Electronics > Beginners

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

<< < (3/5) > >>

james_s:
Interesting, now that you mention it my friend did have a gravity feed hot water system with a booster pump for the shower, I'd never seen anything like it before. I don't actually know the history of showers in the USA however I don't think I've ever seen a house that didn't have one in this region. The oldest house I've been in is the one my uncle has which was built in 1922 and it still has multiple bathrooms that are almost completely original and have showers. I've never seen gravity feed hot water here, water heaters very much resembling what you can buy today were pretty much standard by at least the 1940s, I suspect earlier but I haven't seen one older than that.

richard.cs:

--- Quote from: james_s on January 04, 2020, 06:36:43 am ---Interesting, now that you mention it my friend did have a gravity feed hot water system with a booster pump for the shower, I'd never seen anything like it before. I don't actually know the history of showers in the USA however I don't think I've ever seen a house that didn't have one in this region. The oldest house I've been in is the one my uncle has which was built in 1922 and it still has multiple bathrooms that are almost completely original and have showers. I've never seen gravity feed hot water here, water heaters very much resembling what you can buy today were pretty much standard by at least the 1940s, I suspect earlier but I haven't seen one older than that.

--- End quote ---

UK hot water systems were traditionally* a mixture of large, low pressure, tanks heated by various means*2 and small on-demand gas heaters (this kind of thing: https://i.pinimg.com/originals/af/b9/f1/afb9f1f3d5aee1caee9b49bde0c4217a.jpg). The low pressure gravity fed systems had a number of advantages; it alleviated (real or perceived) concerns about contaminated hot water getting fed back into the public drinking supply, allowed cheap thin-walled copper tanks, and provided much higher flow rates (both hot and cold) for filling baths and similar in an era when baths were almost universally preferred over showers. Until perhaps the 1980s the electric instant showers were mostly associated with cheap bedsits (think one-room apartment) and retrofitting to the homes of elderly people who struggled to get in/out of a bath.

It is possible to have a good shower from a gravity system without pumping but it requires the right design of shower mixer and head, and is more like warm rain than a pressurised jet so not to everyone's preference. Also most thermostatic mixers are too restrictive, not really being designed for it. People who prefer showers tend to end up retrofitting either a pump or more cheaply*3 an instant electric shower of the type described in this thread.

The modern UK approach is instant gas water heating in smaller houses (most of them!), usually a "combi" boiler of around 16 kW which provides enough hot water for one shower, and also provides central heating but not both at once. Electrically heated houses tend to be built with mains-pressure water heaters that would look much more familiar to American eyes, and gravity systems are now mostly reserved for much larger houses where the peak power (and flow rate) demands of multiple showers can't be met any other way than by having lots of stored hot water.

* say 1930s to 1990s with a heavy dependence on income, etc.

*2 "back boiler" in the coal fireplace, later electric resistive heating and from the 1970s large natural gas boilers shared with the heating

*3 say US$100 to buy: https://www.screwfix.com/c/bathrooms-kitchens/showers/cat820272#category=cat820272&sort_by=price

Simon:

--- Quote from: treez on January 02, 2020, 03:58:35 pm ---Please help to explain the temperature  switching in my Aqualisa  “Vitalise S” electric shower?
Its initially  going mad hot when I switch to a cooler setting .
At 18:02 of this video, we see the  likely heater  format of the heater  element selector switches….



--- End quote ---

Please would you stop putting everything in the general chat section. Just sit and think for a moment before posting!

ocset:

--- Quote ---Please would you stop putting everything in the general chat section. Just sit and think for a moment before posting!
--- End quote ---
Thanks, OK, sorry about that.

SolarMan:

--- Quote from: treez on January 02, 2020, 03:58:35 pm ---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?

--- End quote ---

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 147 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/s

This 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).

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod