Overview
Most HVAC control faults that look like failed hardware are actually signal mismatches. A 0–10 V actuator being driven from a 2–10 V output never fully closes. A 4–20 mA transmitter wired into a voltage input reads as a constant extreme. An NTC probe read by a controller that expects PT1000 looks plausible at room temperature and is wrong everywhere else.
This guide covers the signal families you will meet on real projects — voltage, current, resistive and floating — how to wire them, how to scale them, and how to prove the loop works before you leave site.
Key Points
- Four families dominate HVAC control wiring: voltage (0–10 V, 2–10 V), current (4–20 mA), resistive (NTC, PT100, PT1000) and floating (tri-state, 24 VAC).
- Voltage is cheap and simple but degrades over distance. Current is immune to cable voltage drop and can detect a broken wire by itself.
- Scaling is always two steps: normalise the electrical value to 0–100%, then map that onto the engineering range.
- In practice, the largest error source is the wiring, not the sensor — a two-wire PT100 over a normal cable run can carry several degrees of error that a three-wire connection removes.
The Four Signal Families Compared
| Signal type | Typical values | Behaviour over cable length | Wire break detectable | Best suited to |
|---|---|---|---|---|
| Voltage | 0–10 V, 2–10 V, 0–5 V | Degrades with distance and electrical noise | No — 0 V is a valid reading | Devices in the same panel, floor or cabinet |
| Current loop | 4–20 mA | Stable over hundreds of metres | Yes, using the NAMUR NE 43 fault bands | Duct and plant-room transmitters, remote sensors |
| Resistive | NTC 10 kΩ, PT100, PT1000 | Error grows with lead resistance unless three- or four-wire | Partly — open circuit reads as over-range | Low-cost temperature sensing near the controller |
| Floating (tri-state) | 24 VAC open / common / close | Unaffected by distance | No | On/off and three-point valves and dampers |
A fifth option — a digital bus such as Modbus RTU or BACnet MS/TP — carries values and status together and removes most analog wiring problems, at the cost of addressing, configuration and a shared protocol understanding on both ends.
Voltage Signals: 0–10 V and 2–10 V
Voltage control is the default in HVAC because it is simple: the controller outputs a DC voltage, the actuator or valve amplifier follows it proportionally. Two practical limits decide whether it works:
- Output drive capability against input impedance. Add up the input impedance of every device connected to the same output. If the total draws more current than the output can supply, the voltage sags and every device on that channel reads low.
- Shared reference. A voltage signal is meaningless without its common. Mixing a controller common with an actuator common that is referenced to a different 24 VAC terminal is the single most common source of offset, jitter and slow drift.
The 2–10 V variant adds a 2 V offset, sometimes called a live zero. That offset makes a broken wire distinguishable from a genuine 0% demand, which is why many controllers accept both ranges and why you must match the configured range to the device on the other end.
Our 0–10 V transmitters — the DSTE-1001 duct temperature sensor, the DTHE-2001 wall-mount temperature and humidity sensor and the DSAE-2101 duct CO2 sensor — suit runs inside a normal building distribution. For a duct probe a hundred metres away in a plant room, use a current-output transmitter instead.
Current Loops: 4–20 mA
In a current loop the transmitter regulates current rather than voltage, so cable resistance does not change the reading. The loop is limited by the supply: the higher the total resistance, the more voltage the transmitter must be able to drive. The check takes ten seconds:
Maximum total loop resistance (Ω) = (supply voltage − minimum transmitter voltage) ÷ 0.020
For a 24 V supply and a transmitter that needs 12 V, that is (24 − 12) ÷ 0.02 = 600 Ω. If the receiving input presents a 250 Ω sense resistance, roughly 350 Ω of cable and connection resistance remains available — which is a lot of copper, but not unlimited.
- Two-wire (loop-powered) transmitters take their supply from the same pair that carries the signal — two wires in total, lowest cost, and the standard for duct sensors.
- Three-wire devices share a supply but return a separate signal; common in transmitters that need more power than a two-wire loop can supply.
- Four-wire devices are fully isolated, with independent supply and output — the usual choice where isolation is required.
The other advantage of a current loop is diagnostics. NAMUR NE 43 defines the out-of-range bands: a current below 3.6 mA or above 21 mA indicates a fault rather than a measurement. Configure the controller to alarm on those bands, otherwise a broken wire will be interpreted as a perfectly valid low temperature.
Grounding practice: connect the cable shield at one end only, normally the panel, and leave the field end floating. Isolated transmitters make this easier and are worth specifying where drives or large contactors share the cable tray.
Resistive Sensors: NTC, PT100 and PT1000
Resistive sensing is still the cheapest way to measure temperature, and the wiring determines how accurate it can be.
- NTC 10 kΩ is the HVAC workhorse. Its resistance is high, so lead resistance matters less, but the response curve is strongly non-linear and must match the controller’s configured curve.
- PT100 is 100 Ω at 0 °C and changes by roughly 0.385 Ω per °C. That low resistance makes it sensitive to lead resistance: 20 m of 0.5 mm² copper has about 0.7 Ω per conductor, so a two-wire connection adds roughly 1.4 Ω to the loop — about 3.6 °C of error before any real measurement begins.
- PT1000 is 1,000 Ω at 0 °C with roughly 3.85 Ω per °C. The same 1.4 Ω of lead resistance becomes only about 0.36 °C — which is why PT1000 is the better default for long two-wire runs.
- Three-wire and four-wire connections compensate for lead resistance properly. If you need PT100 accuracy at distance, three-wire is the minimum standard, not an upgrade.
A transmitter solves the same problem differently: convert the resistance to 0–10 V or 4–20 mA at the sensor, and the cable no longer carries the measurement as a resistance at all. Sensors with built-in transmitters, such as the DSTE-1001, remove lead-resistance error entirely. For a deeper comparison of sensing technologies, see PT100 vs NTC vs digital HVAC temperature sensors.
Scaling: From Milliamps and Volts to Engineering Units
Every conversion follows the same two steps: normalise to 0–100%, then map onto the range. These are the conversions that come up most on site:
| Signal | Engineering range | Formula | Example |
|---|---|---|---|
| 0–10 V | 0–100% | % = V ÷ 10 × 100 | 6.5 V = 65% |
| 2–10 V | 0–100% | % = (V − 2) ÷ 8 × 100 | 4.4 V = 30% |
| 4–20 mA | 0–100% | % = (I − 4) ÷ 16 × 100 | 12 mA = 50% |
| 0–10 V | 0–100 °C | T = V ÷ 10 × 100 | 7.2 V = 72 °C |
| 4–20 mA | −20 to 60 °C | T = (I − 4) ÷ 16 × 80 − 20 | 12 mA = 20 °C |
| 4–20 mA | 0–2000 ppm CO2 | ppm = (I − 4) ÷ 16 × 2000 | 8 mA = 500 ppm |
| 0–100% demand | 2–10 V output | V = 2 + (% ÷ 100) × 8 | 30% = 4.4 V |
The arithmetic is simple, but it is also easy to invert under time pressure. Our free analog signal calculator converts between volts, milliamps, percent and engineering units, and flags the NAMUR fault bands, so you can check the numbers before you touch the controller.
Floating (Tri-State) Control
Floating control uses two switched 24 VAC outputs — open and close — with a shared common. It is the cheapest way to drive a valve or damper to a position, and it has one structural weakness: without a feedback wire, the controller only knows position by counting run time.
- Drive time matters. A 60 s actuator and a 100 s actuator are not interchangeable in a controller configured for the other.
- Overlapping or missed pulses cause position drift that accumulates over a season. Re-synchronising periodically, or specifying an actuator with position feedback, prevents it.
- Floating is sufficient for isolation valves and on/off terminal dampers. It is not sufficient where the coil must hold a setpoint, or where ΔT is a performance metric.
If you are choosing between on/off and modulating duty, the trade-off is covered in our guide to electric valve selection for HVAC.
Commissioning Checks That Catch Most Faults
- Measure at both ends. Compare the signal at the controller terminal and at the device terminal. More than roughly 0.2 V of difference on a 0–10 V signal means a wiring, load or common problem, not a device problem.
- Confirm the signal type and range. Voltage input, not current input; 0–10 V, not 2–10 V; the correct sensor curve. Write the configured range on the as-built.
- Drive 0%, 50% and 100%. Record the electrical value and the physical position at each step, including the direction of travel.
- Check the loop budget on 4–20 mA. Confirm the transmitter has enough supply headroom, and that the controller alarms on the out-of-range bands below 3.6 mA and above 21 mA.
- Record the fail position. On power loss, does the valve open, close or hold? Confirm it matches the design intent and note it in the handover file.
Five Mistakes We See Most Often
| Mistake | Symptom | Fix |
|---|---|---|
| 0–10 V output into a 2–10 V input, or the reverse | Valve never fully closes or never fully opens | Check the range in the datasheet and match the controller configuration |
| Two voltage signals sharing a common through different 24 VAC legs | Slow offset, jitter, readings that drift with load | Use one reference, or specify isolated outputs |
| 4–20 mA wired into a voltage input | Reading pinned at one extreme regardless of the process | Select the current input, or use a signal converter |
| Long 0–10 V run to a remote duct sensor | Reading sags or becomes noisy in wet weather or when other loads switch | Move to a 4–20 mA transmitter or a bus device |
| Fault bands left disabled | A broken wire reads as a valid measurement | Enable the NAMUR-style out-of-range alarm |
FAQ
Q: Can I run 0–10 V over 100 metres?
A: Not with reliable accuracy. Voltage signals pick up noise and drop across the cable. Put the transmitter at the sensor and send 4–20 mA, or use a bus device.
Q: What is the practical difference between 2–10 V and 0–10 V?
A: The 2 V offset provides a live zero, so a broken wire is distinguishable from a real 0% demand. Many devices accept both, but the controller’s configured range must match the device.
Q: Why does my 4–20 mA loop read correctly at the panel and wrongly at the far end?
A: Total loop resistance has exceeded what the transmitter can drive. Check the budget: (supply − minimum transmitter voltage) ÷ 0.02 gives the maximum resistance in ohms.
Q: Is PT100 or NTC more accurate?
A: PT100 and PT1000 are more linear and more accurate over a wide range; NTC 10 kΩ is cheaper and adequate over a narrow HVAC band. In practice the wiring introduces more error than the sensing element — especially for two-wire PT100.
Q: Where do I start if a zone controls poorly?
A: Measure the commanded signal and the actual position together, at several points across the range. That single test separates a signal problem from a mechanical or hydraulic one.
Summary
Signal choice is a design decision, not a wiring detail. Use voltage for short runs inside a panel or floor, current for anything remote or safety-relevant, resistive sensing only where the wiring supports its accuracy, and floating control only where position accuracy does not matter. Then verify at both ends of the cable before commissioning, and most faults disappear before they appear on a service report.
Work With DNARENS
DNARENS builds the control layer for HVAC projects: FCU thermostats, motorised ball valves, motorised valves, modulating damper actuators and 0–10 V temperature, humidity and CO2 transmitters, manufactured in Quanzhou, China, with OEM and ODM support. Every unit is 100% function-tested before shipment, and third-party certification can be arranged for the destination market on request.
Free tools for your next project: the analog signal calculator, sensor range selector, valve Cv calculator, FCU flow calculator and damper actuator torque calculator — all at danrens.com/tools.
Contact our engineering team with your device list, signal types and target market for matched models and a quotation.
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