Tuesday, August 21, 2012

Q&A: Compare Series vs. Parallel Fan-Powered Terminal Units

There are two types of fan-powered terminal units - series and parallel. Every manufacturer offers both types and special variations such as low profile and quiet units. Although the type of unit to use is often up to personal preference, there are distinct differences:

Series fan-powered terminals have fans that must run throughout the occupied mode in order to deliver ventilation air to the zone:

  • These units act as boosters for the air handler because their fans move the air the rest of the way to the zone. This allows the air handler to run at system pressure far lower than other types of terminal units require. The typical system pressure supplying series fan boxes is 0.50 IN WG.

  • Since the fan runs continuously during occupied periods, they provide constant air motion and more air changes than other types of terminal units.

  • The continuous operation of the fan results in relatively constant sound levels, unlike other types of terminal units that vary air volumes and/or cycle fans.
Parallel fan-powered terminals have fans that only switch on during the heating mode to pull warm return air from the ceiling plenum:

  • Since the unit fan is off during the cooling mode, the box acts like a single duct VAV and simply varies airflow from the air handler to maintain room temperature. Typical system pressures are between 1.00 and 1.50 IN WG.

  • Some engineers do not specify parallel fan units because the fan cycling is often noticeable to occupants.

  • Parallel fan units must include a backdraft damper to prevent primary air from leaking back through the blower into the ceiling plenum. Leakage around the backdraft damper can be an issue and could be considerable when downstream pressure requirements are greater.
An ASHRAE research project (RP-1292) completed in 2007 was conducted to determine which type of fan-powered terminal used the least energy from a whole building perspective. The report said that either unit could be equally efficient when properly sized and applied. This original report only included units with standard PSC fan motors. A subsequent addendum to the report, paid for by a consortium of interested parties, took the newer ECM technology into account in the same energy model. It gave more of an advantage to the series fan units.

 Randy Zimmerman - Chief Engineer

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