Bullshit/snake oil alert re electric heaters

[adam2]

Disscussed in detail on another forum.
Be aware, especialy if you have any older friends or relatives whom might be taken in by this nonsense.
http://www.theiet.org/forums/forum/mess ... erthread=y

[Snail]

Reminds of special 'insulating' paint being advertised on tv a while back and, er, certain other heating products also currently being tv advertised.

[mr brightside]

There's a really tasty thread about it all here...

http://forums.moneysavingexpert.com/sho ... ?t=3549983

I don't work in domestics so this is the first i've heard about it all. I like the bit where a poster claims that salt water has stored energy in it which is released by heating it up, i can see why people get suckered in. Fortunately there was other people on hand to balance out the debate.

Personally i don't like fan heaters because the motor wastes power which could be doing some heating, and when the bearings seize the element cooks the job lot and fills the room with acrid smoke. A cheap convector from Homebase is all i need, or better still i just set fire to something.

[UndercoverElephant]

Personally i don't like fan heaters because the motor wastes power which could be doing some heating...

So...if the energy consumed by the motor does not end up as heat in the room (somehow), where does it go?

I have the same question about "inefficient lighting", at least in winter when the room is cold. If the energy being "wasted" by the inefficient bulb doesn't end up as heat in the room, where does it go?

[Little John]

Personally i don't like fan heaters because the motor wastes power which could be doing some heating...

So...if the energy consumed by the motor does not end up as heat in the room (somehow), where does it go?

I have the same question about "inefficient lighting", at least in winter when the room is cold. If the energy being "wasted" by the inefficient bulb doesn't end up as heat in the room, where does it go?

In the case of the heater, I am guessing that some of it is lost as heat and some of it continues as kinetic energy in air currents in the room. However, those air currents, if they are unable to escape the room, must eventually convert to thermal energy as well. It has to all end up as thermal energy.

All of which is why I have never understood why some electrical heater are described as being more "efficient" than others. The only way they could be more "efficient" is by using less energy and so producing less heat. If the primary purpose of an electric heater is to produce heat, then the more "efficient" ones must be, in fact, less efficient.

Also, I agree about the bulbs. The waste energy definitely must end up as heat in the room. Thus, if an energy efficient bulb is able to produce the same amount of light with less heat produced than an energy inefficient bulb, this will mean the room is slightly less warm than it might otherwise have been, in turn requiring the central heating system to have to remain on for that little bit longer to compensate, in turn negating much of the energy saving made by the more efficient bulb. The only way to counter the above would be by reducing the thermostat setting on the central heating to ensure the saving made by the energy efficient bulb are not lost.

In which case, the simplest solution would have been to have reduced the setting on the central heatings thermostat in the first place and not bother going to the expense of getting fancy energy efficient bulbs.

[mr brightside]

Personally i don't like fan heaters because the motor wastes power which could be doing some heating...

So...if the energy consumed by the motor does not end up as heat in the room (somehow), where does it go?

It goes on turning the motor. It is wasted on bearing friction, carbon brush friction (if fitted), and synchronous slip losses due to power factor and the fact that it is impossible for a motor to be 100% efficient even at a power factor of 1.0

Electrical power greater than the shaft output power has to be supplied to the motor becuase of motor loss. The relationship between supplied power and yeilded power is an expression of motor efficiency.

[Little John]

Personally i don't like fan heaters because the motor wastes power which could be doing some heating...

So...if the energy consumed by the motor does not end up as heat in the room (somehow), where does it go?

It goes on turning the motor. It is wasted on bearing friction, carbon brush friction (if fitted), and synchronous slip losses due to power factor and the fact that it is impossible for a motor to be 100% efficient even at a power factor of 1.0

Electrical power greater than the shaft output power has to be supplied to the motor becuase of motor loss. The relationship between supplied power and yeilded power is an expression of motor efficiency.

Yes, of course it goes on turning the motor. However, it doesn't then just disappear in a puff of smoke (first law of thermodynamics). After the motor has turned, where does the energy used to turn it go to (I'm not talking about the energy lost to heat before it got to turn the motor. I'm talking about the energy that we know was used to turn the motor)?

Don't misunderstand me here. There are, of course, instances where the kinetic energy of a motor may be transferred to something other than heat. For instance, if a motor is used to lift a weight six feet into the air, then the kinetic energy in the motor has been transformed into potential gravitational energy in the suspended weight.

However, in the case of, say, a fan-heater motor, whilst there will be some potential gravitational energy stored in part of the motor as it rises, this is directly compensated for by the other side of the motor which is falling. Thus, there must be little or no kinetic energy, overall, that is transferred from a fan-heater motor's motion into gravitational energy. In which case, all that is left is kinetic energy in the air currents in the room that have formed as a consequences of being pushed around by the the fan and/or, thermal energy from the motor. Putting aside the direct leakage of heat form the motor, the movement of air currents, as they settle down following the motor being switched off, must be finally transformed to heat.

Mustn't they?

I guess what I am trying to say is that my understanding of the laws of thermodynamics tells me that, in a closed system, all energy is conserved via transference and all energy must eventually be transferred to heat for reasons of entropy.

I'm happy to have those laws re-explained to me if I've got them wrong though.

[JohnB]

The problem with heat from lighting is that as hot air rises it stays above head height close to the ceiling. Draughts tend to come in close to the floor, so lights don't heat the parts of the room where the people are. I found this when I lived in my van, with a ceiling that just touches my head. I could be sitting down feeling cold, but when I stood up my head would be hot and my feet cold!

I had a temperature gauge that showed the temperature at seat height and at shoulder height when standing, and there was usually a big difference when I had any heating on.

[Little John]

The problem with heat from lighting is that as hot air rises it stays above head height close to the ceiling. Draughts tend to come in close to the floor, so lights don't heat the parts of the room where the people are. I found this when I lived in my van, with a ceiling that just touches my head. I could be sitting down feeling cold, but when I stood up my head would be hot and my feet cold!

I had a temperature gauge that showed the temperature at seat height and at shoulder height when standing, and there was usually a big difference when I had any heating on.

So, replacing ceiling lights with energy saving bulbs makes some sense. However, anybody consider getting energy efficient bulbs for any lights other then ceiling light should be made aware that their heating energy consumption will probably need to rise by the equivalent of the heat saving in the energy efficient bulb in order to compensate for that heat saving.

[adam2]

All electric heaters are virtually 100% efficient at the point of use.
In the case of a fan heater, the fan uses less than 1% of the total consumption, and in all common circumstances the energy used by the fan motor ends up as heat in the heated space due to friction.

If electric heating is to be used at all, than a powerfull fan heater is often the best option for quick warming of a large cold space.
The fan blows the warm air air around, and given sufficient loading, quicky warms the area.

For longer term use, a fan heater is often not a good choice. Warmed air blown around by a fan is, by definition moving air. Air that is moving tends to feel less warm than still air, and therefore a slightly higher temperature and consequent higher energy use is needed to give the same PERCIEVED temperature.
Also, no real world space to be heated is airtight and warmed air that is blown around by a fan is more liable to escape from the heated space than is more or less static warm air.

I know of someone who uses 9KW of electric fan heating in their living room every winter morning !
This is by no means costly as it is for the last 30 minutes of the off peak tariff. The daily cost is about 25 pence and well worth it to be warm in the early morning, before the stove is lit.

On a smaller scale, my Mother uses a 3KW fan heater for 30 minutes each winter morning for cheap pre-heating on off peak power.

[Little John]

All electric heaters are virtually 100% efficient at the point of use.
In the case of a fan heater, the fan uses less than 1% of the total consumption, and in all common circumstances the energy used by the fan motor ends up as heat in the heated space due to friction.

If electric heating is to be used at all, than a powerfull fan heater is often the best option for quick warming of a large cold space.
The fan blows the warm air air around, and given sufficient loading, quicky warms the area.

For longer term use, a fan heater is often not a good choice. Warmed air blown around by a fan is, by definition moving air. Air that is moving tends to feel less warm than still air, and therefore a slightly higher temperature and consequent higher energy use is needed to give the same PERCIEVED temperature.
Also, no real world space to be heated is airtight and warmed air that is blown around by a fan is more liable to escape from the heated space than is more or less static warm air.

I know of someone who uses 9KW of electric fan heating in their living room every winter morning !
This is by no means costly as it is for the last 30 minutes of the off peak tariff. The daily cost is about 25 pence and well worth it to be warm in the early morning, before the stove is lit.

On a smaller scale, my Mother uses a 3KW fan heater for 30 minutes each winter morning for cheap pre-heating on off peak power.

Thanks for that Adam. I did wonder if a fan heater might be a problem because of it pushing air around. That is to say, in a non-airtight room (which is basically all rooms), some of that moving air might be more likely to escape the room than air that is heated by radiation alone.

[JohnB]

I used a halogen heater, placed close to me and mostly on the 400w setting, for most of last winter. Occasionally I increased it to 800w. I assume they lose some energy in light, but haven't investigated. I was actually fairly comfortable with some warm clothes on as well, and it wasn't hugely expensive.

My alternatives would have been 1kw plus heaters with a thermostat, and I think the low continuous heat was better. My ex lodger certainly used far more than me with a 1-2kw convection heater, but then he wasn't paying the electricity bill!

[UndercoverElephant]

So...if the energy consumed by the motor does not end up as heat in the room (somehow), where does it go?

It goes on turning the motor. It is wasted on bearing friction, carbon brush friction (if fitted), and synchronous slip losses due to power factor and the fact that it is impossible for a motor to be 100% efficient even at a power factor of 1.0

Electrical power greater than the shaft output power has to be supplied to the motor becuase of motor loss. The relationship between supplied power and yeilded power is an expression of motor efficiency.

Yes, of course it goes on turning the motor. However, it doesn't then just disappear in a puff of smoke (first law of thermodynamics). After the motor has turned, where does the energy used to turn it go to (I'm not talking about the energy lost to heat before it got to turn the motor. I'm talking about the energy that we know was used to turn the motor)?

Don't misunderstand me here. There are, of course, instances where the kinetic energy of a motor may be transferred to something other than heat. For instance, if a motor is used to lift a weight six feet into the air, then the kinetic energy in the motor has been transformed into potential gravitational energy in the suspended weight.

However, in the case of, say, a fan-heater motor, whilst there will be some potential gravitational energy stored in part of the motor as it rises, this is directly compensated for by the other side of the motor which is falling. Thus, there must be little or no kinetic energy, overall, that is transferred from a fan-heater motor's motion into gravitational energy. In which case, all that is left is kinetic energy in the air currents in the room that have formed as a consequences of being pushed around by the the fan and/or, thermal energy from the motor. Putting aside the direct leakage of heat form the motor, the movement of air currents, as they settle down following the motor being switched off, must be finally transformed to heat.

Mustn't they?

I guess what I am trying to say is that my understanding of the laws of thermodynamics tells me that, in a closed system, all energy is conserved via transference and all energy must eventually be transferred to heat for reasons of entropy.

Seems that way to me also.

[mr brightside]

All electric heaters are virtually 100% efficient at the point of use.
In the case of a fan heater, the fan uses less than 1% of the total consumption, and in all common circumstances the energy used by the fan motor ends up as heat in the heated space due to friction.

Energy could be transferred into shaft vibration or housing vibration due to rotor imbalance, but i don't know to what extent. As the motor bearings wear or lose lubrication this would get worse. In the situation of a convector versus a fan heater i'd always go for the one with no moving parts if i was after efficiency; although i concede that some losses could be transferred into heat. It's an interesting debate topic whichever way you cut it.

[Little John]

All electric heaters are virtually 100% efficient at the point of use.
In the case of a fan heater, the fan uses less than 1% of the total consumption, and in all common circumstances the energy used by the fan motor ends up as heat in the heated space due to friction.

Energy could be transferred into shaft vibration or housing vibration due to rotor imbalance, but i don't know to what extent. As the motor bearings wear or lose lubrication this would get worse. In the situation of a convector versus a fan heater i'd always go for the one with no moving parts if i was after efficiency; although i concede that some losses could be transferred into heat. It's an interesting debate topic whichever way you cut it.

The point is that all the energy gets lost to heat, in the end. Even the energy that was not initially lost to heat because it turned a motor. Even that energy ends up as heat in the room. It can't simply disappear.

In light of all of the above, I would have thought that the most efficient electric heater (in terms of transferring electrical energy to heat energy) is one that transfers a given amount of electric energy to heat in the shortest period of time possible. In other words, an old fashioned resistance, electric-bar-element fire. The only other improvement you could get over the above would be to mount a small fan near the bar-element in order to distribute the heat round a room quickly. Basically, an electric resistance heater with a fan....

Oh wait......