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I know this statement will cause an immediate uproar in the HVAC Community and it seems I am cutting off my nose to spite my face, because I make my living from Manual ‘J’. Everywhere on the Internet you will find information about the, “Must Have Manual ‘J’, for ALL furnace and air conditioner installations. This is one of those issues easier said than installed and sold. The Internet is the greatest source of information the world has ever seen, BUT everyone needs to keep in mind, not everything on the Internet is the gospel truth which should be believed unconditionally and general statements may not fit your particular situation.
ACCA Manual ‘J’ is the best, and most widely accepted method, for sizing Residential HVAC equipment. Is this true for the replacement, retrofit market? Maybe, but maybe it isn’t. I will explain the reasons I believe this may not be the case. I have over 35 years of experience in the HVAC field and the very consistent truth is, equipment is over-sized and duct systems are under-sized. If I had a nickel for every home I have visited where a 4 or 5 ton air conditioner was installed with 8” x 16” trunks, 12 to 14 - 4” x 10” supplies and 4 - 6” x 30” returns, in a 2500 sq.ft. home, I would be a rich man.
Manual ‘J’ requires accurate information concerning the construction components used to build the home. Accurately determining the insulation values in fully finished walls, correctly determining 15 year old window U-Values and SHGC, finding out what the actual insulation values are for a ceiling, or how about a vaulted ceiling/roof combination, is next to impossible without extensive research, effort and time. Even after you have done the research, put forth the time and effort, you will find older windows have no NFRC Ratings, the ceiling insulation has been extensively crushed, and the settling factor is unknown in the walls and floors. Even in the new construction industry, it has been proven the insulation values are not what the ratings state, installation practices are less than optimal and the overall effective R-Values are not what was documented, predicted, or expected.
Also keep in mind that 40% to 80% of the cooling loads are directly from solar gains through the windows. Changing window SHGC values from 0.35 to 0.45, can change the cooling loads of a home by more than 20%. These best guess values for the Manual ‘J’ could make the loads on a 3000 sq.ft home vary from 2 tons to 5 tons. So what do you do?
Is this the size you will put in? Most of the reputable contractors who went through all the efforts to provide the Manual ‘J’ would say, “This is the size you need and is what we are going to install for you”. Would they be correct and would you have a system which functions perfectly? MOST LIKELY NOT!
What went wrong with these methods? I did everything possible as an educated consumer to select only a Manual ‘J’ providing, reputable contractor? The answer is, your duct system is incapable of providing the equipment capacity the best guess Manual ‘J’ proved I needed. For air conditioning systems, airflow directly determines capacity. If the airflow is not being delivered to support the size of air conditioner installed, the capacity will be reduced, or even worse, the equipment will fail. This can lead to very short equipment life and hefty repair bills, not to mention excessive utility bills.
What are the choices? Either you upgrade the entire duct system to meet the Manual ‘J’ loads or you decrease the air conditioner size to meet the duct capacity. The most common retrofit will fit into the latter category, the air conditioner and furnace need to be sized to the existing duct system capacity to provide the most efficient operation. This practice will also greatly extend the life expectancy of the equipment.
These are the reasons I developed Right-Size to size air conditioners and furnaces for the retrofit/replacement market. Manual ‘J’ loads are easily manipulated when used to select equipment for existing homes. The data entered is consistently inaccurate and the end results will reflect this best guessed equipment sizing. I know of very few homeowners who would purchase equipment from any contractor if they were told they would have to make extensive changes to the ducts system, in their fully finished basement, to meet the Manual ‘J’ loads. It is extremely rare that a duct system in any home over 10 years old will have the duct system sized correct to the equipment currently installed. On the other hand, a good share of existing duct systems, with a few simple modifications, can achieve airflows to size equipment that will maintain the desired indoor temperatures for over 95% of the cooling or heating hours. This is not the ACCA Design recommendation of 1%, but for existing homes, it will typically be much more comfortable than it was with the existing grossly over-sized equipment.
TO Manual ‘J’ or To NOT Manual ‘J’