Fan Powered Terminal Units

Ever wondered how fan powered terminal units keep your building comfortable and efficient? In this video, we’ll break down exactly how FPTUs work — from their internal components to how they control airflow and temperature in both overhead and underfloor systems. We’ll cover series versus parallel configurations, how primary and return air mix, how they perform during winter conditions, and what drives the CFM needed to meet heating loads. Let’s get started.

What is an FPTU?

A Fan Powered Terminal Unit, or FPTU, is part of a variable air volume system that uses a small fan and mixing chamber to blend primary air from the air handler with return air from the plenum. This allows precise temperature and airflow control in individual zones. FPTUs are popular because they improve comfort, maintain proper ventilation, and efficiently provide heating and cooling right at the zone.

Key Components

Inside each unit, you’ll find several major components: a primary air damper with a flow sensor, a fan section—usually with an ECM motor—an induction opening for return air, and often a reheat coil, which can be either electric or hot-water. Some units also include sound insulation, filters, and a controller with temperature and flow sensors. All these elements work together to maintain the right air mix and temperature for your zone.

How Airflows Mix

Here’s how airflow mixing works. The terminal receives cool, dry air from the air handler. It blends that with return air from the ceiling plenum to temper the discharge air. When the space needs heating, the reheat coil adds warmth. The local fan ensures steady mixing and maintains airflow, especially at low primary air volumes.

Series vs Parallel FPTUs

There are two main types of FPTUs — series and parallel.

In a Series FPTU, the fan operates in series with the primary airstream. That means all supply air passes through the fan. The fan runs continuously during occupied hours, delivering a constant discharge volume even when primary airflow modulates. This provides stable ventilation and consistent diffuser throw, which is ideal for interior zones or spaces that need steady air movement.

In a Parallel FPTU, the fan is in a parallel path to the primary air. During cooling, the fan stays off—air flows directly from the duct to the space. When heating is needed, the fan turns on, drawing warmer plenum air across the reheat coil. The result is quieter cooling, lower energy use, and excellent perimeter heating control.”

Dedicated Outside Air Connection for Enhanced Ventilation

Some fan powered terminal units, such as the Titus TFS model with IAQ connection, can be equipped with a dedicated outside air opening to introduce conditioned ventilation air directly into the terminal unit. This design allows a controlled amount of outdoor air to mix with return and primary air at the zone level, helping meet stringent indoor air quality (IAQ) and ventilation code requirements such as ASHRAE Standard 62.1. The dedicated intake enables precise balancing of outdoor airflow and ensures each zone receives the required minimum ventilation CFM, even when the primary air volume is reduced during part-load conditions.

When and Why to Use FPTUs

So when should you use a fan powered terminal instead of a standard VAV box? Typically, it’s when the zone requires heating with limited primary airflow or stable ventilation. Standard VAV boxes can’t effectively provide heat at very low primary airflows. FPTUs can, because they induce warm plenum air and mix it with a small amount of primary air. They’re ideal for perimeter zones, spaces that need constant diffuser throw, or areas with stringent ventilation requirements.

Climate Considerations

Fan Powered Terminal Units are most common in colder climates, like the Northeast, Midwest, and Pacific Northwest, where buildings experience significant heating loads at their perimeters for much of the year. In these climates, perimeter zones lose heat through windows and walls, even while the core might still need cooling. FPTUs are the perfect solution — they pull warmer plenum air and add reheat to maintain comfort without overcooling.

In warmer climates, such as Southern California, Texas, or Florida, you’ll see far fewer FPTUs. Those regions use standard VAV boxes with reheat because perimeter heating is rarely needed beyond what the VAV box with the reheat coil can already provide. Climate drives design: cold regions lean heavily on parallel units for perimeter heating, while mixed climates may use series units for consistent ventilation.”

Winter Design and the Heating Formula

Let’s talk about what happens during peak winter conditions and how airflow relates to heating capacity. The heat delivered to a space is defined by the formula:

Q = 1.08 × CFM × ΔT or Q = (1.2 × L/s × ΔT)

where Q is in BTUs per hour, CFM is airflow, and ΔT is the temperature difference between discharge air and the space. During winter, your minimum ventilation airflow might not be enough to meet the heating load. For example, if you only have 200 CFM (94 L/s) at a 28°F (15.6°C) rise, you can deliver about 6,000 BTU/hr (1.76 kW) — not nearly enough for a perimeter zone needing 12,000 BTU/hr. (3.5 kW)

That’s where FPTUs come in. The fan draws additional warm plenum air, increasing the total discharge airflow to, say, 600 CFM (283 L/s). Using the same formula:

Q = 1.08 × 600 × 28 = 18,144 BTU/hr or (Q = 1.2 × 283 × 15.6 = 5.3 kW)

Now the terminal easily covers the heating load — without requiring extra primary air from the main air handler. This ability to decouple ventilation CFM from heating CFM is the key advantage of fan powered terminals in cold climates.”

Control Sequence of Operation

Control sequences for FPTUs follow a predictable pattern. In cooling mode, the primary damper modulates to maintain zone temperature. The fan stays on continuously for series units, or off for parallel units. In heating mode, series fans keep running while reheat engages. Parallel units start their fan only when the space temperature drops below setpoint. Building automation systems monitor minimum ventilation airflow, fan status, and reheat control to maintain comfort and indoor air quality.

Applications

In overhead VAV systems, parallel units work best for perimeter zones that require frequent heating. Series units are preferred in core zones where maintaining constant airflow and diffuser performance is critical. For UFAD—Underfloor Air Distribution—systems, fan powered terminals can be placed beneath the raised floor to locally mix and heat air near the perimeter.

Selection & Design Tips  

When selecting an FPTU, review manufacturer data carefully. Check airflow ranges, fan power, pressure drop, and coil performance. For quiet operation, use ECM motors and optional attenuators. Always specify a pressure-independent control damper with calibrated flow sensors and confirm your minimum ventilation CFM meets code requirements. Don’t forget about accessibility and orientation — parallel units must be installed level, and underfloor terminals need removable panels for service.

Commissioning & Common Pitfalls

During commissioning, verify primary airflow calibration, fan rotation, and coil operation. Trend zone temperature, primary airflow, and reheat valve position. Common mistakes include not maintaining minimum ventilation flow, short-cycling parallel fans, or overlooking acoustic treatments. Proper setup ensures efficiency and occupant comfort.

Fan powered terminal units play a vital role in modern HVAC systems—blending air, maintaining ventilation, and improving zone control. Whether you choose series or parallel, overhead or underfloor, or design for a cold or warm climate, understanding their operation helps you design smarter and troubleshoot faster.