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HVAC Sensor Range & Output Selector

Pick the application, enter the band you actually need to measure, and get a standard measuring range, the sensor type that suits it and the output signal to specify. Covers temperature, differential pressure, humidity and CO₂.

Application and operating band

The band is what the sensor must read in normal operation — not the alarm limit and not the widest value it might ever see. Typical values are loaded for you and can be edited.

What are you measuring?

Lowest value to measure unit: °C

Highest value to measure unit: °C

Why not just pick the widest range available? Because a transmitter spreads its output across the whole span — a 0-100 °C unit used over an 8-14 °C band puts less than a tenth of its resolution where you need it.

The three rules behind the recommendation

The range must cover the whole operating band. A sensor that clips at the edge of normal operation is worse than no sensor, because the controller cannot tell “at the limit” from “past the limit”.
It should not be much wider than the band. Resolution, accuracy and repeatability are all quoted against full span, so an oversized range multiplies every error by the same factor.
Both ends of the band want some headroom. Enough for start-up and pull-down excursions, but not so much that the useful part of the signal collapses into a narrow slice of the scale.

In practice this means choosing the smallest standard range that still contains your band — which is exactly what the tool does. If your band fills more than about 90 % of the range, step up one size; if it fills less than 20 %, step down.

Typical ranges by application

Application	Typical band	Standard range	Usual signal
Chilled / hot water pipe	5–60 °C	0–100 °C	4–20 mA
Domestic hot water	45–70 °C	0–100 °C	4–20 mA
Steam / high-temp water	100–180 °C	0–200 °C	4–20 mA
Supply / return air in duct	12–38 °C	−20–80 °C	NTC 10 kΩ
Room air	18–28 °C	0–50 °C	NTC 10 kΩ
Outdoor air	−15–42 °C	−50–50 °C	NTC 10 kΩ
Duct static pressure	50–800 Pa	0–1000 Pa	0–10 V
Water differential pressure	20–150 kPa	0–200 kPa	4–20 mA
Relative humidity	25–90 %RH	0–100 %RH	0–10 V
CO₂ (DCV)	400–1500 ppm	0–2000 ppm	0–10 V

Bands are the values seen on ordinary commercial jobs; ranges are the sizes sensor makers normally stock. Treat them as a starting point, then confirm against the project's design conditions.

Element choice in one line each

NTC 10 kΩ — cheapest and the default for room and duct air; non-linear, so it needs the controller's matching curve.
PT100 / PT1000 RTD — linear, stable and interchangeable; the choice for water, steam and anything that has to stay accurate for years.
4–20 mA transmitter — converts any element into a signal that survives long cable runs; needs a loop supply and two wires per point.
0–10 V transmitter — simpler wiring and fine inside a plant room, but keep the run short and the screening good.
Passive elements on a bus — where the controller already speaks a fieldbus, a passive element on a bus module is usually cheaper per point than individual transmitters.