Kitchens – mounting un-vented range hood in plaster and wood lath wall

In general, yes an exhaust vent (or any vent) can turn corners. But there are many other factors to consider.

Different vent materials or dimensions resist air differently. Different ways of turning corners resist air differently. The best place to start looking for info on this is the vent installation manual. It will talk about different ways to construct vents, what types of ducting corners can be used, and what size of ducting you need. It will talk about how many feet of vent the blower can push through, and how many feet to count a corner as (typically 5').

So you know, the shallow box vents have poor airflow. And you have to consider getting through the top-plate of the wall, which would create a huge weak point. When we built our kitchen I was hoping for in-wall ducting. Instead, the installers had to put a 10" circular duct up through the middle of the cabinet above the stove.

When calculating the minimum size of a range hood, there are three things you should consider. The size of the cooking surface, the amount of heat produced by the cooking surface, and the volume of the kitchen.

If the range hood is attached to a wall, you should have 100 cubic feet per minute(cfm) per linear foot. So if you have a 30" wide range, you should have a hood rated at 250 cfm ((30/12)*100 =250). If the hood is over an island, you'll use 150 cfm/linear foot. In this case that same 30" cook top, would require 375 cfm ((30/12)*150 = 375).

Next we'll determine the minimum capacity based on British thermal units(BTU)/hour, by dividing the BTU/hour by 100. For example, if we had a cooktop that produced 40,000 BTUs, we would need 400 cfm. If you are using an electric range (measured in watts), simply multiply watts by 3.41214163 to determine BTU/hr.

The final calculation, will be based on the size of the kitchen. The air in the kitchen should be cycled 15 times per hour, so our formula will be ft³/4. If we have a 10ft x 10ft x 8ft kitchen, (10 X 10 X 8)/4 = 200 cfm.

We'll then choose the largest from these three calculations, and that will be the minimum size hood we need. If you are doing more cooking than the average person, or just want a little more air movement. You can always get a larger hood, this is just the minimum size you should consider.

International Residential Code (IRC), says the minimum intermittent exhaust rate for a kitchen is 100 cfm, while the minimum continuous exhaust rate is 25 cfm.

M1507.4 Local exhaust rates. Local exhaust systems shall be designed to have the capacity to exhaust the minimum air flow rate determined in accordance with Table M1507.4.

So you'll want to make sure the hood is at least capable of achieving these flow rates.