Forced Air

Forced air is the side of the building that moves conditioned air. An air handler pulls in return air, mixes in outdoor air, drives it through filters and coils, and a fan pushes it out through ductwork to the zones. Every station on that path is something a control system measures, sizes, or holds at setpoint — mixed-air temperature, , the outdoor-air fraction, coil capacity, fan speed. These pages walk the air side end to end, from the air handler out to the VAV box, and give you the tools that put a number on each station.

New to it? Take the lessons below in the order they're listed — they follow the air from the air handler out to the terminal boxes, commission it to its design flows, and meet the dedicated-outdoor-air unit that conditions ventilation air on its own. Already have a unit in front of you? Jump to the tools: size a duct, average a traverse, work the minimum outdoor air, or check a coil. Everything here is open, needs no login, and tracks nothing.

Start here — the lessons, in order

1 Air Handlers → 2 Economizers → 3 Building Pressure → 4 Unit ID → 5 VAV Systems → 6 Duct Static → 7 Air Balancing → 8 Dedicated Outdoor Air

LESSON :: AIR HANDLERS READ

Air Handlers

The air path through an air handler, station by station — return, mixing box, filters, heating and cooling coils, supply fan — and the job each one does. The chapter's starting point.

Step 1Air PathMixing → Coils → Fan

LESSON :: ECONOMIZERS READ

Economizers

When an air handler should cool with outdoor air instead of the compressor — the changeover logic, the damper sequence, and the mixed-air target that free cooling chases.

Step 2Free CoolingDamper Sequence

LESSON :: BUILDING PRESSURE READ

Building Pressure

Why a building goes positive or negative — supply vs return vs exhaust, the relief path, and how the controls hold a building near neutral without the doors fighting you.

Step 3Supply / Return / ReliefNeutral

LESSON :: UNIT ID READ

Unit Identification

Naming the box you're standing in front of — RTU vs AHU vs fan coil vs makeup air, single-zone vs VAV — and what the nameplate and the duct layout tell you about it.

Step 4RTU / AHU / MAUNameplate

LESSON :: VAV SYSTEMS READ

VAV Systems

One air handler, many zones that never agree — VAV boxes, the pressure-independent loop, reheat, and how the air handler holds duct static while thirty dampers move.

Step 5VAV BoxesPressure-Independent

LESSON :: DUCT STATIC READ

Duct Static Control

Holding supply-duct static pressure as the boxes open and close — where the sensor goes, the fan-speed (VFD) loop that chases setpoint, and the static reset that saves fan energy.

Step 6Static SetpointFan Reset

LESSON :: AIR BALANCING READ

Air Balancing

Commissioning the air side to design flow — verifying box CFM against a traverse, setting box min, max and reheat, proportional balancing against the index zone, and holding the building slightly positive.

Step 7Flow Rings · K√VPProportional

LESSON :: DOAS READ

Dedicated Outdoor Air

The other way to bring in fresh air — a unit dedicated to 100% outdoor air that decouples ventilation and latent load from sensible, drives its coil deep to a dew-point setpoint, reheats to neutral, and leaves the space to the terminals.

Step 8Latent / SensibleDeep Coil · Reheat

Build one — the simulator

Interactive models for the air side — a live graphic with the function-block program that runs it one tab away, so the sequences, sensor faults, and stale overrides from the lessons move in front of you instead of sitting on the page. The fan coil is the smaller machine to start on.

SIM :: AHU WORKBENCH RUN

DDC Workbench — Air Handler

A single-zone air handler as a live DDC graphic — economizer dampers, hot-water coil, two DX stages — with the function-block program that runs it one tab away. Force sensors, override points through a real priority array, and watch the screen tell on the machine.

Live DDC GraphicEconomizerPriority ArrayWiresheet

SIM :: FCU WORKBENCH RUN

DDC Workbench — Fan Coil

A DX fan coil on the same workbench — a smaller machine that leaves the whole control chain readable. Load a sample sequence or wire your own from a blank sheet, then stage cooling, break the fan proof, and chase the fault from the graphic into the logic.

Live DDC GraphicDX StagingWire Your OwnFault Presets

The tools

Thirteen field utilities for the air side — measure what a duct is moving, size ducts and coils, work the ventilation math, and predict what a speed change does to a fan. No login, nothing to install.