"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