cost of a 15 min shower ?

[Jeff]

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

Regards Jeff

[Jeff]

Christian McArdle wrote :-

I would have thought that you should knock off a bit for stored energy due to more heat exchangers and standing losses ?

Regards Jeff

[Jeff]

Doctor Evil wrote :-

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

[John Rumm]

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).

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.

[Owain]

"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

[bigcat]

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

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

[Doctor Evil]

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 differen 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 littl 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 wate requirements (output of appliance to DHW), then only the input matter 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 runnin time. Look at your gas bill to see what you pay per kW/hr, as i 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". A 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 ar usually the same. No waste heat is pumped out of flue, as with oil an gas boilers.

Stored Water:

The equation gave 8.75 kW, which is fine if all things are equal and n losses. That is what you pay for 10 litre/min of 55C hot water. You ar 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 an other losses. Then there are cylidner standing losses over time. Scale will also hinder all appliances in efficiency as time moves on

-- Doctor Evil