Electric shower goes mad hot when i switch to a cooler setting...why? - Page 1
Forum section: electronics / beginners

author: ocset
author_rank: Super Contributor
timestamp: January 02, 2020, 03:58:35 pm
content: 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….Electric shower teardown…..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?Also, no doubt I am wearing out the switch by switching it in the shower…..16A, 250V switches usually have a 10000 ops lifetime…but does this refer to making/breaking at mains peak, or mains zero cross or elsewhere on the cycle?16A switchhttps://www.infinite-electronic.hk/datasheet/4f-H8350ABBB-B.pdf

author: German_EE
author_rank: Super Contributor
timestamp: January 02, 2020, 05:34:32 pm
content: Best guess:The 'medium' setting has a 1Kw load across 230V and the 'high' setting has a 2Kw load. When you switch between settings both of the heaters are briefly connected producing a 3Kw heater.

author: Cyberdragon
author_rank: Super Contributor
timestamp: January 02, 2020, 06:26:48 pm
content: Quote from: German_EE on January 02, 2020, 05:34:32 pmDesign fault then, the switch in something like that should never be make before break.

author: German_EE
author_rank: Super Contributor
timestamp: January 02, 2020, 06:50:54 pm
content: It's your landlord's shower so, as you said, you can't take it apart. What you can do is the following:1. Make 100% certain that the shower is electrically isolated from the power on both live and neutral.2. Go away, have a coffee, and then check again that the supply is completely isolated from the shower.3. Test the DC resistance of the shower at various settings, look for jumps up and down in the resistance as you alter the control.4. Restore the 230V feed, have another coffee then come back and check your work.If in doubt, stop, being curious is one thing but naked EEVBlog users, water and mains power do not mix.

author: Ampera
author_rank: Super Contributor
timestamp: January 02, 2020, 11:24:22 pm
content: Quote from: German_EE on January 02, 2020, 06:50:54 pmIt sounds like you speak from experience there.

author: johnkenyon
author_rank: Regular Contributor
timestamp: January 03, 2020, 11:39:34 am
content: UK electric showers are crap. In order to get a decent flow at the right temperature during the winter, use of the "high" position is mandatory.The only exception is where the water pressure is so low to start off with, and there isn't sufficient flow to reduce the temperature down to a bearable level.The only time I've ever used the "low" setting is when I wanted a summer cooling shower, but not as cold as the incoming cold water feed (i.e. ca. 15 degC)Put another way - this sounds like equipment not being used as it was designed to be used.

author: ocset
author_rank: Super Contributor
timestamp: January 03, 2020, 01:37:24 pm
content: QuoteThanks, yes you are right, the landlord has pranged the flow control so its always on max flow...he did this so that the shower doesnt fail due to build up of scale........ive been in many rented flats...this is a common hack.Getting a shower really hot by reducing the flow lets scale build up quicker...also, it often results in temperature shut-offs...which mean stationary hot water in the element chamber , and even more scale build up. Temperature shut-offs also cause more wear to the temperature switch contacts....which if switched at mains peak, is only rated for some 1000 operations.

author: richard.cs
author_rank: Super Contributor
timestamp: January 04, 2020, 01:00:59 am
content: Quote from: johnkenyon on January 03, 2020, 11:39:34 amThe higher power ones give ok temperature and flow rate, at least most of the time. Three steps is most common but I've used a two step (plus cold) one which is deeply irritating because the jump in power is too big. In spring and autumn you have a choice between too-hot on high and pathetic-trickle on low.Really to get good flow rate at 40C with winter incoming water needs 10 kW. Plenty of smaller ones out there but they're pretty pants.

author: james_s
author_rank: Super Contributor
timestamp: January 04, 2020, 01:19:09 am
content: Why do they use those electric showers instead of just drawing hot water from the same tank/boiler that provides hot water to the rest of the house?When I visited England the shower at my friend's place was roughly similar to what I have at home. His hot water came from the same boiler that heats the rest of the house which is quite different than the typical USA arrangement but the shower itself was similar. I did see those electric showers in a store and wondered what they were all about though.

author: IanB
author_rank: Super Contributor
timestamp: January 04, 2020, 02:23:19 am
content: Quote from: james_s on January 04, 2020, 01:19:09 amThere are a few reasons. One reason is lack of water pressure in the hot water tank. Older British houses use gravity flow plumbing for the hot (and cold) water system upstairs and there simply isn't enough pressure to run a shower unless you add hot and cold booster pumps to give enough flow. It can be simpler just to run cold water from the mains supply to the shower and use an instant heater.Note that no houses built at that time had showers, they only ever had bathtubs. So showers are always installed as a later upgrade, and instant heat electric showers are simpler to add.Another reason is simpler plumbing. By using an on-demand heater in the shower you only have to plumb in a cold water supply instead of both hot and cold. This is especially useful if adding a shower cubicle to a bedroom as an upgrade. Instant heat also avoids waiting for the hot water to reach the shower when you first start to run it (avoids the whole standing off to one side to avoid the cold water experience while waiting for it to get warm).Modern houses do tend to have pressurized plumbing so the gravity feed problem goes away, but it remains the case that showers were never a standard bathroom fixture in older houses and so there is a huge market in shower upgrades.

author: james_s
author_rank: Super Contributor
timestamp: January 04, 2020, 06:36:43 am
content: 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.

author: richard.cs
author_rank: Super Contributor
timestamp: January 04, 2020, 12:02:26 pm
content: Quote from: james_s on January 04, 2020, 06:36:43 amUK 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

author: Simon
author_rank: Unknown
timestamp: January 04, 2020, 01:06:36 pm
content: Quote from: treez on January 02, 2020, 03:58:35 pmPlease would you stop putting everything in the general chat section. Just sit and think for a moment before posting!

author: ocset
author_rank: Super Contributor
timestamp: January 04, 2020, 02:38:45 pm
content: QuoteThanks, OK, sorry about that.

author: SolarMan
author_rank: Contributor
timestamp: January 04, 2020, 06:12:21 pm
content: Quote from: treez on January 02, 2020, 03:58:35 pmLet'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.KCapacity of the little tank inside the shower which holds the heating elements: .25LMaximum combined power of the shower heating elements (assuming 9.5kW total power for the shower1-aqualisa-vitalise-electric-shower-installation-guide-s-sl-slx-vte8521s.pdf(475.11 kB - downloaded 366 times.) minus 0.5kW for the pump): 9kWSetting 1 power: 3kWSetting 2 power: 6kWSetting 3 power: 9kWTime that you are on setting "3" while passing from setting "1" to setting "2": 0.2 secondsIncoming water temperature: 5 degrees COutlet water temperature while on setting 2: 37 degrees CCALCULATIONS:~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 LFlow 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 powerMaximum volume of water that could have been exposed to the full 9kW power = .25L + .028L = .278LWith 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).

author: Simon
author_rank: Unknown
timestamp: January 04, 2020, 06:44:00 pm
content: sounds like a control algorithm going haywire. For example a poorly tuned PID control will oscillate.

author: Gyro
author_rank: Super Contributor
timestamp: January 04, 2020, 06:55:34 pm
content: Quote from: Simon on January 04, 2020, 06:44:00 pmIt's a standard electric shower, it hasn't got any of that.(It's switches, elements and a water valve)

author: IanB
author_rank: Super Contributor
timestamp: January 04, 2020, 07:09:06 pm
content: 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.

author: German_EE
author_rank: Super Contributor
timestamp: January 05, 2020, 10:29:22 am
content: IanB, I think that you have the correct explanation, somewhere in that shower will be a solenoid operated water valve.

author: ocset
author_rank: Super Contributor
timestamp: January 05, 2020, 12:55:47 pm
content: 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.

author: Simon
author_rank: Unknown
timestamp: January 05, 2020, 03:35:42 pm
content: How about replacing it? If this is a landlords responsibility then they are now duty bound to do it as it's dangerous.

author: SolarMan
author_rank: Contributor
timestamp: January 05, 2020, 08:09:54 pm
content: Quote from: Simon on January 05, 2020, 03:35:42 pmAnd ask your landlord to give you the old one for a postmortem examination

author: tggzzz
author_rank: Super Contributor
timestamp: January 05, 2020, 08:19:25 pm
content: Quote from: blueskull on January 03, 2020, 01:41:52 pmTo be more general, treez, * and mains do not mix.