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N°5 – Stats (VTC to A level)

Stats are simple control mechanisms which compactly regroup the 4 stages of a control regulation system (measurement-comparison-transmission-action).

Their mode of action is often the equivalent of a basic “open/close” electrical contact. This method of control is called “all or nothing.”

The regulating of a heating system by ambient thermostats can mean a succession of on/off actions of the boiler.

  • It’s too hot, the electric contact of the thermostat opens and the boiler stops.
  • It’s too cold, the electric contact of the thermostat closes and the boiler starts.

Stats are used in simple control systems, especially in the field of safety.
There are thermostats (general temperature regulating), aquastats (water temperature), pressostats or manostats (pressures), and hygrostats (humidity), etc.

There are mechanical and more precise electronic models.
In mechanical ambient thermostats and aquastats, detection is carried out by a temperature sensitive device; the comparison is carried out by a balance spring and the action carried out by an opening or closing of an electric circuit.


Indicate on the aquastat below the 4 stages of « measurement-comparison-transmission-action ».


Connect, by drawing, the aquastat below, so that the burner is on.


The system below is in operation (January in the Northern Hemisphere). Is the boiler at operating temperature and why?

The boiler is at operating temperature because the aquastat contact is open and the burner is off.

The thermostatic valve is counted in the « stat » family. Fitted to the emitter inlet, it controls the interior temperature of a heated room by progressive variation of hot water flow. Its operation is therefore not of the “all or nothing” type.

Source DANFOSS                                        Source Hervé SILA

1: Thermostatic bellows
2: Ambient temperature set point control screw thread (by valve head rotation)
3: Compensation spring (allows comparison between the set point and the real ambient temperature).
4: Thrust axis of the thermostatic head towards the clapper valve.
5: Gland seal
6: Clapper return spring
7: Clapper (or valve), its position allows variation of water flow in the emitter.
8: Tap body
9: Flow direction

10: Shows the uncoupling point of the thrust axis and the clapper.
As with all control regulation systems, the thermostatic valve has the 4 stages of « measurement-comparison-transmission-action ».


Considering the identification shown above, which elements conform to the stages of «measurement-comparison-transmission-action»?