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Giotto Classic boiler pressure question

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  • Giotto Classic boiler pressure question

    Recently I cleaned out the Pressure switch & pipe on my classic as the machine was going over 1.5bar causing the relief valve to open. Machine is now working but I notice that I get a fluctuation on the pressure gauge. On initial turn on from cold as the boiler heats the water pressure the pressure goes up to 1.4bar (relay clicks) then pressure comes down to 1bar (relay clicks) & the boiler starts to heat the water again. Pressure goes up to around 1.4bar, relay clicks & boiler turns off & needle gradually falls back to 1bar. This occurs frequently as the machine warms up (first few minutes) then once warm settles at around 1.1bar at the lower end. It still fluctuates up to 1.4bar but not as often. Pretty sure that the pressure switch is old & tired and will look at buying a new pressure switch but could also be a pressure leak somewhere as well. This whole episode got me thinking on the operation of the pressure switch itself (yep always like to know how things actually work).

    Does the pressure switch have a predefined pressure 'zone' built into it, i.e. when I adjust the switch so that it switches off the boiler at say 1.1bar does this also lower the pressure when the boiler switches back on? or does the adjustment only adjust the turnoff point for the boiler?

    I'm intrigued on how this works as there is also a thermostat on the boiler that I suspect also feeds info the the control box for under/over temp. Can anyone shed some light as to how all this works?

    Many thanks

  • #2
    The pressure switch is a microswitch with the actuating force coming from a rubber membrane loaded by the boiler pressure.
    The force required to depress the actuator and stop current flow to the boiler element is higher than the force required to release the actuator and allow current. This and some lag time in the element transferring heat to the water gives the pressure 'zone'.

    Not familiar with the innards of a giotto classic but the thermostat is likely only for over temperature safety, shutting everything down at around 130C and may or may not be resettable.
    Last edited by Jimmytheboot; 17 May 2013, 08:05 PM.


    • #3
      Ah, thanks. Makes sense that the thermostat is for over temp safety shut down. I can see now that an old pressure switch that still works can still start to create problems, as the rubber diaphragm would lose some of it's elastic properties with age. Thanks for the explanation, has confirmed what I was thinking.

      You also highlighted the boiler element that I forgot to consider in all this. Thanks for that.