economic way to heat water

Leo said:
Do we want to get really complicated and start adding in usage patterns and temperature loss over time from the cylinder? So I'm certainly not volunteering... :D

I think the best way to find out for sure is to get an energy meter and monitor for a standard week with each method. You'd need to get this wired in to the immersion circuit though.
Leo
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I agree, Thats why I didn't even attempt it :)
 
aircobra19 said:
Why 60?
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Because its not hot enough to scold , but it is hot enough to kill off any bacteria and germs that may want to live in your cylinder.
Ofcourse you can have any temp you are comfortable with. In public buildings it should be no higher than 60C.
 
DavyJones said:
is this any help?. 1 BTU (British termal unit) is the amount of energy needed to raise the temperture of 1 pound of water by 1 degree F.

1 BTU = 3.412 watts
1 pound = .45359237 KG
1 litre of water = 1.01 kilograms
Degree F = 5/9 degree C

Standard cylinder volume is approx 135L

I think if these are applied in the right way you will get your answer.

Any mathematicians?
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I could be way out, but I'm calculating that at about 73Kw to raise a tank from 20 to 60 celsius :eek:
 
I think you might have a small problem with your maths ;)

135 litres = 136.35 kg
136.35 kg = 300.6 pounds
~300 BTU to raise tank by 1 deg f
if looking at 20 to 60 - this is a difference of 40 degrees celcius, or 22.22 degrees F
So to raise tank by 22.22 F = 6666w, or 6.6 Kw.

How does this compare to the ESB web site: 9.25 to heat a tank, but this is a 120 litre tank and they may be heating it higher. They also gave a 2 month figure that I divided by 8, possibly should be 9.

However, none of this answers the question - the real problem is how much heat is lost during the day and hence needs to go back in to heat up the tank. This is down to the insulation.
 
dem_syhp said:
I think you might have a small problem with your maths ;)

135 litres = 136.35 kg
136.35 kg = 300.6 pounds
~300 BTU to raise tank by 1 deg f
if looking at 20 to 60 - this is a difference of 40 degrees celcius, or 22.22 degrees F
So to raise tank by 22.22 F = 6666w, or 6.6 Kw.

How does this compare to the ESB web site: 9.25 to heat a tank, but this is a 120 litre tank and they may be heating it higher. They also gave a 2 month figure that I divided by 8, possibly should be 9.

However, none of this answers the question - the real problem is how much heat is lost during the day and hence needs to go back in to heat up the tank. This is down to the insulation.
Click to expand...

Where are you getting the 22.22 degrees F from, please?

Oh and by the way I meant 125L not that it matters.
The best way to use your immersion imo is to turn it on 30 mins before you need it. who needs a full cylinder of hot water on stand by, heat as much as is needed.
 
Right, I've been thinking about this - now I'm not an engineer - so open to be proven wrong. I've no figures here - just an explanation.

But, the rate of change of temperature of an object is proportional to the difference between its own temperature and the ambient temperature. (Newton)

We all know this - if you make a cup of coffee and won't drink it for a couple of min, you're better putting in the milk first to minimise the amount it cools down. Versus waiting the couple of minutes and then adding the milk - it will have cooled down more (greater temperature differential) and then when you add the milk, if you compared it to the first cup - it's cooler.

So, if the tank is hot the temperature loss will be at it's greatest - so while you're only heating it by a couple of degrees each time, your rate of loss is maximised and hence you're using up more energy.

Assumptions:
The same amount of energy required to heat it from 20 to 30 degrees, or from 50 to 60 - which appears to be true (and makes sense to me).
Newtons law of cooling applies to our hot water tank - again can't see why not.

Result: Fit a timer + insulate to minimise losses
 
DavyJones said:
Because its not hot enough to scold , but it is hot enough to kill off any bacteria and germs that may want to live in your cylinder.
Ofcourse you can have any temp you are comfortable with. In public buildings it should be no higher than 60C.
Click to expand...

I thought there might be an official guideline or something.
 
DavyJones said:
Where are you getting the 22.22 degrees F from, please?

Oh and by the way I meant 125L not that it matters.
The best way to use your immersion imo is to turn it on 30 mins before you need it. who needs a full cylinder of hot water on stand by, heat as much as is needed.
Click to expand...

Sorry, I was basing it on 20 degrees C to 60 C which is the same figures as other poster. This is a difference of 40C.
To convert to F is:
40 x 5 / 9 = 22.22
 
aircobra19 said:
I thought there might be an official guideline or something.
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There probably is, I was only speaking to a colleage today about it as some pump manufactors state that water no hotter then 55C should enter pump from cylinder. The public building one is in the regs.
 
dem_syhp said:
Sorry, I was basing it on 20 degrees C to 60 C which is the same figures as other poster. This is a difference of 40C.
To convert to F is:
40 x 5 / 9 = 22.22
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Is 40/1 x 5/9 not 72 or if 20C = 68F and 60C = 140F differance is 72F?
 
dem_syhp said:
Sorry, I was basing it on 20 degrees C to 60 C which is the same figures as other poster. This is a difference of 40C.
To convert to F is:
40 x 5 / 9 = 22.22
Click to expand...

Nope it's 40 x 9 / 5 = 72

My calculation is based on
3.412 watts per lb per deg F
7.52 watts per kg per deg F (3.412/.454)
1014 watts per 135kg tank per deg F (7.52 x 135) (I ignored the 1.01 litre/kg)
1826 watts per tank per deg C (1014 x 9/5)
73049 watts per tank for 40 deg C (1826 x 40)

which is 73.05 Kw

It doesn't sound right to me, but the maths add up

Edit: This lad works it out using joules etc, so its probably more accurate - 7.5kw for a 150 litre tank
 
Oops, sorry - why not to do sums late at night.

I'm not going to look at it again till fresh eyes!
 
This is fun;)

(1 BTU/lb °F)(300 lb)(140°F - 68 °F) = 21,600 BTU

1 BTU = 3.412
so
21600BTU = 6314 watts

= 6.31 KW
 
DavyJones said:
This is fun;)

(1 BTU/lb °F)(300 lb)(140°F - 68 °F) = 21,600 BTU

1 BTU = 3.412
so
21600BTU = 6314 watts

= 6.31 KW
Click to expand...
If 1 BTU = 3.412w
then
21,600 BTU must equal 63,000and something watts?
= 63kw which is the same are of nonsense sounding answer j26 came up with!

(Not saying it is nonsense, just that it sounds mad).

Somebody somewhere is missing a carry one!
 
yoganmahew said:
If 1 BTU = 3.412w
then
21,600 BTU must equal 63,000and something watts?
= 63kw which is the same are of nonsense sounding answer j26 came up with!

(Not saying it is nonsense, just that it sounds mad).

Somebody somewhere is missing a carry one!
Click to expand...


Sorry, My mistake. 3.421 BTU's in 1 watt.
 
You need to factor time into these calculations if you're talking costs, so one unit of electricity is 1 kWh, ... so 1 BTU is 2.931 ×10-4 kWh, which correcting the above means 1 watt is 3.412 BTU/h

Which all goes to remind me why I didn't attempt this in the first place :)
Leo
 
Phew - I'm tired just looking over it - thanks! Of course doesn't really answer question of most economical way of heating water. Just validates what the wee spec on the tank should have told us if any of us had looked it up ;)
 
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