Electronics > Beginners

Electric shower goes mad hot when i switch to a cooler setting...why?

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Simon:
sounds like a control algorithm going haywire. For example a poorly tuned PID control will oscillate.

Gyro:

--- Quote from: Simon on January 04, 2020, 06:44:00 pm ---sounds like a control algorithm going haywire. For example a poorly tuned PID control will oscillate.

--- End quote ---

It's a standard electric shower, it hasn't got any of that.  ;)  (It's switches, elements and a water valve)

IanB:
Here's an answer to the "mad hot" question.

Firstly, understand that instant electric showers have two internal control switches: one turns the heater on and off, and the other turns the water on and off.

In a well designed shower, when you turn it off it will first turn off the heater, and then after a short while it will turn off the water. This allows the flowing water to remove residual heat from the heater so it doesn't boil and scale up.

In a poorly designed shower it may turn off the heater and the water simultaneously. If this happens the stationary water stuck inside the heater will get very hot due to the remaining heat in the heater assembly. If you turn such a shower off and then on again in short order you will get a burst of boiling hot water coming out that will make you jump out of the way very quickly.

Now consider turning the heat control from high to medium, or medium to low. It may be that while turning from one setting to another the water flow is temporarily turned off between settings. I don't know why this should happen, but suppose it does. Then you will get the standing water inside the heater assembly being made really hot as above, and then being flushed out when the water is turned back on.

German_EE:
IanB, I think that you have the correct explanation, somewhere in that shower will be a solenoid operated water valve.

ocset:
Thanks but the flow doesnt reduce when i switch from 2 to 1.....(though obviously it does when it eventually  cuts out on overtemperature, but by then the mad hot water has already happened)

I just now wonder if the "2" element switch sticks ON for too long when i go from 2 to 1.
Im wondering if its very old, and the switch contacts are possibly "sticking"  for too long when they should break contact...perhaps they are heavily pitted contacts and just wont break quickly enough.....there being oxides all over  them which kind of temporarily weld the contacts together for a bit...meaning they cant break straight away.

So instead of going quickly from 2 to 1....i am actually going from "2" to "2 and 1 in parallel" then to "1"?   :-//
I believe others above have similar views.

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