It is very important to size the heating and cooling unit appropriately. If the unit is too small, it will not condition the house fast enough, on the other hand, if it is too big, it will condition the house too fast without properly dehumidifying the room. Often contractors use “rules-of-thumb” to figure out the size of the heating unit, without considering some important variables such as, the internal heat gain and loss, moisture content, orientation of the house, ventilation and infiltration, resulting an overly-sized heating system. This issue is called “short cycling.” Undersized heating and cooling equipment is always running and will never satisfy the required temperature. The undersized equipment will not last as long as a properly sized system.

It is more cost-effective to size a heating unit according to the average winter or summer condition while deciding the outdoor design temperature, instead of preparing for the rare “worst-case-scenarios.” Residential Manual J8 methods is not appropriate for high rise, commercial and industrial buildings, a room with an indoor hot tub or swimming pool, and houses that use passive solar methods. 

In any ACCA Certified Manual J load calculation, the following items need to be addressed:

• Use the actual orientation of the house. Solar heat gain depends on the orientation.
• Get the exact construction material R-values
• Properly evaluate the infiltration load and required ventilation load following Manual J procedures.
• Use design conditions recommended by ASHRAE or Manual J. According to ACCA, the indoor dry bulb temperature should be 70oF for heating and 75oF for cooling. The indoor relative humidity (RH) for cooling should be 50% for wet climates and 45% for dry climates.
• Include curtains, drapes and blinds because they can change the r-value of the window. 
• Include overhangs - “By including the overhang benefit in the analysis, the projected cooling load can be reduced to a ton on a 2400 ft2 home” (Addendum A to ACCA Manual J).
• Include the people internal gain from a house, which is assumed to be the number of bedrooms plus one. 

Contractors can save huge amounts of money on costly mistakes when installing heating and cooling systems in a home by hiring a Certified Provider to do the calculations. There are many crucial benefits to the customer in hiring an ACCA Certified Manual J Load specialist: HVAC design experience, ACCA certified software experience, turnaround time, and customer service. WrightSoft Universal 7.0 software is one of the most recognized Load Calculation software programs for EnergyStar, building permit code compliance, LEED Certification, Build Green projects and energy efficient mortgages. 

Make sure your HVAC installer or Manual J Load Calculation provider is not using a cheap, non-ACCA certified software to produce your residential Manual J report or it could be rejected. 

The Air Conditioning Contractors of America (ACCA) Manual J Residential Load Calculation Procedure is the accepted industry standard for the proper sizing and selection of HVAC equipment in residential applications. Manual J outlines the accurate procedure which can be used to determine the heat loss and heat gain for conventional residential structures. Make sure your Load Calculation Reports are based on ACCA's Manual J 8th Edition, inclusive of all four addenda to the 8th edition. 

Lastly, huge price savings can be had in using an ACCA Certified Manual J Load Calculations provider in every home you build by choosing the right sized HVAC equipment. An ACCA Certified Manual J Load Calculation provider can help set you apart from your competition by guaranteeing your client will have the industry standard in home heating and cooling efficiency, keeping their costs and yours to a minimum.

Author Resource: www.load-calculations.com 
Savoy Engineering Group offers ACCA Certified Manual J8 Load Calculations, Manual S HVAC equipment selection and Manual D Duct Design services performed by Masters level engineer utilizing the ACCA Certified WrightSoft Universal software based on Manual J8. Savoy Engineering Group has been providing Manual J, S & D service for 5 years and has completed 3,000+ design projects. We are Fast, Accurate and Affordable!