A50-7138ef, Duct Static Pressure Probe

acceptable system static pressure. If the existing duct system is too small, larger ductwork must be installed.

The duct system and diffusers should be sized to handle the design airflow volumes quietly. To maximize sound attenuation of the unit's blower(s), the supply and return air plenums should be insulated for a length of at least 15 ft from the unit. Direct line of sight from return air grilles into the unit's return should be avoided. If return air is to be ducted to an equipment room, an elbow should be installed within the equipment room. Running a return air drop to near the floor of the room will aid in sound attenuation. Avoid transmitting vibrations generated by the movement of air in the ducting to the walls of the building. This is especially important where ductwork penetrates walls. The maximum recommended return air velocity is 1,000 fpm. Lower return air velocities will result in lower sound power levels. The use of round supply duct plenums should be considered, as it will significantly reduce low frequency sound at the equipment room. If rectangular supply plenums are used, the aspect ratio of the duct should be kept as small as possible (i.e., as close to square as possible). The large, flat surface areas associated with large aspect ratio duct systems will transmit sound to the space, and the potential for duct-generated noise is increased. The max- imum recommended supply air duct velocity is 2,000 fpm.

Units with two fans should have a properly designed “pair of pants” duct connection. An adequate straight length of ducting from the unit should be allowed before elbows are installed. If connecting an elbow directly to the fan outlet, a minimum straight length of 2 fan diameters from the fan outlet is recommended. Elbows should turn in the direction of fan ro- tation, if possible. Abrupt turns will generate air turbulence and excessive noise. Turning vanes should be used in all short radi- us bends. Ensure that ducting does not obstruct access to the unit for routine servicing.

DUCT STATIC PRESSURE PROBE AND TUBING (VAV Only) — On VAV systems, the duct static pressure sensor and tubing are field-mounted. The sensor tubing sensing point should be located near the end of the main supply trunk duct in a position free from turbulence effects and at least 10 duct di- ameters downstream and 4 duct diameters upstream from any major transitions or branch take-offs. Incorrectly placing the sensing point could result in improper operation of the entire VAV system.

Install the factory-supplied duct static pressure probe with the tip facing into the airflow. See Fig. 16.

AIRFLOW

PROBE

a50-7138ef

TUBING

Fig. 16 — Duct Static Pressure Probe

(P/N 39EK20462)

Use 1/4-in. OD approved polyethylene tubing for up to

50 ft (3/8-in. OD for 50 to 100 ft) to connect the probe to the bulkhead fitting mounted above the unit display panel (Fig. 17). Carefully route the tubing from the probe to this bulkhead fitting.

The static pressure control should be adjusted so that, at full airflow, all of the remote VAV terminal boxes receive the minimum static pressure required plus any downstream resis- tance. Control the system to the lowest static pressure set point that will satisfy airflow requirements. Lower static pressure set points reduce total required brake horsepower and reduce generated sound levels.

DUCT HIGH-STATIC (DHS) LIMIT SWITCH (VAV Only) — The duct high static limit switch is a mechanical safety that prevents duct overpressurization. The switch is lo- cated on the side of the VAV low voltage control panel (Fig. 18) and is factory set at 3 in. wg. To make an adjustment using an accurate differential pressure gage, connect low side and high side to gage and pressure source. Place a voltmeter across common and normally open contacts. Rotate the adjust- ment knob (Fig. 19) clockwise to increase pressure setting and counterclockwise to decrease pressure setting. When the bot- tom of the adjustment knob is approximately 1/8-in. from the switch body, the switch will trip at approximately 3 in. wg.

IMPORTANT: Use tubing that complies with local codes. Improper location or installation of the supply duct pres- sure tubing will result in unsatisfactory unit operation and poor performance.

DUCT STATIC

PRESSURE

PROBE

BULKHEAD

FITTING

	EXPN
	STAT
	EDIT
WARNING	ALARM

1 2 3

SRVC

TEST

ALRM

LOCAL	SET	SCHD
OFF
REMOTE

4 5 6

7 8 9

– 0 .

HIST ALGO

CLEAR

ENTER

WARNINGALARM

a50-7267ef

LOCAL

OFF

REMOTE

Fig. 17 — Display Panel Location on Unit Front Panel