Calorifier and Power Consumption

purplerobbie

purplerobbie

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I fitted a new engine to the boat about 12 months ago at the time i didn't really bother about a calorifier.
I have just decided to get one to fit and I'm looking for advice.

The engine is only a 25hp. I'm assuming that if i go to big on a calorifier it will never heat the water and if i go to small there will be hardly any water.

Any idea on what size would be good?

The one i was looking had a 1000w 240v element in it.
If i wanted to heat the water through 12v with an inverter how many amp/hr would it take to power a 1000w element? Is there a lot of loss with an inverter?

Rob
 
My own thoughts these, if the thermostat is working correctly then in theory it doesnt matter how big the calorifier is, the thermostat will divert the coolant to it instead of to the heat exchanger for the heat to go to the oggin. Of course if too big i suppose it could be argued that it would cool the engine too much, i dont think so though. Second point, I asked the question about the electric bit a while ago, the concensus was that the 12v system wouldnt be powerful enough, 1000 watt equals 4ish amps at 240 volts, or 90 ish amps at 12v assuming the invertor isnt inefficient, think ive got the sums right, but no doubt some one will soon put me right if i am wrong!
Stu
 
The size of the calorifier doesn't matter, as long as you can fit it into the boat - I'm talking physical size. On the electric heater a 500 W would be better, because that would fit in most shore-power supplies around Europe. Also, most inverters aren't good for heating elements and will burn out.

Best of luck!
 
I never even considered that 1000w might cause a problem with shore power.
I'm thinking of buying one of the kipor pure sinewave generators would that be ok to power it?
Rob
 
Most shore power in Europe is 13 - 15 Amps. With a water-heater at 1000 W, it would use up most of the shore power. So if you take battery-charging, computer, radio, electric kettle - most things on your boat! - you would soon blow a fuse or circuit-breaker. Best to keep it at 500 W so it will also work if you're using a generator onboard, to save running the engine.

ETA: normal small generator used by liveaboards (600W - 2000W).
 
OK its the higher temp of the element that transfers heat to the water, assuming the element is hotter (and electric elements will be) then the calorifiers capacity only effects the time taken to heat the water! And of course amount of energy used to do so. To explain electrics then heat transfer:

Electrics:
Power in watts is equal to amps multiplied by volts - this is true for an AC circuit with a resistive load (heating element) and is the same for a DC (battery) circuit. So ignoring any losses in the inverter and of course insulation losses to the calorifier, the conversion(s) would equate to:

For AC:
P=I x V
therefore I=P/V or
amps = watts/volts = 1000/240=4.17 Amps

For DC then:
1000/12=83.33 amps!

Therefore you need to select a much smaller heating element and/or allow more time to heat the water!

Heating Water:
FYI: It takes 4186 joules (specific heat capacity (Cp) of water) of energy to raise 1kg of water by 1C.

Also of interest here: one electrical watt is one joule of energy used in one second.

The formular to use to calculate energy used heating water is: Q = m x Cp x dT

Q = Energy in joules
M = mass of water in kg
Cp = 4186 j/kgC as explained above
dT = difference in temp heating water from/to

So:
For say a 25 litre tank (note 1 litre of water weighs 1kg) how much energy would we need to raise water temp by 45C, from say 10C (cold) to 55C (hot):

25 x 4186 x 45 = 4,709,250 joules

Power is normally rated in kilowatt hours, so a 1kW or 1000W heating element delivers 1,000 joules of heat per second or 3,600,000 joules in one hour.

So how long would it take to heat 25 litres (or 25kg) of water from 10C to 55C using a 1000W (1kW) heating element:

4,709,250 / 3,600,000 = 1.3hrs or 1hr 18 mins

It follows:

A 500w element at 12v would draw 41.67 amps and take 2hrs 36 mins to heat 25kg of water from 10 to 55C.

A 250w element at 12v would draw 20.83 amps and take 5hrs 12mins

I trust you get the picture - all as clear as mud! /forums/images/graemlins/confused.gif
 
I suspect that there is an element of misunderstanding as to how calorifiers provide hot water on a boat in the original post.

It would be highly unusual to use 12 volts through an inverter to heat the water. In fact (as other poster's contributions regarding the calculations have shown), it really isn't practical. You would have to run the engine frequently to recharge the batteries and you might as well use the waste heat from the engine to heat the water which leads me to what I was going to explain in the first place.

To resolve any possible confusion, the usual arrangement is:-

When you are plugged into shore power, the calorifier is heated by the immersion heater connected to the mains electricity supply. We have never had a problem with a 1KW heating element. after all its only just over 4 amps at 230 volts.

When you are not plugged into shore power, the calorifier is heated by waste heat from the engine. On our boat this doesn't take very long. Motoring into harbour after a days sail will often produce a tank of hot water. Charging the batteries for an hour in the mornings when we are at anchor will top the hot water up completely.

Regarding size, if its plumbed in correctly then I cannot conceive of there being space on your boat that's big enough to allow you to fit a calorifier that your engine couldn't heat up.
 
Agree entirely with John's summary above. Just to add that we used a 1000w calorifier element throughout a cruise to France, Belgium and Holland and never had trouble with shore power anywhere. As someone above said, it's the same as a kettle uses. We don't have a diesel inboard so also considered a generator but the trouble is that all the affordable ones are under 1KW continuous. The answer, if we go down that route, is to change the element to 750W (via surecal). What we did discover is that our puny 10 litre tank is jolly good, because it has a standard mixer value on the outlet, it equates to about 20 liters of warm water and lasts for a day - or two quickish showers. With hindsight I would have tried to shoehorn a 15 liter tank in, but in any case it shows how small a tank you need.
 
[ QUOTE ]
If i wanted to heat the water through 12v with an inverter how many amp/hr would it take to power a 1000w element? Is there a lot of loss with an inverter?

[/ QUOTE ]

We are liveabords and changed our calorifier from 40 to 29 litres when it sprung 22 leaks after only three years. We also downgraded the emersion heater from 1500 to 750 watts. We can now heat the water from batteries via our 2.5kva inverter and it takes about half an hour and uses about 35 Ah. You will need a large battery bank to support this heavy drain and some serious charging support if you intend to stay at anchor for any length of time. We have wind, solar and a 12 volt diesel generator that charges at 250 amps. Anything is possible - but why not try a cheap solar shower - works very well when the sun is shining.
 
[ QUOTE ]
Most shore power in Europe is 13 - 15 Amps. With a water-heater at 1000 W, it would use up most of the shore power. So if you take battery-charging, computer, radio, electric kettle - most things on your boat! - you would soon blow a fuse or circuit-breaker. Best to keep it at 500 W so it will also work if you're using a generator onboard, to save running the engine.

[/ QUOTE ]

A 1000 watt immersion will only draw 4-5 amps so should cause no problems at all with shorepower unless they are woefully underpowered.

A 500 watt immersion may take a long time to heat the water if the calorifier is a decent size. Go for the 1000 watt, you can always turn other stuff off for an hour while the water heats up if you ever find yourself limited to 5 amps of shorepower.
 
Fit the largest size calorifier in the space available. I have a 25HP engine and the calorifier is about 20" diameter x about 24" high - don't know the capacity. Heats it up well after about 20minutes of engine running.
 
At first glance the heat-up time you suggest could give the impression it's OK, as follows:

29 x 4186 x 45 = 5,462,730 joules

2.5Kva in a DC circuit (KVA=KW) is 2500W
Hence in one hour it produces 2500 x 3600 = 9,000,000 joules

Therefore to heat 29kg of water from say 10C to 55C (by 45C) would take a 2.5kW heater:

5,462,730 / 9,000,000 = 0.61 hours or 36.6 mins.

However you say your heating element on the DC side is only 750W, therefore it is only able to heat 29kg of water by 11.12C in half an hour! As follows:

Energy needed:
29 x 4186 x 11.12 = 1,350,000 joules

Energy used in half hour:
750 j/s x 1800 secs = 1,350,000 joules

This equates well with your electrical statement as follows:

750w/12v = 62.5A running for one hour hence 62.5AH or 31.25AH when running for half an hour!

So beware, your figures only relate to an 11C rise in water temp, which could be a little misleading!

No doubt if you are live-aboard your hot water heating starting point is higher than cold (10C) at the start. This is a factor that must be considered when you select your calorifier: Will I draw-off only a small amount of hot water and then allow good time for the hot water to recover.

This must be taken into consideration when you size a heating element to a calorifier! If you wanted to heat your tank from say cold (10C) to 55C using your 750W heater it require:

29 x 4186 x 45 = 5,462,730 joules

and it would take your heater

5,462,730/2,700,00 = 2.02 hours!

Hence 750/12 = 62.5AH or 125A total current flow in two hours - remember amps are not power they are current flow, only VA or Watts are power!

You now have all the calcs which are fundamental laws - hope they are helpful!

So I would go with others, use shore power to drive the heating element when berthed and if you can (as I do on my boat) heat from the engines when underway!

Using an inverter could lead to battery drain problems if you are not careful!
 
[ QUOTE ]
750w/12v = 62.5A running for one hour hence 62.5AH or 31.25AH when running for half an hour!

So beware, your figures only relate to an 11C rise in water temp, which could be a little misleading!

[/ QUOTE ]

I think what people want of these forums is practical answers - not theoretical suggestions.

Our water heats up in half an hour because:

1 In the summer in the Med the water in the tanks is not 10C - more like 20C.

2 The water does not need to be heated to 55C - more like 40C

3 The whole 29 litres does not need to be heated - just the top part of the tank which is enough to give us two showers.

The 35 Ahrs we use is just 3.33333% of our house battery capacity - so we ain't bovvered about battery drain.

I just wonder how many 2B pencils you used for your calculations - or maybe you don't have them on your Mobo?
 
Well - practical answers are welcome - but YOU need to do a little investigation too ...

The OP is based (according to his profile) in the IoM ... that is some way from the Med, and I can assure you, the base water temp will not be anywhere near 20°C even in the summer ... 10°C is more realistic during the summer and optimistic right now ...


Anyway - as a fully PRACTICAL answer to the OP...

Use SHOREPOWER for the electric immersion heater - ours is 500w with a smallish tank (don't know the size off hand)
Use Engine water whilst away from shorepower - don't go via inverter - you're just wasting energy
 
[ QUOTE ]
Use Engine water whilst away from shorepower - don't go via inverter - you're just wasting energy

[/ QUOTE ] Just how much energy would you be wasting running a 56hp engine just to heat your hot water when at anchor?

I have considered all the options and believe our system is the most efficient and practical and environmentally friendly and the most considerate to the other boats at anchor.

See the archives for more on this in the Liveaboard forum.
 
[ QUOTE ]
Just how much energy would you be wasting running a 56hp engine just to heat your hot water when at anchor?

[/ QUOTE ]
Catch all the hot water coming out of the exhaust and measure it......

Obviously you know all there is to know about it and we should bow to your superior knowledge ....
 
A bit of a dig I think, my reply:

1) These are not theroetical they are fundamental laws of physics. Useful to anyone wanting to size a calorifier, I would hope.

2) The boat mentioned is not located in the Med. Well said others!

3) The thermostat that isolates the heater is more often than not located lower down in the tank, so its not just the top that gets hot and why have a 29L tank if your only heating the top, do you have two stats? Just install a smaller tank!

4) As hot water is drawn-off it is replaced with cold water which cools the water in the tank and activates the stat.

5) If 35 AH is just 3.33% of your total in-house battery bank that makes your bank 1,051AH - Wow

6) No 2B pencils - took 5 mins - just used my HND pass with Distinction, B.Eng(Hons 1st Class) University of London, MSc in Building Services Engineering (Brunel) and 35 years of experience as a Consultant designing M&E Systems to some of the highest containment medical research labs in the world!

My advice is given freely and I hope is informative to others, if I go over the top, please forgive me!
 
Any particular reason to unearth such an ancient thread? The OP needn't have worried about his engine's ability at warming water, a 25hp motor is dumping upwards of 15kW into its cooling circuit at full power and proportionately less down the power range.
 
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