Manual J Load Calculations – Don’t Make Assumptions, Design Does Matter!

The use of commercial software to calculate the heating and cooling loads greatly simplifies the process. Various software applications that provide results certified by ACCA to meet all requirements of MJ8 are available such as WrightSoft Residential.  Heating and cooling load calculations are dependent on many factors.  This blog covers building orientation, AED (Adequate Exposure Diversity), HVAC location and ventilation/infiltration.

Orientation

Solar gains change depending on the time of day and the season.  The orientation (N, NE, E, SE, S, SW, W, NW) of your house must be considered in the cooling load calculation. The sensible heat gain during the summer is impacted greatly by the orientation of the house, overhangs (shading from the sun) and window to wall ratio.

While utilizing “worse-case” orientation may be tempting, most likely permit will reject.  Sometimes the peak cooling load for the worst case orientation is acceptable for system sizing but not always. When the HVAC design moves to the equipment sizing step, Manual S, this “worse-case” assumption may impact the system design.

Building Components – Test for Adequate Exposure Diversity

The hourly load gain through glazing 30% greater than the average hourly load gain is called the AED limit. A house will experience a spike in the block cooling load (heat gain) at a particular time of day – especially if one orientation has a lot of windows. The Test for Adequate Exposure Diversity is a procedure that is best performed with ACCA certified load analysis software due to the complexity of both the theory and the equations.

Most new construction with a minimum U-Factor of 0.35 for glazing will result in a house passing the Test for Adequate Exposure Diversity by not exceeding the 30% maximum. Sometimes a house fails by slightly exceeding the AED limit late in the day. That’s when HVAC design experience in the interpretation of the software output is critical. ACCA Manual J8 Appendix 3 describes the procedure to compensate for a house that does not pass the Test for Adequate Exposure Diversity by compensating for the load only in those rooms with the inadequate exposure diversity without oversizing the whole system. If the variance shown in the test is not addressed, there will likely be a comfort issue in the rooms with the high glazing load.  The easiest solution is to close the drapes in the rooms that have a high summer load.  Often human behavior of closing a drape will resolve any issues in the one or two rooms that may have a high glazing load.

The U-value and SHGC (window performance values) must be known and accurately represented by the data input. Shading provided by the overhang of eaves will reduce the sensible heat gain. If shading is ignored in the load calculation the cooling load will be inflated. However, using insect screens, and internal blinds or shades in the calculation can be problematic.  Just because the HVAC contractor says curtains will be drawn, will the kids actually do this?  Or what if you sell the house and they have different blinds? We at Savoy Engineering Group also avoid using internal shading because it is the #1 place people fudge Manual J load calculations.  External overhangs and porches definitely include, but do not add internal shading unless you want to risk slightly undersizing your system if everyone doesn’t follow the “rules”.

HVAC System Location and Duct Leakage

In an ideal world best practice for HVAC design is to “keep all ductwork within the conditioned space in order to eliminate the duct losses/gains to and from the outside conditions.” But in the real world there are one-story slab-on-grade or houses with unconditioned attics.  Sometimes it is impossible to keep all the ductwork inside conditioned space. Typically an installer will put the HVAC system and ductwork completely in the attic in a slab-on-grade house.  In a single-story house with a basement, the duct system is typically run in the basement, which is considered conditioned space provided the basement walls are insulated or there are supply registers in the basement. When ducts are placed outside conditioned space, the heating and cooling loads are impacted by R-values of the duct insulation and duct leakage.

Per ACCA, we only use Summer EnergyStar Residential HVAC Indoor Design Temperature

Summer EnergyStar Residential HVAC Indoor Design Temperature

Ventilation/Infiltration Factors

Ventilation and infiltration impact both the heating and cooling load by bringing outside air into the conditioned space. The target ventilation and infiltration rate must be accurately represented in the data input of the load calculation. In humid climates, the impact on the latent cooling load added by ventilation/infiltration can be significant.

Each safety factor applied to the outdoor/indoor design conditions, building components, ductwork conditions, or ventilation/infiltration conditions outlined above has its own impact on the heating and cooling loads. But, a more significant impact occurs when the safety factors are combined. The compounding of safety factors will result in an inflated load that cannot be relied upon for the remaining steps of the HVAC design process.