In the illustrated terminal reheat multiple...

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Question 1 of 2066636eee28f7522a1c51ebbb

In the illustrated terminal reheat multiple zone system, what statement represents the functioning of the supply air duct thermostats controlling the preheater steam flow and the cooling coil chilled water flow? Illustration RA-0042

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🔍 Key Concepts

• Relationship between preheater steam valve thermostat setpoint and cooling coil off-coil design temperature in a terminal reheat multiple-zone system • How system design avoids simultaneous heating and cooling in the main supply duct • Direction of action of the duct thermostats that modulate steam and chilled water valves in the illustration RA-0042

💭 Think About

• Look at the sequence of operation: as the supply-air temperature rises from cold toward warm, which coil (preheater or cooling coil) should be in control first, and which one should be fully closed before the other starts to open? • If the preheater is supposed to shut off before the cooling coil starts to open, should its thermostat setpoint be a little higher, the same, or a little lower than the desired off-coil temperature from the cooling coil? • Imagine the duct temperature hovering at the cooling coil design off-coil temperature. In that condition, would you want steam still flowing to the preheater, chilled water flowing to the cooling coil, both, or neither?

✅ Before You Answer

• Verify which duct thermostat symbol controls preheater steam and which controls chilled water to the cooling coil in the diagram. • Check whether the goal described for this system is to prevent simultaneous flows of steam and chilled water in the central unit (preheater and cooling coil) except under changeover or extreme conditions. • Confirm whether avoiding overlap means the preheater must be satisfied (steam valve closed) before the duct temperature reaches the cooling coil design off-coil temperature, or after it passes that point.

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