How to reduce power to a heating device.

[Dave Edwards]

OK, this is a weird one!
I have a grid tied PV system which exports spare power to the grid.
The maximum it will export is around 2.5kW
Now I want to use around 1.5kW, but I want to use it to heat a 3kW element.
So it will be a slow long heating rather than the full 3kW the element normally takes.
I can send the spare power I have to that element no problem, but at the moment because it is 3kW it takes all the spare power and makes up the difference from the grid.
I want this 3kW element to just take 1.5kW of power.
It's much like the 3 position switch on a slow cooker. Low, Medium and High. Somehow different amounts of power are being supplied to that same element.
What device would I need to achieve that?
Would it be a voltage regulator? if I took the voltage down to 110 x 13A that would be around 1.4kW
I can find voltage regulators on the net, but not ones that would supply 13A there seems to be a load of them that convert 240V to 110V but only 300W which is only around 2.75A
Where do I find a voltage converter that will allow a constant output of 110V @ 13A ???
Before anyone suggests it, obviously the sensible thing to do is to use a 1.5kW heating element!
But for what i want, the heating element is a fixed thing. that can't be changed. I just need to be able to feed 1.5kW to a 3kW element.
Any ideas?

 

[davesparks]

Reducing the voltage to 110V won't reduce the power to 1500W it will reduce it down to about 600W and the current will be around 6A.

In basic terms the resistance of the element will dictate the current which flows for the applied voltage, this then dictates the power which dissipates. Its a bit more complex than this due to things like the change of resistance due to temperature.


The easiest way to achieve what you want is to use a solar immersion heater controller which is designed for this exact task, such as the solar iboost.
These use a current sensor on the incoming supply to detect exactly how much power is being exported to the grid and then regulate the immersion heater until there is near zero power being exported or imported.
This basically operates like a 3kW dimmer switch which is electronically controlled.

 

[Wilko]

Hi - just thinking out loud so don’t shoot me - if 2 identical 3kW elements were wired in series across 240V. This would reduce the total load to 1.5kW with 750W in each element.

Some hot water cylinders have provision for 2 elements. Or you could perhaps install a second tank etc. But don’t ask me about how to plumb them .

 

[Dave Edwards]

Reducing the voltage to 110V won't reduce the power to 1500W it will reduce it down to about 600W and the current will be around 6A.

In basic terms the resistance of the element will dictate the current which flows for the applied voltage, this then dictates the power which dissipates. Its a bit more complex than this due to things like the change of resistance due to temperature.


The easiest way to achieve what you want is to use a solar immersion heater controller which is designed for this exact task, such as the solar iboost.
These use a current sensor on the incoming supply to detect exactly how much power is being exported to the grid and then regulate the immersion heater until there is near zero power being exported or imported.
This basically operates like a 3kW dimmer switch which is electronically controlled.

Yes, I know of those devices, and that is what I'm trying to achieve.
But I thought it might not be vastly complicated to make a simpler version that just delivers 1.5kW to the heater. It would sometimes be over the spare amount and so would be made up by the grid, but mostly it would be less than the spare power, so it would be 'free'

Would it really be incredibly complicated to make (or buy) a device that converts 240V @ 13A to 240V @ 6A ???

Actually that really should have been my question all along! I didn't need any of the explanation and detail that makes people fly off at tangents I don't want!

This question is really:

How can I change a power supply from a continuous 240V @13A to 240V at 6A
Is that easily possible and if so with what components?

 

[davesparks]

Yes, I know of those devices, and that is what I'm trying to achieve.

Actually that really should have been my question all along! I didn't need any of the explanation and detail that makes people fly off at tangents I don't want!

Well excusee for attempting to explain the reason I gave my answer instead of just saying no.
And I do apologise for offering what I thought was a helpful suggestion for a product which is commercially available, tried, tested and proven to do exactly what you want with the minimum of wasted powee and will save you money in the long run.

I understand you would much rather take the approach of installing something unlikely to be intended for this purpose which may not necessarily be safe, will probably cost more in the long run and will likely waste a fair amount of the power that you are trying to conserve.

There are devices available which could limit the power consumption of your immersion heater, you would also need suitable test equipment to be able to set them to the power level you require, and would waste an amount of power as heat.

 

[Dave Edwards]

All I want is someone to tell me if I can buy some sort of variable resistor or potentiometer or rheostat (I don't know which, that's the bit of information I need) That will allow me to control a normal 240V 13A supply output to a varying amount of power from 0 to 3kW

 

[telectrix]

if you use any sort of resistance that resistance itself will consume power and dissipate it as heat. davesparks suggestion is the correct one. a cheap solution might be Wilko's idea, the maths works out, but both elements in series would need to be heating the water.

 

[davesparks]

[

Hi - just thinking out loud so don’t shoot me - if 2 identical 3kW elements were wired in series across 240V. This would reduce the total load to 1.5kW with 750W in each element.

Some hot water cylinders have provision for 2 elements. Or you could perhaps install a second tank etc. But don’t ask me about how to plumb them .

Can you still get dual element immersion heaters?

 

[Dave Edwards]

Well excusee for attempting to explain the reason I gave my answer instead of just saying no.
And I do apologise for offering what I thought was a helpful suggestion for a product which is commercially available, tried, tested and proven to do exactly what you want with the minimum of wasted powee and will save you money in the long run.

I understand you would much rather take the approach of installing something unlikely to be intended for this purpose which may not necessarily be safe, will probably cost more in the long run and will likely waste a fair amount of the power that you are trying to conserve.

There are devices available which could limit the power consumption of your immersion heater, you would also need suitable test equipment to be able to set them to the power level you require, and would waste an amount of power as heat.

Hey, davesparks, you have given me really useful information & I thank you for that.
You totally misunderstood my message though. My comment about people flying off at tangents was because someone said I needed to get two tanks with two 3kW heaters etc.. etc. when I had said in my first message that the heating element wasn't a changeable thing in my question.
I said I've heard of the iboost, because I have heard of the iboost. I didn't say that with any underlying nastiness or intent.

I have an electric drill that is maybe 2kW, it has a tiny little wheel on the trigger that alters the power and speed of it. Are you all saying that is heating up because of it's resistance?
It the 3 way switch on a slow cooker. I guess it depends on the total power the device draws when it's flat out?
Partly the reason I wanted to try to do this myself is that I've already made a device that senses when the meter light is on solid.That sensor then activates a relay that switched current to a device. It all works wonderfully until I try to use a device that draws a lot more than is spare. All that happens then is the meter light quickly goes off, so the relay goes off, but I've used power from the grid in the process. So it clicks on and off and sure I'm not using the full 3kW but I'm probably still using 2kW.
If I can reduce the demand to less than 2kW then I'll be able to feed that element with some power when there is spare.

 

[telectrix]

following on from Wilko. a dual element heater could possibly be reconfigured in series. with 1 thermostat instead of 2. maybe contact manufacturer to see if this is possible.

Backer 16"/27" 3KW Dual Element Twin Thermostat Immersion

Standard 2 1/4" 3KW Immersion 16"/27" length Dual Element Twin Thermostat

www.advancedwater.co.uk

 

[Zerax]

I know this might be a crazy idea... but could you not install a 2kW element instead.

But to fly off at a tangent... do you get that much sun in Scotland to make playing around with PV viable ? Here in the warm south of the country, if you're on the grid, they're only really viable with a generous FIT that you can't get anymore.

 

[w0z]

The device you need is another 3kW element connected in series with the first.
No tangents to go off on (or sines or cosines) for this method as there is no reactance.
Efficiency of energy conversion will be close to 100%, and why not arrange a suitably rated switch to short out the second element, and you can label it Normal/Slow (or Slow/Normal).
Alternatively you could use a diode in series with the existing element, the element will only heat on one half of the ac cycle (again with a switch to short the diode as above) but I'm not sure how well your PV inverter would react to the resultant load. Your risk. (link is just one example diode)

 

[Dave Edwards]

I know this might be a crazy idea... but could you not install a 2kW element instead.

But to fly off at a tangent... do you get that much sun in Scotland to make playing around with PV viable ? Here in the warm south of the country, if you're on the grid, they're only really viable with a generous FIT that you can't get anymore.

I'm generating 4000 kWh a year and on an old FIT tariff so I'm getting £2,200 a year income.
I apologise to you and everyone else here. It's my fault for giving information.
I know I could fit a 2kW immersion, I know I could install 2 hot water tanks with 2 3kW immersions.
I should know that if I tell people what I'm doing they are going to offer 'help' from all sorts of different angles and ideas.

I really just wanted to know if you can take a 240V 13A supply and somehow change it to a 240V 6A supply.
I should have just asked that question really.
My Bad!
Trouble is I still don't have the answer really.
It seems variable resistors or the like are going to generate lots of heat and waste power.
Leaves me wondering how other equipment that varies output power seems to do it without those problems.
I guess there's no easy answer.
Thanks for all replies and help and ideas though.
Much appreciated.

 

[telectrix]

most devices that reduce voltage or current are boxes of electronic witchcraft that morf over years of development to minimise the power wasted.

 

[davesparks]

Hey, davesparks, you have given me really useful information & I thank you for that.
You totally misunderstood my message though. My comment about people flying off at tangents was because someone said I needed to get two tanks with two 3kW heaters etc.. etc. when I had said in my first message that the heating element wasn't a changeable thing in my question.
I said I've heard of the iboost, because I have heard of the iboost. I didn't say that with any underlying nastiness or intent.

I have an electric drill that is maybe 2kW, it has a tiny little wheel on the trigger that alters the power and speed of it. Are you all saying that is heating up because of it's resistance?
It the 3 way switch on a slow cooker. I guess it depends on the total power the device draws when it's flat out?
Partly the reason I wanted to try to do this myself is that I've already made a device that senses when the meter light is on solid.That sensor then activates a relay that switched current to a device. It all works wonderfully until I try to use a device that draws a lot more than is spare. All that happens then is the meter light quickly goes off, so the relay goes off, but I've used power from the grid in the process. So it clicks on and off and sure I'm not using the full 3kW but I'm probably still using 2kW.
If I can reduce the demand to less than 2kW then I'll be able to feed that element with some power when there is spare.

I suspect the slow cooker has 2 heating elements, one is switched on at low power, the other is switched on at medium power, both are switched on at high power.

The electric drill is a different type of load, it has a motor in it which can have its speed altered by changing the magnetic fields inside it. The speed is not controlled by simply changing the power input as far as I know, but motors are not really my area so I may be wrong.

Whatever you do I suspect the device you have built will chew through relays pretty quickly as it will often reach the point where it enters rapid on/off switching cycle you have described.