31.3 Calibrating Thermostats, Pressure Switches & Pneumatic Controls

31.3 Calibrating Thermostats, Pressure Switches & Pneumatic Controls

Trade F next calibrates the controls Chapter 27 installed. The outline cluster is air-sensitive thermostats, chilled-water low-temperature cutouts, dual / low-pressure switches, pneumatic controls, proportional thermostats, summer-winter changeover, thermostatic temperature controls, thermostatic motors, and bellows-type devices. Installation was “where it goes.” Calibration is “what temperature or pressure it actually makes,” compared to a thermometer or a gauge, not to the faceplate art. Refrigeration & Air Conditioning Technology, 9th Edition (2021) is the book. FBC Energy Conservation 2023 R403.1.1 still wants the comfort thermostat on an interior wall sensing representative air. Pneumatic control piping remains Class A statutory scope (F.S. 489.105(3)(f)); Class B still owes 3–15 psig theory. Class B still stops at 25 tons / 500,000 Btu for the connected system.

Quick Answer: Calibrate against a real thermometer or a real gauge at the sensing element. Heat anticipator = heating-circuit amps. Freeze stat ≈ 35–38°F, often manual-reset. Dual pressure: high usually manual-reset, low has a differential. Pneumatic main 18–25 psig, branch 3–15 (9 mid-scale). Proportional means a throttling range, not a click. Never jumper a pressure switch as a calibration.

Why calibration is a maintenance item

A thermostat that is 3°F off will short-cycle or never satisfy. A dual pressure switch that was “set by ear” will either nuisance-trip or fail to protect. The exam’s move is: what do you measure, where do you measure it, and which screw is range vs differential. Jumping a bellows “until the gauges look right” is how a high-pressure machine runs without a cutout.

Air-sensitive thermostats

An air-sensitive thermostat senses room or duct air — bi-metal, remote-bulb in the airstream, averaging element, or electronic thermistor. Calibration rules:

• Compare to an accurate thermometer at the element, not to a probe in the sun on the west wall and not to supply-air temperature.
• FBC Energy R403.1.1: at least one thermostat per separate heating and cooling system, interior wall, representative air — not a supply chase, not an uninsulated exterior wall.
• Electromechanical heat anticipator: set to the measured current of the heating circuit (ammeter in series with W). Typical gas-valve circuits are a fraction of an amp (often about 0.2–0.8 A — measure, do not guess). Too low an anticipator setting short-cycles. Too high makes long overshoot. Cooling anticipators are often fixed.
• Electronic stats: offset / calibration in the installer menu; deadband so heat and cool do not fight. Do not use a heat-pump O/B mismatch as a “calibration.”
• Old mercury bulbs (mostly gone) had to be level. A tilted mercury stat is a calibration error, not a new subbase.

Worked anticipator. A gas RTU’s W circuit draws 0.45 A with the valve energized. Set the anticipator to 0.45, not to the highest number “so it never shuts off” and not to the lowest number “so it responds faster.” Then compare the element to a calibrated thermometer at setpoint.

Chilled-water low-temperature (freeze) controls

A chilled-water low-temperature cutout (freeze stat) protects a water coil from freezing when outdoor air is cold, when a chiller overshoots, or when flow is lost. It is not a space thermostat.

• Sensing bulb or capillary in the leaving water or strapped in the coldest coil circuit, metal-to-metal, insulated from false ambient.
• Setpoint typically just above freezing — teaching band about 35–38°F. 20°F is how you split a coil.
• Often manual-reset so the operator finds the flow or damper problem instead of cycling ice.
• Do not substitute a space stat or a low-pressure refrigerant switch for freeze protection on a hydronic coil.

Class B still sees freeze stats on legal ≤ 25-ton chilled-water air handlers. A 30-ton AHU with the same freeze stat is Class A tons. North Florida (Jacksonville, Panhandle) sees freeze-stat work on economizer AHUs; South Florida still uses the same setpoint because a lost-flow chiller can ice a coil in July.

Dual and low-pressure switches

A dual high/low pressure control puts two bellows in one housing (Chapter 27.1). Calibration:

Calibrate with accurate gauges (or a pressure pump / deadweight on the bench). Range screw moves both cut-out and cut-in; differential screw spreads them. Never leave a jumper on either bellows as a “repair.” Raising a high-pressure cutout to hide a dirty condenser is a Section 31.4 fail.

A low-pressure control used alone is the same suction bellows without the high-side. On pump-down, the thermostat drops the liquid-line solenoid; the LP stops the compressor. Calibrating LP as if it were a box thermostat is how you short-cycle a freezer.

Pneumatic, proportional, summer-winter, thermostatic motors

Pneumatic main air is about 18–25 psig after the dryer and filter. The analog branch signal is 3–15 psig: 3 psi one end of travel, 15 psi the other, 9 psi mid-scale. Plenum tubing is UL 1820 (FBC Mechanical 602.2.1.3).

Calibration of a pneumatic thermostat:

1. Tee a 0–30 psi test gauge in the branch.
2. With the space at setpoint, a proportional stat should sit near 9 psig.
3. Direct-acting (DA): temperature up → branch pressure up (typical cooling). Reverse-acting (RA): temperature up → branch pressure down (typical heating).
4. Throttling range (proportional band) is the degrees that swing the branch 3 to 15. A 2°F band hunts; a 20°F band never satisfies. Comfort often lands around 4–8°F — use the sequence.
5. One-pipe stats bleed through a restrictor; a clogged restrictor looks like a bad thermostat. Two-pipe stats have a dedicated supply and branch.

Proportional electric/electronic thermostats are the same idea in 0–10 V or 4–20 mA: output is modulating, not a click. Calibration is zero, span, and throttling range, plus direct vs reverse.

Summer-winter changeover (two-pipe hydronic, some heat pumps, some pneumatics) must have a deadband. Changeover on water temperature (hot water vs chilled water) or outdoor air, not on a random toggle that lets the same coil heat and cool in the same hour. A summer calibration that is perfect will be wrong in winter if the stat is DA when it should be RA.

Thermostatic motors (damper and valve actuators):

• Pneumatic spring ranges are often 3–7 and 8–13 psig so valves sequence (one starts after the other).
• Stroke: 90° on many dampers, linear on globe valves. A stopped linkage is a calibration miss.
• Electric floating (open/close/stop) vs proportional (0–10 V) vs spring-return fail-safe. End switches must actually make.
• Thermostatic (wax/bellows) radiator valves are self-powered; you set the head, you do not feed them 24 V.

Class A owns pneumatic control piping. Class B still has to read 3–15 psig and DA/RA on the exam, including on a 20-ton (legal Class B) AHU that happens to have leftover pneumatic stats from a renovation.

Thermostatic temperature controls and bellows-type devices

A thermostatic temperature control is a remote-bulb or bi-metal switch with a range and a differential (cut-out vs cut-in). Examples: box stats, coil stats, fan-limit combinations, defrost termination (Section 31.4). Calibration:

• Bulb metal-to-metal in the medium you actually care about (air, water, product). A box stat with the bulb in the discharge of the coil reads coil, not product.
• Differential too tight short-cycles the compressor; too wide swings the product.
• Bellows-type controls (temperature or pressure) use a volatile fill and a capillary. Do not kink the capillary. Do not flatten the bellows to “make it trip sooner.” Adjust the spring (range) and differential screws. A bellows that has lost charge will not calibrate — replace the control.

Florida HVAC scenario

A Class A crew in Hillsborough County commissions a two-pipe AHU with a pneumatic proportional stat, a chilled-water freeze bulb, and a dual pressure switch on a 20-ton DX backup (Class B tons, but the pneumatic mains are Class A). They set the freeze stat to 20°F “so it will not nuisance,” jumper the dual pressure switch “until gauges look right,” and leave the pneumatic stat DA in winter with a 2°F throttling range. Result: a frozen coil, a compressor without high-pressure protection, and a hunting damper. Correct: freeze 35–38°F manual-reset, dual pressure calibrated to gauges with high-side manual-reset, branch 9 psi at setpoint, RA for heat, deadband on summer-winter.

Traps: (1) Calibrating a wall stat against supply air. (2) Anticipator at either stop. (3) Freeze stat at 20°F. (4) Jumpered dual pressure. (5) Mixing main 20 psi with branch 3–15. (6) Class B bidding pneumatic mains. (7) Flattening a bellows instead of turning the range screw.