Are all space heaters really 100% efficient?

I was reading your efficient space heaters article (space heater ratings). How can you say all space heaters are 100% efficient? If that was so, then why do space heaters have input and output BTU ratings? For a heater to be 100% efficient, it would have to have a BTU output equal to the BTU input, which, I know for a fact, just don’t happen.

So your article seems to imply just because most efficient space heaters are rated at 1500w (input), that they all will put out the same amount of heat, which I know for a fact, they won’t.

Can you please explain this to me, as I am also looking for the most efficient space heaters. And I know for a fact that the little plastic cased ceramic heater rated 1500w 5120 BTU does not put out any where near the heat of a large wire ribbon heater of the same rating.

Answer from Green Energy Efficient Homes

When considering how much energy efficient space heaters use, you need to bear in mind the first law of thermodynamics, which states that energy can be transformed from one state to another, but cannot be created or destroyed.

Efficient space heaters transform energy from the form of electricity to the form of heat. Any electricity going into the space heater gets used by the space heater and is not lost. We tend to think of devices as being inefficient because, for example, an incandescent light does not turn all of its electrical energy into light – some turns directly into heat. A dryer may not be that efficient because much of the heat it produces from electricity gets vented to the outdoors along with the moisture from laundry. And a refrigerator may not be that efficient if it has poor insulation, a poorly designed compressor, inadequate heat exchange to get rid of the heat being extracted from the fridge interior, and so on.

Because of the first law, all the energy in the electricity powering efficient space heaters is preserved. Most or all of it is converted directly into heat. A small amount may be converted into kinetic energy – this usually means, for efficient space heaters, moving a fan motor, which in turn moves fan blades to blow air on the heating element and thus spread the heat more effectively through stronger convection in the air.

However, even the kinetic energy used to move the fan, and move the air, eventually turns into heat, because the friction within the motor creates heat in the motor itself, and the friction between air molecules being pushed around by the fan eventually turns into heat.

If you look up the BTU ratings and wattage of a range of efficient space heaters, you quickly discover that they all follow the pattern of 1 watt input power produces about 3.41 BTU. British Thermal Units are a measure of heat over a time period (actually, one hour), so a 1,000 watt heater producing 3,412 BTU really means that heater will consume 1 kilowatt hour over a one hour period and produce 3,412 BTU over that one hour period.

To demonstrate the constant relationship between watts of input power for an electric heater and BTU output, I looked up over 25 different efficient space heaters and selected the 17 where I could find both wattage and BTU output. You might wonder why it’s hard to find both wattage and BTU output for all efficient space heaters, and the reason is that many manufacturers and retailers know that there is a constant relationship between watts and BTUs for electric heaters, so they don’t bother publishing both. In some cases I found the BTU output expressed in parentheses, for example “1,500 watts (5,118 BTU)”, confirming my assertion that this is a constant relationship between watts and BTU for efficient space heaters.

In fact if you look up the relationship between a watt hour and a BTU, for example on an online energy conversion calculator, you will discover a constant relationship of about 3.414 BTU per watt.

I’ve included a table below of the 17 efficient space heaters where I could find both wattage and BTU ratings. Only one of these heaters has a BTU/watt ratio outside the range of 3.40-3.47: the ProFusion Heat Quartz Combo Heater, which has a ratio of 4.444, or 6,000 BTU output for 1,350 watts input. I expect this is a result of misreported specifications, since it really is impossible to produce more BTU of heat energy than the equivalent amount of electrical energy – you can’t violate the laws of physics! For example the manufacturer may have measured heat output incorrectly, or measured it with a main and a supplementary heating element operating, but only provided the wattage for the main element. Or they may have mis-entered the wattage as 1,350 (what they claim) instead of 1,750 (which yields a more thermodynamically plausible ratio of 3.43).

You’ll notice in the table that the range of ratios is 3.407 to 3.467, a roughly 1.8% range. (The Duraflame Dartmouth used to have a ratio of 4.444, which is likely because the Amazon description was incorrect, with a higher BTU number and lower wattage number; the Amazon listing has been updated.) The explanation for this is quite straightforward: space heater manufacturers tend to round wattage to the nearest 50 or 100, and may also round their published BTU ratings. A nominal 1,500 watt heater could really be a 1,477 watt heater – just as a 26 oz package of cereal may have slightly more or less than the stated weight.

Heater Type Watts BTU Ratio
Duraflame Vent-Free Metal Stove 1350 4600 3.407
Duraflame Dartmouth Infrared Quartz Heater with Air Purifier Quartz 1500 5200 3.467
Air King 8566 Oscillating Ceramic Heater Ceramic 1500 5118 3.412
Lasko 751320 Ceramic Tower Heater Ceramic 1500 5118 3.412
Lasko 5409 Oscillating Ceramic Heater Ceramic 1500 5118 3.412
Lasko 5435 Max-Endurance Oscillating Ceramic Heater Ceramic 1500 5118 3.412
Lasko 754200 Ceramic Heater Ceramic 1500 5118 3.412
Holmes FamilySafe 1500W Fan-Forced Heater Fan 1500 5118 3.412
Lasko 758000 Cyclonic Ceramic Heater Ceramic Fan 1500 5118 3.412
Fahrenheat Electric Baseboard Heater 1500 5120 3.413
Fahrenheat Electric 1500 Watt Portable, Liquid-Filled Baseboard Heater 1500 5120 3.413
Fahrenheat Portable Electric Hydronic Heater, Model# LFP6152 1500 5120 3.413
ProFusion Heat Ceiling-Mounted Workshop Heater with Halogen Light — 5200 BTU, 1500 Watt 1500 5200 3.467
Caframo Electric Space Heater 1500 5200 3.467
ProFusion Heat Quartz Combo Heater Quartz 1500 5200 3.467
Fahrenheat Commercial Wall Heater 4000 13650 3.413
Ouellet Commercial Fan-Forced Heater 10000 34120 3.412

 

The only way to increase BTU output from efficient space heaters over and above the roughly 3.41 ratio is to use electricity not to produce heat but to move heat from one area to another. This is what heat pumps do: using a refrigeration cycle, they extract heat from the outdoors on a cool day, and pump that heat into an indoor space. A heat pump can be very efficient at moving heat: on a mild day (say, 10C or 50F) they can pump up to 4 units of heat energy into an enclosed space from the outside, for every unit of electrical energy consumed. This doesn’t violate the first law of thermodynamics because heat pumps are not producing heat so much as moving heat from a colder to a warmer place.

The last point you make is that a ceramic heater does not produce nearly the heat of a wire heater even though they have the same wattage. When comparing efficient space heaters it’s important to distinguish energy efficiency with effectiveness for a particular application. As I explain in my main article Energy efficient electric heaters, different types of efficient space heaters can be more effective at different heating applications. For example, heaters with a fan are more effective at quickly dispersing heat throughout a room because they create stronger air currents to move the heat, whereas passive convection heaters such as a baseboard heater use natural convection currents created by the heat itself. Infrared or radiant heaters, meanwhile, do not heat the air but instead the solid (or liquid) objects their infrared radiation strikes, and they are therefore more effective at keeping someone warm as they sit in an armchair (with the heater aimed at them), since they are only directly heating the person and the armchair, not the air in the room. Baseboard heaters can also be less efficient if they are located on outside walls, because the closer the heat source is to an outside wall, and the lower the insulation of the wall, the more of the heat escapes to the out-of-doors.

2 replies
  1. JC
    JC says:

    Great article with a clear explanation of why electric heaters *are* 100% efficient – even a lightbulb is very nearly 100% efficient as a heater.

    Looks like Duraflame revised the specs on the Duraflame Dartmouth Infrared Quartz Heater with Air Purifier, now they claim a more blievable 5200 BTU/Hr on the Amazon product page.

    Reply
  2. Robin
    Robin says:

    Hi JC. Actually a lightbulb IS 100% efficient as a heater, not just very nearly.

    Thanks for pointing out the Duraflame specs have changed, I’ve updated the article to reflect that. They lowered the BTU and upped the watts, which took them down from a physically impossible ratio of 4.44 to a more credible 3.467.

    Reply

Leave a Reply

Want to join the discussion?
Feel free to contribute!

Leave a Reply

Your email address will not be published. Required fields are marked *

You can use basic HTML tags such as <b>, <i>, <p> and so on. Posts containing <a> or "href=" are not allowed.


*Prove you're a real human being - enter the CAPTCHA code