Homemade Wood Stove Heat Exchangers

I use homemade wood stove heat exchangers to rob heat off of my large wood stove in the living room, and provide heat to various applications in my home. These homemade heat exchangers are crude but effective.

Since they are inside the wood stove, their appearance isn't an issue.



The stove I used for this project is basically a large homemade iron box of about 12 cubic feet.

It's a well crafted stove by an individual who knew what they were doing when it comes to welding and metal fabrication, but it wasn't designed for optimal efficiency.

The size of the stove provided the opportunity to install wood stove heat exchangers, and the need for more efficiency fueled my interest in doing so.

Let's look at how, with the help of a good friend, I created and installed wood stove heat exchangers to help me get more heat from this large iron box that is my living room wood stove.

What's a Heat Exchanger?

To begin with, let's define a heat exchanger so it's clear what we're talking about. A heat exchanger is simply a device that allows heat to transfer from one medium to another, without the two mixing.

Think of a radiator on your car. That's a heat exchanger. It transfers engine heat to outside air without mixing it with the coolant inside your engine.

The fins and tubes on window air conditioners are a type of heat exchanger as well. They remove the heat of compression without allowing the air to mix with the refrigerant

Heat exchangers work best when both mediums, the one doing the heating and the one being heated, are in motion. This motion allows for greater exchange of energy between the hot and cold sides. It's also best when there is lots of surface area involved. Greater surface area allows for more efficient heat transfer.

Lastly, the exchange of energy is promoted by use of materials that are good conductors of heat, like metal and liquid. Copper and water come to mind as good conductors.

So, this is why you'll often find water pumps, fans, small copper tubes and fine aluminum fins on heat exchangers and coolers - they all increase the efficiency of heat transfer.

Fabrication and Installation

Having never built wood stove heat exchangers before, it was up to me to select material that would be appropriate for use with a wood stove. Even though copper, brass and thin aluminum fins are excellent for heat transfer, they aren't appropriate materials for an environment of flames, smoke, cinders and ash.

I selected black iron piping. It's not the easiest to work with, but it's tough as nails, can stand up to high heat, doesn't care if it gets dirty or banged around, and can be welded or brazed if necessary.

By making my wood stove heat exchangers from black iron pipe, I could also use standard plumbing fittings, and everything I needed was available as scrap or stocked at the hardware and home improvement stores. No special parts or materials were needed.



The first heat exchanger was fashioned into a serpentine pattern so it would meander back and forth to maximize surface area for the volume of space that it required. I also "stacked" it by making two flat meandering sections and then joining them together. My intention was to place it just under the exhaust of the stove where heat and flames were sure to find it.

Making the heat exchanger was easy, but installing it was a bit trickier. After placing it in position, holes were drilled in either side of the stove, near the top, so black iron supply piping could be fed inside and tightened into the fittings.

Since the opening for the supply pipes were at the top of the stove, they were welded in place to prevent exhaust gases from escaping.

A upside-down candy cane shaped piece of strap iron was also used to support the heat exchanger. It was hooked onto the lower piping and welded onto the inside opening of the flue.

The first of my wood stove heat exchangers was created and installed. It was the first step to success.

After some experience, wood stove heat exchangers of similar design were installed inside the stove - one on the bottom as a grate, and one on each side to absorb heat from alongside of the fire. In addition, a tight fitting heat exchanger was placed on top of the stove, near the exhaust to capture heat from this large flat surface where flames frequented.

Internal wood stove heat exchangers were designed in an open configuration to allow for circulation of hot gases inside the stove. The external unit was designed in a tight configuration to fit the limited space on the stove top, and to maximize the amount of water in the unit.

To increase effectiveness of the external wood stove heat exchangers, plaster of Paris was used to make full contact with the stove top and completely surround the iron piping. This effectively created pipes inside of a "hot rock" to maximize transfer of energy from the surface of the stove to the surface of the piping by using the plaster as a conductor.

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