No plug, no battery: An expert explains what this low-cost stove fan does for a cold living room

It sits on top of the stove like a small metal windmill and needs no plug or battery. Once the stove gets hot enough, the blades simply start turning.

German discounter Netto has been selling a heat-powered StarQ stove fan for €19.99, which is just below $23, this heating season, and the gadget appears to have caught shoppers' attention. Many websites reported that it had already sold out online, although stores could still have stock.

But does putting a small fan on your wood burner actually make the room warmer? Is it worth the hype?

How does it run without electricity?

The clever part is that the fan makes its own electricity from the heat of the stove.

Its base becomes hot while the cooling fins higher up remain relatively cooler. That temperature difference is used by a thermoelectric module to produce a small electrical current, which powers the motor and spins the blades. This is based on what's known as the Seebeck effect.

For the StarQ model sold by Netto, the stated operating range is 70°C to 345°C, or roughly 158°F to 653°F. Other stove fans can have different limits, so don't assume every model starts spinning at the same temperature. That also means these fans won't work properly on every stove. The surface underneath must become hot enough, while also staying below the fan's maximum permitted temperature.

What the stove fan actually does

Warm air naturally rises. With a wood burner, that can mean plenty of warmth around the stove and higher up in the room while the sofa several yards away still feels noticeably cooler.

A stove fan tries to change that pattern by pushing some of the heated air horizontally into the room.

That can be particularly useful in a large living room or an open-plan living and dining area where the stove is an important source of heat. Instead of waiting for natural air circulation to spread warmth around the room, the fan encourages the air to move more quickly.

But there's an important fact: it isn't increasing the stove's heating output. The stove produces the same amount of heat, and the fan merely helps in redistributing some of the heat so that your room feels warmer.

Why one stove builder isn't convinced

German stove builder Björn Keller is much less enthusiastic about the idea.

Speaking to Netzwelt, Keller argued that a good stove produces much of the pleasant warmth people enjoy as long-wave radiant heat. Rather than simply heating the air, radiant heat warms you when it reaches your body, a little like feeling sunshine on your skin.

A stove fan, by contrast, increases air movement and convection. Keller says that the fan can also stir up dust, while doing nothing to change the way the stove itself produces heat.

That doesn't necessarily mean a fan is useless. It means its benefit depends heavily on the type of stove and the room around it.

Final word: Is the €20 stove fan worth buying?

You're most likely to notice a difference if you have a freestanding metal stove in a large or open-plan room and find that one side becomes much warmer than the other.

There's less reason to expect a dramatic improvement in a small, well-insulated room that already heats evenly. The same applies to some soapstone, tiled, or heavily insulated stoves, where the top surface may not become hot enough for this type of fan.

Before placing one on your stove, check both manuals. Make sure your stove permits this type of accessory and follow the fan manufacturer's temperature and positioning limits.

And be cautious with claims that a stove fan will dramatically cut your firewood use. Better heat distribution is plausible; a guaranteed percentage saving is a different claim entirely.

For around €20, the fan may be a useful way of getting existing warm air toward a colder corner of the room. Just don't expect a tiny spinning propeller to turn your stove into a more powerful heater.