Load Calculation

AC Supply is now offering residential load calculation HVAC design services to DFW area contractors. A properly performed Manual J, D and S will ensure that a HVAC system is sized correctly for every room of a home. With today’s technology and tighter building construction the old “rules of thumb” are outdated. Unfortunately, the Manual J design can be complex and time consuming.

“Implementing a quality HVAC system design is the foundation for a professional installation and year round comfort. Understanding the concepts behind load calculations, equipment selection, duct design and room distribution is essential to the design process” (ACCA).

That’s were we come in. We use only ACCA approved software from Wrightsoft for every load calculation.




A properly performed Manual J heat load calculation should be completed on every installation and retrofit and is now code in most municipalities. The Manual J will detail each room and living space with the proper cooling and heating requirements. An oversized system will have shorter run times along with humidity problems while a under sized system will not have the capacity to cool the house and run longer. Both situations will result in costing your client extra money. An ACCA certified Manual J load calculation will ensure that the air conditioner is sized properly for a home. 


The Manual S is used for equipment selection. The equipment selection process uses procedures to properly select the heating, air conditioning and heat pump equipment. This process looks at both the the sensible and latent loads of the structure found in the Manual J calculation and compares it to the performance ratings of the equipment. Equipment size should not exceed 115% of the total heat load and should satisfy both the sensible and latent heat requirements. The Manual S will impact both the total CFM of the system and the size of the duct work required. Because of this, equipment selection will need to be made prior to completing the Manual D duct design.


Manual D is the proper sizing of the duct system. Once the Manual J room details and Manual S equipment sizing have been completed, we can begin to design the duct system for your project. If the duct system is not properly sized, the air conditioning system will most likely not perform as well as it should, have rooms that are too hot or to cold, excessive wear and tear on the HVAC system while costing more to operate. It is important to have the equipment selection made prior to performing the Manual D calculations because each manufacturer’s equipment is different which can impact the supply and return duct work size.


A block load looks at the entire structure as a whole to size the HVAC equipment. This is the minimum that should be performed when replacing a heating and air conditioning system. A block load calculation does not have a room by room breakdown, Manual S or D.