In my shower (my landlord’s so I cant take it apart), I have three temp settings….(1=coldest…2=medium….3 = hottest)…when I am in the shower and switch from 2 to 1…..the shower first goes mad hot, then trips out on overtemperature and so then goes stone cold…….then gradually warms up to setting 1.
I believe the following is happening…would you agree?....
The switch actuator, on its way from 2 to 1, actually transitions through setting 3, making it go mad hot. What surprises me, is that this short transition actually has an effect… being so short in time.
I am wondering if there is some delay in the switching mechanism?
Let's do some calculations to see if changing to setting 3 momentarily should be enough to make the water "mad hot":
There are only three variables which determine the temperature of water coming out of the shower: Input water temperature, power supplied to the heating elements, and the flow rate of water through the shower. I am going to assume that the incoming water temperature is constant and that the flow rate does not change while you change between the various (1,2,3) heating settings (maybe you can check the second of these assumptions?). If you turn the second heating control knob then you most certainly will be changing the rate of flow.
ASSUMED VALUES:
Heat Capaciy of Water: 4180 J/L.K
Capacity of the little tank inside the shower which holds the heating elements: .25L
Maximum combined power of the shower heating elements (assuming 9.5kW total power for the shower
1-aqualisa-vitalise-electric-shower-installation-guide-s-sl-slx-vte8521s.pdf (475.11 kB - downloaded 69 times.) minus 0.5kW for the pump): 9kW
Setting 1 power: 3kW
Setting 2 power: 6kW
Setting 3 power: 9kW
Time that you are on setting "3" while passing from setting "1" to setting "2": 0.2 seconds
Incoming water temperature: 5 degrees C
Outlet water temperature while on setting 2: 37 degrees C
CALCULATIONS:
~on setting 2:
Volume of water per second = Energy Per Second / (heat capacity of water * temper change)
Volume of water per second = (6kw * 3,600J/s/kW) / (4180 J/L.K * (37degressC - 5degreesC))
Volume of water per second = 0.14 L
Flow rate of water = 0.14 L/sThis means that in the ~0.2s that the selector is on setting "3"
.028L = (0.14 L/s) * (0.2 s)will be added to the little tank inside the shower.
Adding this to the volume of the tank, we can find the maximum amount of water that could have been exposed to the full 9kW power
Maximum volume of water that could have been exposed to the full 9kW power = .25L + .028L = .278L
With a flow rate of .14L/s (calculated above), this means that no more than about:
2 seconds worth of water (= .278L / 1.4L/s)should have been exposed to the full 9kW of power.
Of course, ideally we could account for the energy stored in the heating elements but I am pretty sure that would be minimal: They are pretty small and are unlikely to contribute much to heat capacity compared to water which is known for its high heat capacity.
If your shower is pumping out "mad hot water" for much more than about 2 seconds then these calculations suggest that it cannot be only from momentarily activating full power. My best guess is a sticky switch.
How about getting a friend to switch your shower from mode "2" to mode "1" while you keep and eye on your electric meter? If you can see a prolonged jump in power consumption then you can be pretty sure that there is indeed some sort of delay in the switching mechanism (or another, potentially more serious fault).