HVACopediaPressure & temperature controls
Cut-out settings, adjustment directions, differentials and control signals.
Low-pressure control settings (psig, cut-out / cut-in)
| Application | Evaporator temp | Temp difference | R-22 | R-134a | R-404A | R-507 |
|---|---|---|---|---|---|---|
| Beverage, floral, produce cooler | 35 to 38 °F | 15 °F | 41 / 66 | 17 / 33 | 53 / 82 | 56 / 86 |
| Smoked meat, reach-in meat, service deli, seafood | 32 to 35 °F | 15 °F | 38 / 62 | 15 / 30 | 49 / 77 | 52 / 81 |
| Multi-deck fresh meat | 26 to 29 °F | 15 °F | 32 / 54 | 11 / 25 | 42 / 68 | 44 / 71 |
| Frozen glass door, frozen walk-in | −10 to 0 °F | 10 °F | 9 / 24 | – | 15 / 33 | 16 / 35 |
| Ice cream, open frozen food | −30 to −20 °F | 10 °F | 0 / 10 | – | 3 / 16 | 4 / 18 |
A starting point for commercial refrigeration. Cut-out is about the saturation pressure at 10 °F below the evaporator surface temperature you want. Cut-in is about the saturation pressure at the highest evaporator temperature you accept.
High-pressure safety cut-out (psig)
| Refrigerant | Cut-out |
|---|---|
| R-12 | 150 to 160 |
| R-22 | 260 to 270 |
| R-500 | 190 to 200 |
| R-502 | 280 to 290 |
Rule of thumb: set the high-pressure safety about 20% above the normal head pressure. For newer refrigerants use the manufacturer's setting. A high-pressure control is a safety, not an operating control.
Typical pressure range between cut-in and cut-out
| Refrigerant | Difference (psi) |
|---|---|
| R-22 | 22 |
| R-401A | 20 |
| R-404A | 20 |
| R-134a | 16 |
Adjusting a pressure control
- Low-side range screw: clockwise raises cut-in and cut-out together. Counter-clockwise lowers both.
- Differential screw: clockwise raises the cut-out only. It does not change the cut-in.
- High-side cut-out screw: clockwise raises the cut-out.
- Directions can be reversed on some makes, so read the control. After any change, cycle the equipment at least 3 times with gauges on.
- The low-pressure safety must be set below the normal cut-out but above 0 psig, so the evaporator never pulls a vacuum.
- Range moves cut-in and cut-out together (the differential stays the same). Differential changes the gap between them. A larger differential means longer runs and longer off periods. A smaller one means shorter, more frequent cycles.
- Cap-tube systems need a differential big enough for the pressures to equalize during the off cycle.
Thermostat and evaporator reference
| Item | Value |
|---|---|
| Room A/C thermostat cut-out | 56 to 60 °F |
| Room A/C thermostat cut-in | 77 to 80 °F |
| Room A/C differential | 3 to 8 °F |
| Air temperature drop across an evaporator (room A/C) | about 5 °F between inlet and outlet |
| Central A/C temperature split | 16 to 20 °F |
| Freezestat opens | about 33 °F above the evaporator (most close again on a 5 °F rise) |
| Best evaporator coil temperature | 34 to 36 °F |
| Heating thermostat differential | 0.5 to 2 °F |
| Heat anticipator current range | 0.15 to 1.0 A |
| Multistage thermostat, second stage | Closes when the room is more than about 3 °F from setpoint (adjustable) |
Control signals
| Damper position | 0 to 10 V signal |
|---|---|
| Fully open | 0 V |
| 25% closed | 2.5 V |
| 50% closed | 5 V |
| 75% closed | 7.5 V |
| Fully closed | 10 V |
Building controls use 0 to 10 V DC or 4 to 20 mA on light twisted-pair wire. Electronic sensors run on 5 to 15 V. A thermopile gives up to 750 mV.
Take it with you
All of these tables, plus the calculators, are in the HVACopedia app, offline.
Calculation aids and reference figures for licensed technicians. Always follow the manufacturer’s data plate and manual for the exact model, and local codes. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Pressures are typical saturation values and change with load and airflow.