27.1 Pressure, Temperature, Humidity & Bellows Controls

27.1 Pressure, Temperature, Humidity & Bellows Controls

Trade Area D (Equipment Component Installation) is 14 percent of both Class A and Class B. After condensers, compressors, air handlers, ducts, and piping, the outline walks the controls you actually hang on those components: bellows-type temperature controls, chilled-water low-temperature controls, dual high- and low-pressure switches, temperature controls, oil-pressure safety, summer-winter switchover, water regulating valves, pressure regulators and strainers, and humidity / thermostatic controls. The 2026 Air Conditioning CBT books that carry this cluster are Refrigeration & Air Conditioning Technology, 9th Edition (2021), and Florida Building Code — Mechanical, 2023. Chapter 18.2 already split operating controls from safety controls. This section is the install of the bellows, capsules, and valves that make that split real. Calibration is Chapter 31.

Quick Answer: A bellows converts pressure or a charged-bulb temperature into motion that opens a switch or a valve. High-pressure cutout is on discharge (usually manual reset). Low-pressure cutout is on suction (often automatic reset). Oil failure is net oil pressure after a 90–120 second delay. Water regulating valves open on rising head pressure. Freeze stats protect chilled-water and DX coils. Class B still stops at 25 tons / 500,000 Btu in any one system.

Bellows-type temperature and pressure controls

A bellows is a corrugated metal capsule. Internal pressure stretches it; a spring or the atmosphere pushes it back. That motion trips a snap switch, a pilot valve, or a modulating stem. Two sensing methods show up on the exam:

• Pressure bellows piped to a refrigerant or water tap. Discharge pressure, suction pressure, oil-pump pressure, and condenser-water pressure all use this geometry.
• Temperature bellows with a remote bulb and capillary. The bulb is filled with a liquid, vapor, or cross-ambient charge. Heat at the bulb raises the internal pressure; the bellows at the control body moves as if you had piped refrigerant to it.

Charge type matters. A liquid-filled element wants the bulb colder than the bellows and capillary (or the whole element at one temperature) so liquid does not condense in the wrong place and fake the reading. A vapor-filled element wants the opposite: the bellows colder than the bulb. Cross-ambient charges are built for controls whose body sits in a hot mechanical room while the bulb sits on a 45°F coil. Installing a liquid-charged freeze stat with the capillary coiled on a 140°F gas furnace is how you get nuisance trips that ‘fix themselves’ when the helper moves the extra capillary off the heat exchanger.

Strap a pipe bulb with metal clips and heat-transfer compound on the leaving side of the fluid you care about, then insulate over the bulb so room air does not swamp the pipe temperature. Coil-face freeze stats use a long capillary woven across the leaving face so a single cold streak still opens the circuit. Do not leave a 6-foot unused capillary hanging in the blower; that unused length becomes a second sensor.

Dual high- and low-pressure switches

A dual pressure control puts a high-pressure (HP) bellows and a low-pressure (LP) bellows in one housing with a shared switch or two isolated poles. The HP port is a capillary or flare to the discharge (or compressor head). The LP port goes to the suction service valve or a true suction tap — not to the compressor process port if that port is actually oil.

High-pressure cutout is a safety. It opens on dirty condenser fins, recirculated Florida discharge air, loss of condenser water or tower fan, overcharge, or a closed discharge service valve. Most listings want manual reset so someone looks at the condenser before the compressor restarts into a 400-psig brick wall. Low-pressure cutout opens on loss of charge, a closed liquid-line valve, a failed evaporator fan, a plugged drier, or a frozen coil. LP is often automatic reset so a short low-load pull-down does not strand the box, but many commercial racks still use manual-reset LP as a loss-of-charge lockout. The exam item is the function and the port, not a memorized psig that only the listing knows. Do not substitute the practice-bank count of switches for the manufacturer’s cutout.

Worked dual-control read. A 10-ton R-410A rooftop in Sarasota shows HP open, LP closed, condenser fan dead, and 95°F outdoor air. That is an HP trip from no air across the condenser, not an LP loss-of-charge story. Resetting without fixing the fan only proves the bellows still moves.

Oil-pressure safety (oil failure)

Small hermetic compressors usually splash-lubricate. Semi-hermetic and open machines with an oil pump get an oil-failure control (oil-pressure safety). The control does not measure oil level. It measures net oil pressure:

(\text{Net oil pressure} = P_{\text{oil pump}} - P_{\text{crankcase (suction)}})

The oil-pump tap is downstream of the pump. The crankcase tap is suction or crankcase pressure. If suction is 65 psig and the oil-pump gauge is 80 psig, net is 15 psig. If the pump is worn or the crankcase is full of refrigerant foam, net can fall to 5 psig while both gauges still ‘look high’ to a helper who only reads the oil-pump gage.

A time delay of typically 90 to 120 seconds ignores the first moments of start, when the pump has not yet built net pressure. If net pressure is not proven when the delay expires, the control drops the compressor and usually needs a manual reset. Bypassing the oil-failure switch ‘until the part comes in’ is how you buy a crankshaft. A sight glass still belongs on the crankcase for level, and a separator still belongs in the discharge on many racks — neither one replaces the net-pressure switch.

Chilled-water low-temperature controls and humidity / thermostatic devices

A chilled-water coil can freeze in Florida. Night air at 55°F, a 42°F water valve stuck open, and a blower on a dirty filter will ice a face and split tubes. The low-temperature (freeze) control is a bellows or capillary thermostat, commonly opening in the mid-30s °F (about 35°F is the textbook coil-face number — use the listing). Prefer manual reset. Interlock it so the chilled-water valve drives closed (or the pump stops, per sequence) and the compressor / chiller enable is dropped. A flow switch or differential-pressure switch on the evaporator barrel is the other low-temp cousin: no water, no compressor. That is not optional ‘if the DDC already has a point.’

Humidity and thermostatic controls in this cluster are the humidistat (make-on-drop for a humidifier, make-on-rise for a dehumidifier or reheat), the combination thermo-humidistat, and the older bellows space thermostat. Florida’s usual problem is too much moisture, not too little: a humidistat that stages a hot-gas reheat or a dedicated dehumidifier is operating control. A dehumidistat that only kills the compressor when RH is ‘low enough’ can strand a warm, wet classroom — teach the sequence, not a single switch.

Water regulating valves, pressure regulators, strainers, summer-winter switchover

A water regulating valve sits in the condenser-water line of a water-cooled machine. A refrigerant bellows on head pressure opens the valve as condensing pressure rises, and a spring closes it as head falls. That holds a usable condensing temperature on once-through city water or a weak tower. Put a strainer on the water inlet. Once-through potable water still needs backflow (coordinate; F.S. 489.105 is not a plumbing license). Heat of rejection is about 15,000 Btu/h per ton, so condenser flow is about 3 gpm per ton at a 10°F range (Chapter 23). Sizing the valve on 2.4 gpm/ton starves the condenser 20 percent.

Pressure regulators on the refrigerant side are different valves: an evaporator pressure regulator (EPR) holds a minimum evaporator pressure (and therefore coil temperature) on a shared suction; a crankcase pressure regulator (CPR) limits suction pressure into the compressor after a hot pulldown; a head-pressure holdback keeps liquid in the condenser in low ambient. Each gets a strainer or filter-drier upstream. A water-side pressure-reducing valve on makeup is not an EPR.

Summer-winter switchover on a two-pipe hydronic system is the seasonal change from chilled water to hot water in the same pipes. A water-temperature changeover (or a manual switch) reverses thermostat action: cooling on a cold main, heating on a hot main. A deadband around the changeover water temperature (commonly in the 80–90°F neighborhood — use the sequence) keeps the thermostat from hunting while the boiler and chiller fight. This is not a heat-pump O/B reversing terminal, and it is not a license for Class B to install the boiler that makes the winter water (Class A, F.S. 489.105(3)(f)).

Florida HVAC scenario

Gulf Coast Mechanical, certified Class B in Pinellas County, change-outs a 15-ton water-cooled package on a clinic — legal under the 25-ton / 500,000 Btu cap. The helper pipes the dual HP/LP with the HP capillary on the suction valve ‘because it was the closest flare,’ skips the oil-failure crankcase tap and uses oil-pump pressure alone, and sets a water regulating valve with no strainer on a 2.4 gpm/ton pump. Three independent failures: wrong HP port, oil-failure wiring that cannot see net pressure, and a starved condenser that will HP-trip on the first 95°F afternoon. The same drawings show a 30-ton sister package on the imaging suite. That cabinet is Class A even though it uses the same bellows. Class B still owes the theory of the 30-ton dual pressure control; Class B does not contract the 30-ton system.

Traps: (1) Jumping HP/LP or oil failure. (2) Treating oil-pump psig as net oil pressure. (3) Liquid-charged bulbs with capillary in a hot cabinet. (4) Freeze stats on auto-reset over finished ceilings. (5) Water regulating valves without strainers. (6) Class B on a 30-ton water-cooled machine because ‘the controls are small.’