Variable power consumption for immersion heater?

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I have a 40l Quick hot water tank, heated either from engine or a 1200W/ 230v element.
I would like to be able to heat water at anchor using my 1kW generator. Is there a relatively straightforward means of reducing the element's current draw so the genny can cope? I'm not fussed about heating time or electrical efficiency.
The alternative is to replace the element with a 500W version which would be disadvantageous when on shore power.
 

pvb

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Using a 500W element is the way to go. It won't make a huge difference, unless you're in urgent need of hot water on shore power.
 

rogerthebodger

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Have a look at a SCR single phase power control module like the old drill speed control units.


http://electronics-diy.com/1000w-ac-motor-speed-controller.php

1000w-ac-motor-speed-controller.png
 
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OK. What I was wondering, and forgive my ignorance on matters electrical, is whether a switched resistor or some such device could be fitted to the immersion's supply to reduce its current draw as required. The only options are 500 and 1200 W elements and I don't want to fit two.
 

VicS

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OK. What I was wondering, and forgive my ignorance on matters electrical, is whether a switched resistor or some such device could be fitted to the immersion's supply to reduce its current draw as required. The only options are 500 and 1200 W elements and I don't want to fit two.

The resistor will itself produce a large amount of heat... it is not a practical solution .

Rogers suggestion wil work if you can find a suitably rated regulator

A calorifier with two elements is probably the best solution.
 
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lw395

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Have a look at a SCR single phase power control module like the old drill speed control units.


http://electronics-diy.com/1000w-ac-motor-speed-controller.php

1000w-ac-motor-speed-controller.png

A thyristor 'dimmer' works by switching the current on at some point in the sine wave, it then switches off when the sine wave crosses zero.
So, if it's set to half power, it will be on for the latter half of both the + and - parts of the cycle.
This means the peak current is the same as full power. Give or take the effects of inductance.
It will generate spikes of HF noise at the switching point.
I'm not sure it's a recommended way to load a generator.
I'm not sure how the generator's regulator will respond, if it has one.

TBH, there have been times when I'd quite like generators to catch fire in anchorages, so, hey, give it a try..!
 

matthewriches

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Just swap the element out for a smaller one, there isn't a sensible way of doing it on a domestic scale. You'll have to live with the fact that it might take a little longer to warm up on shore power but unless you have an immediate demand then it isn't any big issue. I have fitted under counter water heaters to a couple of large Fairline motorboats to cope with adhoc demand in crew room.
 
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pvb

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As I said in post 2, fitting a 500W element won't make a huge difference. It will still heat about 10 litres of water an hour from say 20 degrees to say 60 degrees. How much hot water do you need?
 

QBhoy

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I’m afraid there isn’t a simple solution to this other than changing the coil to a lower rating...or of course getting a larger generator. If you try and dampen the load demanded by the coil, then you introduce other issues like stored energy and heat...not things found ideal on a boat really.
Not too up to date on such things now...but I hear inverters have come on leaps and bounds recently. Perhaps a solution if your battery bank is up to it. Of course the generator can assist in charging the batteries whilst doing so ?
 

William_H

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I’m afraid there isn’t a simple solution to this other than changing the coil to a lower rating...or of course getting a larger generator. If you try and dampen the load demanded by the coil, then you introduce other issues like stored energy and heat...not things found ideal on a boat really.
Not too up to date on such things now...but I hear inverters have come on leaps and bounds recently. Perhaps a solution if your battery bank is up to it. Of course the generator can assist in charging the batteries whilst doing so ?

No I don't think inverter on 12v is the way to go. "Come on leaps and bounds" maybe but you can't just get power from no where. I think OP answer is as said put ina smaller power element. However if there was room for another element then 2 elements could be switched in series to reduce power. But then why not just alternative element. As said I don't think SCR power controller would work very well. Who knows what it would do to the generator. ol'will
 

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Thanks all. My hope was I could buy an off-the-shelf device to wire into the immersion suppy; seemingly not. The genny (Honda eu10i) has only 230V output.
I have no choice on how much hot water I heat - it is a 40l tank which takes 20 mins to come to showering temperature with the current element. I guess I need to downsize the element and be more patient.
 
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vas

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smaller element as others have said, plus run the engine for the odd case you're in a hurry. Not very smooth but no other option than going 3phase and using inverters to slow things down :eek:
 

rogerthebodger

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Thanks all. My hope was I could buy an off-the-shelf device to wire into the immersion suppy; seemingly not. The genny (Honda eu10i) has only 230V output.
I have no choice on how much hot water I heat - it is a 40l tank which takes 20 mins to come to showering temperature with the current element. I guess I need to downsize the element and be more patient.

You could fit a 230 VAC to 110VAC transformer similar to those used on work sights to power 110 VAC machines. I have a 110VAC steel cutoff machine that I picked up at auction and drive that from a 240 VAC to 110 Vac auto transformer
 

andsarkit

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The easy option is a yellow 110v site transformer and as the power is reduced to 300W you will only need the smallest one - probably 750VA. If you search for torroidal transformers they come in many output voltages and you can select one to maximise the heating power. For example this one https://airlinktransformers.com/product/standard-range-toroidal-transformer-cm1000292 will give you about 700W with 184Vac output. Torroidal transformers do take quite a large surge on the first mains cycle when switched on and you would have to be careful that the generator can cope with this.
 

Kinsale373

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Im not sure that the transformer option will work. The element is effectively a big resistor so W=V*I so is you 1/2 the voltage the current doubles) A 1200W elemement will still try to pull 1200W at 110V so in effect you double the current to the Element 10.4 A ! Is your wiring and the secondary of the transformer rated for that , if not you introduce a fire risk.

Stay away from Power controller's , unless you go for a top of the range expensive unit it will be noisey. Note the noise reducing circuit suggesed in small print for Contoller suggested earlier

Any controls introduced = inefficiency .

The most efficient and cheapest solution is to reduce your element to 500W. This way you add no weight to the boat either,

Kinsale 373
 

rogerthebodger

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Im not sure that the transformer option will work. The element is effectively a big resistor so W=V*I so is you 1/2 the voltage the current doubles) A 1200W elemement will still try to pull 1200W at 110V so in effect you double the current to the Element 10.4 A ! Is your wiring and the secondary of the transformer rated for that , if not you introduce a fire risk.

Stay away from Power controller's , unless you go for a top of the range expensive unit it will be noisey. Note the noise reducing circuit suggesed in small print for Contoller suggested earlier

Any controls introduced = inefficiency .

The most efficient and cheapest solution is to reduce your element to 500W. This way you add no weight to the boat either,

Kinsale 373

You are wrong

The element is a resistor and the resistance stays the same I (current ) = V(oltage) / R(istance) so as the voltage reduces the current will decrease and W = V * I so the if the current halves the wattage will be half
 
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