Flow rate of new shower

[pamela]

The flow rate of the new shower is not fantastic considering the large amount of rewiring that was done. I measured the following flow rates at different temperates.

mains water temp = 12.6 C

setting temp temp-increase lits/min 0 12.6 C 0 C 10.0

4 34.7 C 22.1 C 5.5 7 39.0 C 26.4 C 4.7 8 41.2 C 28.6 C 4.3 9 42.7 C 30.1 C 4.2 10 43.8 C 31.2 C 4.0

Does the 9kW unit seem to be working at full capacity?

From what I remember of my school days, there's some way of using the specific heat capacity of water to actually calculate how much energy the water has gained based on the increase in temperature.

I think a 9kW shower should provide 9,000 joules of electrical energy per second. There's also something about calories in the calculation but I'm not sure where! :-(

From my figures, does anyone know how to calculate if 9kW power is actually getting supplied to the water?

[Vortex11]

As you say 9kJ per second is 9kW

Water heat capacity is about 4.2kJ per kg per degree

So 9kW will increase 1Kg of water by 9/4.2 = 2.1 degrees per second

Your setting 4 is about 0.1 kg per second

So you would expect temp increase of about 20 degrees.

So your figures are plausible.

David

[Adrian]

9,000 joules is 2,151 calories.

1 calorie is the amount of energy to raise one millilitre of water by one degree.

Six litres of water in a minute would be 100ml/sec - so you should get a temp raise of about 21deg at that flow rate, or a bit over 31deg at 4 litres.

That looks fairly close to the figures you've got there...

[Chris Hogg]

Specific heat of water = 4.186 J/g/deg.C flow rate = 4 litres/min = 66.7 g/s

Hence to raise the temperature of the water flowing at 4 l/min by 1 deg.C takes 4.186 x 66.7 = 279.2 J/sec

Hence 9000 J/sec will raise the temperature by 9000/279.2 = 32.2 deg.C

So I'd say your shower was working near-perfectly!

[Graham.]

You can easily measure the volt-drop in the cable and compare it to what is predicted foe the length and size of cable. If you are used to a "proper" shower, a 10kW electric one will seem barley adequate.

[Tim+]

Any minute now, someone will be along to say that you should have fitted a waste water heat recovery unit. ;-)

Tim

[pamela]

Thank you very much David, Adrian and Chris for your calculations. It's also interesting to see your slightly different methods. :-)

If my shower is delivering its full power then is there anything at all that can be done to increase the flow rate (without losing temperature)?

Is my mains pressure satisfactory? It gives a cold flow rate of 10 litres/min.

[Tim+]

Water temperature and flow are reciprocally related when heat input is fixed so you can't increase total flow without a drop in temperature.

Different jet size in the shower head may increase the apparent flow though.

Tim

[Chris Hogg]

In a word, no. You are limited by the physics. If you want hot water at a higher flow rate, you need a higher wattage shower heater. Mira do some 10.8kW showers,

but I imagine you need some seriously heavy-duty cable and possibly its own CU and MCB to cope with one of those. Others will advise, I'm sure.

[Adrian]

There's three variables with an electric shower :- input power, flow rate and temperature rise. They're all inextricably linked.

You want to increase one of them - flow rate - without changing a second

- temperature rise. So... you need to increase the input power, too. You need a bigger shower, which'll probably need bigger wiring.

The only other way to do it, if you don't want to increase the input power, is to reduce the temperature rise. But you don't want to decrease the output temperature... so you need to increase the input temperature...

[John Rumm]

Yup, seems to be near enough spot on.

Forget calories...

You are right, 9kW is 9000 joules per sec, or 60 times that per min. You then divide that by the temperature delta multiplied by the SHC of water (approx 4200 J/kg/C) to get the flow in kg/min (or litres since 1kg of water is also 1L)

9000 x 60 / 31.2 / 4200 = 4.1

[John Rumm]

10.8kW will be better, bit mot much in real terms. If you look at the table here:

None of the electric options are going to perform that well in the winter.

[Tough Guy no. 1265]

What's all the fuss about? When I were a lad they were all 7kW. They were perfectly usable. You don't need it piping hot and you don't need gallons of water. Just choose one, hot or fast, or anywhere inbetween.

[Mr Pounder Esquire]

Contributes the person who has no hot running water in his house !!!!!!!!!!! Tell us Mr Hucker, when you use the toilet do you run your electric shower to hot or do you just lick your fingers clean?

[pamela]

When I were a nipper se'en kilowatt were unimaginable luxury. We ne'er 'ad owt like that. If we 'ad five kilowatt it would 'ave blown our minds. Our Redring Super 8 were 3kW and that were plenty ample.

[Tough Guy no. 1265]

Soap dissolves just as well in cold water. Use the water at the temperature given to you by mother nature.

[Mr Pounder Esquire]

No.

Use the water at the

"I can sleep outside in a temperature of -20C wearing only shorts". (Peter Hucker)

[GB]

That was my thought. For an able bodied person, a 9-10 KW electric shower is perfectly adequate. Not luxurious, but adequate. I regard the flow rate from 20+KW combis as pretty meagre, and an electric shower is going to be under half of that flow rate.

But for a frail disabled person, I don't think it's necessarily good enough. They could get quite cold in the shower, without enough warm water to cover them properly. I'd have thought it better to use a stored hot water solution, suitably pumped if necessary. It's a shame this wasn't discussed at the specification stage.

As it is, I hope there's adequate heating in the bathroom to get it

*really* warm before mum starts her shower.

[Chris Hogg]

+1

A smaller shower head, with fewer holes, will give a better impression of an intense flow of water at a given flow rate through the heater than a great big 'pancake' with lots of holes.

[tabbypurr]

I don't think anyone's ever said you should. It's an option to solve the OP's imaginary problem. I wouldn't know what to do with 10kW. Aeration & pulsing shower heads can also help.

NT