kitControl-Tstat - HVAC Components

Understanding Control Types: Tstat vs PID

Supplement to PID Control Guide

Two Ways to Control Temperature

In Niagara N4, you have two main options for controlling HVAC equipment:

1. Tstat (Thermostat) Component: Simple ON/OFF control

2. LoopPoint Component: Modulating PID control (0-100% output)

Let's understand when to use each one and why it matters.

The Tstat Component (ON/OFF Control)

What it does:

The Tstat component works exactly like your home thermostat. It outputs a simple TRUE or FALSE (ON or OFF) based on whether the temperature is above or below your setpoint.

Key Properties Explained:

Cv (Control Variable): The actual measured value (e.g., room temperature is 22°C)

Sp (Setpoint): Your target value (e.g., you want 21°C)

Diff (Differential): This is the "deadband" that prevents rapid cycling

Action: Direct (for cooling) or Reverse (for heating)

Out: The output - either TRUE (ON) or FALSE (OFF)

Understanding Differential (The Critical Part!)

The differential prevents your equipment from rapidly cycling ON and OFF. Here's how it works:

Heating Example (Reverse Action):

• Setpoint (Sp) = 21°C

• Differential (Diff) = 1°C

• Heater turns ON when Cv drops to 20°C (Sp - Diff)

• Heater stays ON as temperature rises

• Heater turns OFF when Cv reaches 21°C (Sp)

• Temperature coasts up slightly to maybe 21.5°C before falling

• Heater turns ON again at 20°C

Result: Temperature cycles between 20°C and 21.5°C

Cooling Example (Direct Action):

• Setpoint (Sp) = 22°C

• Differential (Diff) = 1°C

• Cooling turns ON when Cv rises to 23°C (Sp + Diff)

• Cooling stays ON as temperature drops

• Cooling turns OFF when Cv reaches 22°C (Sp)

• Temperature coasts down to maybe 21.5°C before rising

• Cooling turns ON again at 23°C

Result: Temperature cycles between 21.5°C and 23°C

Visual Representation of Tstat Operation:

Heating Mode (Reverse Action):

Temperature starts at 19°C (cold room)

19°C → Heater ON

19.5°C → Heater ON (heating)

20.0°C → Heater ON (heating)

20.5°C → Heater ON (heating)

21.0°C → Heater OFF (reached setpoint!)

21.3°C → Heater OFF (coasting from thermal inertia)

21.2°C → Heater OFF (starting to cool)

20.5°C → Heater OFF (still above 20°C)

20.0°C → Heater ON (hit lower threshold!)

...cycle repeats...

Real-World Example: Electric Radiator Control

The System:

Small office with electric radiator controlled by a relay (ON/OFF only)

Tstat Configuration:

• Cv: Room temperature sensor (currently reading 19.5°C)

• Sp: 21°C (comfort temperature)

• Diff: 1°C (prevents rapid cycling)

• Action: Reverse (heating mode)

• Out: Connected to digital relay (TRUE = energize relay = heater ON)

What Happens:

Current temp 19.5°C is below 20°C threshold → Out = TRUE → Relay closes → Heater ON

Temperature rises to 21°C → Out = FALSE → Relay opens → Heater OFF

Room temperature cycles between 20-21.5°C all day

Tstat vs LoopPoint: When to Use Which?

Decision Guide: Which One Should I Use?

Ask yourself these questions:

Question 1: Can my equipment modulate?

NO → Use Tstat

Examples: Electric heater with relay, pump start/stop, fan on/off, compressor staging

YES → Continue to Question 2

Examples: 0-10V valve, VFD on pump, modulating damper actuator

Question 2: How tight does my control need to be?

±1-2°C is acceptable → Tstat is fine (saves setup time)

Examples: Warehouse heating, simple office space

Need precise control (±0.5°C or better) → Use LoopPoint PID

Examples: Laboratory, clean room, critical comfort spaces, process control

Question 3: Is this a critical application?

High energy use OR expensive equipment → Use LoopPoint (better efficiency, less wear)

Examples: Large boiler, expensive chiller, high-capacity AHU

Small load OR cheap equipment → Tstat is acceptable

Examples: Small electric heater, bathroom exhaust fan

Common Mistakes with Tstat Components

Mistake 1: Differential Too Small

Setting: Diff = 0.1°C

Problem: Equipment cycles every 30 seconds! Relay burns out in 2 months.

Fix: Use Diff = 0.5-2°C depending on system thermal mass. Larger mass = can use larger differential.

Mistake 2: Wrong Action Setting

Setting: Heater configured with Direct action

Problem: Heater turns ON when temperature rises! Room overheats then freezes.

Fix: Remember - Heating = Reverse, Cooling = Direct

Mistake 3: Using Tstat for Modulating Equipment

Scenario: You have a 0-10V modulating valve but wire it to a Tstat output

Problem: Valve slams between 0V (fully closed) and 10V (fully open). Temperature swings wildly. Valve actuator wears out quickly.

Fix: If you have modulating equipment, use LoopPoint PID control instead!

Practical Wiring Examples in Niagara

Example 1: Electric Heater with Tstat

Equipment:

• Temperature sensor: SpaceTemp_AI (reading 19.2°C)

• Electric heater relay: Heater_BO (BooleanWritable)

• Desired temperature: 21°C

Tstat Configuration:

• Wire: SpaceTemp_AI.Out → Tstat.CV

• Set: Tstat.Sp = 21

• Set: Tstat.Diff = 1

• Set: Tstat.Action = Reverse

• Wire: Tstat.Out → Heater_BO.In

Result:

Heater turns ON at 20°C, turns OFF at 21°C, room cycles between 20-21.5°C

Example 2: Hot Water Valve with LoopPoint

Equipment:

• Temperature sensor: FlowTemp_AI (reading 62.5°C)

• 0-10V mixing valve: HWValve_AO (NumericWritable)

• Desired temperature: 70°C

LoopPoint Configuration:

• Wire: FlowTemp_AI.Out → LoopPoint.In

• Set: LoopPoint.Setpoint = 70

• Set: LoopPoint.ProportionalConstant = 4

• Set: LoopPoint.IntegralConstant = 0.5

• Set: LoopPoint.Action = Reverse (heating)

• Set: LoopPoint.MinOutput = 0, MaxOutput = 100

• Wire: LoopPoint.ControlledVariable → HWValve_AO.In

Result:

Valve smoothly modulates between 0-100%, maintaining steady 70°C ± 0.2°C

Summary: Quick Decision Matrix

Use Tstat when:

✓ Equipment can only be ON or OFF (no modulation capability)

✓ Temperature swings of ±1-2°C are acceptable

✓ Simple, low-cost equipment

✓ You want quick, easy setup

✓ Examples: Electric heaters, pumps, fans, simple zone control

Use LoopPoint when:

✓ Equipment can modulate (0-10V valves, VFDs, dampers)

✓ Precise control is needed (±0.5°C or better)

✓ Energy efficiency is important

✓ Equipment is expensive (minimize wear from cycling)

✓ Examples: Modulating valves, VFDs, boiler firing rate, precision systems