Arkansas HVACR NewsMagazine November 2020

S tate, National, Chapter News Tech News

money and you begin to get more work with more competitive bids. HVAC replacement remains a 40% gross margin and duct replacement is 70% gross margin job. Therefore, it pays to include duct modifications in your scope of work. Once the amount of duct is determined, you can move to what the air requirements should be. At this stage, it may become clear whether the nominal tonnage installed in the home was correct, or if the system was oversized from the beginning. Generally, homes that are leaky or of pier and beam construction may require as much as one cfm per square foot of air delivery. Homes on a slab will use no more than .8 cfm per square feet. Homes built after 2006 (code compliant) will use .6 cfm and low load homes will be .5 or less cfm per square foot. Of course, you must run a load calc as these numbers are a simple generalization to get us thinking more appropriately on sizing of equipment as the relation of BTU and air flow must be tied together.

choke point for performance. We must maintain adequate surface area to keep static across the intake as low as possible. The basic calculation can be made by dividing the cfm required (400 cfm per nominal ton) by two. A three- ton nominal system should have 600 square inches of free return air. Platform return air can substantially limit air intake to make performance impossible. Floor joist

returns are often undersized and restricted. It is important your staff recognize the issues and formulate solutions. Not all structures are going to

Photo 1: Platform Return

allow substantial improvements; however, your company will go in with eyes open and limitations on performance outlined within the contract and communicated to the customer. Many times, the customer may opt for an inclusive job scope to correct the deficiencies that will solve problems at higher costs. Customers understand lasting solutions sometimes cost more.

“ Four Questions to Meet Performance”.

The first fundamental question is “Can I feed the equipment enough air to get the job done?” Return air continues to remain the number one

Return air, net free area calculation can be made by dividing the cfm required (400 cfm per nominal ton) by two.