On 09/03/2016 13:27, David wrote:
Warning - long ramble.

http://www.theguardian.com/commentis...r-once-a-week-
polluting-environment

Shock horror, gruniad article that is technically illiterate...

Interesting use of statistics.

"The average 10-minute shower uses 60 litres of water. A power shower uses
three times that and a bath about 80 litres. So a family of four each

Well those are all questionable...

The water usage of the shower will of course vary with the flow rate. 6
lpm is tad low for a decent shower - but 8 to 10 lpm would be rated as
fairly good by most.

The average bath will use more than 80L - 100 to 120L is more common.

having a daily 10-minute power shower (I know that is a very conservative

A power shower is unlikely to use 3x the water - unless you are talking
about something with a giant soaker head and additional body jets. Most
people use them to get back to a sensible shower in cases where the
unpumped option is a crap one.

estimate for some teenagers) will consume a staggering 0.25m litres of
water every year. The annual average cost for electricity for four 10-
minute showers per day would be up to about £400, or £1,200 if a power

Well lets say their 60L is right[1], that's 240L/day (although 4 people
showering every day is probably excessive for most houses). That makes
87,600 L/year - so not sure where the quarter million figure comes from.
If we assume a required average temperature rise of 35 deg C, that's and
energy requirement of 876000 x 4200 x 35 = 12.8GJ or 3577 kWh or about
£456 if heated electrically at the price I buy electricity. Obviously
electric heating is not the way to go with that kind of usage. At my
current gas price that would be about £100 (allowing for some
inefficiency in the boiler)

[1] its probably very low - I know if two people have consecutive
showers here, it swallows most of the 210L cylinder, and that is mixed
with additional cold!

shower is involved. Even worse, the power-shower family would be emitting
a staggering 3.5 tonnes of CO2. As we can afford only one tonne of carbon
emissions per person €“ for everything from food to transport €“ if we are
to keep global temperatures below the critical 2C threshold, this would
consume nearly all of the familys carbon budget."

That's a pretty poor para really... I was not aware it was critical to
keep global temperatures below 2C - that's going to need a bit of an ice
age!

I can't be bothered to work out how they derived the CO2 figure.

For all you DIYers, some of these figures may seem a little strange.

Indeed.

"The average 10-minute shower uses 60 litres of water"

Well, at 6 litres per minute this must be a pretty feeble shower -

Its on the low end of nearly ok IME. Its well in excess of what you will
get out of an electric shower in the winter.

although I sized our combi to heat 15 litres per minute so it could
service two showers. However that was 15 litres of hot water, and doesn't
count the mixed in cold.

To be fair, when the combi is working flat out, that will be with a 35
deg temp lift - so you will be using its output at close to final
temperature with very little cold added.

(assuming its a ~35kW combi)

http://www.harwoodandassociates.co.u...low-rates-for-
taps-showers-and-baths/

has some rule of thumb figures, but my brain started to hurt trying to
compare everything.

It does say of power showers "A 12 minute shower with a flow rate of 15 l/m
would use 180 litres of water." which seems to tie in with the "three
times" figure for normal vs power shower.

The figure for a "normal" shower is low, and 15 is a bit high for power
shower in many cases.

Using Google for bath volume, I find
http://www.waterwise.org.uk/news.php...facts-figures-
and-misconceptions
from 2011 which seems to be remarkably similar to this 2016 puff piece but
with the opposite spin.

Although it has an 8 minute shower using 62 litres of water.

that gives nearer 8 lpm which is probably a better figure.

Using the site from above:

http://www.harwoodandassociates.co.u...es-a-bath-use/


"According to BS6700 a standard 1700mm x 700mm bath uses approximately 100
litres of water at 40C. This is split into 60% hot and 40% cold water
when hot water is stored at 60C."

So about 60l of hot water from store or combi.

No, 60L from store, but getting on for 100L from the combi (most users
will demand full output from the combi for bath filling - there is no
real advantage to taking less than full unless you like running baths
slowly!)

(i.e. set the combi water temp limit to 60, and it will not deliver
water hotter than 60. However demand too much flow rate from it and that
temp will fall)

But, of course, the variable volumes are only the start.

"The annual average cost for electricity for four 10-minute showers per
day would be up to about £400, or £1,200 if a power shower is involved"

If you take their figures, those could be plausible - but see above.

Note the words "up to" - often used in shop SALE signs as "up to 50%
reduction" where all but one item has only 5-10% knocked off.

We (like many others) have a gas combi boiler which provides our hot
water. I am struggling to see how I use anywhere near £400 (or £100 per
person) when I shower using a mains pressure gas combi.

It won't be anything like that with gas at a quarter or less of the
price per kWh than lekky.

On flow rates, the Triton Aire fitted to our bath has a maximum flow rate
of 15.5 l/min at 1 bar (or 8 l/min with the flow limiter fitted). I've
grumbled in thread passim about the seemingly weedy flow. Not yet measured
it but 6 l/min for the average shower does seem slow.

Yes, but not by a huge amount.

Googling for flow rates for electric showers doesn't seem to turn up much
from the manufacturer, however:

https://ask-a-saint.silversaints.com...ctric-showers-
testing-the-water-inlet-pressure-and-flow-rate

says "For a 8.5KW or 9.5KW Triton showers the minimum required water
pressure is 1.0 Bar with a minimum flow rate of 8 litres per minute.

The pressure requirement is straight forward. The flow rate however is
not a measure of what the shower can deliver, but just ensuring there is
little chance that other users in the house won't starve the shower of
water, causing its flow rate supply to fall below that which it can
deliver.

See:

http://wiki.diyfaq.org.uk/index.php/...ting_the_Power

You will note that the best electric shower will not even manage 5 lpm
in the winter (assuming you want the water warm that is!)

A Mira 8.5KW or 9.5 KW showers only require a maintained water pressure of
0.7 Bar and 8 litres per minute. This is one of the primary reasons we
always specify a Mira shower over Triton."

Can't really see the logic unless you have a very poor mains pressure.
Most areas will get at least 1 bar of mains pressure.

Which suggests that 6l/min is too low for your average electric shower!

No, its *way* too high.

4 lpm in the winter, rising to 5.75 in the summer perhaps if you have a
10kW shower.

However to give an electricity cost of £100 per year per user they must
surely be using an electric shower (or possibly immersion heater?).

Assuming the cylinder is lagged is does not make much difference.

O.K. - well into a long ramble.

Anyway, so far we may have established that the minimum flow rate for a
modern electric shower is around 8 l/min, and DIYers may well note that

or not...

electric showers are generally held to be not very good compared to a tank
or combi system, which suggests a satisfying shower could require 10 l/min
even with an "Economy" shower head.

I would say, you can go down to 7 or 8 lpm, but yup ten is quite nice.

TL;DR I am a little surprised that the average (I think) flow rate for a
shower at around 10 l/min would fill the average 100 l bath in 10 minutes.

Erm why?

10 x 10 is traditionally 100

Then again although shower time is an alternative Universe I don't think I
spend an average of 10 minutes under the shower.

I find it easy to spend more than 10 mins...


--
Cheers,

John.

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