Why on earth are we doing a Manual J or a Manual D or a Manual S Anyway?

Many local codes now require Manual J and Manual S load calculations for every new home and remodel that changes the load of the house. The fact is, very few contractors know how to properly execute either load calculation. Although advances in building insulation are rapidly changing the profile of the average American home, the typical HVAC contractor still selects an air conditioning system for a home based on the old rule of thumb of 500 sq. ft. per ton. Unfortunately, most contractors are afraid to change this long habit of ‘guesstimating’. This ‘fear factor’ holds many contractors in design paralysis. In a new home, this practice can have disastrous consequences. As the building envelope is made more efficient and the effective R-value is dramatically increased by the use of spray foam and other high efficiency insulation measures such as SIPs, ICF and dense pack cellulose, the size of the air conditioning equipment should logically decrease. A well-constructed high-efficiency building envelope can and does change the rules. Our years of experience indicate that when properly assembled, a super efficient home can be cooled with an A/C unit as little as half the size of one used to cool a conventional home. Consequently, many HVAC contractors mistakenly install oversized equipment in high efficiency homes. We’ve received phone calls from building contractors asking us to do Manual J load calculations for them. Some of their customers are complaining that it’s “raining in their house.” This is clearly not the fault of the insulation application. It is the responsibility of the HVAC contractor to properly match equipment to the heat load of the house, design in the necessary dehumidification functions and to provide adequate mechanical ventilation for today’s tighter, more efficient homes. With our nearly 2 decades of HVAC design and installation experience, we can guide the HVAC contractor involved with your projects to the safe efficient solution.

More than Manual J designers, we are masters of Manual S

Over-sizing is a very real problem, but it isn’t the only problem. Design professionals must also understand the principles of Manual S regarding equipment selection. It is an all too common mistake when completing a load calculation alone to miscalculate the equipment size. Many designers attempt to take the results of a Manual J calculation and erroneously assume that the heat load estimate for the house is the recommended equipment size. They further compound the error by assuming that the nominal capacity of HVAC equipment is equal to its actual installed output.

Bull's Eye

To use an analogy, the results of a Manual J load calculation are like a target with a big red bull’s-eye on it. The Manual S procedure helps us to select the proper bow and matched arrow that enable us to hit the bull’s eye every time. Tradewinds has mastered the art of properly sizing equipment for any location, nation-wide, by considering local ambient design temperatures and making the correct adjustments to equipment cooling capacities so that a perfect match is selected every time. Custom load calculations conducted by an experienced third party like Tradewinds’ Manual J Design Studio will equip you with a design that will provide a comfortable, energy efficient living experience.