PID Controller vs Thermostat — Which Do You Need?

Choosing between a PID controller and a thermostat (on/off controller) comes down to one question: how tight does your temperature tolerance need to be? This guide walks through how each control method works, compares them spec-for-spec, and lines up our current PID and on/off controller ranges so you can match the right device to your process.

The Quick Answer

If your process needs to hold within roughly ±1°C of setpoint, or you're running ramp and soak profiles, heat treatment cycles, or dual heating/cooling control, a PID controller is the right choice. If a tolerance of a few degrees either side of setpoint is acceptable — HVAC, panel climate control, catering equipment, simple heating or cooling triggers — a thermostat (on/off controller) is simpler to set up, and often cheaper, for the same job.

How PID Control Works

A PID controller uses closed-loop control: it continually reads the sensor input (the process value) and calculates an output that varies — rather than simply switching fully on or fully off — to hold the process as close as possible to setpoint. The name comes from the three calculation terms involved: Proportional, Integral and Derivative. Most modern PID controllers, including the Novus range we stock, include an auto-tune function that calculates a working set of PID parameters automatically, rather than requiring manual tuning from scratch.

Because the output is varied rather than simply switched, PID control is normally paired with a fast-switching output stage. Many Novus PID controllers offer a voltage pulse output (typically 5V DC) designed to drive a solid state relay (SSR), which has no moving parts and can switch fast enough to take advantage of the fine adjustments PID control makes. Some models also offer a direct relay output as a second output or alarm.

For a full breakdown of the P, I and D terms, see PID Explained and the Glossary Of PID Terms.

How Thermostatic (On/Off) Control Works

A thermostat, also called an on/off controller, has a simpler output behaviour: it's either fully energised or fully de-energised, with no proportional stage in between. Once the measured temperature crosses the setpoint, the output switches — and an adjustable hysteresis (dead-band) value stops it switching rapidly back and forth as the temperature hovers near that point.

Because the output only needs to switch on and off rather than modulate finely, thermostats are typically built around a direct relay output rated to switch the load itself — no separate SSR needed. This makes installation simpler and, for loads within the relay's rating, removes a component from the system.

See On Off Control Explained and Thermostat Setup for more detail on setup and hysteresis.

PID vs Thermostat: Spec Comparison

	PID Controller	Thermostat (On/Off Controller)
Control method	Proportional-Integral-Derivative — output is continuously varied to hold the process value close to setpoint	Output switches fully on or off, with an adjustable hysteresis band to prevent rapid cycling
Measurement accuracy (sensor input)	Novus N1200-DIO: 0.2% of span with Pt100 input; 0.25% of span ±1°C with J/K/T/E thermocouple input	Novus N322: 0.7°C (1.3°F) with Pt100 input; 3°C (5.4°F) with J/K/T thermocouple input
Achievable control tolerance	Typically ±0.1°C or better once tuned — depends on process dynamics and tuning, not just sensor accuracy	Set by the hysteresis band you configure — commonly ±1–3°C; can't be PID-tuned
Output type	SSR pulse output (silent, no moving parts) and/or relay, depending on model	Direct relay output rated for the load — e.g. N322's 16A SPDT relay, no SSR required
Setup	Auto-tune calculates PID parameters; more configuration options overall (ramp/soak, alarms, communications)	Set a setpoint and hysteresis value — minimal configuration
Best suited to	Tight-tolerance processes, ramp & soak profiles, dual heating + cooling from one controller	Wider-tolerance applications where simple cycling is acceptable
Production Automation price range	£42–£146 (N1030-PR to N1200-DIO)	£24–£62 (Ageon A102 to Novus N322 range)

A note on accuracy: the accuracy figures above are measurement accuracy — how precisely the controller's sensor input reads the actual temperature — not the tolerance the process will hold to in day-to-day operation. A well-tuned PID controller can typically hold a process within roughly ±0.1°C of setpoint or better, because its PID algorithm continuously fine-tunes a varying output rather than simply switching fully on or off. A thermostat's output can't be PID-tuned in the same way: it's either on or off, with a hysteresis (dead-band) setting that determines how far the temperature can drift before the output switches again. That hysteresis is also there for a practical reason — most thermostats switch an electromechanical relay, a mechanical component rated for a limited number of switching cycles over its working life. Switching it too often shortens that life, which is why on/off controllers are set up to cycle only every few seconds rather than continuously. An SSR pulse output used with PID control has no moving parts and doesn't carry the same wear limitation, which is part of why PID control can afford to make much more frequent, finer adjustments.

Some PID controllers can also be switched into on/off mode via a parameter change — the Novus N1030-PR and N1050-PR both support this, so it's possible to buy a PID controller and run it as a simple thermostat if requirements are uncertain, and switch to full PID control later without changing hardware.

Which Should You Choose?

As a general guideline: if your process needs to hold within roughly ±1°C of setpoint or better, choose PID. If a tolerance of roughly ±3°C or more is acceptable, an on/off thermostat is usually the simpler and more cost-effective option. A few other factors worth weighing up:

Load switching speed — if the load switches rapidly (as with an SSR-driven heating element under PID control), an electromechanical relay isn't a good fit; that's why fast PID applications typically pair an SSR-output controller with a solid state relay rather than switching the load directly.

Ramp and soak requirements — if the process needs to hold at intermediate temperatures for a set time before moving on (kiln firing schedules, heat treatment cycles), that's a PID controller feature (e.g. the Novus N1050-PR's 5 programs of 4 segments, or the N1200-DIO's up to 20 recipes across 180 segments) — on/off controllers don't offer profile programming.

Process integration and non-temperature variables — if you need remote monitoring or control via RS485 Modbus, or need to control a process variable other than temperature (level, pressure, weight, humidity, speed), the N1200-DIO's universal input and built-in RS485 cover this; a standard thermostat only handles a simple temperature on/off decision.

Simplicity and retrofit — if you're replacing a failed mechanical or capillary thermostat and the original tolerance was already generous, a digital on/off controller such as the N322 or Ageon A102 is usually a straightforward like-for-like upgrade without the added configuration of full PID.

Our PID Controller Range

The models below are generally the base "PR" variant of each line — the simplest configuration, with one pulse output for driving an SSR plus one relay output, and usually the best starting point. Most lines also offer variants with additional relay outputs, extra alarms, or built-in RS485 Modbus communication — for example, the Novus N1050-PRRR-485 adds two further relay outputs plus RS485 on top of the base N1050-PR, at a higher price. If your application needs more outputs, alarms, or network integration than the base model below offers, contact us and we can point you to the right variant.

Model	Key features	From
Novus N1030-PR	Compact single-setpoint PID, auto-tune, switchable to On/Off mode, PT100/J/K/T input, 1/16 DIN, SSR pulse + relay outputs	£42
Novus N1020-PR (2024)	Compact 1/32 DIN footprint (22.5 x 45mm cutout) for confined panels, USB-C configuration, auto-adaptive PID, pulse + relay outputs — a direct replacement for the CAL3300 or Omron E5GC	£67
Novus N1050-PR	Ramp & soak (5 programs × 4 segments), USB QuickTune configuration, adds Type S thermocouple support	£79
Novus N20K48	Modular process controller, Bluetooth/QuickTune Mobile setup, universal input (temperature plus level, pressure, weight, humidity or speed), 20 programs × 9 segments	£86
Novus N1200-DIO	Premium process controller — pulse, relay and analogue outputs, built-in RS485 Modbus, universal input, up to 20 ramp & soak recipes across 180 segments	£146

See the full PID Temperature Controllers range, split into Basic and Advanced models. PID controllers with an SSR pulse output need a compatible solid state relay to switch the load.

Our Thermostat / On-Off Controller Range

Model	Key features	From
Ageon A102	Entry-level heating-or-cooling controller with IP68 NTC sensor supplied, -50 to +100°C range, 17A direct relay output, preconfigured for cooling with a switchable heating mode	£24
Novus N322 (PT100)	Digital on/off thermostat, 2 independent setpoints, 16A relay output (no SSR needed), -50 to +300°C, 0.7°C measurement accuracy	£54
Novus N322 (J/K/T)	Same as above with thermocouple input — up to 1000°C range with Type K, 3°C measurement accuracy	£59
Novus N322 (PT100, RS485)	PT100 version with Modbus RS485 communication for remote monitoring	£62

Explore the full Digital Thermostats & On/Off Controllers range, including the Ageon Black Line controllers.

Common Applications

PID control is the usual choice for ovens and kilns running multi-stage firing schedules, autoclaves and sterilisation cycles, heat treatment and metal processing, brewing and fermentation temperature control, and processes needing SCADA/PLC integration or control of a variable other than temperature (level, pressure, weight, humidity, speed). See Kiln Temperature Control and Powder Coating Oven Temperature Control for application-specific guidance.

On/off thermostats are the common choice for HVAC retrofit, refrigerated display and catering equipment, general machinery where a simpler cycling response is acceptable, and floor or electric heating driven directly from the controller's relay.

Frequently Asked Questions

Can a PID controller be run as a simple on/off thermostat?
Yes — the Novus N1030-PR and N1050-PR can both be switched from PID to On/Off control via a parameter change, so the same controller can cover either use case if requirements aren't fixed yet.

Is a thermostat always less accurate than a PID controller?
In terms of sensor measurement accuracy, generally yes — see the spec comparison table above for verified figures from our own range. But the more practical difference is control tolerance: a PID controller's continuously-varying output can typically hold a process within roughly ±0.1°C once tuned, while a thermostat's on/off output is set by its hysteresis band, commonly ±1–3°C, and can't be PID-tuned. For a wider-tolerance application that gap may not matter in practice.

Do I need to buy a separate SSR with a thermostat?
No — models like the Novus N322 and Ageon A102 switch the load directly through their built-in relay. A separate SSR is typically only needed with PID controllers that have an SSR pulse output, such as the N1030-PR, N1020-PR, N1050-PR and N20K48.

What sensor types are supported?
It varies by model. PT100 and J/K/T thermocouple inputs are common across both PID and thermostat ranges; the N1050-PR adds Type S thermocouple support, and the N20K48 and N1200-DIO take a wider universal input (including 4-20mA and voltage signals) alongside standard sensor types. The Ageon A102 instead uses its own supplied NTC sensor rather than a PT100 or thermocouple input.

Can either type replace an old mechanical or capillary thermostat?
Yes — both the digital on/off N322 and the PID N1030-PR are commonly used as direct retrofits for failed bimetallic or capillary thermostats, with the choice coming down to how tight the replacement needs to hold tolerance.

Need Help Choosing?

We're happy to talk through your application and suggest the right controller for your process. Contact us if you'd like technical or sales support.