Arkansas HVACR NewsMagazine September 2021

HVACR NewsMagazine September 2021

Tech News

justify installing the equipment I have on hand, or the equipment the homeowner wants, and we perform a “load on the zone. Then we manipulate the Manual J findings to make the space fit the equipment without regard to the total dwelling. The Manual J tells us how much seasonal gain or loss the entire home (room by room) will have in relation to the design criteria, construction methods, construction materials and the geographic location. Once a complete Manual J is performed, we analyze the data, calculate the loads on individual rooms, size the duct for the proper btu delivery in both sensible and latent categories. We then move to Manual S to select the equipment that performs to meet both sensible and latent loads. Manufacturers demonstrate capacity through performance information that adjusts for the region the system is installed. If one system is inadequate due to multiple levels, construction design or size, we choose multiple systems to meet each individual zone. If one zone fails to operate as necessary, the other zones will fail for efficiency and or comfort. This is where the “Goldilocks and the Three Bears” becomes obvious, as air flow has to be “Just Right” , and the evaporator must have the correct capacity to cool and dehumidify. If you remember the story of “Goldilocks”, most likely you are about to begin your Social Security withdrawals. Another forgotten or misplaced skill has been the understanding of air and its current condition or desired condition. There are seven properties and measurements of air. When we know two of the seven measurements, we know all that there is to know using a psychrometric chart. If we measure Dry

Air Flow Through the Evaporator Part Two of Five- Tom Turner

“Goldilocks and the Three Bears” may be an important resource for study.

In part one we discussed the issue of air flow through the furnace or air handler. We discussed the fact that a lot of engineering goes into being sure the correct amount of air flows at the proper speed to deliver mechanical means to push the air for cooling and heating . As contractors, install mangers and installation technicians, we have the power to allow the engineering to succeed or fail. Because we place an air handler or furnace between two points of duct, does not ensure the system will function as designed. It is our job to understand the challenges and assure the customer of an installed product that performs as its engineers designed it to. Design criteria also points to paying attention to the load on the home. May I repeat? “The load on the home”. This indicates a Manual J has been performed and applied. We must avoid the confusion between the “load on a home” and the “load on a zone”. A Manual J is for the entire home , regardless of the size of home or the zones included within the conditioned space.

Many contractors are under the impression a Manual J is the tool to