12kW electric water heaters - could I get some advice please

[Tipsygoldie]

I was hoping that someone could give me some advice please.

I have a combi-boiler and mains fed cold water (with reasonable pressure) but no tanks.
In the past I have always used electric 10.5kW showers which were fine but fairly ugly.
I have just had my shower room redone and was told that I would have sufficient water pressure to run a rainfall shower via a mixer system but unfortunately that is not the case and it is far too feeble.
Having had the rainfall shower head installed and the tiling replaced etc I was hoping that I could find a way to use them.

Is it possible to run a 12kW water heater in series after the combi-boiler to produce higher temp water for the same flow rate and then mix this with mains fed cold water in the mixer to allow a greater water flow rate overall at a reasonable showering temperature? I have 10mm cable in situ - would this need to be increased?

If not is there an electric shower that would produce a sufficiently high flow rate for a rainfall head?

I would be very grateful for any suggestions!! Thanks

 

[OwainDIYer]

You may have sufficient pressure, but not flow.

What is the recommended flow in litres per minute for the rainfall head, and what is the kW rating of your combi boiler?

 

[ban-all-sheds]

Is it possible to run a 12kW water heater in series after the combi-boiler to produce higher temp water for the same flow rate

If the output temperature of the heater gets too high it will shut down. What is your flow rate, what would be the temperature of the incoming water, and what output temperature do you expect/want to get?

Given the flow rate it's a very simple calculation to work out how much a 12kW heater will raise the temperature by - the specific heat of water is 4.19J/gK, and your 12kW heater will deliver 12,000 joules/sec.

and then mix this with mains fed cold water in the mixer to allow a greater water flow rate overall at a reasonable showering temperature?

Probably not, and could well be a disaster. If the mixer restricts the flow of the water through the heater, it will get hotter. So the mixer will restrict it more. And it will get even hotter. Redring do make a special mixer designed for use with an instant heater, but I doubt that's what you have, it's as ugly as sin, and AFAIK it is not thermostatic.

I have 10mm cable in situ - would this need to be increased?

It's a 50A load. A 10mm² cable may be able to carry that, or it may not - it depends on the installation method:

http://www.batt.co.uk/upload/files/4d5.pdf

What is the rating of the protective device for the circuit?

If not is there an electric shower that would produce a sufficiently high flow rate for a rainfall head?

What flow rate do you need?

What temperature rise do you want?

 

[Steve]

I would have a thermal store fitted. Your combi boiler can still be used for the other taps but the thermal store will be able to cope with the demand of this shower and at mains pressure. But these are expensive.

An alternative but a bodge is a bog standard cylinder with a pumped outlet - but the shower will also then require a pumped gravity fed cold feed too - the mains pressure cold will force water back into the hot circuit otherwise. I had a power shower (don't think they sell these anymore) which contained a dual pump and a mixer. This required the cold storage tank to be seperate and larger than the hot storage tank so that the hot would always run out first (if this ever happened within its 15 minute run cycle). You would need space for two storage tanks and a cylinder. Which defeats the point of a combi boiler.

I have to say having always used electric showers myself, I am very happy with my new thermostatic mixer shower supplied by a combi. But we have a standard shower head.

I have stayed in a Radisson Blu hotel which had a rainfall shower head but its output was terrible and only about half of the outlet holes had water coming out of them. Hard to get these right it appears.

 

[ban-all-sheds]

It might be that the hotel shower was one which deliberately mixes air into the spray to reduce the amount of water used.

Should be fine - everyone knows that air is just as good for washing as water.