Form #: SIDOM.4 Page | 9 Revised: 03/19
Controls Setup
General
The LMHS single duct VAV terminal is designed to supply a varying quantity of cold primary air to a
space in response to thermostat demand. Some units have reheat options to meet heating demand
requirements as well. Most VAV terminals are equipped with pressure compensating controls to
regulate the response to the thermostat independent of the pressure in the supply ductwork. To
balance the unit it is necessary to set both the maximum and minimum set points of the controller.
Each control option has specific procedures for balancing the unit.
Set Points
Maximum and minimum airflow set points are job and unit specific. Where maximum and minimum
airflow levels are not specified on the order, default values are noted on unit label.
Field Adjustment of Minimum and Maximum Airflow Set Points
Each unit is equipped with an amplifying inlet airflow sensor that measures a differential pressure
proportional to the airflow. The relationship between the inlet airflow pressures and CFM is shown
in Figure 6 –Krueger Inlet Airflow Sensor Chart (pg. 11). This chart is also attached to each unit.
The controls on most new projects are Direct Digital Controls (DDC). These controls require that
flow parameters be loaded during start-up to translate the sensed pressure into a measured flow
rate. There are several conventions in use for representing this flow factor; however, there is no
universally accepted method.
1.) Magnification Factor - The magnification factor may be expressed as the ratio of either
velocity or pressure of the output of the sensor to that of a pitot tube.
For example, a velocity magnification may be used. All Krueger probes develop an
average signal of 1” WG @2625 fpm. This gives a velocity magnification of
4005/2625, or 1.52.
The magnification factor may be a pressure factor. In this case, the ratio of
pressures at a given air velocity is presented. For a velocity constant of 2626, at
1000 fpm, this is 0.1451 / 0.0623 = 2.33.
2.) K-Factor: The ‘K-factor’ may be represented in two ways –
It may be a velocity K-factor, which is the velocity factor independent of the inlet
area (2625 fpm/in. WG for all Krueger inlet airflow sensors, Linear and LineaCross).
Alternatively, it may be the airflow K-Factor, which is the product of the velocity
factor and the inlet area. For an 8 inch Krueger unit this would be 2625 * 0.349, or
916. A separate factor is required for each size. The following Table 1 contains the
K-Factors for all Krueger VAV terminal inlets.
CFM=K√∆P
i. CFM= ft.3minute
⁄
ii. ∆P= Pressure Differential ("WG)
iii. K=Sensor Constant
