No doubt it'll vary according to the type of geyser, but I suspect 1.5 hours is more than enough in most instances - at least judging by the time it takes for our geyser to recover from a full flush (about 45 minutes, but it's quite small - and old).
No doubt Duncan could give us a clue - assuming a 2kW heating element how many litres of water can be warmed 10 degrees in 10 minutes?
(Gee - in days gone by I knew the numbers to work that out off by heart. Getting old )
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Insulating certainly needs to be a priority for all households. The R150 investment will definitely save you money and pay for itself relatively quickly. Not too sure about solar - everything that I've seen is very expensive.
I'm a little unsure why this guy (in the comment above) is saving so much energy (and therefore rands). There are a couple of things that can play a factor like,
- Insulation
- Thermostat setting
- Usage
I've been trying to find some equations to do a quick calculation as to real savings and turning geysers on/off. Temperature setting will definitely play a role.
I can't see how the temperature of the element affects efficiency - it is a resistive heater and is nearly 100% efficient, regardless of temperature. It would be on for longer to heat from say 50º to 65º than from 63º to 65º, but I can't see how this effects efficiency.
The lower the temperature of your geyser the slower it will cool down.
Regardless of the energy issues, it is still important to try to turn your geyser off during peak times (07h00 - 10h00 and 18h00 - 20h00) to alleviate the load it adds to the electricity network. If you can turn it off all day and only turn it back on after 20h00 that will make a significant difference to load shedding.
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PS. For those of you confused by this there is a difference between energy and power.
Energy is what you pay for and is measured in kilowatt hours (kWh). It is a measure of how long you have been using a certain number of kilowatts. So if your kettle is a 2kW kettle and you run it for 1 hour (yikes!) you will use (2kW * 1hr = 2kWh).
In this case the kettle's power rating is 2kW.
Eskom has a near infinite supply of energy but can only deliver a certain amount of power. Let's just say Eskom's power capacity was 20kW, that would mean that only 10 people could run their kettles at the same time, but there would always be enough energy to boil water, as long as only 10 people did it at the same time.
If 11 people tried to boil their kettle at the same time Eskom would not be able to provide the power for them to do so, even though they could provide enough energy.
This is exactly the problem that we have at the moment. Eskom has plenty of energy, but a limited capacity to supply power (their capacity is about 40000MW).
I'm going to cheat here a bit....
Working backwards....45ºC change consumes 7.838kWH, which means 5.7ºC/kWh, or rather 0.17kWH/ºC.It takes a 3kW, 150l hot water cylinder (HWC), or geyser, element to heat the water from 20 ºC to 65 ºC in the order of 2 hours 40 minutes and consumes about 7.838kWh
From Eskom's residential DSM page
So to work out how long it would take to heat from temperature a to b
time to heat = 0.17 divide by (element kW) times (b - a)ºC (approximately)
The resistance is not constant. The resistance is lower at lower temperatures, higher at higher temperatures. I've been messing around with some maths...
Consider the voltage as constant in the following.
Applying V = I * R, half the resistance produces double the current.
Doubling the current doubles the power demand applying P = V * I
Lower resistance at the appliance would also increase the effect of line resistance, energy dissipated at a non-target point, but that "should" be nominal.
EDIT: With an element resistance of the order of 10 - 20 ohms, line resistance could well be a factor.
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While the element is on the resistance will be just about constant, as it is significantly hotter (internally where the resistive part is) than the water. When it is off, the resistance should be quite a bit lower (resistance increase with temp).
I'm guessing that the kilowatt rating is rated when submersed in water, and the resistance will only change slightly with variations in water temperature.
e.g. with P = V^2/R, a 3kW element is about 17.6 ohms with V=230V. If that resistance were to change to 17ohms, the power rating would be 3.1kW. I reckon that in reality the resistance might only change by 0.1ohms depending on the water temperature.
Last edited by duncan drennan; 07-Feb-08 at 08:08 AM.
If I get a chance, I'm going to tong a geyser off a cold start to see how the current changes over time. About the easiest practical test.
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So at work if just turn on the geyser for 1.5 hours in the morning there is enough hot water for the whole day, we just wash coffee cups etc., and this will help with load shedding and costs.
Only stress when you can change the outcome!
the geyser in my house takes 1hr 15 min to heat up... i shower every morning and evening.. my electricity account was R450 per month but since the increase will go up...i have installed lots of timers for customers and 90% of them are on override or i have disconnected and removed it...because of the Inconvenience they cause.
the other way of looking at this geyser is like someone mentioned the other day they turn down the thermostat to 50... but now they run 100 litres into the bath and dont add cold...think about it if it was 65-70 you would only take 60 litres from the geyser and add cold... instead of 100 litres meaning the geyser will heat up to temp quicker and switch off again using less elctricity...there are just too many variables...bottom line put in a solar panel and you will pay out your nose for it but in 50 years you will save money.
or you could do like the old quick shower way... water on... get wet... water off...soap...water on rinse water off...so if you really want to be conservative this way you save water and electricty...or better still get big drum mount it in your back yard put a fire under it and bingo you go hot water no electricity used...
and say this again all because some fools didnt do their job right we must now suffer...if the planning was done correctly 10 years ago we wouldnt be waiting 7 years to get the problems sorted out...and if you gona get all conservation sensitive on me...let talk about cellphones are we gona stop using them because they are harmful to your heath and polluting the enviroment all the batteries from them which are gona need to be disposed of etc etc...motor cars trucks etc etc are a far bigger problem and i dont see anyone driving 4 people to work in the morning or cycling to work...one man/woman one car/4x4...4x4 tyres noise pollution how many women do you see going to the malls...dropping kids off etc in big 4x4 when those tyres are worn where will they be dumped???? i could just go on and on so if you want save the enviriment look at everything we humans do every day to kill the enviroment...we chase the animals out of their enviroment then want to shoot the monkeys because they are pests and steal food out your kitchen... because you cut down the paw paw tree or mango tree and built a house where they got their food from....let me stop here.
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