Is Your Ductwork Leaving Rooms Hot and Humid?

A decade-long study by NIST (National Institute of Science and Technology) found many heating and cooling systems perform far below customer expectations. The biggest homeowner complaints were an unexpected lack of comfort and high utilities. Additionally, NIST determined over 50 percent of heating and cooling energy created by a central furnace or air-conditioner may be wasted due to poor ductwork layout, sizing, and workmanship. When a customer experiences hot or cold rooms within the home, the failure is focused on the heating and cooling equipment, when the actual problem may be ductwork related. This article explores some of the causes of uneven heating and cooling within a home and second order effects to health, safety and comfort.

Airflow

A pervasive problem in many homes is simply inadequate airflow to specific areas resulting in hot and humid rooms in the summer, and cold rooms in the winter. Heating and cooling equipment is sized based on the amount of heat a home loses in the winter and gains in the summer. This heat transfer is determined by an ACCA (Air Conditioning Contractors of America) Manual J “block” load calculation.  A block load determines the total BTUs (British Thermal Units) of heating and cooling that must be produced by a furnace or air-conditioner. Therefore, a block load determines the size of the home’s heating and cooling equipment.

After the furnace and air-conditioning equipment is sized properly, the next challenge is delivering the heat or cool to the correct locations inside the home. Each individual room will gain or lose heat in different amounts and at different rates due to numerous factors such as room orientation, air-tightness, insulation, and the number and performance of exterior doors and windows to name a few. As an example, a large living room with floor-to-ceiling windows oriented to the west will need significantly more cooling in the summer than a small bedroom with a single window facing the north. In order to determine the number of BTUs to provide to each location, a room-by-room load calculation must be performed. This is a time-consuming detailed extension of the Manual J “block” load.

Once the amount of BTUs has been calculated for each room, the designer can by extension calculate the amount of airflow, measured as CFM (Cubic Feet per Minute), required for each individual room in the home. This information informs the ductwork system design, specifically, the layout of main supply and return trunks, number of branch supply vents per room, placement of registers and grilles, and sizing of the ductwork system.

Challenges

The majority of homeowners and residential production builders choose the lowest possible bidder. Given the market’s fixation on low price as the sole buying criteria, coupled with the hours of design time and corresponding cost in accurately sizing equipment and calculating and drawing correct ductwork, the majority of residential HVAC contractors use inaccurate or incorrect general rules of thumb or worse yet just guess. Due to a lack of science-based design, the best-case scenario is comfort issues, rooms that are too hot and humid in the summer or cold in the winter. Worst case, comfort issues may be symptoms of more costly or dangerous problems such as humidity control problems that lead to condensation, mold and potentially structural rot of wood components.

Home Performance Group Heating and Cooling Services in Kansas City

If you are building a new home or finishing a basement, make sure your contractor is trained and experienced in performing Manual D ductwork design. Be prepared if they ask you to pay for it: Manual D analysis and design is time-intensive and require several hours or more to properly document all the necessary information and complete the calculation and analysis. 

At Home Performance Group, we continue to invest in technical training and up-to-date software so that we can correctly layout and accurately size our customer ductwork systems. We have performed numerous designs for our clients and prospective clients. Our in-home consultations are no cost and include a copy of the duct design with a system purchase. Alternatively, if you choose not to buy from us, we can provide you the ductwork layout and design for a nominal fee.

If you are interested in a no-cost in-home consultation, schedule with a Solutions Advisor today. 

Article by Larry L. Motley Jr., 26 Jul 2021

Larry is a graduate of both Wentworth Military Academy and Missouri Western State University earning a double bachelor’s degree in Economics and Finance. Additionally, he maintains six professional tradesman licenses in two states and advanced credentialing in green technology, project and program management, and process improvement. Larry is a three-time combat veteran having served in Operation Iraqi Freedom, Operation New Dawn, and Operation Inherent Resolve. He continues to serve through a value-based building science company focused on providing clients the best design, highest quality installation, and most honest repair services in the community.