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Author Topic: How is the energy efficiency of space heaters calculated?  (Read 6977 times)

Offline onsk

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I see that heating devices such as space heaters have varying energy efficiency ratings. Now, as far as I know, what usually makes electric appliances less efficient is loss of energy in the form of heat, while more efficient ones are so because they loose less more energy to the environment while doing the work they are supposed to do. But as the purpose of electric heaters is to spread heat, it seems to me that the more 'inefficient' in the usual sense they are, that is, the more they loose the energy that goes into them to the environment, the more efficient they are. So what makes some of them less efficient than others? What is inefficiency in such devices?
« Last Edit: 08/11/2011 20:55:50 by chris »


 

Offline Geezer

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Re: How is the energy efficiency of space heaters calculated?
« Reply #1 on: 06/11/2011 16:55:56 »
You're right. It's a load of technobablle mumblespeak invented by marketing people.

It's pretty difficult for an electric heater to be anything other than 100% efficient in terms of energy in versus energy out. The only reason I can think of why some of the energy going in does not come out as heat is if the heater also has a fan to distribute warm air. In that case, some of the energy in is spent in doing work on the air, but even then, all the energy going into the heater is spent on heating the room, so it's still 100% efficient.
 

Offline JP

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Re: How is the energy efficiency of space heaters calculated?
« Reply #2 on: 07/11/2011 00:53:38 »
They can vary in their efficiency at heating a space, or at heating a space in a short amount of time.  Radiant heaters in particular are poor at heating spaces, since they beam their heat out in a direction, and most of it gets deposited on whatever that beam hits.  This is great if you're sitting in their line of fire, but not so great if you leave the room and they end up heating up your wall, rather than the room...
 

Offline CliffordK

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Re: How is the energy efficiency of space heaters calculated?
« Reply #3 on: 07/11/2011 01:50:35 »
Personally, I've started using a small space heater rather than central heating in my house.  It gets the heat where I am at, and I don't have to heat the whole house.

I've been told that the ceramic heaters are most efficient, but I haven't verified it.  They do allow good fan based air circulation.

For large area heating, a heat pump may be more efficient than an electric heater.  However, their efficiency drops as the temperature drops (which is when you need them the most).  In theory, a water based heat pump is less dependent on the ambient temperature.  However, that might depend a bit on the cost of pumping the water up from the well.

In the Northwest, the sun also has a tendency to disappear about the time one might want more solar heating.
 

Offline Geezer

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Re: How is the energy efficiency of space heaters calculated?
« Reply #4 on: 07/11/2011 02:26:09 »
Well, yes, but presumably a space heater is intended to heat the space :D. Radiant heaters heat the objects in the space, which tends to give people "corned beef leg" syndrome.

I think the story about ceramic heaters is a load of codswallop slightly suspect. They might create a larger surface area so that the the heat transfer is accelerated, but if you want to heat an enclosed volume of air to a particular temperature, you have to put in a certain amount of energy, and that's going to cost the same amount no matter how you do it. One good option is to install a ceiling fan and run it very gently in "suck mode" to prevent all the warm air getting stuck near the ceiling.

Personally, I rather like those inexpensive oil-filled electric heaters. They seem to provide a nice even temperature, and you can crank them down to a minimum to provide just enough heat in rooms that are not occupied. 
 

Offline CliffordK

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Re: How is the energy efficiency of space heaters calculated?
« Reply #5 on: 08/11/2011 06:40:03 »
I was thinking a bit...

There are different types of heating.

I like my 200W Ceramic heater with what looks like a computer fan to blow warm air towards me.  However, I wonder if some of the heat is wasted as the hot air from the heater may displace local warm air... 

Perhaps another option would be something like a heat lamp that would be a heavy IR heater...  with the intention to warm surfaces (me).

The radiator type heaters are good for heating a closed room.

I have an ordinary space heater in my pumphouse that is set to kick on between 30°F and 35°F.  Presumably any type of heater would be effective, although it might as well be quick acting once it hits the threshold.

Personally I don't use electric blankets, but perhaps there would be benefits of just heating a wrap around one's body.

Of course, the other option is to put on more clothes, coats, blankets, whatever.
 

Offline Geezer

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Re: How is the energy efficiency of space heaters calculated?
« Reply #6 on: 08/11/2011 08:19:08 »
I suppose it's a tradeoff between preventing a lethal case of exposure, and comfort. A pair of electric trousers would probably be a good way to minimize expenditure, but they might not be too comfy.
 

Offline imatfaal

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Re: How is the energy efficiency of space heaters calculated?
« Reply #7 on: 08/11/2011 09:24:34 »
I want some electric trousers to go with my NOxSocks
 

Offline onsk

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Re: How is the energy efficiency of space heaters calculated?
« Reply #8 on: 08/11/2011 17:14:00 »
Back to original question regarding how energy efficiency of heat-spreading devices is measured and what inefficiency could mean in a device whose purpose is to spread heat, it seems that different heating appliances should perhaps be rated  with reference to their purpose, whether this be to keep a whole space warm, or a portion of it (in the case of heaters that emit a directed beam). I suppose that one could come up with standards for rating how well this is done (say, measure how much electricity is consumed until a a point at a certain distance from the device in a standardized space reaches a certain temperature), but I am still curious as to what the actual ratings given to electric heating devices mean (or, in other words, in what consists the difference between two similar devices with different energy ratings).
 

Offline Geezer

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Re: How is the energy efficiency of space heaters calculated?
« Reply #9 on: 08/11/2011 17:52:15 »
but I am still curious as to what the actual ratings given to electric heating devices mean (or, in other words, in what consists the difference between two similar devices with different energy ratings).

If they are using the term "efficiency", they better specify exactly what their definition is. If they don't, they are waffling, and you have to conclude they are only referring to thermal efficiency.

The energy received from the power outlet can only do a few things that I can think of;

Make heat
Pump air
Make visible light
Make sound
Generate electric fields

The sound, light and electric fields should be pretty much negligible, and even if they are not, most of their energies will be dissipated as heat inside the room. That leaves heat and pumping air. The pumping air bit is usually very small in relation to the heating bit (you don't want a howling gale because it will only remove heat from your body more quickly), so all, or nearly all, of the energy supplied produces heat in the "space".

That means any space heater is very efficient at heating the space. It will be at, or close to, 100% efficient.

I've seen some ads in the US for electric space heaters that make outrageous claims (and they are also extremely expensive). I'm sure the ads are worded very carefully too, but I suppose you can only go so far to protect people.

Caveat epmtor!
 

Offline damocles

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Re: How is the energy efficiency of space heaters calculated?
« Reply #10 on: 08/11/2011 20:22:50 »
In terms of heat out/energy in efficiency, the argument that most heating devices are 100% efficient is a sound one.

But there is one instance where that is not the case: a heater with a flue. In this case

heat efficiency = heat into room/(heat into room + heat up flue)

So the 100% efficiency does not apply to wood/coal fires or gas fires with flue.

Here in South-Eastern Australia where we have a mild climate and the world's highest per capita greenhouse gas production, the vast majority of the little space heating that is needed is achieved with gas fires, and wood fires make up a significant part of the balance.
 

Offline CliffordK

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Re: How is the energy efficiency of space heaters calculated?
« Reply #11 on: 08/11/2011 20:50:24 »
There is debate about the use of unvented gas heaters.  Probably not a good idea with liquid fuels (kerosene&Heating Oil).  But, it is a viable option with propane/methane/butane heaters, especially catalytic heaters.  In which case they would also approach 100% efficiency.

For vented heaters... most of them use heat to propel the exhaust gases up the chimney.  However, I was wondering if it would be appropriate to attempt to recover 100% of the thermal energy, and use a fan to propel the exhaust gases up the chimney instead.  Wood, of course, has issues of creosote buildup.
 

Offline Geezer

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How is the energy efficiency of space heaters calculated?
« Reply #12 on: 08/11/2011 21:10:38 »
Quite so, but the OP does seem to be referring to electric heaters.

Flying off on a tangent (as usual) - I don't know if this would apply where you live, but it would cost me about the same whether I heat my home with electricity or gasoline (assuming I actually had a stove that could safely burn the stuff.)
 

Offline CliffordK

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How is the energy efficiency of space heaters calculated?
« Reply #13 on: 08/11/2011 21:18:31 »
Flying off on a tangent (as usual) - I don't know if this would apply where you live, but it would cost me about the same whether I heat my home with electricity or gasoline (assuming I actually had a stove that could safely burn the stuff.)
Further off tangent  :-\

Odd...  it costs half or a quarter as much to power a car on electricity as gasoline.

What gives?

----------------------------------

Maybe I can answer my own question.  The gasoline engine wastes a lot of energy with heat production, and friction of more moving parts.  Then cooling systems to get rid of the excess heat.  The electric motor doesn't have all of this.

Heating one's house, one wants to conserve as much heat as possible.
« Last Edit: 08/11/2011 21:22:00 by CliffordK »
 

Offline damocles

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How is the energy efficiency of space heaters calculated?
« Reply #14 on: 08/11/2011 22:55:03 »
Quite so, but the OP does seem to be referring to electric heaters.

Flying off on a tangent (as usual) - I don't know if this would apply where you live, but it would cost me about the same whether I heat my home with electricity or gasoline (assuming I actually had a stove that could safely burn the stuff.)

Where I live, and this applies to Melbourne area only, methane (mains supply) < lignite briquettes (yuk!! messy) < propane (bottled gas) similar to mains electricity < wood, coke, heating oil = kerosene (commercially supplied). Some have access to non-commercial bio-methane or wood, which might change this ordering, and with proper building design for solar heating and heat storage, the need for heating can be nearly totally removed in our generally mild climate (coldest month average about 6-13°C daily min/max).

On a slightly different matter, one of our two flued gas heaters has its air intake feeding into a heat exchanger inside the flue so that the air that is used to burn the gas is pre-heated, less heat escapes with the exhaust gases, and heat efficiency is improved in that way.
« Last Edit: 08/11/2011 23:04:16 by damocles »
 

Offline Geezer

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How is the energy efficiency of space heaters calculated?
« Reply #15 on: 08/11/2011 23:41:08 »
Maybe I can answer my own question.  The gasoline engine wastes a lot of energy with heat production, and friction of more moving parts.  Then cooling systems to get rid of the excess heat.  The electric motor doesn't have all of this.

Heating one's house, one wants to conserve as much heat as possible.

Right! IC engines are only about 25% efficient in terms of work done, but they are very good space heaters. The price of 1MJ is about the same whether I buy it as gasoline or electricity (and electricity is really cheap here). The price per MJ of propane is not so different from gasoline either. It sort of makes sense. Energy prices have to be (or ought to be) competitive.

 
 

Offline damocles

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How is the energy efficiency of space heaters calculated?
« Reply #16 on: 09/11/2011 00:09:45 »
From Geezer:
Quote
(...snip...)

 The price of 1MJ is about the same whether I buy it as gasoline or electricity (and electricity is really cheap here). The price per MJ of propane is not so different from gasoline either. It sort of makes sense. Energy prices have to be (or ought to be) competitive.

 

This is true to a large extent, and has accounted for the fading and near extinction of some of the sources I mentioned in my last post. But there are other factors that operate.

Context: mains methane vs more expensive bottled propane depends on availability and/or cost of installing mains infrastructure.

Aesthetics: to many people the flickering glow of a genuine wood fire is an important emotional/nostalgic feature that outweighs, at least sometimes, the inefficiency and extra expense. They find it comforting. Many bars and restaurants here have installed wood fires for this sort of reason.

Convenience: a heater that can be activated by an electric switch or a gas tap is often preferred to one where fuel will have to be carried and ash disposed of, in spite of the fact that in many of our rural areas the latter is cheaper.

But by-and-large "survival of the fittest" applies to energy sources, and account for the near extinction of briquettes and kerosene around here.
 

Offline CliffordK

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How is the energy efficiency of space heaters calculated?
« Reply #17 on: 09/11/2011 00:54:37 »
Aesthetics: to many people the flickering glow of a genuine wood fire is an important emotional/nostalgic feature that outweighs, at least sometimes, the inefficiency and extra expense. They find it comforting. Many bars and restaurants here have installed wood fires for this sort of reason.

Wood, of course, is a renewable resource.  Methane can be recovered from various sources, but the majority of it is a fossil fuel.  Electricity comes from mixed sources, depending on one's location.  Here in the Pacific Northwest, 2/3 to 3/4 of our electricity comes from renewable resources (generally hydroelectric).  But, that is not true nationwide.

There is a variety of efficiencies of wood burning apparatuses.

Wood stoves generally tend to be fairly energy efficient.
Wood fireplaces tend to be far less energy efficient.
 

Offline CZARCAR

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How is the energy efficiency of space heaters calculated?
« Reply #18 on: 09/11/2011 12:18:37 »
Well, yes, but presumably a space heater is intended to heat the space :D. Radiant heaters heat the objects in the space, which tends to give people "corned beef leg" syndrome.

I think the story about ceramic heaters is a load of codswallop slightly suspect. They might create a larger surface area so that the the heat transfer is accelerated, but if you want to heat an enclosed volume of air to a particular temperature, you have to put in a certain amount of energy, and that's going to cost the same amount no matter how you do it. One good option is to install a ceiling fan and run it very gently in "suck mode" to prevent all the warm air getting stuck near the ceiling.

Personally, I rather like those inexpensive oil-filled electric heaters. They seem to provide a nice even temperature, and you can crank them down to a minimum to provide just enough heat in rooms that are not occupied. 
"suck mode" blowing up pressurizes the ceiling above the fan & accelerates heatloss? I shine an incandescent spotight [50w] on me 4 heat
 

Offline Geezer

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How is the energy efficiency of space heaters calculated?
« Reply #19 on: 09/11/2011 16:51:13 »
 
suck mode" blowing up pressurizes the ceiling above the fan & accelerates heatloss?

Probably has little effect on heat loss. It evens out the temperature of the air in the room without creating a draft.
 

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How is the energy efficiency of space heaters calculated?
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