DIYbanter - View Single Post - electric cold-feed showers.

wrote:
Hi,
My current shower is electric with a cold-water only feed and is
getting rather old. I think it is 7.5 Kw but I'm not 100% sure. We
are finding that at this time of year it is barely able to heat the
water to an acceptable temperature. We have very good mains pressure
and usually have to reduce this by running a tap at the same time as
the shower in order to give the unit time to heat the water before it
leaves the heat echanger. I am thinking about replacing it but if I do
this it would have to be like for like at the moment. By that, I don't
mean I want to replace one poor shower with another but if I do replace
it now, then the new one will also have to be electric cold-feed only.
I know people will want to try and convince me about Mixer showers and
Venturi showers but I haven't got the time to start messing around with
the plumbing at the moment. I want to take the old unit off the wall
and put a new unit in its place, otherwise the job will have to wait
for a while and we put up with cold showers.

So the question is, can anybody recommend any reasonably priced
electric cold-feed only showers which might be able to cope better than
the one we have now? I was thinking that a higher wattage shower might
have a better chance of heating the very cold water before sending it
to the showerhead. B&Q currently have a 9.5 Kw Gainsborough at £64.98
or a 10.5 KW Gainsborough at £89.00. Are either of these any good or
should I avoid them like the plague ? I don't mind spending more if I
have to, the momey will not be waster as whatever I install now will
almost certainly be moved into the En-suite when I eventually get
around to building it.

Basic physics prevents you getting anything massively better. Instant
electric showers are just about 99% efficient - essentially all the
electricity they consume goes into heating the water passing through,
with only small amounts lost through heating the wall they're mounted on
and the surrounding air. Given that, uprating from a 7.5kW to a 10.5kW
one is adding another (3/7.5)*100% = 40% of available power - which can
be traded off either as 40% more flow at the same temperature, or a 40%
greater temperature rise at the same flow rate, or anywhere inbetween.

Then there's upgrading the cable to supply 40% more power; it's likely
your existing cable is sized for a 7.5kw (just about 30A) load, while a
10.5kW 'monster' will be pulling about 42A. To a first approximation,
this usually means replacing a 6mmsq run with a 10mmsq one (and uprating
the MCB); proper calcs need doing to be sure, though, depending on
circuit length and routing (especially if it goes through thermal
insulation...)

Now, if a plumbing-change-free solution is all you feel up to for now,
then a heftier-rated shower with the cable upgrade is your only
solution; just don't expect magic. And you may want to look into putting
a flow restrictor in-line with the new (or even the old!) shower, simply
to reduce the flow rate through it to give it more of a chance to heat a
necessarily smaller dribble of water to a more acceptable temperature.
You may find a more modern instant-leccy shower includes an automagic
flow-rate-controller, too, so in winter you get water that's just as hot
as in summer on a given setting; just less of it.

'O'-level physics follows: pushing the temp of 1 litre of water up by 1
degree Celsius needs 4.2kJ. A kJ is 1 kW/second; so, over a minute, each
kW of shower-rating can raise a litre through about 15 degrees (since it
takes 4.2, call it 4, seconds to raise it through each degree), or more
usefully for back-of-the-envelope purposes, half a litre through the 30
degrees needed to take 'cold-but-not-freezing' (10 degrees C) to 'warm
but not hot especially by the time it's been through the air and hits
your skin' (40 degrees). Which gives us the rule-of-thumb that flowrate
of showerable-temp water in litres/minute is half the kW rating of the
heater - whether that's an instant-electric or a combi (after exhausting
any pre-stored reserve). Of course, shower and combi mfrs play fast and
loose with their 'flow rate' figures - even if they do give one, it's
almost always for an unrealistically low temperature rise (assuming,
say, a 20-degree feed - "it's summer!" - and a 35-degree output "it's
summer, you *want* a refreshing, cooling, not-too-hot shower!". (Indeed,
one of our regular physics-defying brochure-regurgitating posters will
be along soon to p-d and b-r; let's not encourage him by arguing.)

HTH - Stefek