It's easy to generate thermal energy during the summer. There's less demand for hot baths, clothes drying and space heating so you can just use a solar thermal water heating system. Come November, it's more difficult – especially in most of North America (like the Northeastern parts of the US and Canada).

From November to March, most homeowners with off-grid systems use propane to generate their thermal energy. On-grid systems use more fuel during the winter, too, but if you are using a wood heating system, do consider wood stoves with the heat recovery concept.

The idea is to store up some energy for a cold day to save you from purchasing fossil fuels to supplement your burning. Those experienced with wood stoves will testify that the smallest unit generates lots of energy to the area around it. The challenge is to capture and store some of that intense thermal energy for redistribution when it's needed.

A wood stove with a heat recovery system is the best and simplest way to capture this energy. There are many such commercial models like the Heartland Oval cookstove. Alternatively, you can use a wood-burning boiler inside or outside. For the more adventurous who want to reduce their carbon footprint to zero, the challenge is to convert an existing wood stove without affecting home insurance policies or violating any local building code regulations.

Regardless of using a commercial boiler or a modified stove, the aim is to store and deliver the harvested energy. 

Externally Mounted Heat Exchanger

With an exposed top, you can see the space where you can add a heat transfer pipe to the stove. Just drill two holes in the stove's back plate to allow the heat exchanger's ½-inch copper pipe to go through. The copper pipe is dry fitted, cut and soldered to the wood stove as a long loop. Its aluminum fin supports should be removed before lighting the stove. Mount the heat exchanger before routing the plumbing supply pipes to the stove. Commercial pipe hangers can be used to route exposed pipes up the basement wall or ceiling and covered using pipe insulation. When the heat exchanger is connected to the plumbing system, it has to be tested for leaks. Then use high temperature black paint on it for better absorption. Such an external unit can add some energy to a home's hot water system. An internal heat exchanger in the firebox would also help.

You can also opt for a flue-mounted heat exchanger , which takes thermal energy from the exit pipe and pushes warmed air into the room.

Internally-Mounted Heat Exchanger

This internal unit has a non-finned, stainless steel tube which resists high temperatures placed straight on the firebox of the wood stove. It can operate at very high pressures which exceed this application's requirement. The high position of the heat exchanger in the firebox ensures that it does not affect filling the stove and leaves a large space around the catalytic converter. (The flue gases cooling around the catalytic converter tend to reduce efficiency while increasing smoke