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 in the window jam 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 impacts the cooling load more heavily. 

  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 the contractor spent the extra few moments to identify the NFRC rating of the window, they may have been 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 over-sizing equipment and IAQ issues like high humidity, etc.  Also, this can change the system design process when selecting equipment and designing ductwork in the ACCA Manuals S and D.  Or, maybe the window faces another direction, 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.


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Comment by David Williams on June 21, 2012 at 8:34am

We're in the same ballpark but we might be on different teams. More likely due to location than ideology.  I also rely on thermal load calculations and am particularly vested in solar impacts in a heat dominant climate.  I have  modeled, designed and built  passive solar structures for about three decades.  The impact of solar gains will contribute a great deal to overall energy use (both ways depending on climate and design),  but I have found that there are very few tools that correctly model usable gains.  Several years of research and monitoring has shown that most tools give no heat contribution value to passive solar gains , which have offset nearly half of the required "heat" energy requirements of homes that we have monitored.  This is substantial and has done much to discourage the use of passive solar design.  Our climate, at a 65F base has 8543 HDD and 205 CDD.

Comment by Christopher Morin on June 11, 2012 at 4:13pm

Thanks for reading my blog Linda!  Sorry you feel so negative towards the Load Calculation process.  I firmly believe that  there is no substitute for measuring, but an educated guess on Infiltration and duct leakage is not exactly a WAG.  If the home turned out to be semi-loose instead of average, I doubt it would change the size of the equipment. Not to mention that where I live, and most of the country, requires a Manual J (v8) Load Calculation as part of the Building Code.  A process that was developed by the most successful HVAC contractors in the country, refined over the last few decades, and ANSI accredited - in my mind has real value!  If you are not using load calculation software to size equipment (Heat loss/gain), dare I ask your method(s)?

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