How much power for water heating

[PaulRainbow]

A typical 15-20 litre calorifier will be fitted with a 1kw 240v element, this will be a 100a draw from an inverter. It takes around 1 1/2 hours to heat 20 litres of water, that's 150ah or about 1900wh.

That equates to 7.5ah, or 95wh per litre of water.

Therefore, the Anker 757 @ 1229Wh (only 980wh safely usable) is not going to be suitable.

The Bluett1 AC200P @ 2000wh has a safe usable capacity of 1600wh, which falls short of the required 1900wh to fully heat the OPs 20 litres of water, although it does come pretty close.

£1700 cost, 27kg to lug back and forth to the boat each day, 20 litres of mildly hot water ?

or

1 litre of diesel through the engine, 20 litres of hot water and batteries charged to 80% ?

 

[PetiteFleur]

If you have a diesel warm air heater, then the Bobil water heater on post no #44 looks a good deal and much cheaper than a separate battery pack.

 

[AngusMcDoon]

£1700 cost, 27kg to lug back and forth to the boat each day, 20 litres of mildly hot water ?

It would be less hefty to lug the water already heated.

 

[simonfraser]

realy depends how much warm water you need
i use a kettle on my diesel hob, enough for washing up or put into the shower bag i use to wash with
the shower water only needs to be luke warm to rinse with, so add cold to the boiled water

 

[geem]

A typical 15-20 litre calorifier will be fitted with a 1kw 240v element, this will be a 100a draw from an inverter. It takes around 1 1/2 hours to heat 20 litres of water, that's 150ah or about 1900wh.

That equates to 7.5ah, or 95wh per litre of water.

Therefore, the Anker 757 @ 1229Wh (only 980wh safely usable) is not going to be suitable.

The Bluett1 AC200P @ 2000wh has a safe usable capacity of 1600wh, which falls short of the required 1900wh to fully heat the OPs 20 litres of water, although it does come pretty close.

£1700 cost, 27kg to lug back and forth to the boat each day, 20 litres of mildly hot water ?

or

1 litre of diesel through the engine, 20 litres of hot water and batteries charged to 80% ?

Assuming a cold water temperature of 10degC heated to 40degC. 20 litre calorifier and a 1kw immersion heater the heat up time is 42 mins
Also, if you have a vertical calorifier then you often don't need to heat all the water as you will get stratification. You draw off the hot water from the top of the tank leaving the cooler water at the bottom. It doesn't work well on large horizontal calorifiers. This can further reduce the run time and power needed as you may only need half a tank.

 

[ylop]

Assuming a cold water temperature of 10degC heated to 40degC. 20 litre calorifier and a 1kw immersion heater the heat up time is 42 mins
Also, if you have a vertical calorifier then you often don't need to heat all the water as you will get stratification. You draw off the hot water from the top of the tank leaving the cooler water at the bottom. It doesn't work well on large horizontal calorifiers. This can further reduce the run time and power needed as you may only need half a tank.

But he wants hot water not luke warm, and heating water tanks to only 40C is ideal breeding ground for bacteria. I'm sure there are boats with large enough vertical tanks to get stratification but not sure its a strategy for a typical yacht install - presumably in anything other than a flat calm the tank becomes mixed anyway!

 

[Graham376]

I tried a 1.2 kwhr genny on my Bene 381. It stalled. I believe my heater is 1.5 kw. I have since been given a Honda 2.0i which is rated continuous 1.6kw. I t grunts a bit but copes admirably. So you probably need an over 1.5 kw provision with a 2kw surge? So this would probably work, but follow Paul Rainbows advice

We have the same Honda genny but I think our immersion element is around 750W, it copes easily although quite noisy, about 15-20 mins for enough hot water for 2 showers. We always use the gas kettle for washing up water, with galley just inside companionway and hatch open, no condensation problems.

 

[PaulRainbow]

Assuming a cold water temperature of 10degC heated to 40degC. 20 litre calorifier and a 1kw immersion heater the heat up time is 42 mins
Also, if you have a vertical calorifier then you often don't need to heat all the water as you will get stratification. You draw off the hot water from the top of the tank leaving the cooler water at the bottom. It doesn't work well on large horizontal calorifiers. This can further reduce the run time and power needed as you may only need half a tank.

Why assume 10 degrees ?

How much stratification would you expect in a 20 litre calorifier, where the typical height is about 500mm ?

Why on Earth would anyone heat water to 40deg C ? Should be between 60-65. Surecal (for instance) maintain about 85c.

 

[Graham376]

Why on Earth would anyone heat water to 40deg C ? Should be between 60-65. Surecal (for instance) maintain about 85c.

For a shower on a boat, what's the point of heating water to higher temperature than needed and then adding cold water to cool it to usable temperature? Different in a house situation with long pipe runs and associated losses.

 

[PaulRainbow]

For a shower on a boat, what's the point of heating water to higher temperature than needed and then adding cold water to cool it to usable temperature? Different in a house situation with long pipe runs and associated losses.

Depends on how much you like to promote bacteria in your water .

Also, 20 litres of 40 degree water gives you 20 litres of shower temp water, first time you use it the water that you use gets mixed with an equivalent amount of cold water, so what's left in the tank isn't very hot at all. Modern calorifiers are set to heat to a higher temperature and fitted with a thermostatic mixer valve. This gives a much larger volume of usable hot water, whilst reducing the risk of bacteria breeding in the calorifier.

 

[Graham376]

Depends on how much you like to promote bacteria in your water .

We've had this boat for 19 years and always fill with chlorinated water. No sign of bacteria so far, even though tank water temperature most likely at 25-30 degrees in summer. With living aboard for extended periods, far higher throughput of water than weekenders.

 

[ylop]

We've had this boat for 19 years and always fill with chlorinated water. No sign of bacteria so far, even though tank water temperature most likely at 25-30 degrees in summer. With living aboard for extended periods, far higher throughput of water than weekenders.

you know you can't see bacteria!

 

[Graham376]

you know you can't see bacteria!

But we would soon suffer the effects if there was any, we always drink tank water.

 

[ylop]

But we would soon suffer the effects if there was any, we always drink tank water.

Ah if only immunity and microbial activity was as simple as that! With hot water systems your biggest issue is probably Legionella - you can drink it (don't gargle) and will be fine. Put it through your shower and inhale it and you may get a horrible form of pneumonia known as Legionnaires' disease. In reality lots of people breath it in and don't get sick, or only get a flu like illness (who in the last 3 years has had a cough and fluey feeling and thought - I wonder if it could be my water tank?), but a small percentage get big sick. Depending on your age, gender, other health conditions it is quite likely to kill you. You need to get the bug in your tank to start with, then you need to grow it to "harmful" levels, then you need to breathe it in - those three things usually don't come together that often, but companies spend a lot of money protecting against these risks because it does happen - it kills at least a few dozen people each year in the UK. In some ways a yacht can be ideal for that. Water sources are variable, quite often supplied through bits of pipe that get dropped on the ground (= potential for contamination) or are not of drinking water quality sitting stagnant for ages ( = easy for 1 bug to grow in the pipe and become 1 million before it even gets on board) and with long runs from the mains ( = low residual disinfectant). Tanks often sit unused for long periods (= reduction in disinfectant dose) and pipes are often DIY'd from non water quality parts or stuff imported on the cheap ( = ideal nutrient sources for bugs), corrosion is not uncommon (Legionellae love a bit of iron), we try to conserve water ( = less flushing out of the pipework) and power (= lower heating temperature in tanks to kill bugs off).

Now I'm not trying to instil some paranoia, I've never actually heard of a case linked to a private yacht, but just about the worst thing you could do is run a hot water tank cool. The lack of a link to leisure yachts may be as much to do with the fact that there isn't one big exposure infecting lots of people; many cases will go undiagnosed (or just labelled as pneumonia) because people only start hunting for the specific cause when there is an unusual cluster of cases. Certainly for someone trying to "design" a system of heating their water efficiently they should be thinking about how to do that safely which means in a way that doesn't encourage bacteria to grow if its going through a shower head, i.e. getting it well above 50 deg C. (you will not want it coming out the shower at 50C due to the scalding risk so need to mix hot and cold to bring it back down as close to the outlet as possible).

 

[V1701]

A litre and a half of nearly boiling water is going to give about a gallon of washing up temperature water, how much do you need to "tackle the washing up"?

 

[geem]

Why assume 10 degrees ?

How much stratification would you expect in a 20 litre calorifier, where the typical height is about 500mm ?

Why on Earth would anyone heat water to 40deg C ? Should be between 60-65. Surecal (for instance) maintain about 85c.

You only need to hear enough for a shower.and washing up. Why heat it to 65. Our 27 litre calorifier heated to 60-65 last two days for two people. A shower and washing up each day.
Our vertical calorfier is only 600mm high but still stratifies. 5 litres of hot water gives a navy shower. 20 litres per day at 40/45degC is ample in my experience

 

[PaulRainbow]

You only need to hear enough for a shower.and washing up. Why heat it to 65. Our 27 litre calorifier heated to 60-65 last two days for two people. A shower and washing up each day.
Our vertical calorfier is only 600mm high but still stratifies. 5 litres of hot water gives a navy shower. 20 litres per day at 40/45degC is ample in my experience

Others may have different experiences to you.

For instance, you failed to answer "why assume 10 degrees", the OP wants to use his boat on the mooring in the Winter, he's based East Coast UK. His water tank will be substantially lower than 10 degrees and the hot water will lose temperature much faster then someone in a warm/hot climate.

I have been full time live aboard on the East coat of the UK for several years, i know what it takes to heat water here and how long it lasts.

 

[QBhoy]

although it’s great to see all the clever mathematical explanations on the power required to heat 20l…I’m afraid it’s nothing to do with this really. Quite simply, the capacity of the tank is irrelevant. It’s the watts required to power the element in the tank. Doesn’t matter if it’s heating 1 litre or a 1000 litres.
most I’ve seen or know will require a continuous rating supply of about 1600w+ with a greater initial surge expected too. So upwards of 2000w would usually be required if think.
Your average kettle in the kitchen will likely need 1800w or more to run, for some perspective.

 

[PaulRainbow]

although it’s great to see all the clever mathematical explanations on the power required to heat 20l…I’m afraid it’s nothing to do with this really. Quite simply, the capacity of the tank is irrelevant. It’s the watts required to power the element in the tank. Doesn’t matter if it’s heating 1 litre or a 1000 litres.

You need two things, enough current to power the element and enough capacity to heat the water to the required temperature, so tank size is completely relevant. You could have a 2000w supply, but if your batteries are only 5ah you won't be putting much heat into your 1000 litres of water.

most I’ve seen or know will require a continuous rating supply of about 1600w+ with a greater initial surge expected too. So upwards of 2000w would usually be required if think.
Your average kettle in the kitchen will likely need 1800w or more to run, for some perspective.

I gave some real World figures in post #21, no "clever mathematical explanations ", when i said "A typical 15-20 litre calorifier will be fitted with a 1kw 240v element, this will be a 100a draw from an inverter. "

Current inrush from an immersion heater element is negligible and irrelevant.

Your typical domestic kettle is 3000w, not 1800w. You need to specifically hunt for anything lower than 3kw, we use a 1kw one onboard because a standard 3kw one uses too much of the available 16a marina supply.

 

[QBhoy]

You need two things, enough current to power the element and enough capacity to heat the water to the required temperature, so tank size is completely relevant. You could have a 2000w supply, but if your batteries are only 5ah you won't be putting much heat into your 1000 litres of water.


I gave some real World figures in post #21, no "clever mathematical explanations ", when i said "A typical 15-20 litre calorifier will be fitted with a 1kw 240v element, this will be a 100a draw from an inverter. "

Current inrush from an immersion heater element is negligible and irrelevant.

Your typical domestic kettle is 3000w, not 1800w. You need to specifically hunt for anything lower than 3kw, we use a 1kw one onboard because a standard 3kw one uses too much of the available 16a marina supply.

Not the intention to warrant a response like this really.
About the kettle. Read it again. I said that “or more”
And I’d also just insist that a 2000w element…requires 2000w…the size of the tank or the length of time it takes to heat it, is a another subject to ponder over all together. Just stating the facts ! He can have all the batteries he likes…but if the inverter isn’t able to produce a continuous output in line with the requirements of the element to power it…I don’t need to state the obvious result.
All the best.