Immersion Heaters - reducing kW drain

[Sui Generis]

I have a calorifier with a standard (short) domestic immersion heater fitted which draws 3kW at 230V, ie c13A.

Some French pontoon power supplies trip out at about 6amps thus in such situations it overloads and trips out the system.

Is there a way of reducing the power demand of the immersion to a sensible load - say 4A, 230V, 920W?

 

[jackho]

I believe you can get a 1500w replacement domestic one which seems a logical choice. Shouldn't be a big job to replace.
Jackho

 

[Anonymous]

Breakers have two devices inside them, thermal and magnetic. An electronic power controller (dimmer) will certainly overcome the the thermal but I am not certain about the magnetic. I think that you will be OK. You will need one that is good for the whole load, e.g. 3kW. Maybe a disco lighting shop? Or a power tool controller? Have a Google.

 

[VicMallows]

An electronic dimmer will do the job .... but you'll have trouble easily finding one that will handle 3kW.

Even 1500W will trip many French supplies (well, the electricity is usually free!).. I have a 500W electric kettle for that reason.

At one time you could get domestic immersions with a dual 'high/low' setting (switch on top) but suspect even the low setting might still be too much.

Vic

 

[Strathglass]

Marine thermostats are available with much lower current requirements.
I think they are in the region of 1Kw.

They would draw an approximate current of 4 Amps which would match your 6 Amp max current available.

I purchased one fron ASAP. I can look up the details if you wish.

www.asapsupplies.com

part no 518707, 7" long 1.1 Kw,

part no 518706, 10" long 1.1Kw.

both under £30.

Iain

 

[Sui Generis]

Thanks Iain, I had a look at that but I understood that the diameter was less than the domestic (2 1/4 as opposed to 3 1/2). I shall look again. Also thanks to VicMallows and Jackho; I've been down that path.

I've thought of dimmers but the ones I've come across aren't man enough, I've thought of dropping the voltage by two thirds but I'm not convinced that that will solve the problem.

Thanks for your input - much appreciated /forums/images/graemlins/laugh.gif

 

[Anonymous]

RS Components sell suitable dimmer modules for around £60. You could probably source at a better price if you shop around. Google.

 

[Strathglass]

The one I got was definately normal domestic size as I used the nice thick gasket that was supplied with it when fitting the immerser into a new hot water tank in a friend's house.

Iain

 

[DickB]

Try Penguin Engineering on Hayling Isalnd. I recently got one (1200w) with a 1.1/4 inch thread. Works fine. About 5amp. Cost £11

 

[VicS]

If you were to use a transformer you would not have to drop the volts by tas much as 2/3 as power is proportional to volts squared (W=V²R). In fact only to 230 X root(920/3000) ie to 127 volts so a 230/110 volt transformer would do the trick giving 3000 X 110²/230² = 686 watts. Still a bit more of a reduction than you wanted though. Also a transfomer rated at a bit more than 686VA would be a fairly hefty lump and expensive.

someone check my maths please

 

[philip_stevens]

Rob,
You could get a 1.1kw immersion from ASAP, go you your local scrap yard, or council dump, get a normal sized good or duff immersion heater, and then find a good metal worker and get the smaller thread tapped inside it, or the smaller immersion heater brazed onto the larger old boss.

Getting it tapped inside an old immersion boss is a better bet, as you could change it yourself if it goes duff.

 

[VicS]

Further thoughts Perhaps a transformer is not so expensive. A year ago a 3kVA transformer was £53.42 from Screw fix. no indication of weight though.

I don't know if your original thoughts were of dropping the volts by a series resistance if so the square relationship comes into play again. This time because power is proportional to current squared (W=I² R) reducing the current to 4 amps would reduce the power dissipated by your heater to 3000 X 4²/13² ie only 284 watts. Not only that, 636 watts would be dissipated by the resistance (The remainder of the 920 watts drawn from the supply) Ok if you wanted cabin heating as well but 284 watts is not much for heating the water.

 

[colvic]

ASAP will supply very quickly; If you get one with dual thermostats they are wired in series and you may find that the safety one, which on ours was non adjustable, tripped after a couple of hours motoring...too hot, so if you find that when you plug into the mains after crusing you've no hot water, then that's the problem.
There is a reset button on the top of the housing.


Phil

 

[Strathglass]

An auto transformer could be user. A 1Kw one would be adequate but would be about 7" square and quite heavy.

My maths are as follows.(I stand to be corrected if wrong)

A 3Kw element will have a resistance of about 17 ohms.

At 110volts from a transformer the current through the element will be about 6.2 Amps and 680 watts of power would be supplied to the water.
If we assume some ineficiency in the auto transformer then less than 800 Watts would be required from the shore supply. Thus the current drawn would be less than 4 Amps.

I personally would not be happy using a resistive series dropper at mains voltage within any water going craft.

Much better using a 1.1 Kw immerser from ASAP. They are certainly available with a standard domestic sized flange, I have one.

As Colvic has said it has a dual thermostat with a reset button on the non adjustable over temperature thermostat.
A bit if a pain but it has ensured that I have fitted the immerser in the calorifier in a very accessable position.

Cheers

Iain

 

[nedmin]

If you get problems with tripping out on overheat with a double thermostat use the one without the trip and fit a skin thermostat,these clamp on the side of the tank.If you get the tank up to normal temp.Set the skin one slightlly higher so you still have the safety overheat facility.You need the one that carries the heater current,not the electronic one.In Holland they fit 500watt heaters(as mine) initially thought it would not be big enough but works well,and I can run it off my 800w gennie! Most immersion heaters used to have a bi-metal trip built inside that would reset itself,have they done away with them must be called progress.!! Dont use an auto transformer as if you get a fault they can be lethal.

 

[Trevethan]

Try MCC boats. bought a 1kw from them -- uses standard immersion spanner -- unsure of thread diameter though

 

[VicS]

Yes your arithmetic is Ok. I chose not to calculate a value for the resistance and approximate it to 17 ohms hence my 686 watts compared with your 680 watts.

An auto transformer would be acceptable, I think, but I assume the one in the Screw fix catalogue is an isolating transformer.

I dont think, however, that I agree with your logic regarding the efficiency of the transformer. If I understand you correctly you are saying that in order to deliver 680 watts, 800 watts would be drawn from the supply. That gives us an efficiency of 680/800 = 0.85 or 85%. I would have thought the transformer was much more efficient than that. If that were generally the case then since the power reaching your house may have been through as many as 6 transformers since it left the generator the transmission system efficiency would be only 0.85 x 0.85 x 0.85 x 0.85 x 0.85 x 0.85 which is 0.38 or 38% Although I imagine a portable transformer will not have quite the same efficiency as one on the national grid it will surely be more than 85%

If you know know differently then I bow to you superior knowledge though.

All this is pretty academic though.

I agree that if 1kw heaters are available and can be fitted in place of the original then that is the way forward.

 

[Sui Generis]

Thanks to everyone for your help. It looks as though ASAP is the most cost effective option.

Cheers

 

[Strathglass]

Yes Vic,
I was just being lazy in the right direction just to be safe. As you say it will be much higher.
From my past recollections on transformer calculations somewhere between 95 and 99%. That is from over 30 years ago, in the days about the start of TTL.

As you pretty academic

Cheers

Iain

 

[VicMallows]

Unfortunately TTL is more like 40 years ago! Scary, isn't it.

(the other) Vic