How much electrity needed to heat fresh water.

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Beneteau381

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Im doing a little project to fit a 120 ltr hot water tank with a calorifier driven by a diesel water heater. It has an auxiliary 3kw element in it. Question is, how many kilowatts (hours) needed to raise the temp from 5 deg C to 65 deg C using just the 3 kw immersion heater?
S
 
Beneteau381 said:
Im doing a little project to fit a 120 ltr hot water tank with a calorifier driven by a diesel water heater. It has an auxiliary 3kw element in it. Question is, how many kilowatts (hours) needed to raise the temp from 5 deg C to 65 deg C using just the 3 kw immersion heater?
S
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We had to do these calculations when I was at marine engineering college but I have long forgotten the method.

But will this help?

How much electricity does my immersion heater use

How to calculate the kW required to heat a volume of water in a particular time.
 
1 kw is 1000 joules per second
The specific heat capacity of water is 4187joules
Ie to raise the temp of one litre ( one kilo) of water by one degree C, apply 1kw to it for 1second
Or 1/3 of a second for your 3kw element.
And 1/3 x 120 litres = 40
So, 40 seconds for 120 litres raised 1deg C

To raise 60deg C from wherever to wherever between 0 and 100, it’s 40 seconds x60deg
= 80 minutes
( it’s been a while for me too , good question !)
 
Blueboatman said:
1 kw is 1000 joules per second
The specific heat capacity of water is 4187joules
Ie to raise the temp of one litre ( one kilo) of water by one degree C, apply 1kw to it for 1second
Or 1/3 of a second for your 3kw element.
And 1/3 x 120 litres = 40
So, 40 seconds for 120 litres raised 1deg C

To raise 60deg C from wherever to wherever between 0 and 100, it’s 40 seconds x60deg
= 80 minutes
( it’s been a while for me too , good question !)
Click to expand...
I think you need to multiply your times by 4.2

I make it about 3 hours
 
Beneteau381 said:
Im doing a little project to fit a 120 ltr hot water tank with a calorifier driven by a diesel water heater. It has an auxiliary 3kw element in it. Question is, how many kilowatts (hours) needed to raise the temp from 5 deg C to 65 deg C using just the 3 kw immersion heater?
S
Click to expand...
I make it 8.36 kWhr
or 2 hr 47 mins for a 3 kW heater
 
LittleSister said:
The above calculations ignore the heat loss from the calorifier while the heating takes place. So need to add a fudge factor to the 80 minutes.

Heat will be lost both externally, and to the pipe work (and contents) of the engine coolant/water heating circuit.
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Its for my house. Basically, heating oil/kerosene, is cheaper than gas when burned in an Autoterm water heater. For producing hot water its more cost efficient than a gas boiler. Then mains to top up if necessary.
 
AngusMcDoon said:
((4.187 * 120000 * 60) / 3000) / 3600

((specific heat capacity of water (J/gC) * mass of water (g) * temperature rise (C)) / heater power (J/s)) / seconds in hour

= your 2 hours 47 minutes result (excluding losses)
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Changing the parameters a bit, the 5 kwatt Auto term uses about half a ltr of oil per hour. So at 84p per litre at the moment? Connect that to the hot water tank calorifier and less again? Basically changing my house hot water heater to a boat heater makes sense?
 
AngusMcDoon said:
((4.187 * 120000 * 60) / 3000) / 3600

((specific heat capacity of water (J/gC) * mass of water (g) * temperature rise (C)) / heater power (J/s)) / seconds in hour

= your 2 hours 47 minutes result (excluding losses)
Click to expand...
I could also use my planned solar panels for the house to feed an inverter from battery storage to augment the electric element part?
 
nice set of calcs :D
just one Q, do you really need to go up to 65C? Regularly having this discussion with my wife.
She loves showering at a water temp I cannot stand, so she wants 60C and I'm happy with 40C :rolleyes:
OK, ours is a gas combi with a 2lt internal storage, so no water tank, but curious if it's energy efficient to heat higher and use less or heat less and use more (I assume you mix it with cold to bring it down to a bearable temp...)
 
vas said:
nice set of calcs :D
just one Q, do you really need to go up to 65C? Regularly having this discussion with my wife.
She loves showering at a water temp I cannot stand, so she wants 60C and I'm happy with 40C :rolleyes:
OK, ours is a gas combi with a 2lt internal storage, so no water tank, but curious if it's energy efficient to heat higher and use less or heat less and use more (I assume you mix it with cold to bring it down to a bearable temp...)
Click to expand...
I said 65deg just to get some figures, you are quite right, dont need to go so high, however, there are regs about temperatures in such systems to deal with bugs such as legionella
 
vas said:
interesting, isn't your water chlorinated already when reaching your house?
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Indirect unvented systems are one of the things that authorities are on yptop of because of the risks of legionella. Years ago I would have done this installatin without a thought. Today, it has to be signed off by a registered plumber. The fitting of the Autoterm isparticularlyinteresting. Whats that mate? is the usual reaction. Part J of the regs covers oil fired boilers so not too onerous BUT it needs sig ing off
 
thanks, bookmarked this, will read later. Don't think we have such a detailed legislation here (but I'm not a registered plumber!)
 
Beneteau381 said:
Ta v much so about just under £3. Basically cheaper than using the ancient combi.
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No. Even with an ancient combi it will cost you more than twice as much ££ to heat water with mains electricity.
Lecky costs circa 39p/ kWh. Mains gas is circa 12.3p/kWh at an assumed 90% efficiency. If your combi is a little less efficient then you are still far better off using gas. Heating on kerosene will cost circa 9.3p/kWh so only a bit cheaper than gas at the moment
 
My rusty and erroneous sums aside -ahem, much obliged - ?
It’s a great opportunity to review solar water heating as well as PV ?
 
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