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Static Pressure at Rated Catalog Points


ARI Standard 880-98, “Air Terminals,” and ASHRAE Standard 130 1996, “Methods of Testing for Rating Ducted Air Terminal Units,” lay out the guidelines for testing VAV terminal units and generating catalog data for same. ARI 880 establishes the discharge air pressure requirements for the test. They can be found in Table 3, on page 9. Note 1 states, “All fan tests for radiated and discharge sound in fan powered terminals shall be run at 0.25 in. water [0.0623 kPa] discharge static pressure or at the minimum recommended discharge pressure, whichever is higher.” Since no manufacturer has a recommended discharge in excess of 0.25” WG, that is where all the data is run for all manufacturers’ catalogs.


Some manufacturers’ catalogs have the fan curves stopping at .15” to .2” WG discharge static pressure. They were all like that 3 years ago, except for Redd-i’s and Nailor’s. Nailor’s fan powered VAV catalog was produced nearly 5 years ago. It was the 3rd one to demonstrate fan design that included fan operation at 0 discharge static. Magna-Flow was the first and Redd-i was the second. All Nailor fan powered VAV terminal units are rated and designed to operate at 0 discharge static pressure. Most of our competitors’ units are not designed for this condition. Zero discharge static pressure puts extreme demands on the motors. Their rotational speeds decrease and their workloads increase. This causes high temperature generation at the motor cores. Most fractional horsepower motors cannot stand this operating condition. Consequently, discharge static has to be added to the unit to speed up the motor and lessen its workload. This, in turn, reduces its relative heat rise. This is why you often see series fan powered terminal units laid out with balancing dampers on the discharge side of the unit. Our competitors’ units will not work without them. It is important to note that this is one of the areas where our competitors are copying us. Titus has now redesigned their motors for 0 discharge static to meet our capabilities. It is interesting to note, however, that their turndown rations are still small enough that they sometimes need balancing dampers to reach low end performance. It is gratifying to note that Nailor does not need balancing dampers to improve our turndown ratios nor to keep our motors on line.


The obvious problem with balancing dampers on the discharge of the fan powered VAV terminal units is the noise that is subsequently generated, some by the higher operating speed of the fan and some by the damper. One of the big problems with discharge dampers on series units is that the dampers are generally overused and set much higher than necessary. This causes excessive noise and can completely disable the speed controller function – another reason not to select terminal units that need discharge balancing dampers.

Increased static above the 0.25” WG cataloged will increase the discharge noise generation. However, increasing to 0.5” WG will not increase it enough to be noticed in the field. You should estimate an increase of approximately 2 dB per octave band over the catalog data @ 0.5” WG discharge pressure.


For the quietest performance, select Nailor 35 SST units in the mid to lower fan curve range, keep your discharge duct velocity low, and do not lay out units with discharge dampers.

PRODUCT: 35S, 35SST, 37S, 37SST AND 35P

Source: Nailor Product Bulletin, July 11, 2000