This idea was mentioned in another thread. In principle, if one put an electric shower on the oputput of a combi, you'd egt even more heat input to the water, and thus get even more flow rate to give a nice luxurious shower. And of course, with 2 heating systems in series, either one can break down and the shower still works.
Is this ever done? Is it workable? Why isnt this a common arragement? I realise todays lec showers arent designed for this, but with a suitable model it looks like a sound idea at first whatnot.
Displaying a lack of any knowledge here, my complete guesses are...
I reckon, cost (TCO-buy/install/run/maintain) would be a big factor ... two devices doing the same job would cost twice as much (approx).
Pressure issues (esp. in low pressure area) two seperate heating devices would probably result in less pressure at the head (each has to have a pressure drop right?).
With added complexity you also decrease the overall system MTBF ... might be a non issue in practice though.
... but maybe a combi with an integrated electric "boost" might work and is essentially the same thing and eliminates some of the install/maintain costs.
I doubt you'd find a generally-available leccy shower which would work in this arrangement.
Biggest single reason is that they're all (AFAIK) made with a single-pipe feed, assumed to be cold, which the element heats to the lower of the desired set temperature and as warm as it can get it for the flowrate. When your combi's delivering water hotter than the setpoint, there's no way for the single-pipe leccy shower to blend it with cold, so you get scalded. Ouch.
A number of less show-stopping reasons occur to me also, such as: (a) the showerhead's designed for a 'dribble' rather than a 'soak me' flowrate, so even if your combi might be up to a more satisfying rate of, say, 12l/min, the leccy shower will end up limiting it to the usual catpee rate; (b) the electronics in some might just shut down the feed if the incoming temp's too great; (c) any design relying on the cold incoming keeping a PCB cool will be najjered; (d) you wouldn't get the 'backup' property as the typical leccy shower shuts its valves when the leccy supply fails.
None of which rules out a cunning custom design involving combi, blending valve, instantaneous-leccy-heater, and a Maxwell's Demon to put all the energetic mole kewls in the right place... but it's not gonna happen by sticking a Triton onto a Worcester, or other unnatural unions of 'normal' gear... AFAICS.
I'd have thought electric showers are designed so the minium flow and inlet pressure does not produce dangerously scaling water with ambient temp inlet.
Once a combi is added on the inlet this no longer is the case, esp as they are often non modulating and will dump a finite amount of heat regardless of flow.
AFIAK electric showers cut out on low pressure differential across the shower and not high temperature. If so then selecting a high temperature (and hence low flow) on the shower in summer could/would result in the water coming out at skin stripping temps.
Not something to try unless all the pitfalls are thought of and accounted for, I could never recommend someone to try it.
Maybe a blending valve on the outlet of the combi set to 25 C, and a cut out thermostat on the inlet to the shower.
However the combi will still tend to heat the water more at low flow, and the valve would throttle the combi 'til it cut out. Also the valve might be damaged by inlet water that is too hot.
If there is a stored water tank it might be better to take the cold main through a coil and to a blending valve before the shower, even if the tank is at ambient it would still do some preheating.
As others have said, an electric shower is not compatible. Few people are dissatisfied with a shower from even the smallest of combis. Large flowrate combis are available, or if you have a low flowrate combi and want a drencher then add another combi and work them in parallel.
As backup the combi outlet can be run through an instant under unit electric heater. When the combi is down the electric heat is turned on giving at least some hot water. Do a Google on IMM for this.
You can have an electric boiler on the outlet of a combi operated by a flow switch. I don't know what the makers would say. They are tall and thin and take up little space. But they cost as much as a combi, so may as well twin the combi's up, and cheaper to run too.
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much explanation there. So although it could be done with a suitable design, there are many complications, all extra expenses, when a suitable combi could just be used.
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