When completing your site survey for Manual J Load Calculations, significant time can (or should) be spent identifying window values. Did you know that most windows these days come with an NFRC rating, a tag located on the window when new or in the window jam (sometimes) that you can reference on NFRC.org? This identification number can tell you important details like window coatings, u-value, light transmittance, air leakage, and solar heat gain coefficients (SHGC). Not identifying the correct values can sway your load calculation thousands of Btu's per hour in both directions, in heating and cooling. Of course, the SHGC only impacts the cooling load.
There are a few things that can influence the solar heat gain through a window or glass door. You can manage these gains by installing awnings and/or exterior shades, but the easiest way is to use windows with low SHGC values. Rated on a scale of zero to one, the lower the value than the less btu/hr gains from the suns rays make it into your home.
Using a leading load calculation software program, I was able to create a few scenarios to display the importance of spending the extra few moments at the customer's home during the sales process and site survey. When adding a 4'x6' bay style, facing south, vinyl frame, double-pane, and clear window, the cooling load was increased by a total btu/hr of 1,045. If spending the time to include the shading default of medium color blinds at 45 degrees, and an outdoor insect screen (50%), then the load of the window can be minimized to only 790 btu/hr.
If you spend the extra few moments to identify the NFRC rating of the window, you may be able to establish there is a low-e coating on this particular window. When including only the coating, not the blinds and insect screen, the cooling load of the window was minimized to just 688 bu/hr. You can see how missing this feature, multiplied by all of the windows in the home, can contribute to improperly oversizing equipment and possible IAQ issues like high humidity, etc. Also, this can also alter the system design process when selecting equipment and designing ductwork (ACCA Manuals S and D). Or, maybe the window faces another direction (East or West), and this creates a spike in the "adequate exposure diversity" that could otherwise require zoning.
On the bright side, no pun intended, you can see how installing blinds and insect screens could deflect the sun's rays just enough to possibly make an uncomfortable room bearable under high loads. When a homeowner is uncomfortable in particular rooms, it always turns into an HVAC Contractor's nightmare to appease them. Instead of blaming the airflow, ductwork, or even system sizing, maybe you should look to those wide open skylights and bay windows. Sure, I like the sun, but sometimes you have to compromise in order to feel "comfortable" when it is 100F and sunny outside.