# Manual J Is A Building Science Engineering Calculation

Manual J is a building science related engineering calculation that helps us put value on the structure’s ability to resist the loss and gain of heat in your specific climate conditions. The information we input describes the elements of the design of the structure such as window energy efficiency, insulation values, orientation of the house, etc. and therefore the output of the calculations tells us the rate of loss and gain of heat measured in Btus per hour.
In the human body the rate of energy we burn is related to our metabolism. We need a certain amount of calories to keep up with the rate we burn energy or we gain unnecessary weight or lose it too fast (I should have such a problem). The right balance is in not just the bulk we eat but the energy content of the food we intake. Your Heating and Air Conditioning system is the same way. We measure the Btus of heat the house is losing or gaining and we find the right machine to remove or produce the right amount of heat for best efficiency and comfort.

The Manual S calculation is the part where we measure the calories (Btus) of the machine we put in to match the house’s rate of energy transfer.  All AC equipment is rated in tons (and Btus per hour). The nominal rating is the number of tons listed on the equipment label. The manufacturer produces what is called a detailed cooling performance chart for each combination of indoor and outdoor equipment that allows us to accurately determine the true performance capacity of the heating and cooling equipment for your local climate conditions. Believe it or not, Harrisburg Pennsylvania is the chosen reference city used by all manufacturers that report equipment performance and efficiency to the voluntary rating organization called AHRI. Where the average summer heat is higher than Harrisburg PA we must take away from the nominal rated capacity. If the humidity is higher than Harrisburg Pennsylvania then we must also take away from the equipment’s ability to remove humidity in the summer time. That is a crucial adjustment that most HVAC contractors don’t even know about. Getting this right will make all the difference in whether or not your heating cooling system will be the right match for your home or business.
Finally, we must assess the ductwork to make sure that the duct plan can carry the amount of air to heat and cool each area that is conditioned in your house. Most average ductwork is designed at about 30% less in capacity than is normally needed. That is due to erroneous myths and traditions that rule the residential HVAC industry. There is more on this on elsewhere on our Tradewinds Appropriate Technologies website www.tradewinds-at.com.

Bottom line:
When the building envelope is super insulated the HVAC game changes radically. If your house for example house has enough nominal tons installed to equal a ratio of 500 sq. ft. per ton, then it is most likely design based on tradition rather than good science. If the AC was properly designed and the insulation of the attic and upstairs sidewalls changed then it is imperative to reevaluate the design to make sure you have the right size system that can handle the right balance of heat AND humidity control in your local climate zone.

If the equipment is over-sized by more than about fifteen percent of the required design calculations then you are likely to experience improper humidity control, uneven room temperatures and equipment short cycling which leads to premature equipment failure.

So how do we get started?

My estimate will be based on the sq. footage of the house and I will provide a proposal in writing for your approval before commencing any work that will cost you anything. I will need the house plans, the window energy specs and, if a system is already  installed, the as built duct design plus the HVAC equipment listed. If the responsible parties involved for whatever reason cannot supply this,  a site visit may be necessary for an additional fee. That also will be quoted as needed in advance for your approval before you incur any cost.

Thank you for your interest in our services.

Scroll to Top