This graph illustrates the relationship between the temperature of the hot deck air and the outside air temperature. The hot deck becomes warmer as the outside air becomes cooler, at a fixed ratio, set in the hot deck controller. On drawing two you see that the control loop controlling the hot deck does not communicate with the space heating requirements. The loop simply produces the degree of heating required for the maximum anticipated heating requirement of the occupied space. There is no compensation for load variations in the occupied space. The graph also illustrates the mixed air temperature (cold deck temperature in the winter). On drawing two you see that control loop controlling the mixed air does not communicate with the space cooling requirements. The loop simply produces the degree of cooling required for the maximum anticipated cooling requirement of the occupied space. There is no compensation for load variations in the occupied space. This arrangement of ignoring the actual requirements of the zones tends to inject too much heat into the system. The system has to introduce extra cooling to compensate for the portion of the heat that is not actually required to address the heat loss. At a domestic level, this is similar to running your furnace 60% of the time while the building heat loss only requires the furnace to run 50%. The house would over-heat unless you either reduced the run time to 50% or opened your windows to compensate for the extra heat generated by the excessive heat production. You would keep your windows closed and reduce the furnace run time. Most multizone systems actually follow the other path, bringing in extra cooling to get rid of the excess heat. DAY NIGHT NIGHT NIGHT NIGHT DAY DAY DAY DAY NIGHT ORIGINAL DESIGN TEMPERATURE GRAPH FOR HOT DECK AND COLD DECK 55°F DESIGN MIXED AIR TEMPERATURE 8.85
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