You're standing in front of an air-side unit you've never seen, and the nameplate is behind a locked panel. Which three questions does the lesson say you can answer from ten feet away?

Where does it sit, what's in the cabinet, and where does its air come from and go.. Siting, cabinet contents, and connections are readable with your hands in your pockets — and between them they corner nearly everything on a commercial roof or in a mechanical room. Design numbers live on the schedule, brand and age on the nameplate; both are paper, and paper is step two — sometimes a lying step two.

One cabinet observation separates a built-up air handler from packaged DX equipment faster than anything else. What is it?

Insulated pipe pairs entering the casing vs compressors and condenser fans onboard.. An air handler is a borrower: bare coils fed by pipe pairs from a plant that lives elsewhere. Packaged equipment brought its own plant along — compressors, a condenser section, often a burner. Location is only a habit (built-up units turn up on roofs too), and the pipes outrank the address.

A rooftop unit has an intake hood, a burner section, and a supply duct into the building — but you walk around it twice and there is no return duct at all. What is it?

A makeup-air unit (or DOAS) — a 100% outside-air machine.. No return duct means nothing recirculates: the unit conditions outside air only, replacing what hoods and exhaust fans throw away. An RTU's return does drop through the curb — but you'd still find a mixing section behind the hood, and the MAU has none. The missing duct is the whole identification.

Two copper lines — one fat and insulated, one skinny and bare — run from a fan unit above a ceiling, out through the wall, to a boxy fan-equipped unit on a pad outside. What are you looking at?

A split system — that's the refrigerant lineset between the indoor and outdoor halves.. A chilled-water pair would both be insulated and would head for a plant, not a fan-equipped box on a pad. Fat-insulated plus skinny-bare is the refrigerant signature — suction line insulated, liquid line bare. Follow the lineset and you've identified both ends at once.

A VFD on the supply fan is enough, by itself, to call a unit VAV.

False. False. The VAV signature is a conjunction: a drive on the fan, AND terminal boxes with actuators out along the supply duct, AND a duct-static setpoint the fan serves. Modern constant-volume units carry drives too — for balancing and soft starts — so follow the supply duct: boxes say VAV; diffusers straight off the trunk usually say constant volume, though newer single-zone units often vary fan speed to the zone's own load with no boxes at all (single-zone VAV).

You can reliably tell a heat-pump RTU from a straight-cooling RTU by walking around the cabinet.

False. False. Same curb, same hood, same cabinet — the reversing valve that makes it a heat pump lives inside the refrigerant circuit. The tell is paper: a model code one letter off its cooling-only sibling, a nameplate whose heating section names the compressor instead of a burner. Some questions only the nameplate can answer.

A packaged unit's model string carries the digits 060, following the common thousands-of-BTU/h convention (60,000 BTU/h ≈ 17.6 kW). What nominal cooling capacity is that in tons? Enter the answer in tons.

5 tons. 60,000 BTU/h ÷ 12,000 BTU/h per ton = 5 tons (about 17.6 kW). It's a convention, not a law — placements differ by manufacturer and some strings encode airflow instead — so treat the digits as a hypothesis and the mechanical schedule as the arbiter.

Over the phone: "RTU-3's design airflow? Check the schedule." Where do you actually look?

The equipment schedule — a table on the mechanical (M-series) drawings, one row per unit.. Three unrelated things share the word. Design data — airflow, static, coil capacities, electrical, location, serves — lives in the mechanical schedule on the drawings. The BACnet Schedule object and the occupancy calendar are runtime artifacts; neither knows what the unit was designed to do.

A service call in an unfamiliar building. The BMS and the drawings say one thing; the ceiling bay says another. What's the story? BMS GRAPHIC AHU-2 M-SCHEDULE AHU-2 · PENTHOUSE · 30,000 CFM (51,000 m³/h) IN FRONT OF YOU 2 ft (0.6 m) box · fan + piped coil condensate pan · grille in the tile below

The box is a fan coil; the graphic filed it under the wrong family and the paper never caught up.. A two-foot (0.6 m) box with a fan and a piped coil is a fan coil, whatever the graphic says. Thirty thousand CFM (51,000 m³/h) is a built-up unit the size of a small room — it cannot hide in a ceiling bay. When the name, the prints, and the box disagree, believe the box; then start auditing the paper.

Documentation has failed you completely, and you need to match BMS points to real cabinets in July without disturbing the building. The lesson's method?

Command the heating valve — the off-season output — and watch the controller's output LED (or meter the terminals) at the cabinet.. Command the season that's asleep — after checking it really is: with the boiler verified off and the loop cold, a heating-valve command moves a signal, not the building (plenty of plants run hot water all summer for reheat, so confirm before you trust it). Killing fans and pulling disconnects announce themselves to the occupants, not just to you. Keep it brief, command a specific output rather than a demand, skip life-safety and freeze-risk paths, and release every override — then watch it actually return to auto.

Practice · Unit Identification

Unit Identification Quiz Air Systems

Read the Unit Identification lesson first, then name the boxes from memory — ten questions down the lesson walk: the three field questions, the tells that separate packaged from built-up from makeup air, the split system's lineset, the VAV conjunction, tonnage out of a model number, which "schedule" your caller actually meant, and the off-season override that makes an unlabeled system announce itself. Each reveal links back to the section it came from.

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