On Wed, 27 Aug 2003 20:54:08 GMT, "Mike Hibbert"
wrote:

"Andy Hall" wrote in message
.. .
On Wed, 27 Aug 2003 20:11:32 GMT, "Mike Hibbert"
wrote:

Hi all,

I'm guessing that this is a pretty daft idea as I have seen no-one else
suggest it. I'm looking at shower options at the moment, and I live in an
area with fairly low water pressure. One of the easiest options would be
to
have an electric shower, but I have hear that the water pressure would be
even worse in winter as the shower would need to heat the water from a
lover
ambient temperature.

So, why not mix the water with hot water from the combi? I could have a
feed
fron the hot and cold water supply going into one pipe and then feeding
the
electric shower. The water would be pre-warmed and therefore could be
made
the same all year round. If the electric shower wasn't raising the
temperature too much it woudn't have such a problem delivering more water
would it?

Like I say, this is based purely on my own "sort of" logic rather than
any
fact, so shoot me down gently!

Cheers
Mike


I'm not sure that it's going to help you.

Electric showers are normally rated at up to about 10kW and the
pipework and shower head designed to give a fairly low flow rate.
This is because 10kW is a relatively small about of heat to add to
water to raise it from cold water mains temperature to the normal
working temperature of around 40 degrees.

You can actually do the sums. Let's say the water is at 10 degrees
coming in. You need to raise the temperature by 30 degrees, so using
Heat energy = mass x specific heat x temperature rise, we have

10000 = M x 4200 x 30

since the specific heat of water is 4200 Joules/kg

A litre of water weighs approx. 1kg so you can work out that this
amount of energy will be enough to heat approx. 0.08 litres per second
or about 4.75 litres/minute.

This equates to a disappointing shower.

A typical combi may have a power rating of 20-30 (possibly even as
much as 45kW), so under the same circumstances of needing to raise the
water temperature by 30 degrees will give you 2 to 4.5 times the flow
rate, which becomes a lot more sensible.

However, if the issue is that the water pressure, or more importantly
the flow rate is poor, then adding in more heating of the water is not
going to help.

If the issue were that the combi is under-rated and water supply is
adequate, then theoretically, adding in another source of heat would
be interesting. However, I am not sure that water regulations or the
manufacturers of electric showers would make this a feasible option.

With a flow rate issue because of the water supply, the options would
be to upgrade the water supply pipe from the street main (you would
have to pay for this unless the supply rate is *extremely* low, or to
have some form of cold water storage tank and a shower pump.


Thanks Andy, I have already looked at getting a cold water supply and then
pumping it into the combi, this could well be the best bet, but there are
lots of pitfalls for me with that (primaily being a total lack of plumbing
skills - but a willingness to learn!). I would still want to have mains tap
water at some of the sinks so would have concerns about the hot being under
pressure and the cold being much less (at the moment it is about 1 bar or
11 litres per min).
Your other option though sounds good, would I have to get intouch with the
water board and they would do the work? Presumably, this woudl mean I have
to go onto a water meter afterwards?

Cheers for the help

Mike


Another thing that you could do, if you have the space is to add in
what amounts to a conventional hot water system on a small scale.

This would consist of a roof tank for the cold water and a smallish
hot water cylinder. You would need to run the coil of the hot water
tank by connecting it across the central heating circuit and using a
motorised diverter valve. Your existing CH controller may be able
to handle the control aspect of this but otherwise a fairly
inexpensive one will do. The principle would be that the CH would
work as it does today, but that a thermostat on the cylinder which
would operate when the water needs heating, would move the motorised
valve over so that the CH water goes through the cylinder coil, and
would also fire up the boiler. In this way of working (which is
standard for non-combi systems) the boiler really doesn't know that
it's being asked to provide heat that's going to a HW cylinder rather
than the radiators.

You could use the hot water from this cylinder just to run the shower
or possibly the bath as well if you wanted. Both the hot and the
cold water for the shower would be derived from the roof tank and a
pump could be included to boost the pressure and flow at the shower -
all completely safely. The combi output could continue to feed all
the other taps that in any case don't usually need as much flow rate,
and also you would have a mains derived supply at both taps in such
cases - i.e. equal pressures. I agree with you that it doesn'[t
seem to safe an idea to have high pressure hot and low pressure cold.

The results that you could get at the shower would be limited only by
the size of tank and cylinder - so you need to think about what flow
rate you want (10 -15 litres/min is good) and how long you want the
shower to run before the cylinder ran out of hot water. Another thing
that this arrangement does is to allow you to have the hot water
stored at 60 degrees. Thus even if the boiler is a bit undersized it
doesn't really matter too much and if you mix hot water at 60 with
cold water at 10 or less, you are not limited by what the combi can do
directly.

All told in materials, not including shower and pump, this should be
under £200 - 250. A good shower pump by Stuart Turner can be had for
about £150.

In effect, this approach removes the limitations of the water supply
and of the combi, if any.

It's worth seeing whether the water supplier will do anything,
although the requirement from Ofwat is minimal on what they are
required to provide. If you want more then you would have to pay
for new pipework from the road, which depending on what's involved may
not be cheap. As you say, if they can find a way to get you on a
meter, they will.


..andy

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