners of 15,000 BTU or greater and the average BTU of a gas range has increased dramatically over the last 10 years.

So the real question is: Can an Over-The-Range Microwave handle these gas ranges?

First, you are probably not going to burn down your house with an OTR. However, you will not exhaust properly if you love to use the top of your range. There are now alternatives to the over-the-range microwave for the avid cook.

The burners on a gas stove produce a lot more heat than those on an electric range, so a kitchen with a gas stove requires a larger capacity range hood vent fan.

To calculate the fan sized needed for a gas stove, combine the BTU ratings for all the burners on your stove do not include the oven's BTUs (gas burners range from 5,000-15,000 BTU per burner, with an average of about 10,000 BTU per burner and a total of about 40,000 for a standard 4-burner stove), then divide by 100 to find the minimum CFM needed for a kitchen with a gas stove. For example:

40,000 BTU Gas Range ÷ 100 = 400 CFM Range Hood Fan or Higher.

Range Hood Ductwork

The size, shape, length, turns, and cap on the range hood ductwork adds resistance which reduces the amount of air the vent fan can move, requiring additional CFM for the fan.

When using smooth, round 8” diameter, metal pipe, add one CFM per foot of pipe, plus 25 CFM for each elbow, and 40 CFM for a roof cap.

For example, if the vent pipe was 10’ long with two elbows and a roof cap, you would need to add 100 CFM more to the fan size ratings above:

10 Pipe Length + 25 Elbow + 25 Elbow + 40 Roof Cap = 100 CFM

Calculating Range Hood CFM Vent Fan Size

To make the final calculation, take the larger of the CFM rating for stove width, room size, and stove burner. Add the additional CFMs needed for the ductwork to arrive at the minimum CFM range hood to buy.

In the examples used above, if your kitchen has a 30” stove (250 CFM minimum) in a 16’ x 16’ x 8’ room (512 CFM minimum), and a 40,000 BTU gas stove (400 CFM minimum) you would want a fan rated at 512 CFM or higher, plus 100 CFM for the ductwork for a total of 612 CFM or more.

Room Size

You should also take into account the size of your kitchen in cubic feet when calculating the size range hood fan needed, since a larger kitchen needs more venting to clear the air than a smaller room.

A range hood should be able to exchange the air in the kitchen at least 15 times per hour or every four minutes. For example, if your kitchen is 16’ long x 16’ wide with an 8’ ceiling, it would contain 2,048 cubic feet of space:

16’ wide x 16’ long x 8’ high = 2,048 cubic feet

To find the fan needed for your size kitchen, multiply the number of cubic feet in the room by the number of air exchanges (15), then divide by the number of minutes in an hour (60).

For example:

2,048 cubic foot room x 15 air exchanges = 30,720 cubic feet moved per hr

30,720 cubic feet ÷ 60 minutes = 512 CFM range hood fan or higher

An easier way to make the calculation above is to divide the number of cubic in the room by four minutes:

2,048 cubic foot room ÷ 4 minutes = 512 CFM range hood fan or higher


Over the last 20 years, the kitchen has evolved from a single, small room to a great room encompassing the once formal dining room and kitchen. This new great room has opened new alternatives to place your microwave.

Let's look at a few:

Microwave Drawer

Microwave drawers slide out with a touch of a button and are great in spaces like islands and cabinets.


Microwave drawers are all produced by Sharp. But with brands like Jenn-Air, Thermador, and Bosch, they are better-looking and can be mounted flush to your wall.

*Microwave drawers are sweeping our nation's kitchens for good reason. The short end of a kitchen's island typically goes unused. The 24-inch Sharp microwave drawer oven is a smart use of that space.

Speed Oven

Speed ovens are really convection microwaves. They have the ability to cook as a microwave, convection oven, or both.

Speed ovens can be placed anywhere unlike microwave drawers, because they fold down and do not slide out. The more powerful 220 volt products need to be placed in a wall cabinet.

*A speed oven is a regular convection oven but is smaller in size and cooks faster. Speed ovens combine convection cooking and microwave technology to speed up the cooking process. Today Miele, Jenn-Air, Bosch, and GE make speed ovens.

Under Cabinet Microwave

GE has a 12-inch deep microwave designed to be placed under a cabinet. It is a bit dated looking, but not bad as a last resort. You can also build it into a cabinet (just use a trim kit, not shown below).

*Countertop models can either sit on the counter or tuck into a shelf or opening in the cabinetry for a built-in look. Models with a mounting kit preserve work space by hanging on the wall, within cabinetry, or under the countertop. Be sure to follow the manufacturer's instructions when mounting your microwave.


Kitchen range hoods that are vented to the outside are a great way to remove heat, odors, moisture, and smoke from your home when cooking. Over the Range Microwave and Hoods are great for convenience and functionality. However, you should look at alternatives if you like to cook or if you will be buying a new range of 55,000 - 60,000+ BTU’s in the future.

The OTR does not have sufficient capability to handle all that smoke, grease, and heat with every burner turned on.

However, there are suitable alternatives if you plan your kitchen accordingly.


What is recirculating venting on a microwave?

Recirculating microwaves refers to the type of venting that a particular unit employs to move the air in the microwave. External A microwave with an external venting system vents to the outside of your home through duct work. Convertible These microwaves vent externally.

Does over the range microwave need to be vented?

Virtually all building codes require a ventilation system installed over the kitchen stove. This ventilation system has a light that illuminates the stovetop surface and a fan that removes smoke and steam from the air. Over-the-range microwaves have a light and fan built into the bottom of the appliance.

Can a microwave vent a gas range?

All current over the range microwaves can be installed over gas cooking products up to and including 5 burner stoves. As long as the 66" minimum from the floor to the top of the microwave or an 18" minimum clearance is achieved form the top of the range to the bottom of the microwave is followed there will be no performance or warranty problem. This includes Advantium ovens and convection microwaves.

source: https://codes.iccsafe.org/content/IRC2015/chapter-15-exhaust-systems


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