HVAC Sensors and Control Signals

A control system is only as honest as its signals. A thermistor set to the wrong curve, a transmitter scaled wrong, or a shield grounded twice can send a technician after the wrong fault for days. Advanced controls work means you can stand at a panel with a meter and prove what a signal should be, what it is, and where the difference comes from. Panels can hold line voltage alongside low-voltage wiring, so this work is for trained technicians, following lockout rules.

What You Will Be Able to Do

• Identify the common signal types and how to measure each.
• Test temperature sensors against a chart.
• Work out what a voltage or milliamp signal should read.
• Test status inputs and actuators.
• Find wiring, grounding and noise problems.

Signal Types

Temperature Sensors

A thermistor is a resistor that changes value with temperature. Most HVAC thermistors have a negative curve, so resistance falls as temperature rises. A 10K thermistor reads 10,000 ohms at 77°F.

Curves differ between sensor types, and this is a classic trap. A 10K Type 2 and a 10K Type 3 both read 10,000 ohms at 77°F. At 32°F, the Type 2 reads about 32,650 ohms and the Type 3 about 29,490 ohms. A sensor with the wrong curve in the controller looks perfect at room temperature, and can be about 6°F off at freezing. Always check the sensor type set in the controller. Naming varies by brand, so go by the curve data.

An RTD, or resistance temperature detector, works the other way. Resistance rises with temperature. A common platinum type reads 1,000 ohms at 32°F and changes about 2.1 ohms per °F.

Because an RTD changes so few ohms per degree, wire resistance matters.

Worked example: a two-wire 1,000-ohm RTD sits on a 500-foot run of 18 AWG wire, so the signal travels 1,000 feet out and back. That wire has about 6.4 ohms of resistance. Dividing by 2.1 ohms per °F gives a false reading about 3°F high. A 10K thermistor loses almost nothing to the same wire, because it changes hundreds of ohms per degree near room temperature.

To test a temperature sensor:

1. Measure the actual temperature with a trusted thermometer, right beside the sensor.
2. Disconnect the sensor at the controller.
3. Measure resistance, and compare it with the chart at that temperature. An ice bath gives you a fixed test point at 32°F.
4. Open reads near infinity, and a short reads near zero. Most controllers show an extreme value for each.
5. If the sensor is good at the sensor end but bad at the controller, the fault is in the wire.

Do not use a software offset to make a bad reading look right. An offset is only for a small, known calibration difference on a good sensor. An offset that hides a fault makes the next diagnosis harder.

Voltage Signals

Voltage signals are scaled, so you need the range of the transmitter and the range set in the controller.

Formula: value = low end + (signal minus low signal) divided by (signal span) x value span.

Worked example: a duct static transmitter gives 0-10 V for 0 to 2 in. w.c. You measure 6 volts. Value = 0 + (6 divided by 10) x 2 = 1.2 in. w.c. If the screen shows 2.4 in. w.c., the controller scaling is wrong. It may be set for a 0 to 4 in. w.c. range.

A 2-10 V signal starts at 2 volts. A signal of zero volts on that range means a broken wire or a failed device, which is useful because it does not look like a normal low reading.

Milliamp Signals

A 4-20 mA signal is a current loop. Current stays the same around the loop, so long wires do not change the value much. That makes it a good choice for long runs and noisy areas.

The bottom of the range is 4 mA, not zero. That is called a live zero. A reading under 4 mA usually means a fault, such as an open wire or a dead transmitter, instead of a low value.

Worked example: a transmitter covers 0 to 100% humidity on 4-20 mA. The span is 16 mA. You read 12 mA. Value = (12 minus 4) divided by 16 x 100 = 50%.

Many controllers read a loop as voltage across a resistor. A 250-ohm resistor turns 4-20 mA into 1 to 5 volts. A 500-ohm resistor turns it into 2 to 10 volts. To check a live loop without breaking it, measure the voltage across that resistor. Reading 3 volts across 250 ohms means 12 mA.

Status Inputs

Status tells you what a device really did, as distinct from what it was told to do. A command with no matching status is a fault condition.

A current switch proves that a motor is drawing current. If it is set too low, a motor with a broken belt can still trip it, because the motor draws some current with no load. Set the switch above the no-load current, and test it against a real belt loss if you can. An airflow differential pressure switch proves air is moving. On a variable frequency drive, the current switch must be one built for that duty.

To test a status input, watch the point on the screen while you open and close the contact. Then check the actual device at the equipment.

Actuators and Outputs

Know how each actuator works before you judge it.

Check these in order:

1. Is the point in override?
2. Is the output signal at the controller correct?
3. Is the signal reaching the actuator?
4. Does the actuator have power?
5. Does the shaft move, and does the damper or valve move with it?

An actuator can run its full stroke while a slipped coupling leaves the damper still. Look at the damper blade or valve stem. Check the failure position too: a spring-return actuator must go to the position the sequence calls for when power is lost.

Pneumatic controls use air pressure, commonly 3 to 15 psi, and still run in many older buildings. A leak, dirty air or a failed transducer between the digital controller and the air line can cause problems that look like a controller fault.

Wiring, Grounding and Noise

• Keep low-voltage signal wiring away from motor and drive wiring. Variable frequency drive output cables are especially noisy.
• Use shielded twisted-pair cable where the manufacturer calls for it.
• Ground a shield at one end only, unless the manufacturer says otherwise. Grounding both ends can cause a ground loop, a false current in the shield.
• Do not share a 24 VAC transformer between devices unless the polarity is kept the same. A reversed common leg can short through a controller and damage it.
• Check every terminal for tightness. Many intermittent faults come from one loose wire.

Warning: Control panels and VFDs can hold line voltage, and VFDs store a charge after power is removed. Lock out power and verify it before you touch wiring. Testing live signals is for trained technicians with rated meters.

Worked Diagnosis

A variable air volume fan runs at 100%, and the screen shows duct static at 0.4 in. w.c. with a 1.2 in. w.c. setpoint. Occupants complain of noise.

1. A manometer at the duct probe reads 1.9 in. w.c., so the real pressure is high, and the screen is wrong.
2. The transmitter output reads 5.6 mA at the controller.
3. On a 0 to 4 in. w.c. range, 5.6 mA means 0.4 in. w.c. The transmitter and scaling agree with each other.
4. The transmitter's tubing is cracked and pulling room air, so it senses far less than the duct.

The signal path was honest, but the sensor was not seeing the duct. You replace the tubing, and the fan slows down and settles at setpoint. Replacing the controller would have done nothing.

Common Mistakes

Check Your Understanding

On a 2-10 V humidity signal covering 0 to 100%, what do 6 volts mean? (6 minus 2) divided by 8 x 100 = 50%.

What current should a 0 to 100 psi current-loop transmitter send at 25 psi? 4 plus (0.25 x 16), or 8 mA.

A controller reads a 4-20 mA loop across a 500-ohm resistor. At 12 mA, what voltage do you expect? 0.012 x 500, or 6 volts.

You put a 10K sensor in an ice bath and read about 29,490 ohms. Which curve is it? Type 3. A Type 2 would read about 32,650 ohms at 32°F.

A 4-20 mA loop reads 23 mA. What does that tell you? The loop is over range or shorted, so do not treat it as a high value.

Key Takeaways

• Know the signal type before you pick a test.
• Thermistor curves differ, and both 10K types read the same at 77°F.
• Wire resistance matters on an RTD and hardly at all on a 10K thermistor.
• Scale every signal, and compare it with a real measurement.
• Below 4 mA on a current loop means a fault, not a low reading.
• Check the device itself, because a shaft can turn while a damper does not.