22.4 Geothermal, Pneumatic & DDC Controls, TAB & Smoke Tests
Key Takeaways
- Ground-source heat pumps use a closed loop (horizontal trench, vertical borehole U-bend, or pond coil) or an open loop (well water through the heat exchanger and back to the aquifer or a discharge well). Closed-loop pipe is typically fused HDPE with a listed antifreeze; open-loop water quality and disposal are not a Class B plumbing grant.
- Pneumatic control piping is Class A statutory scope (F.S. 489.105(3)(f)); Class B does not pick it up. Main air is roughly 18–25 psig; the signal is 3–15 psig. Plenum pneumatic tubing must meet FBC 602.2.1.3 / UL 1820.
- DDC (direct digital control) is sensors, controllers, and actuators on a network (often BACnet). Low-voltage HVAC control wiring is inside both Class A and Class B scope; a new 277/480-volt feeder is not.
- TAB: total pressure = static pressure + velocity pressure. For standard air, V (fpm) ≈ 4005 × √VP (in. w.c.). Pitot tube plus manometer, velometer, flow hood, tachometer, and hydronic differential gauges are the instruments.
- FBC Mechanical 2023 606.2.1 (and NFPA 90A 2015): supply systems greater than 2,000 cfm get a listed duct smoke detector downstream of the filters and ahead of branch connections. Return risers serving two or more stories and any portion of a return system greater than 15,000 cfm get detectors at each story (606.2.3). Activation shuts the air-distribution system down (606.4) unless it is a smoke-control mode.
22.4 Geothermal, Pneumatic & DDC Controls, TAB & Smoke Tests
The last Trade C install bullets on this chapter are geothermal heat pumps, pneumatic controls, DDC, testing, adjusting, and balancing (TAB), smoke detectors, and fire-department smoke tests. Chapter 20 already installed air-source heat pumps. Chapter 18.2 already said pneumatic control piping is Class A. Chapter 30 will dwell on instruments. This section is the install: loop in the ground, air in the tube, code on the detector, numbers in the TAB report.
Quick Answer: Closed geothermal loops are horizontal, vertical, or pond; open loops use well water. Pneumatic piping = Class A; 3–15 psi signal on an 18–25 psi main. DDC low-voltage is both classes. V ≈ 4005 √VP. Supply > 2,000 cfm gets a duct smoke detector downstream of the filters (FBC 606.2.1 / NFPA 90A 2015). Grease ducts still get the 100-watt light test (Section 22.3).
Geothermal heat pumps — the loop is the condenser (and the evaporator)
A ground-source heat pump (GSHP) is a water-source heat pump whose “tower” is the earth or a well. In cooling, the refrigerant rejects heat into the loop; in heating, it extracts heat from the loop. The reversing valve, backup heat, and energy-code strip rules from Section 20.2 still apply. What is new is the loop type.
| Loop | What is buried or pumped | Florida install note |
|---|---|---|
| Closed horizontal | HDPE pipe in trenches, typically several feet below grade | Needs land; watch the water table and existing utilities |
| Closed vertical | U-bend in a borehole, often on the order of 150–400 feet | Common on tight Florida lots; grout the annulus |
| Closed pond / lake | Coiled pipe submerged (deep enough not to freeze or cook at the surface) | Retention ponds exist; boat anchors and liners still matter |
| Open loop | Groundwater through the refrigerant-to-water heat exchanger, then to a return well or approved discharge | Water quality, fouling, and disposal — not a license to run potable building plumbing |
Closed-loop pipe is typically high-density polyethylene, heat-fused, pressure-tested, and filled with water or a listed antifreeze (propylene glycol is the usual HVAC fill; do not invent an automotive cocktail). Purge air until the loop is clean; air in a vertical U-bend is a permanent insulator. Size the heat pump from expanded data at the entering-loop temperature, not from an air-source SEER sticker. Florida ground and pond water are warm; cooling-mode EWT is not a 45°F wish.
Direct-exchange (DX) geothermal puts refrigerant copper in the earth. That is a different listed product, still a 608 circuit, and still inside 489.105 capacity. Do not confuse DX earth loops with HDPE water loops.
Class B may install a 20-ton GSHP; Class B may not install a 40-ton geothermal plant on one loop (any one system). Plastic pipe, fusion, and well work that is not HVAC loop (potable wells, sanitary) stay outside the HVAC grant. Chapter 28.3 returns to geothermal loops on the electrical/plastic-pipe side; this section is the heat-pump install.
Pneumatic controls — air as the analog signal
Pneumatic controls use compressed air instead of 24-volt wires. A main air supply from a control compressor, dryer, and filter typically sits around 18–25 psig. The branch (signal) to a damper or valve actuator is the classic 3–15 psig analog: 3 psi is one end of travel, 15 psi the other, 9 psi mid-scale. Direct-acting controllers increase branch pressure as the sensed variable rises; reverse-acting decrease it. Normally-open and normally-closed damper springs fail to a known position on loss of air — that fail position is a life-safety choice on outdoor-air and smoke dampers, not a shop preference.
Receiver-controllers, one-pipe versus two-pipe thermostats, restrictors, and pilot-bleed amplifiers are the textbook hardware. Calibration is Chapter 31; install is clean, dry, oil-free air, leak-tight tubing, and the correct fail spring.
F.S. 489.105(3)(f) lists pneumatic control piping in Class A. 489.105(3)(g) (Class B) does not. Class B candidates still owe the 3–15 psi theory; they do not bid a pneumatic remodel as Class B. FBC Mechanical 602.2.1.3: combustible pneumatic tubing exposed in a plenum shall be listed to UL 1820 (peak optical density not greater than 0.50, average not greater than 0.15, flame spread distance not greater than 5 feet). Vinyl tubing that is not plenum-rated does not belong above a corridor ceiling used as a return plenum.
DDC — silicon instead of 18 psi
Direct digital control (DDC) replaces the pneumatic analog with a controller, sensors (temperature, humidity, pressure, CO₂, current), and actuators (damper motors, valve motors, VFDs) on a network. BACnet (ASHRAE 135) is the common open protocol on Florida jobs; proprietary buses still exist. Sequences (economizer, occupied OA, demand-controlled ventilation, pump lead-lag) live in software. That does not waive FBC 403 outdoor-air minimums or FBC 606 smoke shutdown.
Low-voltage HVAC control wiring is inside both Class A and Class B (F.S. 489.105). Line-voltage to a new AHU disconnect is still the electrical fence from Chapter 21.1. Sensors need a listed location (mixed-air sensors in a mixed-air stream, averaging elements on large coils, outdoor sensors out of discharge and sun). Actuators need a stroke that matches the damper and a fail position on smoke and outdoor-air dampers. A DDC graphic that shows 2,000 cfm of outdoor air does not replace a TAB reading.
TAB instruments and the 4005 equation
Testing, adjusting, and balancing proves the system that was designed in Chapter 17. Total pressure (TP) = static pressure (SP) + velocity pressure (VP). A pitot tube facing the flow plus a manometer (or digital manometer) reads VP and SP. For standard air (0.075 lb/ft³):
(V \approx 4005 \times \sqrt{VP})
with V in fpm and VP in inches of water. Traverse the duct (ASHRAE / AABC / NEBB grid), average the velocities, multiply by inside area to get cfm. A rotating-vane velometer or hot-wire anemometer reads face velocity at a grille; a flow hood (balometer) reads outlet cfm directly. A tachometer confirms fan rpm before you blame the sheave. Hydronic TAB uses differential pressure across a circuit setter and the manufacturer’s flow chart — the 500-constant from Chapter 18 is the design number; the readout is the measured number.
Worked pitot. VP = 0.25 in. w.c. (V = 4005 \times \sqrt{0.25} = 4005 \times 0.5 = 2{,}002) fpm. A 16×12 duct (inside 1.33 ft²) carries about 2,670 cfm. If the schedule was 2,000 cfm, you are high — close the fan inlet, drop rpm, or open other branches; do not crush one damper until the reading looks pretty and the next room starves.
TAB reports list design versus actual for supply, return, exhaust, and outdoor air. Kitchen makeup from Section 22.3 is a TAB item: 2,400 cfm exhaust wants about 2,400 cfm makeup, measured, interlocked. Smoke-control systems have their own special-inspection path; they are not a comfort TAB with a fog machine.
Smoke detectors and fire-department smoke tests
FBC Mechanical 2023 Section 606 is Florida’s duct-detector section and it names NFPA 90A:
- 606.2.1: To prevent recirculation of dangerous quantities of smoke, a detector approved for air-duct use is installed on the supply side as required by NFPA 90A. Listed detectors sit downstream of the air filters and ahead of any branch connections on supply systems greater than 2,000 cfm. Exception: not required in the supply system where all portions of the building served are protected by area smoke detectors on a fire-alarm system, and that system complies with 606.4.
- 606.2.2: Multiple air handlers sharing a common supply with combined capacity greater than 2,000 cfm — each supply system gets detectors per 606.2.1 (with a fan-powered terminal exception).
- 606.2.3 Return air risers: Where return risers serve two or more stories and serve any portion of a return system greater than 15,000 cfm, install smoke detectors at each story, upstream of the connection between the riser and other ducts or plenums.
- 606.3: Install per NFPA 72. Detectors are not required on fans whose sole job is to exhaust to outdoors. Provide access.
- 606.4: Activation shuts down the air-distribution system, unless the system is part of a smoke-control system, in which case it switches to smoke-control mode. 606.4.1: Where a fire alarm is required, duct detectors are connected to it and typically report as a supervisory signal, not a general evacuation alarm.
NFPA 90A 2015 is the same idea: supply-side detection on systems over 2,000 cfm; additional return-side detection on large, multi-story returns. NFPA 90B 2024 does not import a 20-ton office AHU into dwelling rules. Duct detectors do not replace area detectors (NFPA 72): if the fan is off, the duct detector sees nothing.
Fire-department smoke tests on this outline are the witnessed tests, not a contractor’s private fog:
- Grease-duct leakage / light test (FBC 506.3.2.5, Section 22.3) — often stood over by the fire inspector.
- Smoke-control (atrium, stair pressurization, zoned exhaust) — special inspection, often with theatrical smoke after TAB of those fans.
- Duct-detector functional test: introduce listed smoke or magnet/test gas per the detector listing and prove AHU shutdown and the supervisory signal.
A comfort TAB “smoke stick” at a diffuser is an air-pattern tool. It is not a 606 test and not a 506.3.2.5 test.
Florida scenario
A certified Class A shop in Duval County turns over a Jacksonville medical office: two 8,000 cfm indoor AHUs (each > 2,000 cfm) with supply-side UL 268A duct detectors after the filters, before the first branch, tied to the fire alarm as supervisory and to fan shutdown; a 20-ton closed-loop vertical geothermal heat pump (inside Class B tons, but this qualifier is Class A) with fused HDPE U-bends, purged and glycol-filled; DDC on BACnet with 24-volt actuators (low-voltage = HVAC); and a leftover pneumatic outdoor-air damper on the older AHU-3 that only Class A pneumatic piping may extend. TAB reads VP 0.16 in. in the 24×12 main ((4005 \times \sqrt{0.16} \approx 1{,}602) fpm) and reports cfm versus schedule, including outdoor air. The fire inspector witnesses the Type I hood 100-watt light test in the café and the duct-detector shutdown on AHU-1. A Class B shop could have taken the 20-ton geothermal and the DDC, but not the pneumatic extension, and not a 40-ton geothermal loop.
Traps: (1) Skipping the supply duct detector because the unit is “only 3 tons” when it is 3,500 cfm. (2) Putting the detector before the filter. (3) Calling duct-detector trip a general fire-alarm evacuation. (4) Class B running new pneumatic mains. (5) Using 4005 × VP instead of 4005 × √VP. (6) Calling a comfort smoke stick a grease-duct light test.
An 8,000 cfm indoor air handler serves a Florida medical office. Where does FBC Mechanical 2023 Section 606 require the listed duct smoke detector, and what does activation do?
A TAB technician measures 0.25 in. w.c. velocity pressure with a pitot tube in a 16×12 duct (1.33 ft² inside). Using the standard-air formula, which result is correct?
Which statement about geothermal loops, pneumatic piping, and DDC on a Florida HVAC ticket is accurate?