If you have been around this industry very long, you are familiar with rules of thumb. In the residential system design world, rules of thumb included X #of BTU per square foot or per cubic foot. Of course, the cubic foot rule was more accurate because it could take into account air changes! If you were in a hurry you could just use the 2-35 method of sizing. (2 windows down as you drove by the home to be designed at 35 mph) Easiest of all, you could assume the original designer did their job properly and simply replace the unit with a like sized one.
Those days are gone, or at least they should be! Today there are a plethora of software to help a contractor run the heat loss/gain calculations, make the proper equipment selection and run the duct design calculations. The millennial generation wouldn’t know any other way, as they grew up on computers. Yet even with today’s sophistication, you can have multiple designers come up with different calculations for the same structure. The reasons for that are no different than when running paper-based Manual J calculations in the old days. You remember the expression GIGO? Garbage in garbage out. The accuracy of the software is dependent on the user entering the correct data. Does that mitigate the value of computer-driven software? Absolutely not, as always it simply mandates that the designer enter the proper values.
In order to have proper operation, is it then simply sufficient to have a competent designer properly load the software with the correct calculations? Many contractors might say no, but in real life the answer is most often yes. What is the difference? Verification. After a properly designed system has been installed, it should be verified through airflow measurements, static pressure readings, duct loss calculations, BTU and other verification calculations. How often is this done? Typically never. By adding this one step, you can add real value for your customers while separating yourself from your competition in a way that you can charge for. Not everyone will be willing to pay for this, but by offering the service to your customers you set yourself apart as the right professional to do the work on their home!
For more information, see “Does Proper Design Guarantee HVAC System Performance” by David Richardson in the October 20, 2014 ACH&R news.Image courtesy of buildingdoctors.com