What size AC do I need? The answer from Manual J

Residential Equipment is typically expressed in tons where 1 ton = 12000 btus.  The most common sizes being 2, 2.5, 3, 3.5, 4 and 5 Ton units.  With the rise of variable speed units and mini split systems you can find many variations of systems from 7000 but up to 5 tons.  So what size does your ac need in the midst of a Texas summer?  I know there are many who will say 1 ton to every 500 sq ft; however, that rule can get you into trouble with modern construction.  An improperly sized air conditioner not only costs you money in efficiency but it leaves humidity in your home leading to numerous other issues.  1000 sq feet with lots of  windows and a black shingle roof will need much more cooling than 1000 square feet with minimal windows and a white metal roof even if everything else is the same.  So if not by square feet, how do I find the proper size for my ac?

Manual J is best described as a calculation method used on residential homes to tell Hvac contractors what the heating and cooling load of a building is.  It is a detailed method that takes into account the type of construction, insulation, windows, direction of the home, the climate the home is in, the type of appliances in the home, and most everything else you can think of that will cause your home to gain or lose heat.  In energy code it is required by 42 states to be used in the design of any residential hvac system, many of the remaining 8 states have counties, cities and municipalities that require it.  It is the only ACCA approved method of heat load calculation.

If. you are in needing one on an existing home and live in our service area just call us at 817 864 1412 and we will set up an appointment to come gather the information that we need to run the calculations.  If you are building new or live outside of our area we will need you to email us at wacey@brazosriverac.com a layout or set of plans as well as provide information on construction such as insulation, window values, etc...

We supply many other contractors and homeowners Manual J, D, S and are happy to help more!  

Our local service area includes the cities of Granbury, Tx; Glen Rose, TX; Weatherford, TX; Stephenville, TX; Lipan, TX and surrounding areas.

Once you have performed a manual J you are going to be staring at loads and worksheets with but values and very little explanation.  The manual J will give you a suggested size for equipment but actual equipment selection can become a very different animal.  Remember not all equipment is made equal.  For instance a Trane 4 ton unit is not really 48000 btus it will be slightly over or under depending on matchup, and a carrier 4 ton will also be different.  After you take into account the actual numbers on the equipment you need to take your total btus and multiply by a sensible heat ratio to find the required size for cooling/heating at peak design conditions.  Once you have that number you have to consider the airflow needed to get the btus distributed to the different rooms and finally the type of equipment that will accommodate the different load swings that will come with the changing conditions.

In short you do the Manual J not to finish your system design but to give you a solid foundation to build upon with the Manual D and Manual S calculations.