Wood stove heat exchanger, what is it and how does it work?

What is a heat exchanger?

A heat exchanger is a device that can transfer heat from one medium to another (either gas or fluid). A common type of heat exchanger is a car radiator, which transfers heat from hot engine coolant (fluid) into the air (gas) that is passed through the cooling fins of the radiator.

What is a wood stove heat exchanger?

An air to air heat exchanger installed in the flue of a wood stove is warmed by the exiting hot air rising in the flue, which heats the tubes inside the heat exchanger. The heat is then extracted by blowing cooler air through the heat exchanger tubes.

The most common type of slow combustion wood stove heat exchanger is installed in the flue. Without a heat exchanger, hot air from the fire is drawn up the flue, and although the flue will become warm, much of the heat within is lost to the atmosphere where the flue terminates. A lot of the heat exiting the flue is reclaimable if a heat exchanger is installed in the flue.

There are other types of wood fire place heat exchangers, including grates and plates that we won’t cover in this article. These fire place type heat exchangers will usually rely on the thermal mass of these devices being heated by the fire. The thermal mass then radiates heat in all directions instead of being lost up the chimney.

Wood stove heat exchangers come in many different shapes and sizes.

How does a flue mounted wood stove heat exchanger work?

Designs of flue mounted wood stove heat exchangers will vary, but the principle of all designs is the same.

The heat exchanger is installed in line with the flue. All of the hot air being drawn up the flue from the fire passes through the heat exchanger. Inside the heat exchanger are either sealed coils or straight pieces of tube. The inside of the coils or tube are hollow and exposed to the room atmosphere.

The outside of the coils or tube are heated as they are exposed to the hot air rising in the flue. As the tubes warm, a fan will blow room air through the tubes, quickly heating that air, before it exits the tubes and circulates into the room. As the coils or tube are air tight sealed, smoke from within the flue cannot transfer to the inside of the coils or tube.

Blowing air through the tubes helps to keep a temperature difference in the tube, therefore increasing radiation and making the heat exchanger more efficient.

This video of a double barrel wood stove heat exchanger gives a good insight as to how flue mounted heat exchangers work.

How does a wood stove heat exchanger save money and keep me warmer?

Consider a wood stove (without a heat exchanger installed) burning wood in a room. Given a certain amount of wood and air the fire will heat the wood stove to a certain temperature and radiate a certain amount of heat into the room. Hot air generated from the fire is drawn up the flue and out to the atmosphere.

When a heat exchanger is installed in the flue that (previously wasted) hot air warms the heat exchanger which is used to deliver more heat into the room.

This situation provides an opportunity to either, produce more heat from the same size fire, create a smaller fire to produce the same amount of heat, or some combination of more heat and smaller fire.
As the price of firewood continues to escalate the payback time on a heat exchanger by saving the amount of wood burnt is relatively quick.

FAQ

Why install a wood stove heat exchanger?

A large proportion of the heat generated by a slow combustion wood stove is lost up the flue. This is effectively wasted energy for which fuel is being burnt. Reclaiming the lost energy with a heat exchanger means that less wood can be burnt for the same heat output. A heat exchanger provides more heat output from the fuel being burnt.

Are there other types of heat exchangers for wood stoves?

Yes, some wood stoves have an inbuilt heat exchanger built into the fire box. This is usually in the pathway for the airflow out to the flue. These work similar to flue mounted in that they heat tubes, but in some designs these tubes have small holes, so are not fully sealed.

The small holes allow any unburnt smoke into the tubes. The difference is that because these tubes are so close to the fire they get very hot. When oxygen rich cold air is introduced, there is further combustion inside the tube. Any smoke entering the tubes is re-burnt. These are a very clean design and produce little no no smoke if well maintained.

You can check out some great ideas for double barrel wood stove designs here.

Should i build my own water heat exchanger?

Heating water with a wood stove is a great idea to save energy costs. You’re already running a wood stove. So, you may as well use some of that energy to heat the water as well, right. Of course its a great idea, but not for the DIY’er.

When water is heated it expands and if released into atmosphere can take the form of very hot steam. A vessel that is not suitably engineered and strong enough can become very dangerous (essentially a steam bomb) when put under the pressure of heated water. Professionally engineered water heater designs have been tested over time, are made from stainless steel, and are pressure tested.

We do not recommend making your own water heater if you value your safety.