Alternative supplies for a mixer shower

[ban-all-sheds]

The failure of my electric shower has meant using the diverter on the bath mixer taps to get a shower.

Works pretty well, so we've decided to get a proper shower mixer plumbed in.

But as we have a combi, and no stored hot water, I'm a little concerned about being left with no way to get a warm wash should the boiler fail. I was going to still keep an electric shower on the wall, but as it would seldom (hopefully never) be used, and would possibly be a problem to seal properly if we have a fixed overhead shower head, I was thinking about an alternative.

Which is to have an electric instant water heater remote from the bath, connected to the same mixer, through a manual 3-port 2-way valve.

i.e. in a boiler-failure situation I'd switch the valve to allow water to flow through the heater.

Can anybody see any problems with this?

 

[securespark]

Back-flow issues?

 

[TicklyT]

One snag is temperature regulation. Instantaneous heaters tend to have a fixed power input, and the user regulates the water temperature by adjusting the flow rate, with higher flows giving a cooler output.

Shower mixers adjust the proportions of hot and cold water making up the same fixed flow rate. They increase the flow rate of hot water for a hotter output.

Thus temperature control could prove erratic, to say the least.
Imagine turning down the temperature at the mixer. That will reduce the hot water flow rate. So the hot water temperature will increase as the flow reduces, returning the shower to the same temperature. That will carry on until you reach the minimum flow or maximum temperature at the heater, when the unit will shut down completely, leaving you under a cold shower.

Conversely, turning the temperature up at the mixer will increase the water flow through the heater, which reduces the hot water temperature.

 

[ban-all-sheds]

Back-flow issues?

How/where?

 

[ban-all-sheds]

Tickly - thanks for that.

Some digging around I've done today has unearthed the fact that the makers of these things (or at least the 2 I've found so far - Redring and Hyco), and a seller, advise against thermostatic mixers. Hyco say "Do not fit a thermostatic mixing valve with the inline. Unpredictable performance will result." And your explanation tells me why.

Redring do sell their own mixer, but it isn't thermostatic, and I guess it keeps the hot flow constant and varies the cold.

It also looks like nothing I'd want on my bathroom wall...

Oh well.