The term fan powered induction unit—also known as fan powered terminal units with sensible coils, dedicated outdoor air systems, or fan powered terminal units—attempts to define a product that delivers sensible cooling to occupied zones. Like all HVAC equipment, there is a time and place for everything, and sensible cooling terminal units are no exception.
I, personally, see this product as an extension of sensible cooling in general. Starting with chilled slabs or ceilings that deliver relatively low capacity and moving up to active chilled beams with a four to one induction ratio conditioning typical office spaces, then fan powered units delivering higher capacities are for owners that may not be not what is going into their space.
This equipment can be purchased from most variable air volume (VAV) fan powered box manufacturers. The conventional series and parallel fan powered boxes are the framework for the sensible unit, with two differences noted from a manufacturer’s point of view: first, the primary air inlet typically delivers ventilation air and handles latent loads in the space. Second, a cooling coil is located on the units’ induced air inlet either with or without heating capabilities. Though these variances begin to explain a manufacturer’s point of view, they do not fully cover the range of issues found in the field during start-up of the equipment.
From an engineer’s point of view, there are 10 to 15 differences depending on how you count them. My two favorites are the following:
The first difference is out of the box ECM motor and fan turndown capability. The ECM motors and fans hit a floor based on the factory’s default programming and fan selection. We have reprogrammed a number of boxes to allow another 5-10 percent turndown in smaller rooms. Unfortunately, set points are not typically repeatable, since the ECM motors try to automatically adjust for static pressure fluctuations. As a result, the prefunctional commissioning may pass, only to fail later on during final testing, depending on the determination of the commissioning agent.
Building automation system controller is another key difference. This will more than likely be a turbo charged version of the standard VAV box controller—a custom programmable advanced application controller (AAD) in lieu of a VAV box application specific controller with minimal setup. Remember, that we have already added an ECM motor with modulation capabilities, a cooling coil, modulating control valve(s), and possibly humidistat and dew point calculations. None of those things reside with standard VAV equipment, which often surprises owners when they receive final bids. To better understand the control prices, owners should equate the system controls architecture to fan coils or small air handlers, versus VAV.
Basically, we are rediscovering the science behind the original floor mounted induction units sporting low primary airflows with sensible cooling coils in the space. This vintage equipment functioned with inaccurate pneumatic controls because a solid psychrometric analysis is like gravity—It works every time. The moral of the story is that fan powered induction unit manufacturers’ literature provides only part of the story, as the entire system still needs to be fully engineered to operate correctly with the central plant and building itself.
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