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Author Topic: Is it more efficient to have a 2nd hot water tank?  (Read 5359 times)

OPS

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OPS  asked the Naked Scientists:
   
Hello Chris,

With the cost of electricity escalating dramatically in SA, I am considering adding a 50L geyser in parallel to my 150Litre geyser as there are only 2 of us in the house, one showering in the morning and the other one bathing at night and seems crazy keeping 150L always readily available. So my thoughts are to have 50L with a nice big element of 3Kw on a timer in the morning set long enough to heat up to the right temperature and coming on in the evening as well. Then if hot water is needed any other time having the 3Kw element will heat 50L quite quickly. The 5OL high pressure cylinder will be fitted in parallel to the 150L cylinder  which will be bypassed with stop cocks and generally switched off but available if we have guests.

A friend of mine believes that heating the 50L of water will take the same energy to heat irrespective of whether it is in a 50L cylinder on its own in the 150L cylinder. My thoughts are that in the bigger cylinder there is more water to heat over and above the 50L.

What are your thoughts on this and would fitting another 50L cylinder be a waste of time?
 
Thanks in advance,
 
Graham

What do you think?
« Last Edit: 12/09/2010 17:30:03 by _system »


 

Offline Geezer

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Is it more efficient to have a 2nd hot water tank?
« Reply #1 on: 12/09/2010 20:43:49 »
I'd be a bit nervous about having a couple of geezers hanging around just to make hot water. However...

The answer to your question is probably "maybe".

It really depends on how well insulated the hot water storage tanks are. If they didn't lose any heat at all, you could heat the water at any time and when you eventually used it, you would get all the energy back that you put into it.

On the other hand, if the heat can "leak" out of the tank, the sooner you use it, the better. In that situation you don't want to heat any more water than you are about to use, so that might favor the second small tank approach.

One way to avoid the heat loss problem is to use an instant water heater. It only heats the water as it is used, but it will require a rather hefty power service connection.

Another approach that really might reduce your power bill substantially is to use a heat pump water heater. They extract heat from the ambient air (and cool it down) to heat the water. I understand the savings can be very significant. The only problem is these units are still rather expensive, so it could take you a long time to break- even on the investment.

Depending on where you are in SA, a solar water heater might also be a good option. They probably work best in areas where there is little chance of freezing.
 

Offline tommya300

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Is it more efficient to have a 2nd hot water tank?
« Reply #2 on: 12/09/2010 20:56:51 »
Keeping this in mind...
Every time an electrical circuit is energize, there is an initial spike that draws the most and then levels out. The more iterations of energizing the circuit add up collectively.
Now you have 150 L tank of hot water, it will take longer to cool that quantity down, than the 50 L will.
The 50 L will end up drawing more power collective.

Keeping the plumbing the way it is and place a day duration timer 15 minute divisions, on the 150 L Hot water heater.
Set the duration time for about 2 hours at a time 2 hours before the first usage in the morning.
When the water heater is not in use it will hold hot water all day long even after you both have used it. This way the on off iterations for a recovery is just in that 2 hr window of opportunity. This 2 hours can be adjusted to a shorter time it is up to you.

I found it to be cost affective adjustable and simple.

 I recall in my personal experience
900 dollars later, I had a heat pump it was the most efficient recovery just increased the electric bill 10 dollars a month. Not bad you say??? wait there is more...
The reservoir was an electrically disconnected, well insulated, hot water tank. The system reduced fuel consumption of the oil furnace to zero in the summer. Hot water jacketed furnace.
4 years later the heat pump failed like any air conditioner.
I removed it and set the hot water heated electric. So much for the savings...
 
« Last Edit: 12/09/2010 21:33:53 by tommya300 »
 

Offline Geezer

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Is it more efficient to have a 2nd hot water tank?
« Reply #3 on: 12/09/2010 21:28:56 »

Every time an electrical circuit is energize, there is an initial spike that draws the most and then levels out. The more iterations of energizing the circuit add up collectively.
Now you have 150 L tank of hot water, it will take longer to cool that quantity down, than the 50 L will.
The 50 L will end up drawing more power collective.


Tommy, I disagree. There is no significant "spike" effect with electric heating elements, and even if there was, the heat produced would simply go into heating the water, so there would be no wasted energy.

All the heat produced by an electric heating element goes into heating the water. The only way to improve the efficiency of the system is to prevent the heat from escaping before you make use of it. You can do that, either by using the water sooner, or by preventing the heat from escaping with good insulation.

The best way (IMHO) is with a really well insulated storage tank that's controlled by thermostats to maintain the water a more or less constant temperature. I have one. It does not lose any significant amount of heat. I know it doesn't because it's in a small closet under the stairs, and the air temp in the closet is virtually the same as the air temp outside the closet, and the outer surface of the tank feels quite cool.

It's only turned off when we are on vacation, although even that's a bit unnecessary and I doubt that saves much money at all.
 

Offline tommya300

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« Reply #4 on: 12/09/2010 21:41:51 »

Every time an electrical circuit is energize, there is an initial spike that draws the most and then levels out. The more iterations of energizing the circuit add up collectively.
Now you have 150 L tank of hot water, it will take longer to cool that quantity down, than the 50 L will.
The 50 L will end up drawing more power collective.


Tommy, I disagree. There is no significant "spike" effect with electric heating elements, and even if there was, the heat produced would simply go into heating the water, so there would be no wasted energy.

All the heat produced by an electric heating element goes into heating the water. The only way to improve the efficiency of the system is to prevent the heat from escaping before you make use of it. You can do that, either by using the water sooner, or by preventing the heat from escaping with good insulation.

The best way (IMHO) is with a really well insulated storage tank that's controlled by thermostats to maintain the water a more or less constant temperature. I have one. It does not lose any significant amount of heat. I know it doesn't because it's in a small closet under the stairs, and the air temp in the closet is virtually the same as the air temp outside the closet, and the outer surface of the tank feels quite cool.

It's only turned off when we are on vacation, although even that's a bit unnecessary and I doubt that saves much money at all.

I have never seen any  mechanical swithing from off to on in any electrical circuit, that there is no initial power spike. The duration is short but the magnitude is large. Adding these short burst of engergy will be collective. The readings of the spike can be detected on an oscilloscope. Then the switch rings and decays to operating level.
OK let's dismiss the spikes.
Mostly all the new water heater are to manufacturer's code and heat insulation factors are also part of this code. The tank built in our time  are questionable.  Why have it recover immediately when sufficient hot water is available to burn your hands. Set it for a short duration and it will last all day.
I done it and it works! My wife uses it early in the morning and that is when it is about to shut down.
I wash and use an abundance and at 9:00 at night it is just hot enough for 10 hot water bottles
« Last Edit: 12/09/2010 22:14:18 by tommya300 »
 

Offline Geezer

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Is it more efficient to have a 2nd hot water tank?
« Reply #5 on: 12/09/2010 22:08:47 »

I have never seen any  mechanical switching from off to on in any electrical circuit, that there is no initial power spike. The duration is short but the magnitude is large. Adding these short burst of energy will be collective. The readings of the spike can be detected on an oscilloscope. Then the switch rings and decays to operating level.


I think what you are referring to is "inrush current". A water heating element is basically a resistor. There is no "inrush current" because there is nowhere for the current to rush to. There is nothing to "charge up". (It has very little inductance and very little capacitance, so it does not store energy in an electric or electromagnetic field.)

You may also be thinking of old-fangled light bulbs. They are resistors, but their resistance is very low until they heat up, so the initial current is rather high. But even if a heating element was like that (it's not), the heat would still end up going into the water.

Think about it in terms of energy. Even if there was some gigantic surge, where would the energy go? It has to go somewhere. If it's significant, and it's not going into the water, we should be aware of it showing up somewhere else.

 

Offline yor_on

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Is it more efficient to have a 2nd hot water tank?
« Reply #6 on: 12/09/2010 23:46:34 »
OPS. What you could do, considering that you live in SA which are quite a hot country, would be to weld a rectangular flat box maybe, not to thick? Lean it to the the suns path, if you get my drift, and paint it black. Either have something heavily insulated inside the house that will keep the heat gotten from the circulation made as the sun heats it making the water move through convection and possibly steam, or just let it flow free and use it to heat up another fluid that keeps the heat better heat exchanger

The point is that you have a lot of sun and that a black surface will get very hot, the rest is just pipes and circulation, and insulated storage.
 

Offline tommya300

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Is it more efficient to have a 2nd hot water tank?
« Reply #7 on: 13/09/2010 00:09:03 »

I have never seen any  mechanical switching from off to on in any electrical circuit, that there is no initial power spike. The duration is short but the magnitude is large. Adding these short burst of energy will be collective. The readings of the spike can be detected on an oscilloscope. Then the switch rings and decays to operating level.


I think what you are referring to is "inrush current". A water heating element is basically a resistor. There is no "inrush current" because there is nowhere for the current to rush to. There is nothing to "charge up". (It has very little inductance and very little capacitance, so it does not store energy in an electric or electromagnetic field.)

You may also be thinking of old-fangled light bulbs. They are resistors, but their resistance is very low until they heat up, so the initial current is rather high. But even if a heating element was like that (it's not), the heat would still end up going into the water.

Think about it in terms of energy. Even if there was some gigantic surge, where would the energy go? It has to go somewhere. If it's significant, and it's not going into the water, we should be aware of it showing up somewhere else.


To avoid any confusion that I may have instigated...What is this about?

http://www.bradfordwhite.com/images/shared/pdfs/manuals/45837D.pdf

"Heating Element Sequencers:
Heating element sequencers are available in order to stage the activation of
the heating elements thereby, reducing the inrush current to the water
heater. The sequencers will control one or two contactor coils depending
upon the water heater voltage, phase, and KW."
 

Offline Geezer

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« Reply #8 on: 13/09/2010 03:21:09 »
Ah! Those are some industrial strength heaters.

The sequencing arrangement prevents the current drawn from the supply from going from zero to maximum in a single step. Instead, the current goes up in steps with a delay between each increase. For the kinds of currents involved with these serious heaters, that's a good idea as it prevents the transmission system from "seeing" a big step function in current. It gives the transmission system an opportunity to recover between each step.

They are not doing it for the benefit of the water heater. It does not make any difference to it whether the elements are turned on simultaneously or not. The sequencing is for the benefit of the power transmission system.
 

Offline tommya300

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Is it more efficient to have a 2nd hot water tank?
« Reply #9 on: 13/09/2010 03:36:56 »
ok understand
 

Offline CPT ArkAngel

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Is it more efficient to have a 2nd hot water tank?
« Reply #10 on: 13/09/2010 03:48:24 »
you should have only one 100L tank... The problem is that every time you will need the 150L tank, you have to heat all the 150L even if you use only a small part of it...
« Last Edit: 13/09/2010 04:59:37 by CPT ArkAngel »
 

Offline Geezer

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Is it more efficient to have a 2nd hot water tank?
« Reply #11 on: 13/09/2010 06:56:15 »
you should have only one 100L tank... The problem is that every time you will need the 150L tank, you have to heat all the 150L even if you use only a small part of it...

That's only a problem if the tank is not well insulated. If it's well insulated, you are only paying to heat the water you actually use. The only disadvantage to having a large, well insulated, tank is that you might tend to take longer showers because the water doesn't run cold so quickly.
 

Offline Bored chemist

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Is it more efficient to have a 2nd hot water tank?
« Reply #12 on: 13/09/2010 07:05:42 »
you should have only one 100L tank... The problem is that every time you will need the 150L tank, you have to heat all the 150L even if you use only a small part of it...

No you don't. You heat,and use, the top of the tank first.
 

Offline Geezer

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Is it more efficient to have a 2nd hot water tank?
« Reply #13 on: 13/09/2010 07:37:58 »
you should have only one 100L tank... The problem is that every time you will need the 150L tank, you have to heat all the 150L even if you use only a small part of it...

No you don't. You heat,and use, the top of the tank first.

Good point!

BTW, have the general public in the UK come to accept that the technological marvels known as thermostats can actually be relied upon to turn an immerser off without human intervention, or is the following nightly ritual still in vogue:

Mabel: "Fred, you did remember to switch off the immerser before you came to bed?"

Fred: "B*gger!"
 

Offline tommya300

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Is it more efficient to have a 2nd hot water tank?
« Reply #14 on: 13/09/2010 08:14:55 »
you should have only one 100L tank... The problem is that every time you will need the 150L tank, you have to heat all the 150L even if you use only a small part of it...

No you don't. You heat,and use, the top of the tank first.

Good point!

BTW, have the general public in the UK come to accept that the technological marvels known as thermostats can actually be relied upon to turn an immerser off without human intervention, or is the following nightly ritual still in vogue:

Mabel: "Fred, you did remember to switch off the immerser before you came to bed?"

Fred: "B*gger!"

Fred was far ahead of Mabel! He put in an electrical timer that he set to turn on the heating elements for only a couple of hours at about 3:00 am in the morning.
 He noticed that there was enough of hot water in the reservoir to last them for the morning cleaning duration of use, and still have hot water to wash the dishes after the evening meal.

He also remembered to relieve any sediment, by draining and flushing out the reservoir yearly, preventing the accumulation reducing the heating efficency.
He also knew to install the water heater in the laundry room, just on the other side of the shower wall.

Use cold water laundry cleaning was Mables idea.
« Last Edit: 13/09/2010 08:53:31 by tommya300 »
 

Offline CPT ArkAngel

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Is it more efficient to have a 2nd hot water tank?
« Reply #15 on: 13/09/2010 15:44:49 »
I supposed that the 150L tank heater was permanently closed unless you need it, because if not, there is no point of getting another 50L tank...
 

Offline Geezer

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Is it more efficient to have a 2nd hot water tank?
« Reply #16 on: 13/09/2010 17:55:49 »
you should have only one 100L tank... The problem is that every time you will need the 150L tank, you have to heat all the 150L even if you use only a small part of it...

No you don't. You heat,and use, the top of the tank first.

Good point!

BTW, have the general public in the UK come to accept that the technological marvels known as thermostats can actually be relied upon to turn an immerser off without human intervention, or is the following nightly ritual still in vogue:

Mabel: "Fred, you did remember to switch off the immerser before you came to bed?"

Fred: "B*gger!"

Fred was far ahead of Mabel! He put in an electrical timer that he set to turn on the heating elements for only a couple of hours at about 3:00 am in the morning.
 He noticed that there was enough of hot water in the reservoir to last them for the morning cleaning duration of use, and still have hot water to wash the dishes after the evening meal.

He also remembered to relieve any sediment, by draining and flushing out the reservoir yearly, preventing the accumulation reducing the heating efficency.
He also knew to install the water heater in the laundry room, just on the other side of the shower wall.

Use cold water laundry cleaning was Mables idea.

I guess that proves my point. Obviously, they don't trust thermostats and good insulation  :D
 

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Is it more efficient to have a 2nd hot water tank?
« Reply #16 on: 13/09/2010 17:55:49 »

 

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