HVAC System Design and Sizing
Current Practice in New Homes:
The Texas Energy Code Field Study shows that new homes built in Texas have less duct leakage than new homes in most other Field Study states and more efficient equipment than other states. Both of these findings bode well for air conditioning efficiency in Texas, but the study also found that the average home in Texas has an oversized air conditioner. The typical system is sized for a ton of air conditioning for each 588 square feet of living space. In comparison, other states in the Field Study have higher square footage per ton. The Air Conditioning Contractors Association (ACCA) says that systems sized according to their design criteria, ACCA Manuals J, D and S, should usually have at least 800-900 square feet per ton. Better windows, tighter envelopes and ducts and LED lighting all lead to tighter, more efficient homes, so old “rules of thumb” based on leaky envelopes and inefficient equipment will lead to grossly oversized HVAC systems and should never be used.
Why it Matters:
There are a couple of good reasons for right sizing air conditioners. The first is simple economics; larger systems cost more. If an air conditioner is oversized the builder pays more to have it installed and passes the cost on to the homebuyer. Oversized systems also have larger motors and have more start-stop cycles increasing energy cost. More importantly though, oversized air conditioners do not dehumidify as well as right sized systems, Oversized air conditioners only take a few minutes to cool a house, especially when the temperature is in the 80s to mid-90s when the humidity is likely to be highest. On oversized systems the temperature in the house reaches the thermostat set point and shuts off before enough air flows over the coil to really reduce the humidity level in the house. This high humidity makes the occupants uncomfortable and they set the thermostat lower, but that still doesn’t make the system run long enough to reduce humidity, so the house gets colder but the occupants don’t really get more comfortable. Right sized systems run longer, allowing them time to reduce the humidity and make the home more comfortable. On hot days right sized systems just run longer with each cycle and with the high efficiency equipment and tight ducts contractors are installing today there is little danger of the system not being able to cool properly, even when the temperature exceeds 100⁰.
Energy Code Requirements:
The 2015 IRC section M1401.3 and IECC R403.7 (and all energy and mechanical codes going back at least 20 years) require heating and cooling equipment to be sized in accordance with ACCA Manual S based on building loads calculated in accordance with ACCA Manual J or other approved heating and cooling calculation methodology. The code also requires that duct systems be designed and installed in accordance with ACCA Manual D and Chapter 16 of the IRC.
Guidelines for Performing or Reviewing Manuals J and S:
Occupancy: For Manual J calculations occupancy must be one person per bedroom plus one. For example, a three bedroom home always has an occupancy of four (one each for three bedrooms plus one additional). Occupancies may not be increased to accommodate expectations for entertainment with higher occupancies. In such cases an additional small system may be designed and installed.
Envelope Leakage, Duct Leakage and Insulation: The State Energy Code sets requirements for envelope leakage (infiltration), duct leakage and insulation levels for walls, floors and attics. Manual J inputs for new homes must be at least equal to code requirements. Higher duct or envelope leakage and lower insulation levels will increase HVAC sizing beyond what is needed. The input for envelope tightness should be “tight” on new homes.
Indoor Design Temperature: ACCA Manual J specifies indoor design temperatures of 70⁰F in winter and 75⁰F in summer. Check to ensure that these are the design temperatures used.
Outdoor design temperatures: ASHRAE and Manual J specifie the outdoor design temperature for different locations. Outdoor design temperatures are based on thirty-year historic climate data and are the temperature that is exceeded only one percent of the total hours in a typical year, 88 hours per year. Make sure the Manual J input for outdoor design temperature is the recommended temperature for your location.
Climatic Data for Cities in Texas and Oklahoma
|City||HDD65||CDD50||Heating design temp 1%||Cooling design temp dry bulb 1%||Cooling design temp wet bulb 1%|
Climatic Data from ASHRAE 90.1, 2013 Normative Appendix D
Well designed and installed HVAC systems make homes in Texas and Oklahoma more affordable, efficient and comfortable. Using the ACCA HVAC design tools or the equivalent, as required by the code, is the only acceptable method for designing HVAC systems.