Your hot water tank

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Your hot water tank

Post: # 17084Post Lozzie »

I have been told several times before that an electric hot water tank uses less electricity if left on all the time, than turning it off and then on later or the next day, to heat up the whole tank.

Does anyone here know if this is true or not?
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Post: # 17095Post Wombat »

This sounds like fodder for Stew!

I suppose, as with everything, it depends! from what I can think of it would depend on the amount of use the system gets and the level of insulation. ((you can always wrap your tank in fibreglass batts or whatever to improve insulation).

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Post: # 17097Post Muddypause »

Complete bollox, if I may use such an epithet here.

Bear in mind that such a hot tank will be thermostatically controlled - inside is a temperature sensor that turns the heater off when it is hot enough, and back on when it cools a little. Now, if you had a perfectly insulated tank - one that lost absolutely no heat to the air (in fact this is virtually impossible) - then in fact it wouldn't make any difference. Heating a tankfull of water, with no losses, uses the same amount of energy, regardless of how often the heater is turned on.

In reality, the heater will come on when either you use some hot water, and cold water comes into the tank to replace it, or when it loses heat to the air.

Now imagine you had a tankful of hot water, and you let it cool down for a few hours (losing its heat to the air), till it was luke warm. Now you turn the heater on, and consume some electricity to heat it back up again. If you had left the tank on all the time, it would still have been losing heat to the air, and this heat would still have needed replacing. Now, there is a rule in physics that says the hotter something is, the faster it loses its heat. So, keeping the temperature elevated means it is losing heat faster than the cooling tank, so you need more electricity to replace the heat that is being lost. The cooling tank will start off losing heat at the same rate, but then lose it more slowly as it cools, so overall, the hotter tank is losing heat at a faster rate.

In both cases it will take the same amount of energy to heat any fresh cold water that enters the tank to replace hot water that you have used.

The real difference is that a tank with the heater on all the time will always provide you with water at max. temperature; the cooling tank will often be at a lower temperature.

The best thing you can do is to insulate the tank as well as you possibly can to reduce heat loss (I've got two insulating jackets on mine).
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Post: # 17098Post Wombat »

Stew to the rescue!

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Post: # 17099Post Muddypause »

I should point out that I failed my physics exams.

Twice.
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Post: # 17100Post Wombat »

Buggered if I know how!

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Post: # 17110Post Boots »

Hey Stew...If your theory is correct...

That holding or insulating hot water leads to heat loss, thus requiring MORE power to maintain the regulated temp.... Why are you doing the two jacket thing?

The ratio of cold to hot in the heating process I'm not sure of. Maybe it would be more effective to remove small amounts at a time... ?

Here's what I do.. thinking I am saving power on hotwater... We allocate hotwater time to certain timeframes (so that water is being drawn in bulk for showers, washing dishes etc at the same time and power is then used to regulate rather than replace water for the bulk of the day). I figure everytime we turn on the hot water tap, we kick off the replace and reheat process, which draws power. (My water is also pumped with an electric pump so I get a double draw) so I draw once a day and the kids fill the sink for dishes while I shower which then fills the bath (subject to a couple of top ups as each gets in). We wash clothes in cold, and bathing and cleaning are our regular hot water draws.

So still not sure... but am now thinking it would be more efficient to actually remove small amounts of water, rather than bulk as we do, as the water would heat quicker wouldn't it?... If minimal cold was being added to bulk hot... :? But then I have to factor in the extra pump that kicks in... :pale:

I also ensure hot water taps are clearly marked to avoid waste, as visitors always seem to fiddle about. :roll: Why do they suddenly go colourblind in other peoples kitchens or bathrooms?

Oh and don't forget the water in your kettle is useful for other things during the day - not just a cuppa - there is often hot/warm water sitting there on your bench/stove, when folks reach for the hot water tap.

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Post: # 17119Post Lozzie »

Wow, what a response, I could write an article on just this subject!
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Post: # 17122Post Boots »

Go on then... :mrgreen:

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Post: # 17124Post Lozzie »

I will, when I have finished writing the other ones on my list!
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Post: # 17129Post Millymollymandy »

I'm still completely in the dark - so what is the answer then? :oops: Or isn't there one?

I have a 2nd hot water tank that does bathrooms that we don't use and the cellar that I put on twice a week because I can't wash my hands in freezing cold water. The main tank is on every night during off peak and that does the bathroom that we use and the kitchen.

They are insulated inside the tank (if that makes sense) so the water stays warm for up to 3 days. I'm assuming I am saving money here by only putting it on twice a week.

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Post: # 17130Post Boots »

:mrgreen: Lookin forward to it!

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Post: # 17144Post Muddypause »

Boots wrote:Hey Stew...If your theory is correct...

That holding or insulating hot water leads to heat loss, thus requiring MORE power to maintain the regulated temp.... Why are you doing the two jacket thing?
To reduce heat loss. The only issue here is that of heat loss - the amount of energy used in actually heating the water is the same. The only reason you woud use any electricity once the water is up to temperature is because heat is lost to the air. We cannot make a tank that loses no heat, but we can try to reduce it to a minimum.
The ratio of cold to hot in the heating process I'm not sure of. Maybe it would be more effective to remove small amounts at a time... ?
Water has a well established Specific Heat Capacity (as does everything else). This shows that it takes 4.2 Joules of energy to heat a gram of water through 1 degree - in English, it takes a little more than one unit of electricity to heat 10 gallons through 20 degrees plus heat losses.

It doesn't matter what the start or end temperature of the water is, if you heat this much water by this amount, it uses this much energy. You cannot change this, no matter how you use the water, or at what time. The only things you can do to reduce energy consumption, is to a) reduce the amount of hot water you use, and b) reduce the amount of heat you lose to the air. In keeping the temperature elevated while it is in storage, you will inevitably lose more heat to the air.
Here's what I do.. thinking I am saving power on hotwater... We allocate hotwater time to certain timeframes (so that water is being drawn in bulk for showers, washing dishes etc at the same time and power is then used to regulate rather than replace water for the bulk of the day
You could try measuring your energy consumption (over, say, a week), and then compare it to your consumption if you only heat the water prior to use. Keep to the same water usage schedule, so you are doing a fair comparison.
So still not sure... but am now thinking it would be more efficient to actually remove small amounts of water, rather than bulk as we do, as the water would heat quicker wouldn't it?... If minimal cold was being added to bulk hot... Confused But then I have to factor in the extra pump that kicks in... pale
A small amount of water will heat quicker than a large amount of water, but 2 x small amount will take 2 x energy needed. Remember that if you add cold water to hot, the cold water will get warmer, but the hot water will also get cooler, and the maths works out that you still have to heat the equivalent of the cold water you added in order to bring it back up to temperature. The only difference would involve the heat losses, because water at a lower temperature will lose heat at a slower rate.
I also ensure hot water taps are clearly marked to avoid waste, as visitors always seem to fiddle about. Rolling Eyes Why do they suddenly go colourblind in other peoples kitchens or bathrooms?

Oh and don't forget the water in your kettle is useful for other things during the day - not just a cuppa - there is often hot/warm water sitting there on your bench/stove, when folks reach for the hot water tap.
hmm..Is it more effective to fill the kettle from the hot tap before putting it on the stove... no, let's not go there for now.
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Post: # 17146Post Boots »

:mrgreen: Tis certainly a head turner, this!

Feelin Dizzy...

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Post: # 17179Post ina »

Just to add my little bit: I've found out that I have plenty of hot water for "normal" use (i.e. shower, washing up) if I have the immersion heater on for about 2 hours a day. I suppose I would need a bit more if I wanted a full bath. I used to have it on either all the time (which meant I always had very hot water - which I didn't really need); then cut it down to overnight, but the two hours seem to do just as well. No idea whether I'm saving any energy that way, but I don't need to add as much cold water when I have a shower.

I can save a lot of heating if I use the cold water tap for washing my hands: It takes ages for the hot water to come through the tap anyway, so by the time it's getting through warm, I've finished washing my hands, but the hot water has gone from the tank, been no use to me, and still has to be heated up again! Admittedly, that might be a bit unpleasant in winter. It just needs getting used to... I draw the line at brushing my teeth with cold water, though! :mrgreen:
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