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

Q&A: What is End Reflection and How Will Discharge Sound Ratings for Terminal Units Change in 2012

There is an acoustical phenomenon known as end reflection that is regularly encountered in HVAC systems. It occurs whenever air flowing in a duct reaches an outlet and suddenly expands to fill a room. Although it might not be obvious to the casual observer, sound doesn’t necessarily travel in the same direction as airflow. The greater the degree of expansion, the more sound is reflected away from the room.

An acoustician might say, “End reflection is the acoustic energy in an acoustic test duct that is prevented from entering the test space by the impedance mismatch created by the termination of the acoustic test duct”. In layman’s terms, whenever a rapid air expansion occurs, some portion of the sound energy generated by the supply device (i.e. terminal unit, fan system, air handler, room fan coil, etc) travels upstream back towards the source. This is end reflection.

All terminal unit manufacturers test their products in accordance with ASHRAE Standard 130 ‘Methods of Testing Air Terminal Units’. This standard provides testing procedures for both radiated and discharge sound. End reflection has been known to affect discharge sound readings for many years, so the standard was amended in 1994 to specify that discharge ducts in discharge sound tests must terminate flush to the inside wall of the test chamber. This was necessary because the further a discharge duct projects into the test chamber, the more end reflection occurs, effectively lowering the sound levels measured within the test chamber.

Test data measured in accordance with ASHRAE Standard 130 is used to produce catalog data in accordance with AHRI Standard 880 ‘Standard for Performance Rating of Air Terminals’. The latest version of this standard (880-2011) went into effect on January 1, 2012. It requires that manufacturers calculate the end reflection loss (ERL) and add it back to the rated discharge sound power levels of terminal unit products. A formula based on ASHRAE Research Project RP-1314 is used to calculate the ERL for the dimensions of the discharge duct used during the sound test. Although the calculated ERL is most accurately applied to 1/3 octave sound data, manufacturers may apply it to existing full octave sound data through 2014.

Here’s how the ERL is calculated:

First determine D_e, the equivalent duct diameter (ft). If the discharge duct is round, simply use the duct diameter. In the more likely situation that the discharge duct is rectangular, the equivalent duct diameter must be calculated as:

D_e = SQRT [(4 x A) / (144 x π)]

where:

A = cross sectional area of duct (in²)

So for a terminal unit with a 15 in by 12 in discharge duct:

D_e = SQRT [(4 x 180) / (144 x π)] = 1.26 ft²

Now

ERL = 10 log [1 + (0.7 x  C_o/π x f x D_e)²]

where:

C_o = Speed of sound in air (use 1128 fps)

f = Octave band center frequency (Hz)

D_e = Equivalent diameter of the duct (ft)

 

So the end reflection loss of a 15 in x 12 in discharge duct is:

2^nd Octave band (125 Hz) = 5 dB

3^rd Octave band (250 Hz) = 2 dB

4^th Octave band (500 Hz) = 1 dB

5^th Octave band (1000 Hz) = 0 dB

6^th Octave band (2000 Hz) = 0 dB

7^th Octave band (4000 Hz) = 0 dB

 

Adding this to the existing discharge sound levels of a fan-powered product would likely raise the NC level by 6 points. The smaller the discharge duct is, the greater the correction will be. Since research has shown that low frequency corrections tend to become overstated as duct sizes get very small, the maximum correction is limited to 14 dB.

 

So what does all this mean?

It means that every manufacturer will need to update all published terminal unit discharge sound performance data and selection software to meet the latest standards. Discharge sound levels will increase for all terminal units and smaller units will see the largest increases. The effect on large units could be negligible.

 

Will the actual discharge be higher?

No. The product will perform exactly as it did before, but now all of the sound energy will be properly accounted for. It would be fair to say that under the previous standard, discharge sound was in many cases being understated.

In order to change the certified performance listings posted on the AHRI website, all participating manufactures were required to resubmit all of their products to the program. It will probably be months before all of the changes are complete and posted on their new website. Although AHRI has agreed to publish a full page announcement in trade magazines to explain why these changes are necessary, it has not yet been sent out for membership approval.

Randy Zimmerman - Chief Engineer