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 and behind a visual screen, their appearance isn't an issue...just their effectiveness.

The stove I used for this project is basically a large homemade iron box of about 12 cubic feet. The photo above shows the big iron box that has become my living room wood stove. It's two feet high, two feet wide and three feet long.

It's a well crafted stove made by an individual who knew what they were doing when it came to welding and metal fabrication, but it wasn't designed for optimal efficiency. Notice the two tiny air intakes on the access door of the stove. These are entirely inadequate for such a large stove.

The size of the stove provided the opportunity to install wood stove heat exchangers inside, and this works well as creosote will build up due to extraction of heat from the hot gases. As you're probably aware, creosote is flammable, but if it catches on fire inside the stove, that's exactly the right place for a fire...inside the firebox.

Let's look at how, with the help of a good friend, I created and installed wood stove heat exchangers to help me extract 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 water-to-air heat exchanger. It transfers engine heat via liquid coolant, 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 outside 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 good thermal batteries, like liquids.

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 (and firewood and ash clean out tools).

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 with ease.

By making my wood stove heat exchangers from black iron pipe, I could also use standard plumbing fittings, high temperature pipe dope, 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. I placed it just under the exhaust of the stove where heat and flames come in contact with it as they make their way up the flue.

The photo below shows the first of the homemade wood stove heat exchangers for my stove. It's positioned directly below the exit to the flue.

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 the stove.

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 toward 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 the large flat external surface.

Photo below shows an example of a side heat exchanger. An identical three loop pattern heat exchanger is located on the opposite side of the stove.

Internal wood stove heat exchangers were designed in an open configuration to allow for circulation of hot gases inside the stove.

Below is a view of the lower heat exchanger that doubles as a grate for the burning wood.

You'll also note two angles tubes in the upper left and right corners of the stove door opening. These are tubes that direct fresh air from the sliding intake vents on the side to the upper portions of the stove to promote complete combustion of hot gases.

The external heat exchanger was designed in a tight configuration to fit the limited space on the stove top, and to maximize the amount of water inside the piping.

Photo below shows the external heat exchanger, plumbed in series with the internal heat exchangers.

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 and encapsulate 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 of heat.

You'll also note in the photo above that several modifications were made to the air intake of the stove. In the lower right corner of the stove is a rotating window air vent and there is an identical one on the other side. See detail to the right.

There is also a sliding air vent on both sides of the stove. Both of the sliding vents allow air to enter in the upper portions of the firebox to help burn the gases inside the stove. The cover for the sliding vents are not shown in the photo.

So, there you have it, homemade wood stove heat exchangers that when combined with a pumping and piping system can carry hot water to whatever application requires hot water. My primary application for these wood stove heat exchangers is heating the house by using a water-to-air heat exchanger installed in the furnace air intake plenum.

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