HVAC Designing a Perimeter System

After the heat loss or heat gain has been calculated, the sum of these losses or gains will determine the size of the duct systems and the heating and cooling unit.

The three factors that ensure proper delivery and distribution of air within a room are location of outlet, type of outlet, and size of outlet. Supply outlets, if possible, should always be located to blanket every window and every outside wall (see Fig. 17-5). Thus, a register is recommended under each window.

The outlet selected should be a diffuser whose air pattern is fan shaped to blanket the exposed walls and windows.

The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) furnishes a chart with the locations and load factors needed for the climate of each major city in the United States. The chart should be followed carefully. The type of house, the construction materials, house location, room sizes, and exposure to sun and wind are
important factors. With such information, you can determine how much heat will be dissipated. You can also determine how much heat will be lost in a building. You can also determine how much cooling will be lost in a building. The ASHRAE Handbook of Fundamentals lists the information needed to compute the load factors.

Calculate the heat loss or heat gain of the room; divide this figure by the number of outlets to be installed. From this you can determine the Btu per hour required of each outlet. Refer to the performance data furnished by the manufacturer to determine the size the outlet should be. For residential application, the size selected should be large enough so that the Btu per hour capacity on the chart falls to the side where the quiet zone is indicated. There is still a minimum vertical throw of 6 ft where cooling is involved.

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