Ductwork Design

Since 1950 Midland has been building custom sheet metal ductwork for residential homes

Planning on remodeling the basement? We can make the heating/cooling trunk line and supply pipe disappear.

Finishing off the second story? Midland knows how to build the right size supplies and returns to heat/cool your new space, while taking up as little space as possible to get from the basement to the second floor.

The old gravity furnace is taking up the whole basement? Let Midland replace your furnace and build you a new custom distribution system that connects to all of your existing grills/register. You will not have to make any cosmetic changes to your living area.

Duct Design Dynamics  

One hundred CFM, cubic feet per minute, is the basis for duct design. For heating with a gas furnace, 100 CFM represents approximately 8000 BTU per hour. For air conditioning, 100 CFM represents about 6000 BTU per hour.

The standard eight inch deep duct is designed around this 100 CFM standard. Every two inches of width equals 100 CFM. A six inch round duct also equals 100 CFM. Begin with two inches of duct width for friction loss, and the formula for 1000 CFM will be 2 + (2x10)=22 for an 8 inch by 22 inch duct which will supply ten six inch rounds. Before the ducts can be designed, the size of the furnace and/or air conditioner must be determined. The above 1000 CFM would match an 85000 BTU gas furnace with 30,000 BTU of air conditioning, also referd to as 2 1/2 tons; a ton being 12000 BTU of cooling. The 8 by 22 duct would feed 10 six inch round outlets.

What goes out must come back, so the return system must be the equivalent. Returns are usually fewer in number and of larger size but must equal 1000 CFM total.

Here's a chart of the relationships to show duct size air flow relationships

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