Air Handlers Air Systems
Every forced-air system in a commercial building — whatever the nameplate says — is built around the same move: pull air out of the space, refresh a slice of it with outdoor air, clean it, heat or cool it, and push it back. The box that does all of that is the air handler. This page follows one parcel of air through that box, station by station, because once you can walk the path, every AHU graphic, every mixed-air alarm, and every "why is my supply temp weird" call gets easier. One question, start to finish: what happens to air as it passes through an air handler?
Standing in front of a unit and not sure what to call it — AHU, RTU, makeup-air unit, fan coil? Telling the family apart is its own question, and it's getting its own page. Everything below is the anatomy they all share, drawn as a generic built-up air handler — learn the path here and you'll recognize it inside every one of them.
The Air Path, End to End
Here's the whole answer in one picture. Air leaves the space through return grilles and rides the return duct back to the unit. Just before the unit, some of it is thrown away — pushed out the exhaust / relief opening — and the rest drops into the mixing box, where fresh outside air joins it. The blend — mixed air — passes through the filter, then across the heating coil and cooling coil where its temperature (and on a wet cooling coil, its moisture) is changed, and the supply fan pushes the finished product — supply air — back out to the space. Then it does it again, all day, every day.
Seen a rooftop unit? · same drawing
A packaged RTU is this exact diagram folded into a weatherproof box on the roof. The stations are all there — mixing dampers behind the intake hood, filter rack, coils, supply fan — just smaller and pre-assembled at the factory. The one real difference: instead of hot- and chilled-water coils fed by a plant, an RTU usually carries a gas heat exchanger and a DX cooling coil — the evaporator of its own onboard refrigerant cycle, with the compressor and condenser sitting in the same cabinet. Walk an RTU and a built-up AHU in the same afternoon and you'll find them in the same order.
Where Three Ducts Meet: the Mixing Box
The first station is the one that confuses people, because it's the only one where air is traded rather than treated. Three dampers live here: the return-air damper (how much of the building's air gets reused), the outside-air damper (how much fresh air comes in), and the exhaust / relief damper (how much leaves the building to make room for it). Why bring outside air in at all? Not for temperature — for people. Occupied buildings need a steady dose of fresh air for ventilation, and codes set the minimum. And what comes in must go back out: if the unit pulls in outside air but nothing leaves, the building inflates like a slow balloon — doors whistle, lobby doors stand open on their own. Where that relief air goes, and what happens when the accounting is wrong, is a page of its own coming in this chapter; for now, know the trade happens here.
On a plain unit at minimum ventilation, the arrangement is simple: the OA damper is held at its minimum position — enough opening for code ventilation, no more — the RA damper is mostly open, and the relief damper is cracked to match. The three are linked and move together. Modulating them past minimum to cool the building with free outdoor air is the economizer sequence — a genuinely separate decision with its own logic and its own failure modes, and it gets its own page next in this chapter. Everything on this page holds the dampers at minimum.
Worked example · mixed-air temperature
The blend obeys a straight weighted average. Say the return air comes back at 75 °F, it's a 35 °F morning outside, and the dampers are at a 20 % minimum. Then 80 % of the mix is return and 20 % is outdoor: in °F, 0.8 × 75 + 0.2 × 35 = 60 + 7 = 67 °F mixed air (in °C: 0.8 × 23.9 + 0.2 × 1.7 ≈ 19.5). That's the number the MA-T sensor should read — and when it doesn't, the mismatch is telling you where the dampers actually are, whatever the BMS command says. The temperature half of mixing is that simple; the moisture half is where it gets interesting, and Psychrometrics Basics covers it — or blend real streams in the Air-Mixing Calculator.
Filter, Then Coils
Out of the mixing box, the blend hits the filter first — always first, because everything downstream is expensive. A coil is a dense stack of thin fins spaced tight; load it with dust and its heat transfer collapses while its pressure drop climbs. The filter takes that hit instead, and it announces its own condition while doing it: as the media loads up, the pressure drop across the filter rises. That's why there's a ΔP switch or sensor straddling the rack — it's the BMS point behind every "filter alarm," and it's a number worth trusting more than the calendar.
Then the coils — the stations that actually change the air. The heating coil raises the temperature and does nothing else; the moisture in the air rides through untouched. The cooling coil is the two-job station: it drops the temperature, and when the fin surface runs colder than the air's dew point, water condenses out of the airstream onto the fins — that's why there's a drain pan and condensate line under it, and why a plugged one shows up as a ceiling stain two floors down. A common design target off the cooling coil is around 55 °F. And here's the tie back to the water side: on a built-up unit these coils are hydronic loads — the same two-way valve, same supply and return piping you met in Load Piping, just wrapped around air instead of sitting in a mechanical room. Size one against a real load in the Coil-Sizing Calculator.
The Supply Fan: What Actually Moves the Air
Everything upstream conditions the air; none of it moves the air. The supply fan is the only mover in the box, and in the arrangement drawn here — draw-through, the most common — it sits downstream of the coils, pulling air across them and pushing it into the supply duct. Two things worth filing away. First, the fan adds a little heat of its own: the motor's work ends up in the airstream, so discharge air runs about 1 °F warmer than the air leaving the coil — which is why the DA-T (discharge-air temperature) sensor never quite agrees with the coil math. Second, on any modern unit that fan rides a VFD, so its speed can follow the building's needs. How the unit decides how fast to run — the duct-static-pressure loop — is its own control story, and it closes this chapter as its own page.
That's the whole walk: return in, some traded for fresh at the mixing box, filtered, heated or cooled (and maybe dried) at the coils, pushed back out by the fan. Every question the rest of this chapter answers hangs off one of these stations — when the dampers should open past minimum (economizers), where the relief air goes (building pressure), what the boxes at the far end of the supply duct do (VAV systems), and how the fan knows how fast to run (duct static). This page is the map; the rest are the territories.
Walk the Unit with a Probe
Those stations aren't just anatomy — they're literally the points on the BMS graphic: RA-T, OA-T, MA-T, DA-T, in air-path order. Reading them left to right IS walking the unit. Try it below: set the outdoor temperature, pick what the coils are doing, and watch each station transform the air. The dampers stay pinned at a 20 % minimum, like the mixing-box section above — and notice you can't make the mixed air dangerously cold that way, no matter how brutal the morning. Then try the last preset.
Why is the fraction pinned at 20 %? Because deciding when to open the dampers past minimum — cooling the building with free outdoor air — is the economizer's job, and the economizer gets its own page. Here the point is what minimum position buys you: with 80 % of the mix being 75 °F return air, the mixed air stays comfortably warm even on the coldest morning on the slider — the minimum position is winter-safe by arithmetic. The failure preset is what it looks like when that arithmetic is taken away. (Temperatures only here — the moisture half of mixing lives in Psychrometrics Basics.)