I can work it out for my electric shower i.e. 9.5 kw @ 10p per kw/hour =
95/4 = 23.75p for a 15 min shower.
But how would that compare to a WB 40 kw combi or a 28kw system boiler and
high recovery cylinder / megaflow / thermal store ?
I would imagine the combi being chaper to run but its 40 kw !!!

Regards Jeff

Jeff wrote:
I can work it out for my electric shower i.e. 9.5 kw @ 10p per
kw/hour =
95/4 = 23.75p for a 15 min shower.
But how would that compare to a WB 40 kw combi or a 28kw system
boiler and
high recovery cylinder / megaflow / thermal store ?
I would imagine the combi being chaper to run but its 40 kw !!!

Regards Jeff


No idea about cost but unless your washing your partner as well, 15
minutes seems far too long to me!

"Jeff" wrote in message
...
I can work it out for my electric shower i.e. 9.5 kw @ 10p per kw/hour =
95/4 = 23.75p for a 15 min shower.
But how would that compare to a WB 40 kw combi or a 28kw system boiler and
high recovery cylinder / megaflow / thermal store ?
I would imagine the combi being chaper to run but its 40 kw !!!

But you still use the same energy. So assuming boiler is say 85% efficient
you can multiply the number of kilowatts used by 1/0.085 or whatever then
multiply by the cost per kW of the gas and you'll find it's about one-third
the price.

And why DO you need a 15 minute shower ? ;-)

"Mike" wrote in message
...

"Jeff" wrote in message
...
I can work it out for my electric shower i.e. 9.5 kw @ 10p per kw/hour =
95/4 = 23.75p for a 15 min shower.
But how would that compare to a WB 40 kw combi or a 28kw system boiler
and
high recovery cylinder / megaflow / thermal store ?
I would imagine the combi being chaper to run but its 40 kw !!!

But you still use the same energy. So assuming boiler is say 85%
efficient
you can multiply the number of kilowatts used by 1/0.085 or whatever then
multiply by the cost per kW of the gas and you'll find it's about
one-third
the price.

so, disregarding heat exchanger losses etc you are saying that the amount of
energy reqd to heat a volume of water is constant ?


And why DO you need a 15 minute shower ? ;-)


Hmm, guestimate, got me thinking now, big job though, 6'6" and a dirty
occupation :-)

Regards Jeff

"Jeff" wrote in message
...

so, disregarding heat exchanger losses etc you are saying that the amount
of
energy reqd to heat a volume of water is constant ?

I'm sure somebody will pick me up on something but yes.


And why DO you need a 15 minute shower ? ;-)

Hmm, guestimate, got me thinking now, big job though, 6'6" and a dirty
occupation :-)

Ah - not a two person shower then :-(

"Jeff" wrote
| so, disregarding heat exchanger losses etc you are saying that the
| amount of energy reqd to heat a volume of water is constant ?

Yes.

Can I guess you're under 35 and/or didn't do physics at school?

| And why DO you need a 15 minute shower ? ;-)
| Hmm, guestimate, got me thinking now, big job though, 6'6" and
| a dirty occupation :-)

1. Turn water on. Get wet. Turn water off.
2. Apply soapy stuff and lather.
3. Turn water on. Rinse. Turn water off. If not clean, repeat from 2.

15 mins in shower. Water running for about half that.

Owain

Jeff wrote:

"Mike" wrote in message
...

"Jeff" wrote in message
...
I can work it out for my electric shower i.e. 9.5 kw @ 10p per kw/hour =
95/4 = 23.75p for a 15 min shower.
But how would that compare to a WB 40 kw combi or a 28kw system boiler
and
high recovery cylinder / megaflow / thermal store ?
I would imagine the combi being chaper to run but its 40 kw !!!

But you still use the same energy. So assuming boiler is say 85%
efficient
you can multiply the number of kilowatts used by 1/0.085 or whatever then
multiply by the cost per kW of the gas and you'll find it's about
one-third
the price.

so, disregarding heat exchanger losses etc you are saying that the amount of
energy reqd to heat a volume of water is constant ?

Yes.
What might reduce the electric shower costs to near gas levels (?) would
be to fit a heat exchanger to take the output water from the shower,
and use it to heat the incoming cold.

Owain wrote:

15 mins in shower. Water running for about half that.

Or failing that splash out the 20p and enjoy a decent shower without
being a tight wad! ;-)



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

"Ian Stirling" wrote in message
...
Jeff wrote:

"Mike" wrote in message
...

"Jeff" wrote in message
...
I can work it out for my electric shower i.e. 9.5 kw @ 10p per
kw/hour =
95/4 = 23.75p for a 15 min shower.
But how would that compare to a WB 40 kw combi or a 28kw system
boiler
and
high recovery cylinder / megaflow / thermal store ?
I would imagine the combi being chaper to run but its 40 kw !!!

But you still use the same energy. So assuming boiler is say 85%
efficient
you can multiply the number of kilowatts used by 1/0.085 or whatever
then
multiply by the cost per kW of the gas and you'll find it's about
one-third
the price.

so, disregarding heat exchanger losses etc you are saying that the
amount of
energy reqd to heat a volume of water is constant ?

Yes.
What might reduce the electric shower costs to near gas levels (?) would
be to fit a heat exchanger to take the output water from the shower,
and use it to heat the incoming cold.

Oh dawd. You'll start Drivel off again with his concentric copper pipes.

"Mike" wrote in message
...

"Jeff" wrote in message
...
I can work it out for my electric shower i.e. 9.5 kw @ 10p per kw/hour =
95/4 = 23.75p for a 15 min shower.
But how would that compare to a WB 40 kw combi or a 28kw system boiler
and
high recovery cylinder / megaflow / thermal store ?
I would imagine the combi being chaper to run but its 40 kw !!!

But you still use the same energy. So assuming boiler is say 85%
efficient
you can multiply the number of kilowatts used by 1/0.085 or whatever then
multiply by the cost per kW of the gas and you'll find it's about
one-third
the price.

And why DO you need a 15 minute shower ? ;-)

Because it takes that long in a 9.5kW electric-trickle shower to get the
soap rinsed off!

Mike wrote:

"Ian Stirling" wrote in message
...
Jeff wrote:

"Mike" wrote in message
...

"Jeff" wrote in message
...
I can work it out for my electric shower i.e. 9.5 kw @ 10p per
kw/hour =
95/4 = 23.75p for a 15 min shower.
But how would that compare to a WB 40 kw combi or a 28kw system
boiler
and
high recovery cylinder / megaflow / thermal store ?
I would imagine the combi being chaper to run but its 40 kw !!!

But you still use the same energy. So assuming boiler is say 85%
efficient
you can multiply the number of kilowatts used by 1/0.085 or whatever
then
multiply by the cost per kW of the gas and you'll find it's about
one-third
the price.

so, disregarding heat exchanger losses etc you are saying that the
amount of
energy reqd to heat a volume of water is constant ?

Yes.
What might reduce the electric shower costs to near gas levels (?) would
be to fit a heat exchanger to take the output water from the shower,
and use it to heat the incoming cold.

Oh dawd. You'll start Drivel off again with his concentric copper pipes.

I fear this is one area where he might actually be right.
Gotta happen eventually.

Owain wrote:
"Jeff" wrote
| so, disregarding heat exchanger losses etc you are saying that the
| amount of energy reqd to heat a volume of water is constant ?

Yes.

Can I guess you're under 35 and/or didn't do physics at school?

| And why DO you need a 15 minute shower ? ;-)
| Hmm, guestimate, got me thinking now, big job though, 6'6" and
| a dirty occupation :-)

1. Turn water on. Get wet. Turn water off.
2. Apply soapy stuff and lather.
3. Turn water on.

Scream in pain as you are scalded by the first slug of really hot water
out of the shower because it's a cheap electric one that doesn't have
very good temperature control;-)

Rinse. Turn water off. If not clean, repeat from 2.

15 mins in shower. Water running for about half that.

Owain

I often have a 15 minute shower - some times even longer! I find it a
good way to unwind after a stressful day in the office!
CM.

A shower that takes 10 litres/min of 55C hot water will consume 150 litres in 15 minutes. The shower will give approx 13 to 14 litres/min with added cold


No. of Kilowatts = Quantity of water in litres x temp rise in degrees C x
Specific heat of water / No. of seconds in one hour.

Specific heat of water = 4.2

So to heat a 150 litres in 1 hour will take:

150 x 50 (raising water from 5C to 55C) x 4.2 /3,600 = 8.75 kW

Apply this formula to all the permutations.

On Thu, 10 Feb 2005 01:11:41 -0000, "Owain"
wrote:

"Jeff" wrote
| so, disregarding heat exchanger losses etc you are saying that the
| amount of energy reqd to heat a volume of water is constant ?

Yes.

Can I guess you're under 35 and/or didn't do physics at school?


Since I detect a rather patronising tone, I must be a pedant and
insist that you are wrong. :-)

The amount of energy required to heat a volume of water depends on the
temperature of the water and the pressure. Thus 4186 J/Kg/K for water
is at standard temperature and pressure or STP as we used to write at
school. (273K ish, 1 atmosphere).

I might add that water takes a lot of heating, which is something to
do with the bond geometry. Thus I suggest we start showering in
liquids that need less energy to heat.

Markus.

On Wed, 9 Feb 2005 23:37:40 -0000, "Mike" wrote:


And why DO you need a 15 minute shower ? ;-)

Personally, I think the higher the pressure of the shower, the longer
I am likely to stay in there. It's more fun than if you have a
dribble.

Like why do you need a one hour massage? Won't 5 minutes do? :-)

M.

wrote in message
oups.com...

Owain wrote:
"Jeff" wrote
| so, disregarding heat exchanger losses etc you are saying that the
| amount of energy reqd to heat a volume of water is constant ?

Yes.

Can I guess you're under 35 and/or didn't do physics at school?

| And why DO you need a 15 minute shower ? ;-)
| Hmm, guestimate, got me thinking now, big job though, 6'6" and
| a dirty occupation :-)

1. Turn water on. Get wet. Turn water off.
2. Apply soapy stuff and lather.
3. Turn water on.

Scream in pain as you are scalded by the first slug of really hot water
out of the shower because it's a cheap electric one that doesn't have
very good temperature control;-)

Don't do cheap....... do quality (triton t100) but wish i'd gone for
thermostatic control ;-)

Regards Jeff

"Grumps" wrote in message
...
"Mike" wrote in message
...

"Jeff" wrote in message
...
I can work it out for my electric shower i.e. 9.5 kw @ 10p per kw/hour
=
95/4 = 23.75p for a 15 min shower.
But how would that compare to a WB 40 kw combi or a 28kw system boiler
and
high recovery cylinder / megaflow / thermal store ?
I would imagine the combi being chaper to run but its 40 kw !!!

But you still use the same energy. So assuming boiler is say 85%
efficient
you can multiply the number of kilowatts used by 1/0.085 or whatever
then
multiply by the cost per kW of the gas and you'll find it's about
one-third
the price.

And why DO you need a 15 minute shower ? ;-)

Because it takes that long in a 9.5kW electric-trickle shower to get the
soap rinsed off!

Too true - took about 25 mins tonite, been playing with a 350 kva diesel
genny for most of today :-(

Regards Jeff

Doctor Evil wrote :-

A shower that takes 10 litres/min of 55C hot water will consume 150
litres in 15 minutes. The shower will give approx 13 to 14 litres/min
with added cold


No. of Kilowatts = Quantity of water in litres x temp rise in degrees C
x
Specific heat of water / No. of seconds in one hour.

Specific heat of water = 4.2

So to heat a 150 litres in 1 hour will take:

150 x 50 (raising water from 5C to 55C) x 4.2 /3,600 = 8.75 kW

Apply this formula to all the permutations.

So are we saying that to do this in 15 mins requires 35 kW ?

so what are the relative costs of electric / combi / stored ?

Regards Jeff

Jeff wrote:
Doctor Evil wrote :-


A shower that takes 10 litres/min of 55C hot water will consume 150
litres in 15 minutes. The shower will give approx 13 to 14 litres/min
with added cold


No. of Kilowatts = Quantity of water in litres x temp rise in degrees C
x
Specific heat of water / No. of seconds in one hour.

Specific heat of water = 4.2

So to heat a 150 litres in 1 hour will take:

150 x 50 (raising water from 5C to 55C) x 4.2 /3,600 = 8.75 kW

Apply this formula to all the permutations.


So are we saying that to do this in 15 mins requires 35 kW ?

Kind of, Dr. Drivel was surprisingly numerate for once (all those
lessons from Andy Hall must be beginning to sink in), but was being a
little sloppy with units. (he came out with the right answer by
including the "in 1 hour" phrase in the answer).

We should be talking energy here and not power. So it takes 8.75kJ of
energy to get that amount of water that hot. It does not matter how long
you take to do it (within reason).

(Power does become important when you talk instantaneous water heating,
because you are then interested in how quickly the system can get heat
into the water).

so what are the relative costs of electric / combi / stored ?

It is probably simpler to do the sum looking at the total cost of energy
required and factoring in the efficiency of the delivery method.

Total cost = fuel cost per kJ * energy required * efficiency

The cost of fuel you can get from your bill.

The energy is the total kg of hot water used multiplied by 4200 and the
temperature rise. (divide by 1000 for kJ)

The efficiency will depend on the type of heating. Leccy shower
efficiency is going to typically be just over 99%

For a modern high efficiency condensing combi combi you may get
efficiencies of 90+% at the shower head. For the stored system with
equally decent boiler, it will be slightly less - but will depend on
things like how well your hot water cylinder is lagged plus a few other
system design issues.

Which is all a complex way of saying the gas heating will get you a
better shower at about half the cost for in real terms. Obviously a less
efficient boiler will eat into that saving.



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

"Jeff" wrote
| 15 mins in shower. Water running for about half that.
| Can I guess you're over 65 and/or did economics at school? :-)

No and no, but I do live in Scotland.

Owain

Markus Splenius wrote:

Since I detect a rather patronising tone, I must be a pedant and
insist that you are wrong. :-)

The amount of energy required to heat a volume of water depends on
the
temperature of the water and the pressure. Thus 4186 J/Kg/K for water
is at standard temperature and pressure or STP as we used to write at
school. (273K ish, 1 atmosphere).


pedant
If youre showering ni a room at 273K youre gonig to use a lot more
energy.
/pedant


I might add that water takes a lot of heating, which is something to
do with the bond geometry. Thus I suggest we start showering in
liquids that need less energy to heat.

Markus.

Any ideas?

Petrol wouldnt need heating at all, its an effective grease solvent.
Poisoning and explosion are potential side effects.

Alcohol would, but the energy savings would be lost by unwillingness to
get out the shower in many cases.

Engine oil might work, but again savings would be lost by the fact that
its impossible to get out the shower, or to turn the knob off.
Extensive bruising might be noticed as a side effect.


NT

Take no notice to the Rum Bugger, he has only half a clue.

Energy - how much it costs you. Quite easy.

A combi:

Look at the makers specs: input and output to DHW which maybe different to output to CH. A combi delivers 14 litres/min @ 35C temp rise. So, main temp is 10C you will get 45C to DHW. You may want to add a little cold, so you may have 15 litres/min. This is all by the way.

Once you have chose a combi that is suitable for your hot water requirements (output of appliance to DHW), then only the input matters when it comes to costing. A combi that gives 40kW input (what you pay) will be 10kW for 15 mins if the burner is full on all of the running time. Look at your gas bill to see what you pay per kW/hr, as it varies.

The gap between input and output gives the efficiency of an appliance.

Electric Shower:

Same with an electric shower as a combi, 1/4 of the rated "input". As it is in kW, and you are charged so, again look at your bill.

Electric showers are about 99% efficient so, the input and outputs are usually the same. No waste heat is pumped out of flue, as with oil and gas boilers.

Stored Water:

The equation gave 8.75 kW, which is fine if all things are equal and no losses. That is what you pay for 10 litre/min of 55C hot water. You are right in that to get to 55C in 15 mins you multiply by 4.

The 8.75 figure does not take into account cylinder heat exchanger and other losses. Then there are cylidner standing losses over time. Scale will also hinder all appliances in efficiency as time moves on.