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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.
Please email Rick at firstname.lastname@example.org with any suggestions and comments.
updated May 2011