20.3 Air-Cooled vs Water-Cooled Systems

20.3 Air-Cooled vs Water-Cooled Systems

The last two Trade Area C items in this chapter are installing air-cooled systems and installing water-cooled systems. Chapters 21 and 23 pick up large chillers, remote condensers, and centrifugal machines. Here you must name the heat-rejection path, size the condenser water when there is water, keep Florida wind and clearances honest, and apply the Class B 25-ton fence. Refrigeration & Air Conditioning Technology, 9th Edition (2021), FBC Mechanical 2023 (including Section 908 cooling towers, evaporative condensers, and fluid coolers), and F.S. 489.105 are the references.

Quick Answer: Air-cooled condensers dump heat to outdoor air. Water-cooled condensers dump heat to water, almost always a cooling-tower loop, at about 2.4 gpm per ton for a 10°F range. Both need listing clearances and 301.15 wind resistance. Class B installs either type only up to 25 tons / 500,000 Btu in any one system.

Air-cooled heat rejection

An air-cooled condenser is a coil plus a fan. Refrigerant condenses inside the tubes; outdoor air carries the heat away. Every split condensing unit in 20.1 and every air-cooled package or heat pump in 20.1–20.2 is this machine. Install rules that actually change capacity:

• Listing clearances on sides, top, and discharge. A 12-inch side yard when the IOM says 24 inches recirculates hot air into the entering face, raises condensing temperature, cuts capacity, and trips high pressure on a 95°F Florida afternoon.
• Discharge path. Do not bury an RTU in a penthouse well or a decorative screen that returns its own 20-degree-hotter plume to the coil. That is the P-h failure from Chapter 17, now as an install defect.
• Pad and wind. 304.10: slab 3 inches above grade (or 6 inches suspended). 301.15: resist FBC Building wind pressures with listed straps, clips, or through-bolts. HVHZ (Miami-Dade/Broward) adds product-approval discipline; the rest of Florida still does not get a loose condenser on pavers. 301.16: flood elevation.
• Airflow across the coil. Dirty fins, bent guards, and landscaping that grows into the intake are maintenance (later chapters) and also install if you set the unit in a hedge on day one.
• Package versus remote. A packaged air-cooled RTU has the condenser in the same cabinet as the evaporator. A remote air-cooled condenser sits on the roof with refrigerant piping back to an indoor compressor — still air-cooled, still 301.15, still nitrogen-purged piping (20.1). Class B’s 25-ton cap applies to the system, not to “the small box on the roof.”

Air-cooled machines do not need condenser-water pumps, towers, or 908 blowdown piping. They do still need 307 condensate from the evaporator (indoor coil or package drain pan). Do not confuse condenser-fan water (there is none) with evaporator condensate.

Water-cooled heat rejection

A water-cooled condenser transfers refrigerant heat into a water stream. Common construction is tube-in-tube, shell-and-tube, or shell-and-coil (Trade D will install the vessel; Trade C is the system). That water then goes to:

1. A cooling tower (open recirculating — the usual HVAC plant),
2. A closed-circuit fluid cooler (coil in the tower, process water stays closed),
3. Rarely, once-through city or well water — wasteful, generally inconsistent with energy and water rules, and still requiring backflow protection on a potable makeup (301.11 points plumbing connections to the Florida Plumbing Code).

FBC Mechanical Section 908 governs cooling towers, evaporative condensers, and fluid coolers: support, drainage of overflow and blowdown to an approved location (not a parking-lot nuisance and not a mystery sanitary tap), makeup water, and placement so discharge air and drift do not dump onto intakes, neighbors, or the tower’s own inlet. Tower fans and cabinets on a Florida roof are wind-exposed appliances under 301.15. Indoor water-cooled package units still need a water-regulating valve or pump, strainer, and drain; they do not magically become “no outdoor equipment” jobs if the tower sits on the roof.

Range and approach are the two tower numbers the exam likes.

• Range = condenser-water entering the tower minus leaving the tower (which is also condenser leaving minus entering at the machine). A common design is 95°F in / 85°F out, a 10°F range.
• Approach = tower leaving water minus outdoor wet-bulb. If leaving water is 85°F and ambient WB is 78°F, approach is 7°F. You cannot leave below wet-bulb; a 1°F approach is a fantasy on a budget tower.

Condenser-water flow for water, at a stated ΔT, is the same 500-constant as Chapter 18:

(\text{GPM} = \dfrac{\text{Btu/h}}{500 \times \Delta T})

At 10°F range, each 12,000 Btu/h (1 ton) needs

(12{,}000 / (500 \times 10) = 2.4\ \text{gpm})

Worked example. A 20-ton water-cooled packaged unit — legal Class B, right at the cooling cap if it is one system — at 10°F range:

(\text{GPM} = 20 \times 2.4 = 48\ \text{gpm})

Or (240{,}000 / 5{,}000 = 48) gpm. Pipe, pump, and tower fill have to make 48 gpm at the real head, not “a 3/4-inch hose because it is only 20 tons.” If the same machine is specified at a 15°F range, flow drops to (240{,}000 / 7{,}500 = 32) gpm — but only if the condenser and tower are rated at 15°F. Inventing a bigger ΔT to save pipe is how you leave the manufacturer’s table.

A 30-ton water-cooled package is 360,000 Btu/h cooling — Class A. Class B still answers the 2.4 gpm/ton question; Class B does not install that 30-ton system. Two independent 15-ton water-cooled packages, each with its own loop, can be Class B if they are truly separate systems.

Side-by-side and Florida constraints

Clearances on water-cooled indoor packages still include service access (306) and manufacturer tube-pull space. Guards (304.11) still apply on roofs. Vibration isolation (301.13) still needs supplemental restraint so a spring-mounted tower does not walk in a hurricane — isolation without restraint is the 301.13 trap.

Air-cooled is simpler to install and dominate Florida residential and light commercial work because wet-bulb is high (towers lose approach) and freeze-up is rare but scaling, Legionella control, and tower water treatment are real operating costs. Water-cooled still wins on large plants (Chapter 21) where a 95°F air-cooled condensing temperature would crush kW/ton. The exam item is: name the sink, size the water if any, anchor against Florida wind, stay inside the license.

Electrical reminder. Condenser fans and tower fans are HVAC equipment. Power wiring stays inside F.S. 489.105: dedicated HVAC circuits, single-phase disconnect replace/reconnect, low-voltage controls. A three-phase 20-ton tower starter that is not those exceptions is an electrical-contractor conversation even when Class B may set the 20-ton package.

Florida HVAC scenario

A certified Class B qualifier in Duval County (Zone 2A) is asked to replace a failed air-cooled 20-ton RTU on a medical office with a water-cooled 20-ton package “because the new one is quieter.” Cooling tons stay at 20 — still Class B. The drawings now add a roof cooling tower, 48 gpm at 10°F range, makeup with no backflow, blowdown piped to a sanitary vent, and the tower set on isolation springs with no seismic/wind snubbers. Four install problems: (1) 301.15 / 301.13 — springs without restraint in a wind-borne-debris region; (2) 908 drainage — blowdown is not a sanitary vent tap any more than evaporator condensate is; (3) 301.11 — potable makeup needs a plumbing-code backflow preventer, which is not an HVAC license to run new potable water; (4) tower clearance so discharge does not re-enter the intake. If the salesman “upsized” to 30 tons while he was at it, Class B is over regardless of gpm math. Keep the 20-ton, size 48 gpm, restrain the tower, dispose of blowdown legally, and do not sell a sanitary connection as a convenience.

Traps: (1) Calling a package RTU water-cooled because it has a 307 condensate drain. (2) Using 4 gpm/ton on a 10°F-range problem. (3) Once-through city water as the default. (4) Ignoring 301.15 on towers. (5) Class B on a 30-ton water-cooled “because water-cooled is indoor.”