Truth About Switching Geysers Off?

[Zoomzoom]

Wow - how did I only just see this monster thread. Didn't we talk about this last year, and the year before I started writing an article about it, then gave up because there was too much vitriol.

To summarise:
If everything is working right - turning off your geyser within a 24 hour period should NOT save you money... Geysers should stay warm.
But there are reasons why turning off your geyser can save you money:
1. You let it cool ALL the way down - say if you're going on holiday. If it cools all the way down and stays there for days and days - you'll save by not paying for the standing losses. These can be between 0-4kWh/day depending on the state of your geyser.
View attachment 367255
from http://blog.homebug.co.za/?p=34
2. Your geyser is leaking, badly insulated, etc. etc. If your geyser doesn't hold its temperature for a few days (when you aren't using it), then there's probably a leak or some other problem (someone stealing your hot water?!)

3. You are using water at a COLDER temperature. Yes in the morning you notice whether your shower is hot or not, but do you really notice if it's 60 or 50 degrees when you are rinsing a cup, or washing your hands... not really. So if you don't switch your geyser on, you're using colder water. 20L of water at 60 degrees uses about 1 kwh. If you're only heating it up to 40 degress, it will probably use about 0.5kwh. Maybe someone in your house sits there with the hot water tap running when they brush their teeth... I dunno!? If the geyser's gone cold already, then at least you didn't pay to heat it - hurrah!
(I say probably because it depends on the temperature of your tap water...)

4. (most likely if not 3) - You went on a saving spree at the same time and also switched off the lights a bit more, used the heater less... and all those things added up you decided to allocate to your clever geyser move!

BTW - if anyone tells you turning off a geyser will mean using MORE electricity, they are DEFINITELY wrong... everyone else, can be right, in the right situation...


PS - Legionella is very real, generally under reported and just not well understood in SA (lots of people die of other preventable things I guess!?)... You don't need to heat to 70 degrees every day, just once a month should do it to prevent any build up.
http://www.enviropaedia.com/topic/default.php?topic_id=151
"The most concerning issues surrounding Legionella within South Africa are the general lack of awareness and the lack of statistics pertaining to the diagnosis of Legionnaire’s disease. These two factors result in the true scale of the problem being unquantifiable and largely unknown!"
They only decided to start taking it seriously in 2013 with the SANS 893 standard: https://www.sabs.co.za/Media/docs\SANS_893.pdf

I wouldn't say it is at all vitriolic, just that there are two camps of firmly held opinion, not just here, but out in the wider 'expert' universe as well, with no metered tests to say this geyser on all the time used this much vs the same geyser off at regular intervals used that much to prove it one way or the other, so as a person trying to work out what is best in terms of saving and what to install ... it's almost impossible to come to a conclusion.

 

[DWAAS]

Well my wife swears it's definitely a saving to switch off in the morning and back on in the late afternoon

 

[noxibox]

Just out of interest.

If you look on a tin of formula they tell you to boil the water and then let it cool down to 70 deg before putting the formula in. So at 70 there is still stuff that can "grow".

The reason is that formula is for babies. We sterilised dummies too. Then their immune systems become more robust and they put everything they can find into their mouths.

So there might be some very slim chance of catching the disease from your domestic water supply, but it does not seem a particularly common source of infection. When it comes to children are we to keep sterilising all possible sources of infection, given that they can potentially catch this and other things from multiple sources? Does any reliable evidence exist that water in the home is a major source of infection?

See when I read this kind of thing it leads me thinking it's over cautious and over blown.

"Outbreaks are commonly associated with buildings or structures that have complex water systems, like hotels and resorts, long-term care facilities, hospitals, and cruise ships. The most likely sources of infection include water used for showering, hot tubs, decorative fountains and cooling towers (parts of centralized air-conditioning systems for large buildings)."

I mean has anyone here ever met or even heard anyone being infected with this?

Nope. I certainly never have, but you'll hear all about it in every geyser thread.

All the research I've ever seen indicates it is almost unheard of in a private domestic environment.

2. Your geyser is leaking, badly insulated, etc. etc. If your geyser doesn't hold its temperature for a few days (when you aren't using it), then there's probably a leak or some other problem (someone stealing your hot water?!)

There must be some cooling. The amount will vary depending on the current temperature of the water.

 

[SauRoNZA]

All the research I've ever seen indicates it is almost unheard of in a private domestic environment.

My exact findings so far.

My geyser is staying on 55 thanks and will go on in the evenings only.

And weekend mornings.

 

[richjdavies]

... it's almost impossible to come to a conclusion.

If you want to save money, you may as well switch if off... that's the conclusion. You can't use MORE by turning off. Anyone who says you can has lost the plot. You can use pretty much the same... but you can't possibly use more.

From the people I've seen/helped, the real savings all come down to #3 above -- you use less hot water (i.e. all day long you use water at 40 degrees, because the geyser is off) rather than 60 degrees. You only notice when you shower - at which point it's back up to 60.

 

[Devinity]

Hi,

There is a lot of conflicting information out there - some say switch your geyser off for some hours per day will save and others (including Eskom) say doing that actually increases power use. Then the municipality wants to put a timer on your geyser so it only goes on for a few hours (at their time not what suits you!) and that is supposed to save electricity but does it?

Does anyone know what the real story is?

I tried switching off my geyser for about 3 months... I ended up using more electricity than when I just left it on.

 

[neilronaldson]

Its easy, test it. I tried it makes almost zero difference to your bill.
The problem is this, if you leave it off for a few hours, you now have to heat up 150 L tank from ambient temperature 20 deg C up to about 60 deg C. So it takes a long time, however, if you keep it on the geyser only runs for a short period. The longer its running at maximum power, the more it uses.
Try it with a kettle. Put 1 Litre of cold water into it and run the kettle. Time it. Then, leave the kettle to cool down for about 10 minutes, turn it on. Time how long it takes. Thats pretty much what your geyser is doing.

+1 here

your geyser is like an ERN - turning your geyser off is treating it like a kettle with cold water...

 

[Zarkon]

Saved plenty of money with my roomate,when we only switched it on once a day to heat up. Might not work for a family with a housewife. It will save u if it's just two people and both work during the day. I tested with leaving it on,and when we forgot to turn it off,was very clear it was pulling a lot of units.

Why it worked for us,water was still warm when got home from work,so 30-40min on brought it back up to it's temps. So like i said works great for a couple or student,won't work for a family who uses far more warm water.

 

[Zoomzoom]

Saved plenty of money with my roomate,when we only switched it on once a day to heat up. Might not work for a family with a housewife. It will save u if it's just two people and both work during the day. I tested with leaving it on,and when we forgot to turn it off,was very clear it was pulling a lot of units.

Why it worked for us,water was still warm when got home from work,so 30-40min on brought it back up to it's temps. So like i said works great for a couple or student,won't work for a family who uses far more warm water.

Why the assumption a 'housewife' (how sexist can you get?) will automatically use more water than two stinky male students who never wash dishes, clothes, or clean house ... hmmm ... okay maybe I can see where the extra water goes.

 

[SauRoNZA]

Why the assumption a 'housewife' (how sexist can you get?) will automatically use more water than two stinky male students who never wash dishes, clothes, or clean house ... hmmm ... okay maybe I can see where the extra water goes.

I think he was referring more to the fact that someone is home during the day who needs hot water.

 

[Arthur]

I tried switching off my geyser for about 3 months... I ended up using more electricity than when I just left it on.

You had cold showers for three months!? Yikes!

 

[me_]

1. You let it cool ALL the way down - say if you're going on holiday. If it cools all the way down and stays there for days and days - you'll save by not paying for the standing losses. These can be between 0-4kWh/day depending on the state of your geyser.
View attachment 367255
from http://blog.homebug.co.za/?p=34

You need to remember that official standing loss figures are calculated from a starting temperature of 45 degrees.
If you increase the starting temperature to 60, the losses will be quite a bit more.

From the table, a A-grade 150l tank has a standing loss of 0.9kw @ 45 degrees, this equates to roughly 1.35kw @ 60 degrees.
A G-grade 150l tank has a standing loss of 3.7kw @ 45 degrees which equates to 5.5kw @ 60.
So losses are approximately 50% higher with a starting temp of 60 degrees compared to 45 degrees.

This was my point earlier where I said using a timer where you stop heating the water before you use it so it remains colder afterwards will result in far lower losses.
At the same time, keeping the geyser on so it keeps the geyser at maximum temperature means you keep losing at the maximum amount. It's a compound interest type effect.

 

[richjdavies]

From the table, a A-grade 150l tank has a standing loss of 0.9kw @ 45 degrees, this equates to roughly 1.35kw @ 60 degrees.
A G-grade 150l tank has a standing loss of 3.7kw @ 45 degrees which equates to 5.5kw @ 60.

Can I ask how you worked that out?

I was under the impression the 45 degrees you're talking about is the 'heating' degrees... i.e. air temp + X = water temp... but


and finally:

I tried switching off my geyser for about 3 months... I ended up using more electricity than when I just left it on.

That may be true, but it isn't because you switched off your geyser
It can't use MORE when it's off... yes, it's gotta heat the water back up... all at once, but it would have done the SAME amount of work, keeping it warm in the first place. Think of it in this simplified way:

Your geyser loses say 1 degree an hour - starting at 60 degrees, going down to 20 degrees.
KEEP IT ON: every hour you heat it back up 1 degree, so it stays hot - over 24 hours you need to add 24-degree-hours of heat, over 48 hours, you've heated it up 48. It stays at 60 degrees all the time.
TURN IF OFF: after 24 hours, its gone from 60 degrees down to 36 degrees, so you have to heat it back up 24 degrees... so 24 degree-hours of heat... but if you leave it off for 48 hours, it goes all the way down to cold i.e. 20 degrees... so now to heat it up, you need to heat 40 degree-hours

So worst case - short time, water didn't get 'cold' - Keeping on EQUALS Turning off (24 = 24)
Best case - longer time, water cools to same temperature as its environment - Keeping on USES MORE than turning off. (48 > 40)
There's no situation when switching off can use more...

 

[Arthur]

Can I ask how you worked that out?

I was under the impression the 45 degrees you're talking about is the 'heating' degrees... i.e. air temp + X = water temp... but


and finally:

That may be true, but it isn't because you switched off your geyser
It can't use MORE when it's off... yes, it's gotta heat the water back up... all at once, but it would have done the SAME amount of work, keeping it warm in the first place. Think of it in this simplified way:

Your geyser loses say 1 degree an hour - starting at 60 degrees, going down to 20 degrees.
KEEP IT ON: every hour you heat it back up 1 degree, so it stays hot - over 24 hours you need to add 24-degree-hours of heat, over 48 hours, you've heated it up 48. It stays at 60 degrees all the time.
TURN IF OFF: after 24 hours, its gone from 60 degrees down to 36 degrees, so you have to heat it back up 24 degrees... so 24 degree-hours of heat... but if you leave it off for 48 hours, it goes all the way down to cold i.e. 20 degrees... so now to heat it up, you need to heat 40 degree-hours

So worst case - short time, water didn't get 'cold' - Keeping on EQUALS Turning off (24 = 24)
Best case - longer time, water cools to same temperature as its environment - Keeping on USES MORE than turning off. (48 > 40)
There's no situation when switching off can use more...

Right. Until it gets to ambient, your geyser loses heat whether you leave it on all the time, or whether you switch it off all day. Either way, that heat loss has to be made up by powering the heating element. It makes a negligible difference whether you make up the heat loss in lots of little spurts all day, or whether you make it up in one big spurt at the end of the day/early morning. Assuming of course you use the same amount of hot water.

Whatever you do, it takes 4.187 kJ to heat one litre of water by one degree C. This we know to the hundredth decimal place. If your 150-litre geyser's insulation is such that the heat loss is one degree C per hour (at 60 C), then the water temp will drop by 12 C in 12 hours. If you power off at say 07h00, then at 19h00 the same day the temp will have dropped to 48 C. Let's assume constant 1 C loss per hour (it's very slightly less the cooler it gets). To get it back to 60 degrees C you'll have to use an exact amount of energy: 150 x 12 x 4.187 = 7 536.6 kJ.

If you leave the geyser on all day, and the thermostat kicks in at 59 degrees (say), it means it'll kick in twelve times during the day (once per hour, when temp drops from 60 to 59 C), each time having to get the temp from 59-deg to 60-deg, so 150 x 12 x 4.187 = 7 536.6 kJ.

The only variance - and it very slight - is that hotter water cools at a slightly faster rate than cooler water. In modern insulated geysers, the delta rate T (60 C vs 48 C) is +- 2%. Trivial to the point of being barely measureable.

 

[Zoomzoom]

I think he was referring more to the fact that someone is home during the day who needs hot water.

Then say that, not the sexist twaddle that he said.

 

[The_Traveller]

Then say that, not the sexist twaddle that he said.

Arghhh shuddup you wind-up merchant ...

 

[TedLasso]

My Electricity bill in a complex during the summer months was around 450.00. This was with the geyser turned on all the time. In the winter months the bill shot up to 800-900 (due to the eskom tarriff used by the BC) ... that pissed me right off. I started turning off the geyser at the DB board. We have been a family of two, and the geyser is turned on every day for about 2 hours (5-7am) ... our electricity bill is now lower than 360 a month. Granted we need hot water for bathing. What's left over is used for dishwashing, etc for the rest of the day.

It saves me money!

 

[krycor]

If you want to save money, you may as well switch if off... that's the conclusion. You can't use MORE by turning off. Anyone who says you can has lost the plot. You can use pretty much the same... but you can't possibly use more.

From the people I've seen/helped, the real savings all come down to #3 above -- you use less hot water (i.e. all day long you use water at 40 degrees, because the geyser is off) rather than 60 degrees. You only notice when you shower - at which point it's back up to 60.

I think it's all about how I use hot & warm water and the availability requirements, the age of the geyser and insulation & location there of.

If u do all the prep work with a brand spanking new one you 'may' get slightly above or below the turning off power usage but not quite.. The turning off crowd, myself included, get the close to least power usage on avg for an avg home with out the latest and greatest tech. You milage may vary but yah.. I stick to my timer for now where changing infrastructure is not possible due capital cost & rent

 

[me_]

Right. Until it gets to ambient, your geyser loses heat whether you leave it on all the time, or whether you switch it off all day. Either way, that heat loss has to be made up by powering the heating element. It makes a negligible difference whether you make up the heat loss in lots of little spurts all day, or whether you make it up in one big spurt at the end of the day/early morning. Assuming of course you use the same amount of hot water.

Whatever you do, it takes 4.187 kJ to heat one litre of water by one degree C. This we know to the hundredth decimal place. If your 150-litre geyser's insulation is such that the heat loss is one degree C per hour (at 60 C), then the water temp will drop by 12 C in 12 hours. If you power off at say 07h00, then at 19h00 the same day the temp will have dropped to 48 C. Let's assume constant 1 C loss per hour (it's very slightly less the cooler it gets). To get it back to 60 degrees C you'll have to use an exact amount of energy: 150 x 12 x 4.187 = 7 536.6 kJ.

If you leave the geyser on all day, and the thermostat kicks in at 59 degrees (say), it means it'll kick in twelve times during the day (once per hour, when temp drops from 60 to 59 C), each time having to get the temp from 59-deg to 60-deg, so 150 x 12 x 4.187 = 7 536.6 kJ.

The only variance - and it very slight - is that hotter water cools at a slightly faster rate than cooler water. In modern insulated geysers, the delta rate T (60 C vs 48 C) is +- 2%. Trivial to the point of being barely measureable.

I wouldn't call the losses "trivial". Well many of us would like to believe we have A rated geysers, the truth is most people will most likely have D rated or less efficient.
A 150l D rated geyser will have a standing loss of 2.1kw @45 degrees. I haven't been able to determine what the average ambient temperature is that is used while testing, but I would assume in the region of 15 - 20 degrees so I'll use 18 degree ambient for my calculations.

We know that 2.1kw = loss of 12 degrees over 24 hours. We can use this to calculate our coefficient constant for the geyser of 0.0245
If we were to heat the geyser to 60 (instead of 45), the standing loss over 24 hours would be 18.66 degrees or 3.3kw, but that would be without the element kicking back in again.
If we were to assume the geyser kicked in every hour, then the standing loss would be 1.02 degrees per hour which is 24.48 degrees per day which is 4.3kw per day.
That's not a reality as most geysers will only kick in when the temp drops by 5 degrees so a more realistic average would be to say that the geyser kicks in on average every 4 hours. This would equal a loss of 3.92 degrees every 4 hours which is a total loss of 23.52 degrees per or 4.1kw per day.

If your geyser is on a timer and you use the hot water shortly after the geyser switches off to bring the average temp down and you had an average temp of around 45, you would only experience a loss of 2.1kw per day. Therefore the standing loss difference is 2kw (almost 100% of standing loss). That is hardly trivial.

In our house, once we're finished showering in the morning, the geyser is usually around 36 - 38 degrees. At this temperature, our standing loss will only be 8 degrees or 1.4kw.

 

[Arthur]

In our house, once we're finished showering in the morning, the geyser is usually around 36 - 38 degrees. At this temperature, our standing loss will only be 8 degrees or 1.4kw.

Good points. As an aside, my "trivial" point is in reference to the 'Newtonian loss', ie the different rate of cooling at different temps.

But whatever your standing loss, when you switch on again in the evening/night you still have to make up the heat loss (whatever it is) to get back to 60 deg C.