Mechanical Engineering – What’s that?
Traditionally, Heating, Ventilation, and Air Conditioning (HVAC) contractors have used rule of thumb based on square footage to determine what size furnace to install in a home. Duct systems have typically been designed on site, with layout and duct sizing based on reference tables. These long-standing trends have led to several common problems found on most existing forced air heating and cooling systems:
- Furnace sizing: the rules of thumb used by contractors err on the side of being way too conservative. HVAC contractors are incentivized to oversize systems for several reasons. First and foremost, many HVAC contractors fear the worst case scenario of a customer call-back that the furnace won’t adequately heat the home. In addition, larger equipment is more expensive, and most HVAC contractors make better margins on equipment than on labor. For more information on equipment oversizing
- Undersized return ducts: Generally, duct tables offer more accurate reference than rule of thumb furnace sizing. However, a properly sized duct system connected to an oversized furnace will make excessive noise, run less efficiently, and place a strain on mechanical equipment. These problems are exacerbated when a leaky duct system is sealed.
- Mis-sized supply ducts: when no design forethought is put into a duct system, it is up to the installer to guess what materials to bring to the job, and duct sizes may be dictated based on what materials are available. The end result will often yield temperature differences from room to room, and high velocity entering small rooms
Engineering the Furnace with Manual J
The best practice for installing a high performance HVAC system is to use sizing methodology developed by the Air Conditioning Contractors of America (ACCA). Manual J Residential Load Calculation is the standard for determining the heating and cooling load of a home. Manual J examines a number of factors including location, orientation, insulation, and airtightness to estimate how much heat a home gains during the summer and loses during the winter. Furnace and air conditioner sizing are based on these calculations. Manual J has been incorporated into many software programs used by home performance contractors, including EnergyPro and EnergyGauge.
Engineering the Ducts with Manual D
Once the appropriate furnace size is selected, the ducts can be designed using Manual D Residential Duct Systems. Manual D uses the same inputs as Manual J, but allocates heating and cooling loads to each room, determining the amount of air needed to heat and cool each room, and consequently, what size and design of duct, boot, and grille should be installed. The most common software used for Manual D is Wrightsoft Right-D, which allows the user to draw the home’s floorplan and experiment with different system designs.
Manual J is Easy, Manual D is Hard
Manual J is often incorporated into energy audit software, so a home performance contractor is often able to determine the proper furnace size for a home during an initial energy audit with very little, if any extra work. Drilling down to individual rooms using Manual D typically requires redundant input, as Wrightsoft Right-D does not offer a full range of functionality needed for energy audits. In addition, Manual D can be very time-consuming when working around pre-existing design limitations in existing homes, and the resulting duct designs may be more labor intensive to carry out. For this reason, many contractors charge an additional fee to perform Manual D, and a system based on Manual D design may be more expensive than an installer-designed duct system.
Copyright September 2014 by Advanced Home Energy
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