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Control a dual duct VAV box with Xenta controllers

Issue

How can I control a dual duct VAV box with Xenta controllers?

Environment

Xenta 102-AX

Dual Duct heating/cooling

Cause

The Xenta 102-AX is an application specific controller designed to vary the volume and outlet temperature of a single terminal unit. It does not, by default, control a dual duct, where a constant source of cold air and hot air and delivered in separate ducts.

Resolution

You could conceivably use a single Xenta 102-AX with the airflow pickup on the discharge of the box instead of the inlet. Then connect the hot deck damper as a 2-out PWM heat stage.

A more elegant solution is to use a pair of Xenta 102-AX controllers. One 102-AX is connected to the cool deck and setup normally. Either its heating airflow setpoint is set to match the cooling low flow or heating airflow is set to 0 if you wish the cool deck to shut off in heat. The "Effective Setpoint Source" must be set to "Norm of Heating and Cooling Setpoints" so that the setpoint sent to the hot deck controller will not bounce from heating to cooling when room conditions change. For this reason, it might be preferable to use an STR-202 so that there is no digital display to confuse the occupant.

The second 102-AX is connected to the hot deck, space temp is bound from the first and nvoeffectivesp is bound from the first to nvisetpoint of the second. Configure input 1 as a duct temp and connect a 4.7Kohm resistor from it to ground. This will simulate a duct temp of 110°F and keep it in warmup mode. As with the cool deck, you have a choice whether to set the warmup low flow to 0 or to some minimum position. Set your occupied setpoints to the same values as the cool deck controller. (It's actually using them in the setpoint calculation, but for simplicity's sake, set them the same. For a more complete description, consult the reference manual concerning nviSetpoint.)

You should also bind nvoOccupancyStatus from the cool box to nviOccCmd on the hot deck box. That way if someone puts the box in to after hours override, they'll both operate.

As mentioned before, the room sensor (STR-200/202/250) is connected to the cooldeck controller. You'll need to connect 3 conductors to a 1/8"TRS jack to the hotdeck controller in order to connect the MSTAT for calibration purposes. Otherwise you'll have to crawl into the ceiling to connect it.

This arrangement is based on having airflow sensors on the inlets of both the cold and hot deck.

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