Are you still using rules of thumb to size HVAC equipment? This one size fits all approach can get you in trouble with your clients and code officials. With a variety of climates, construction types and building materials to account for, there is no accurate way to confidently determine the HVAC load without completing a Manual-J load calculation. This process does not have to be difficult, but without the proper tools and training it can be daunting.

You can complete load calculations by hand, using long detailed worksheets involving lots of long formulas and math, or you can use powerful software designed to help you quickly and efficiently create accurate calculations. One example of the way that using Manual-J software (Wrightsoft Rightsuite Universal and Elite RHVAC) allows for the input of unique project variables is in the way that it accounts for the building envelope.

Manual-J Load Types

Load calculations with Manual-J take into account all the variables that will affect the need for heating and cooling the structure. The seven different load types that are factored in include:

• Envelope
• Design
• Block and peak loads
• Room
• Duct
• Ventilation
• Summer dehumidification

Of these seven, the most significant factor in determining the load on most houses will be the envelope load. Its also one of the easiest to understand and quantify. The envelope load is the sum of the building component loads which include the ceiling, floor, walls, fenestration, and infiltration.

In addition, sensible (dry-bulb temperature) and latent loads (moisture) come into play as the envelope is the first line of defense against the outdoor environment. Lastly, for cooling purposes it also includes internal loads (occupants/appliances).

Consider these two houses at right. One is made up almost entirely of glass and will have significant  fenestration heat gain and loss. While the other more traditional craftsman plan will have a much different envelope load profile with mostly insulated opaque panels.  These two houses illustrate how much the envelope can differ and how using a rule of thumb just isn’t going to cut it.

The difference between glass and solid wall is an obvious example. But you also have to account for all the minor changes in building practice and materials. For instance, do you think a house with Zip wall sheathing and taped seams will have the same amount of air infiltration as a house with OSB and housewrap. Not likely. Adjusting a rule of thumb to meet these differences is guessing at best.

Envelope Loads in Manual-J Software

Manual-J load calculations using either Wrightsoft Rightsuite Universal or Elite RHVAC allow you to account for these variations by taking into account climate and the total resistance to heat flow through the building envelope. The inputs in the  Manual-J software range from simple and straightforward stock building material libraries to direct inputs of u-values that you’ve calculated on your own. Our Manual-walks you through exactly how to account for the envelope to ensure an accurate load calculation that will make your clients and code officials happy.

Most Manual-J training course available today will provide you with either a building science primer and review of the book, or conversely, will provide you with just an overview of a software package. Most of these coming in one or two day live events with little time for comprehending the material. Our Manual-J Online Training is different because we give you the total package in a format that allows for in-depth comprehension. We provide an in-depth building science primer, tips and tricks from years of experience with completing load calculations, a step-by-step guide to using the software, and the opportunity to perfect the Manual-J through practice exercises.  

To register for our online Manual-J training, click on your software preference here:
Wrightsoft Rightsuite Universal or Elite RHVAC