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Welcome to Ricks Heat Load Page

The first thing to determine in designing a system is the amount of energy required for heating and cooling. The best method is Manual J8 load calculation method. The Version 8 came out in 2001 to replace the antiquated 20 year old Manual J method. The version 8 takes into account the layout of the duct, color and type of roof, location of the air return and equipment. If a unit is in the attic, it is wasting approximately 15% of its energy. This effectively drops the SEER rating of a system 2 ratings downward. For example a 15 seer system functions as poorly as 13 seer system! Hopefully Austin Energy will reduce rebates based on this to properly reward people on effective efficiency and not theoretical.
These heatload calculations take time but it is the prescribed method that calculates how large a system your house actually needs for heating and cooling.

This method can be expanded to also determine the heating & cooling requirements for each room. In 2007 I completed a house with distinct zones that needed cooling only part of the day. By Zoning, I was able to reduce the size of the AC because of knowing how much heating & cooling was required as well as time of day. The house needed most of the AC during the daylight hours upstairs with very little in the downstairs Northside Bedroom area. I was able to achieve over 1000 feet per ton using this technique. We worked with the owner and builder during the construction and have a system with a filtered fresh air intake and control over all areas of the house. Since 2010, the new ultrascroll 2 stage compressors have made zoning even better. With alot of design time and careful attention to duct design I have a house that is 3300 feet, 3 story using a single 3 ton 2 stage system. Most of the time it runs in the 2 ton mode achieving a respectable 1650 feet per ton.

Manual J8 + Exposure Diversity PDF

Most of the AC contractors use a per square foot number to avoid this long calculation. Square footage does not take into account window type, number of people in a room, number of computers, window direction, quality of construction, duct loss, color of roof, lifestyle, age of kids, etc. A properly sized system is more energy efficient, removes more humidity from the air, is less susceptible to mold growth than an oversized unit. A properly sized system will also last longer because it will run the optimal amount of time putting less stressful conditions on the equipment.

To see an actual printout of a Manual J8 load calculation look at this house layout complete with the duct design, the airflow per room and many other items of interest.

Manual J8 Detailed PDF

Please email Rick at austintexasrick@yahoo.com with any suggestions and comments.

Email: Ricks Heating & AC

updated May 2011