Mains half cycle skipping to regulate heater element

[ocset]

Thanks Gyro,
Though sorry because I should have made it clear that I am only speaking about “electric showers” here…….ie, that only have a cold inlet. (no mixing)
The shower in question (2 posts up) has three flow settings, and a large number of temperature settings. Such a wide range of temperature setting could only be economically feasibly produced by burst fire mains operation, with variable burst lengths. Would you agree?

[Gyro]

Yes I was aware that you were talking about an electric shower. I was saying that the same wax cartridge technology can be used (as is used in a mixer type) in order to maintain temperature across different flow rates as you switch between different sized shower heads - one of the features.

I suspect that the three flow rate settings are in fact three coarse (step) flow adjustments accompanied by three selected element power configurations and the fine control is a finer continuously variable flow rate adjustment. As I said, a wax capsule would probably take care of the rest. It would have to be configured that way, otherwise on flow setting 3 it would freeze you and on setting 1 it would take your skin off!

You can achieve a lot of variation with a three circuit element and sufficient flow control. I don't see anything to suggest a more advanced electronic system.

[nali]

The shower in question (2 posts up) has three flow settings, and a large number of temperature settings. Such a wide range of temperature setting could only be economically feasibly produced by burst fire mains operation, with variable burst lengths. Would you agree?

No, I'd concur with Gyro. Look at the drawing of the internals in the instructions; the temperature control goes into what looks very much like a mechanical mixer/regulator.

[ocset]

Having re-looked at the shower of post #23, I now agree with your words. It’s the flow rate that is being  adjusted to give a wide range of temperature outputs.
However, there is a very big problem with this. It means that people may end up using a very low flow rate, which is very bad because it results in scaling up of the shower system….also, if your shower is on high power and high flow rate, then you quickly adjust the flow rate down…it can result in significant scalding…as well as temporary thermal shut down of the shower.
So this is why mains burst fire control, using a fixed flow rate, (or perhaps just two very similar flow rates)  would actually be the best way to control an electric shower.
Please see point 13 (page 3 ) of the  “Mira Décor” shower……

Mira décor shower manual (£400 electric shower)
http://resources.kohler.com/plumbing/mira/pdf/1365393-w2-a-mira-decor-installation-and-user-guide.pdf

…this is diabolical “DO NOT switch the shower off and back on while standing in the water flow.”
….when soaping up in an electric shower..it is essential to be able to switch the shower off….then back on again when you wish to rinse off the soap….
Burst fire mains control is the way forward.

[Gyro]

Sadly, scaling is inevitable in hard water areas. Even an indirect hot water cylinder fills up with limescale, and the heat exchanger coil in that case is only heated to 60-80'C by the boiler. So called Combi boilers need water softeners and still suffer short lives in similar situations, kettle need descaling frequently etc.

The universal problem with all electric showers is that the water flow through the small heating chamber is so fast that the element dissipation an surface temperature  must be ridiculously high in order to 'flash heat' the water.

If a low temperature indirect water cylinder scales up, then you basically have no chance with and electric shower, no matter what the control method you use - you simply can't heat the water fast enough at low element temperatures. The 10.8kW output and surface temperature of an electric shower element makes an ordinary domestic kettle element look like a toy.

With regard to scald risk (I note that the specification for the Mira shower that you keep referencing says:

Factory set to safe max. temperature: No

Higher quality shower models maintain the water flow through the heating chamber for a few seconds after the element is turned off, to draw off the excess heat and bring it down to a safe temperature (which also helps with a little with scaling). Burst fire simply isn't going to help you with this - you need to dump the residual heat of the element / heating chamber into flowing water when you turn it off.

[ocset]

Thanks, yes I  see your good points.
What I would add is, that I have lived in some 35 different rental accomodations, and in virtually all of them, the electric shower had been pranged by the landlord to always run on maximum flow.
The reason for this is because adjusting the flow rate of an electric shower can lead to problems…..because if  one restricts the flow rate too much, too suddenly,  then the  heater element overheats…and even though there is a thermal shutdown, it never really operates fast enough, and so cumulative damage is done to the heater element and other parts.
Another point about slow flow rates is that it increases limescale build-up….more so than faster flow rates.
This is why shower temperature adjustment by flow rate adjustment is a poor, el-cheapo solution to the problem.

[Gyro]

Yep, electric showers are a pain in the neck generally, even the ones with low flow shut-off.

The one that maintained water flow, until the heat in the thermal mass of the element/heating chamber had dissipated, was in a holiday cottage that we rented in June. It seemed to fix the scald problem completely.

Unfortunately it's a bit far to go back and check the make and model.