Fire & Water: Some Thoughts on Wood Stove Design and Efficiency

Sometimes I have to get away from the climate stuff for awhile. This is one of those times.

Also, each year at this time my wife asks how we can get our swimming pool to warm up quicker this spring. Even after 20 years, global warming hasn’t helped a darn bit.

She also always mentions wood heat as a possibility. I have always discounted the idea as too involved a project.

Well, this year we’re gonna git ‘er done. Last year I built a homemade solar pool heater. This year we are going to add some of that concentrated, carbon-based fuel to our energy portfolio.

After all, we DO have lots of wood available to us behind our house. Mature hardwoods, and the old trees just fall over and rot. I believe one of our white oaks dates to before our country WAS a country.

So, how to make a wood stove that can heat swimming pool water? Over the years, I’ve had enough experience with wood burning fireplaces, free-standing wood stoves, thermodynamics, radiative and convective heat transfer, buoyancy of heated air, etc., that I think I could help come up with a good stove design.

And ‘Uncle Lou’ (my wife’s sister’s husband) up in Sault Sainte Marie, Michigan has a lifetime of building and welding and fixing and fabricating. So, he’s helping me design a stainless steel wood stove with an outer water jacket that I’ll pump pool water through to heat the pool. We will use stainless steel to help keep iron out of the pool water.

Meanwhile, I’ve been reading about the newer EPA-certified stove designs – which is all you can buy anymore — that provide a hotter fire with more complete combustion of wood, rather than losing the gases and smoke out the chimney like the older “smoke dragon” designs do. I had no idea that (dry) wood could be so completely burned that there is little or no smoke at all. Cool!

The modern advance in wood stove technology is, simply put, to create a hotter fire with sufficient oxygen supply to burn all the wood and its byproducts.

To achieve this, the firebox is better insulated, and a pre-heated supply of air is made available in the upper portion of the firebox through perforated stainless steel secondary burn tubes so the wood gases and smoke can burn.

I’m sure many of you have these stoves, which are the only ones sold for inside residential use anymore. The secondary burn tubes produce beautiful, “ghost” flames, helping to ignite the wood gases and smoke that used to just go up the chimney.

So, this got me to thinking about the optimum stove design that would provide maximum efficiency, that is the maximum amount of heat energy from burning the wood transferred into your home (or my swimming pool water).

The goal is pretty simple: burn the wood and its gases as completely as possible and let as little heat escape out the chimney as possible. But even after hundreds of years of experience, people are still debating the best way to accomplish that.

I was thinking about the efficiency of a car engine as an analogy…but it is totally wrong. 100% efficiency for a car engine would be for all of the energy created by burning fuel to go into the mechanical work of pushing the pistons, turning the engine, and creating motion, with zero waste heat.

The wood stove is just the opposite, though. We want to create as much “waste” heat as possible, with as little mechanical energy as possible used to “push” the air through the system.

So, what are the limits to a 100% efficient wood stove?

First, you must recognize that you have to lose SOME heat out the chimney. It is the warm air in the chimney which provides the buoyancy (lift) needed to draw more air into the firebox. But the greater the volume of air flowing out the chimney, and the higher its temperature, the lower the efficiency of the stove for heating purposes.

Next, the higher temperatures required in the firebox for more complete combustion means more insulation, which means a reduction in the rate of heat flow to the room — which is opposite to the whole point of heating with a wood stove in the first place.

Now, I realize a hotter fire which is burning fuel more completely might actually lead to an increase in heat transferred to the room….but, all other things being equal, more insulation MUST, by itself, reduce the rate of heat flow compared to less insulation. Simple thermodynamics.

It’s an interesting dichotomy, trying to increase the efficiency of these stoves. On the one hand you need to MINIMIZE the loss of heat from the firebox in order to attain the high temperatures required for more complete combustion. But you also want to MAXIMIZE the loss of heat by the stove to the room. That’s the whole point of using the stove.

But this really isn’t a dichotomy if you realize that you are only insulating a portion of the stove – the firebox – to achieve the high temperatures and more complete combustion. If you can then route the hot gases leaving the firebox through a different part of the stove before going up the chimney, you then have the opportunity to extract the extra heat you generated from more complete combustion at the higher temperatures created within the (well insulated) firebox.

In other words, the firebox portion of the stove is primarily the energy generation portion of the system, and the rest of the stove that the hot gases pass through is the heat recovery portion of the system.

What is needed is a way to provide the hot gases leaving the firebox a greater opportunity to transfer their heat through the stove to its surroundings. A longer path through the stove, with multiple baffles conducting heat to the outside of the stove, would be one way to accomplish this.

Another would be to have a system of fins inside. Either way, you need to get the hot gas to come in contact with as much stove inner surface as possible, to maximize conduction of the heat to the outside of the stove, before all the heat goes up the chimney.

Now, obviously, you can’t remove so much heat from the exhaust that the air in the chimney is no longer buoyant, because then you will lose the stove’s “sucking” power for the fresh air it needs to burn the wood. An insulated chimney will help keep those gases as warm as possible through the entire path length of the chimney.

The air supply is of particular interest to me. (After all, I am a meteorologist. We know air.) Why doesn’t a bonfire, with an unlimited supply of fresh air, not burn all of the wood gases and smoke completely? It’s because as soon as a flame develops, it gets turbulently mixed with cooler ambient air, reducing the temperature of the mixture below what is necessary to burn the wood gases and smoke.

An analogy is the entrainment of environmental air into a convective cloud, which reduces the clouds ability to produce precipitation…a key component of the atmosphere’s heat engine.

So, in the modern wood stove they put tubes heated by the fire in the firebox to deliver an additional “secondary” source of air – very hot air — to the upper part of the firebox where the hot gases and smoke naturally collect. The pre-heating of the air is necessary for combustion of those gases to occur.

But after thinking and reading about this, I don’t really see the need for a distinction between “primary” and “secondary” air sources for a wood stove. All that is needed is a sufficient supply of pre-heated air to the whole fire. The secondary burn technology seems to me to be a retrofit to fix a problem that could just as easily have been fixed by reworking the primary air supply.

So, Uncle Lou and I have been discussing a way to preheat ALL of the air that enters the firebox, one that includes as its first ‘stop’ the window in the door, since a steady stream of hot fresh air is also needed to keep the window clean.

Of course, this is all in the design phase right now. Unfortunately, as Bert once told Ernie on Sesame Street regarding building a lemonade stand, “It’s easy to have ideas. It’s not so easy to make them work.”

So, if you don’t hear a progress report in a month or two, you’ll know the project was a failure. At least I don’t have to worry about burning the swimming pool down.

So, now the REAL stove experts out there can chime in and tell me where I’m wrong in my newbie analysis of wood stoves. It’s OK…I’m used to it.