There are certain energy efficiency claims that, for an energy nerd, would appear as a spaceship flying onto the radar screen a la James Bond, You Only Live Twice. The hair on the back of the neck starts to twitch – know what I’m sayin’?
When I saw this headline – 6 Energy-Saving Portable Electric Space Heaters – cue the SPECTRE spaceship eating spaceships coming on the screen and closing fast. But first, the author of this article talks about wasteful fireplaces. I guess, yes, if it’s a big 1950s walk-in fireplace, but these are fighting words. I have never seen discussion on woodstoves. First, you have to “design” for heat transfer. Don’t try to hide the thing by packing it in the wall and keeping the stove pipe out of site. That’s where all the heat comes from for Pete’s sake! A tall stove pipe in a big room can be a contemporary feature of a modern home. My pipe runs about 20 feet through the conditioned space. That is a lot of heat transfer surface.
If you were to look up woodstove efficiency, you will find that they average around 70% efficiency, but that is for the stove only and not the stove pipe. Mine is a Jotul Oslo rated at 63% and a very low 3.0 grams of particulate emissions per whatever. It’s low per the almighty, all-knowing EPA.
What is magic about a woodstove that is not part of most alternatives is radiation. Any mechanical engineer worth anything knows radiation heat transfer is proportional to the difference between temperatures to the fourth power. Very hot surfaces discharge massive amounts of radiant heat. For instance, the surface of the glass in the picture shown is about 1000F. The top of the stove is 700F. The base of the stove pipe is 500F.
I have a Fluke 62 Mini infrared thermometer for these measurements. You can’t claim to care about energy efficiency without an IR thermometer (I probably just alienated every reader). But seriously dudes, you can use this to do your own home tightness testing, and it only costs about $100. The other thing is if you claim to be a master chef, it is enormously beneficial for making stovetop sauces and sweets. It’s very accurate and doesn’t get in the way of stirring.
A plot of woodstove external surface temperature is shown nearby, and you can see how fast heat is ejected from these “systems” from flame to exhaust. The distance in feet is from the base of the pipe, up. If the system isn’t at least 80% efficient, I’ll eat the splinters in the bottom of the wood tote.
Moving on to electric heating devices: these manufacturers have outsmarted us energy efficiency nerds. They have made these wasteful, inefficient sources of heat look cool and moreover, people think they are efficient. Although I don’t have Consumer Reports’ budget to buy these things, test, and tear them apart, I am 98% sure of the following.
- Fact: The core heat supply of these is electric resistance, or what I affectionately call toaster coils.
- Fact: Toaster coils are 100% efficient in converting electricity to heat.
- Fact: Fuel-source energy is converted to electricity and transported to your home at about 32% efficiency. This is essentially the stem to stern efficiency of these things – 32%, not 100%.
Therefore, these things are NOT efficient, unless you consider 32% heating efficiency to be, well, efficient.
- Fact: A $25 cheap and crappy toaster heater from your Home Depot, Lowes, Menards, etc., is just as efficient as the $600 masterpieces and likely provides the same Btu per hour of heat output.
So, cut the crap. If everyone used these things the grid would crash.
However, I will throw them a bone for being the right choice in the right application. Consumers who spend significant periods of time in relatively small spaces, such as the bathroom getting ready for work in the morning or the bedroom, these can help save energy cost and probably emissions.
However, consider that the conversion from heat source energy at a power plant to heat a home with a toaster coil wastes about seven times as much heat as a 90% gas-fired furnace in your home does, and Btu for Btu costs about four times as much money.
 72 cent natural gas and 11 cent electricity