Pressure switch and wattage temperature control of water heater
It will contain a diaphragm and a spring applying opposing pressure to a load-spreading plate or washer on the dry side of the diaphragm (to prevent rupture).

Also why don't they just use a flow switch instead of a pressure switch?

&quot;Just&quot;? A flow switch is more complex, more expensive and more likely to jam due to foreign material ingress.

I plan to replace it with a flow switch hence electronic question.

Talk to your home insurance provider first.




The thermostat seems connected to the low medium high switch. Shoudn't it be connected to some sort of relays?

Relays would be required if the switch and thermostat contacts were not capable of switching the current. Cost can be saved by avoiding them.

Haven't you come across heaters where high setting means full 6.5 kW heat used, perhaps in old style heater?

I'm not sure what your question is.
The sensible thing to do would be to use two heaters and have the two switches cam operated as follows:
2.5 kW | 3.5 kW | Result
-------+--------+--------
 Off   |  Off   | 0.0 kW
 On    |  Off   | 2.5 kW
 Off   |  On    | 3.5 kW
 On    |  On    | 6.0 kW


If the pressure switch or flow switch malfunctions will the heater still give say 6 kW full power?

There are two other levels of protection. The rotary switches should normally be off and the thermostat contacts would open when the temperature rises.

If yes, this can cause too high temperature and melt the case.

Melting the case is not the problem. Generating steam and the risk of explosion is. Water expands to 1760 times its volume when converted to steam.

If not and the thermostat purpose is to control heater power what kind of circuit must it be?

It's a mechanical switch. If temperature goes above the rated temperature then the contacts open. If the temperature goes below the rated temperature then the contacts close.

The heater unit has only few components, which should the one that regulate heater power, usually?

Thermostat. 100% on or 0% on.

Or how does the thermostat or switch regulate current? You have seen this done?

On - off control. Yes, I have seen it done on water heaters, electric ovens, clothes irons, washing machines, hair driers, waffle makers, central heating boilers and pumps, dishwashers, ... and they're just things around the house.