23.3 Air Handlers, Evaporators, Heat Exchangers, Receivers & Heat Strips

23.3 Air Handlers, Evaporators, Heat Exchangers, Receivers & Heat Strips

The rest of this Trade D cluster is the air side and the vessels that are not compressors: air handlers, evaporators, warm-air heaters, heat exchangers, receivers, and heat strips — including wiring, firestats, and fire codes. FBC Mechanical 2023 Section 307 (condensate), NFPA 90A 2015, NFPA 90B 2024, FBC Energy Conservation 2023 Section R403.7.2, and F.S. 489.105 are the code spine. Refrigeration & Air Conditioning Technology, 9th Edition (2021) is the component book.

Quick Answer: An AHU is filter → coil → drain pan → blower, sometimes heat strips. 307.2: 3/4-inch drain, 1/8 in/ft, no direct sanitary, plus a 307.2.3 overflow method in an attic. A receiver is high-side liquid storage; an accumulator is low-side slug protection. Heat strips need airflow proving and a firestat, and they are not primary heat in Climate Zone 2.

Air handlers, evaporators, filters, blowers, drain pans

An air-handling unit (AHU) is a cabinet that moves and conditions air. The exam’s install order is not decorative:

1. Return / mixing box (if used),
2. Filters (upstream of the coil — a coil is not a filter),
3. Evaporator or hydronic coil (the heat exchanger that cools or heats the air),
4. Drain pan under any coil that can condense,
5. Blower,
6. Heat strips or a heating coil when specified, with listed clearances to the coil and to combustibles,
7. Supply discharge.

Filters. Access doors that actually open in the finished ceiling. High MERV raises external static pressure (ESP); if the blower cannot make the extra inches, you lose airflow, lose capacity, freeze the coil, and overheat strips. Do not trap a filter rack behind a fire damper you cannot reach.

Blowers. Residential AHUs are often forward-curved direct-drive or belt. Commercial units add backward-inclined and plenum fans. Set rotation, belt tension, and pulley alignment (Chapter 32). External static is duct plus accessories; it is not “the nameplate CFM no matter how you choke the return.” Florida latent loads want coil face velocity in a range that dehumidifies without blowing condensate off the fins — textbook wet-coil values commonly sit near 400–500 fpm, and high face velocity is a blow-off risk.

Evaporators. A direct-expansion (DX) coil boils refrigerant. A chilled-water coil is a hydronic heat exchanger. Superheat and TXVs are Chapter 26; Trade D’s job is to set the coil level, pipe the correct inlets (refrigerant distributors down, water connections per the marked headers), and give the pan a chance to drain. Warm-air heaters on this outline are the heating side of the AHU or a forced-air furnace/heater: electric strips, a hydronic coil, or a fuel-fired heat exchanger. F.S. 489.105 still does not let HVAC run new LP/natural-gas piping inside the building except disconnect/reconnect on a changeout. A cracked furnace heat exchanger is a carbon-monoxide hazard — replace the listed appliance; do not “seal” it with high-temperature caulk. NFPA 90B 2024 is the warm-air book on the 2026 CBT list for the residential-scale systems it covers; NFPA 90A 2015 is the commercial air-conditioning and ventilating book.

Drain pans — FBC Mechanical 307.2 (the same numbers Chapter 19.4 used, now on the AHU):

• Condensate drain not less than 3/4 inch, not smaller than the pan outlet, sloped 1/8 inch per foot.
• No direct connection to the sanitary DWV (307.2.1.1). Indirect to an approved location / existing safe waste — which also matches the HVAC statutory condensate fence.
• Where overflow can damage the building (attics, plenums, finished ceilings — Florida’s default AHU location), 307.2.3 requires one of the listed methods: an auxiliary pan with a separate drain (1-1/2 in. deep, 3 in. larger than the unit, 24-gage galvanized is the code-pan description already used on this outline), an equipment overflow drain to a conspicuous point, an auxiliary pan with a UL 508 shutoff, or a UL 508 device in the pan/overflow.
• Downflow coils (307.2.3.1) take a pan-mounted water-level device, not a float taped only to the primary drain line.

Primary pans need slope and a trap when the outlet sees fan suction (negative plenum). A dry trap is an air leak; a missing trap on the return side can pull the drain dry and blow water. Secondary pans drain to a visible point — a wet stain on a foyer ceiling is how you learn the auxiliary drain was capped.

Heat exchangers

Heat exchanger on Trade D is the generic vessel or coil that moves heat between two streams without mixing them (unless it is a failed furnace exchanger). Types you must install as listed:

Do not treat a receiver as a heat exchanger. Do not run potable building water because a double-wall coil appeared on the schedule — 489.105 still excludes potable water lines.

Receivers versus accumulators

These two vessels are the exam’s favorite swap.

A receiver sits on the high side after the condenser (and usually after a condenser-liquid valve). It stores liquid refrigerant so the condenser can drain, the charge can vary with load and ambient, and the system can pump down (close a liquid-line solenoid; the compressor pulls refrigerant into the receiver/condenser and shuts on low pressure). Horizontal and vertical receivers carry a liquid level, often a sight glass, a king valve, and pressure relief piped per Chapter 11. Size is a manufacturer/charge problem: a pump-down receiver is often asked to hold the full charge without liquid backing into the condenser at the worst condition. Critically charged capillary or small-orifice splits often omit a receiver; adding one without redesigning the charge is how you flood the condenser.

An accumulator sits in the suction line, low side, between the evaporator (or reversing valve on a heat pump) and the compressor. It protects the compressor from liquid slugging. Heat pumps use them because the reversing valve can send liquid toward the compressor during changeover. A U-tube with a small oil-return orifice meters oil back while leaving liquid behind to boil. An accumulator is not a spare receiver, not a filter-drier, and not a muffler.

Heat strips, wiring, firestats, fire codes

Electric heat strips (duct heaters / AHU heat kits) are resistance elements in the airstream. Power:

(1\ \text{kW} = 3{,}412\ \text{Btu/h})

A 5 kW kit is about 17,060 Btu/h. A 10 kW kit is about 34,120 Btu/h. A 15 kW kit is about 51,180 Btu/h — well under the Class B 500,000 Btu heating cap by itself. Still check the system heating total if strips combine with a gas section or a large hydronic coil.

Worked current. A 10 kW kit at 240 V single-phase draws (10{,}000 / 240 \approx 41.7) A of heater current before the blower. Continuous loads and the blower FLA are already rolled into the AHU MCA as configured — use the nameplate MCA/MOCP, not a memory of “40-amp breaker for 10 kW.”

FBC Energy Conservation 2023 Section R403.7.2: electric resistance shall not be the primary heating system in Climate Zone 2 (most of Florida, including Jacksonville, Orlando, Tampa, Tallahassee, and Pensacola). Zone 1A (Broward, Collier, Hendry, Lee, Miami-Dade, Monroe, Palm Beach) is the very-hot-humid row — still size to Manual J / R403.7, and do not treat strips as a license to skip a heat pump where the code path wants one. Strips as supplemental / emergency heat on a heat pump are the usual legal use in Zone 2.

Firestats and limits. NFPA 90A 2015 and NFPA 90B 2024, plus the listing (UL 1995 equipment, UL 1996 duct heaters), require airflow proving so elements do not energize in still air, and overtemperature protection that is not the room thermostat. Typical listed kits:

• A primary automatic-reset limit,
• An independent secondary manual-reset limit or fusible link — the outline’s firestat language,
• A sequencer or electronic control that stages strips and often brings the blower on.

A firestat is a safety. It is not a night setback. If it trips, find the lost airflow (dirty filter, closed damper, failed blower) — do not jumper it. Clearances to combustibles, to the coil, and to filters follow the listing; strips too close to a plastic filter or a liner are a fire-code problem, not a “tight attic” excuse.

Wiring and 489.105. Heat kits are line-voltage. Single-phase 240 V is common on residential kits; three-phase appears on commercial. HVAC may replace, disconnect, or reconnect power wiring on the line or load side of a dedicated existing disconnect on single-phase, and may repair or replace power wiring, disconnects, breakers, or fuses on dedicated HVAC circuits with a circuit-breaker lock, plus all low-voltage control wiring (sequencer, firestat, thermostat W/E/AUX). A new building feeder from the panel to a new heat-strip disconnect is electrical-contractor work. Do not land 60 A of strips on a 15 A lighting circuit, and do not treat 24 V thermostat wire as the heater circuit.

Florida scenario

A certified Class B firm in Orange County (Zone 2A) replaces an attic 4-ton AHU with a 10 kW heat kit. The primary pan gets a 3/4-inch drain at 1/8 in/ft to an indirect receptor — not a sanitary tee. Because overflow would ruin the ceiling, they add a 307.2.3 auxiliary pan, 24-gage, 3 inches larger, with a visible secondary drain and a UL 508 float that kills the unit. Filters sit upstream of the coil with an access panel. The 10 kW kit is 34,120 Btu/h — supplemental on a heat pump, not the primary heat (R403.7.2 in Zone 2). The kit’s airflow switch and manual-reset firestat stay in the circuit. Low-voltage W/E and the existing dedicated single-phase HVAC disconnect are HVAC work with a breaker lock; the electrician already had a correctly sized dedicated circuit. Across town, a receiver on a water-cooled package is piped on the liquid line after the condenser with relief outdoors; the accumulator is on the suction of a heat-pump AHU. Swapping those two vessels is the fastest way to slug a compressor or to lose pump-down.

Traps: (1) Receiver ↔ accumulator. (2) Direct sanitary condensate. (3) No auxiliary pan in an attic. (4) Drain-line-only float on a downflow coil. (5) Jumping a firestat. (6) Electric resistance as primary heat in Zone 2. (7) New panel feeders as HVAC work. (8) Filters downstream of a wet coil with no access.