Manual J Calculation is important when choosing the right HVAC

Ask the Contractor

We are in the market for a new HVAC system. Our current system is 18 years old and is a 10 seer. I am well aware of the change in efficiency and improvement of HVAC units and it is important to have a high-efficiency system installed. In meeting with HVAC contractors, 2 companies automatically suggested that we install a 15 seer rated system, without any sort of review of our home. The 3rd company suggested that a Manual J calculation be performed to determine our heating/cooling needs. We would like to hear from you as to what you think we should do. Tom and Sally, Cottonwood

A Manual J is the short industry name for Heat Load Calculation or Residential Load Calculation and is performed in order to determine how much cooling and heating a home needs to stay warm in the winter and cool in the summer. If anyone is considering the purchase of a new HVAC system, your contractor SHOULD perform a Manual J load calculation for your home without hesitation. A Manual J will indicate the properly size unit that is required for your home.

It has been suggested by two contractors, that you should install a 15 seer rated unit. Would you buy a pair of size 12 shoes because they were the coolest grooviest, hippest shoes ever and although your standard shoe size is 9.5, you are buying these larger shoes because someday your feet might grow? Most homes because a Manual J is not performed or the homeowner thinks the higher the Seer rating is more beneficial are oversized because a Manual J was not performed. A larger system is not better and will not match the task at hand and can cost you more money to operate. A larger than necessary system means that you are paying more to operate larger fans, larger compressors, a larger system will not usually run long enough to reduce the humidity in the home and moisture problems can occur and the larger than necessary system will frequently turn on and off and a larger system is more expensive to install. Efficiency, comfort and air quality are jeopardized with a larger than necessary system.

A Manual J is performed with a computer program. All of the homes relevant data is entered into the software program such as the square footage of the home, the insulation, window types, orientation of the home and other information such as where the home might gain or lose heat. After entering this information, the Manual J report provides how much cooling and heating the home needs in Btu per hour for each room, zone and the entire home. The heat load calculation will tell you how much heat is required on the coolest day of the year in the middle of the night. The cooling load indicates the amount of heat that can be removed on the warmest day of the year. And then there is the latent cooling load which indicates the amount of moisture that the system should be able to remove and all three of these calculations will indicate the size of the equipment that should be installed in your home.

Load calculations take time, detail and the contractor that performs a Manual J and does not rely on rule of thumb is a good guy. Keep in mind just because your neighbor has a 15 seer unit, does not mean your home needs one. Every house is different and how the home is situation on the lot and window layout can drastically influence the heating and cooling load.

If considering an HVAC replacement, it is also good to know that with a Manual J there are two different types of load calculations; Whole House or Block or Room-By-Room calculations. A Whole House calculation is based on your home being one entire large room and this is done when there may not be a necessity to modify the existing duct work and when you might be looking at replacing apples to apples system. The Room-by-room calculation provides calculations for every room and the amount of heating and cooling required for each room and this method is important if you are considering duct work modifications. This is a good method to use to balance the airflow to each room after a system has been installed, where if the Whole House method was used the balance could not be achieved.

It is important to remember there is no rule of thumb when considering the installation of a new HVAC system. A rule of thumb approach can yield an improperly sized system, higher energy bills, uncomfortable living conditions and more service calls. If you are building a new home, due to the ever growing presence of new improved building materials, advanced insulation methods, new improved ventilation systems, make sure your builder shows you the Manual J that was performed on your home and that your system was sized correctly.

Remember to tune in to YCCA's Hammer Time twice each weekend Saturday and Sunday morning 7:00 am on KQNA 1130 am/99.9 fm or the web Listen to Sandy and Mike talk about the construction industry and meet your local community partners and contractors.

If you have a question about a home improvement project or need a specific contractor or want to know about hiring licensed contractors, Call Sandy, Executive Director at Yavapai County Contractors Association - 928-778-0040. Don't Start Without YCCA!


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