Secret to How Heat Powered Wood Stove Fans (Eco Fans) Work

Is it really possible that the heat from a wood stove can create electricity?
Enough electricity to power a small fan?

What is an Eco Fan?

Yes its true. Eco fans, aka heat powered wood stove fans, aka stove top fans, are powered only by the heat generated by the wood stove. They do not use any source of electricity.

An Eco fan sits on top of the wood stove and as the stove comes up to temperature the fan blades start spinning. Some manufacturers claim that an Eco fan pushing warm air away from a wood stove will help heat a room up to 18% faster. The other benefit of an Eco fan is that by moving air around (and creating a convection effect), the wood stove does not have to run as hot and therefore you can use less wood.

How do eco fans work?

There are two types of Eco fan designs, a Stirling Engine, and a Thermoelectric Generator. Both Eco fan designs work by exposure to temperature differences, where one part is exposed to warmth (as per a wood burning stove), and the other part is kept cooler.

The Stirling Engine Eco Fan
The Stirling Engine, (or Stirling Generator), has been developed with many different working designs based on the fundamentals of expanding and compressing fluid or gas in a closed cycle.

Two pistons connected together contain the same gas. The gas is expanded when heated forcing the first piston in a direction that forces the opposing piston to compress. As the fluid or gas in the hot side starts to cool it’s force is lessened, but there is now a force on the second piston that contains the compressed fluid or gas. The second piston the moves in the direction of the first piston, repeating the cycle.

This cycle continues so long as there is a heat source that continues to expanding the fluid or gas in the first piston.

A rotating crank connected to the pistons turns as the pistons move. Fan blades are attached to the crank to make Stirling Engine eco fan.

The Thermoelectric Generator Eco Fan
The Thermoelectric Eco Fan uses inexpensive thermoelectric generator (TEG) modules to provide a small amount of electrical power. The TEG is an electrical device made of two dissimilar metals separated by positively and negatively charged semiconductor material.

When heat is applied to one side of the TEG and there is a sufficient temperature difference to the other side and electrical power can be generated through the force of thermal insulation. The generation of electrical power in this method is known as the Seebeck coefficient.

A TEG when connected between two heat sinks, one side placed directly on top of the hot wood stove, and the other side thermally insulated to create a temperature difference will create enough electricity to power a small DC motor. Multiple TEG’s connected in series will create a higher voltage, and therefore more power to spin a motor fast enough to connect it to fan blades and push enough air around to assist with heating a room. An this is the simple no input power design of a modern wood stove powered Eco fan.

FAQs

We answer some common questions about Eco Fans.

Do wood burner fans really work?

Yes they work very well. They are great for anyone conscious about the use of electricity, and looking to save money on electrical and wood costs.

Are Eco Fans better than Electrical Fans?

That depends on what you want to achieve. If blowing the most amount of air is your goal then a powered fan will outperform an Eco fan. However an Eco fan is perfectly designed for its job. Best results to heat a room quickly are by using low speed fans to gently push air around. To much air flow speed and the air can create a cooling effect, like a breeze.

For heating a room quickly, an Eco fan is a great option.

Are two stove fans better than one?

Two Eco fans can be better than one, but you might want to try before you buy. It depends on many variables including the room configuration, size of the wood stove, and how you setup the air flow. Many people have run two Eco Fans with great results.

Our suggestion is try one, if you like the results, but you you can see there is still cold areas of the room you want to push air into, and the area immediately around the wood stove is still blazing hot with extra heat, then try adjusting the fan to push air into that colder zone. If that works to better warm up the cooler area, then you would likely benefit from a second Eco Fan.

Where to place a fan on a wood stove?

An Eco fan sits on the top surface of the wood stove. So long as the surface gets hot enough the Eco fan can sit anywhere on the top. Just make sure it has enough distance to spin the fan blades without hitting the flue or anything else you might put on top such as a kettle.

Face the Eco fan towards the direction you want the air to flow. Best results may be to setup an airflow circuit in the room. Directing the warm air directly towards where a person may sit may not be the quickest way to heat a room.

Why does my stove fan not work?

The thermoelectric generator will fail if it gets too hot. This can happen when a heat sink is undersized for the heat put out by the wood stove.

Another problem (mainly with larger models) is not getting the stove hot enough, but this is rare. Most people will build a fire hot enough to power the Eco fan.

How do i repair my Eco fan?

An eco fan is a relatively simple device, with only a few parts. However it does require some special skill, materials, and parts to make (or repair). The most common problem will be failure of the TEG module. If the unit has multiple modules each will have to be tested.

The good news is that TEG modules also work in reverse. That is, when an electrical power source is applied to a TEG module one side will warm and the other side will cool. If you can separate the TEG module from the heat sink try connecting a 9 volt battery across its leads. Do not apply power for too long as the TEG module will burn out if not connected to its heat sink.

Also be careful because they can generate a lot of heat. Once you sense some heat its time to remove the power source. Continue testing all modules in this way until you find the faulty unit.

Replacement modules can be found at most electronic stores. Make sure you also pickup some heat sink compound so that you can get a good thermal connection from the TEG to the heatsink.

DIY stove fan – How to make a heat powered fan

Well, there’s just so many ways. This video shows a clever design and warns of the problems with over heating. If your keen to make your own Eco fan, this video is a good starting point.