Stovetop Heater

One of my ongoing projects is building our own travel trailer from scratch. We call it the Gypsy Wagon because it is shaped kind of like a traditional one. It is 16 feet long and 6 feet wide on the inside which is not a lot of room. I wanted to have heat in it but there really isn’t room for a space heater or a wood stove. The typical option for RV’s is a furnace. This could be installed underneath the bed platform and the heat could be delivered to the cabin by a duct and a fan. If you read my previous post about why I don’t like forced air furnaces you will understand why I decided not to do this on the gypsy wagon.

Instead I made an air-to-air heat exchanger unit that sits on top of one of the burners on the cooking stove. They say to not use your stove as a heater and I agree somewhat. It is an open flame and puts its exhaust gases right into your living space. This is not much of a problem if you have a large well-ventilated space but this is a small tight space so it needs a vent. If you have a vent you need some way to keep all the heat from going up the chimney. The solution is a heat exchanger.

 

A heat exchanger takes the heat of the exhaust gases and transfers some of it to fresh air that is circulated. Metal conducts heat well so I made the heat exchanger out of steel. The heat exchange elements are 1” diameter steel tubes.  They are welded into a section of 6" diameter steel tubing that sits on top of one of the burners on a gas or propane stove. To transfer heat efficiently you need to expose as much surface area as you can to the hot gases so there are seven tubes in my little device. The more tubes the better. Just be sure to provide space around each one so the exhaust gases can get all the way to the chimney vent.

You should either make short legs for the bottom or cut scallops out of the 6" tubing so that combustion air can easily get the the flame on the stove burner.

looking down vent at heat exchange tubes

I bought a small humidifier unit at a thrift store and took off the fan part of it to circulate fresh air through the heat exchange tubes. This fan only uses maybe 15 or 20 watts so that’s not much of a power load. You could use a computer cooling fan, too. They don’t use much power. I mounted the fan so it would blow air through the heat exchange tubes which are heated by the gas flame from the stove.

For the vent, I bought some stock components that allow me to connect a flexible 3” diameter aluminum duct to the top of the unit. Dryer vent will work as will any non-combustible flexible duct. I can stick it out of a window or make a separate, closeable, vent hole in the roof of the trailer. The vent tube needs to have at least 2 feet of vertical rise to vent properly. The gases from the stove burner will not get out if the vent goes down. The will simply escape from under the heat exchanger unit and into your living space,

It works quite well. It doesn’t take much heat to warm up the small space in the gypsy wagon. It is only 96 sq feet of floor space. When you don’t need heat you can take the unit off of the stove and store it away. Just remember that this thing will get hot and you will want to wait for it to cool down before moving it.

The Science of Efficient Heating with Wood

Did you know that burning wood in some fireplaces can actually make your house colder? All wood-heating appliances are not alike. Manufactured wood stoves built before 1988, when the EPA began controlling emissions from wood stoves, can be quite inefficient. New and clever ideas for wood heaters may have serious downsides. If you understand the chemistry and physics of wood burning, draft, and radiant heat you will be able to make better decisions when choosing a wood heating appliance. Here is some of the science behind creating an efficient wood heater.

It takes three components to produce fire: combustible material (like wood); oxygen; and sufficient heat to ignite it. Our atmosphere is about 15% oxygen. That doesn’t seem like much but it is enough for us to breathe and to sustain fire. Wood can ignite at temperatures lower than 500 degrees Fahrenheit, if dry enough and exposed for long enough, but the easiest way to provide the ignition requirement is to apply a flame via a match or lighter.

percentages vary with type of wood and variable moisture

Wood is mostly made of cellulose, hemi-cellulose (a collection of complex sugars related to cellulose), lignin (a complex polymer of aromatic alcohols), and a small percentage of extractives (fatty acids, resins, and terpenes). All these complex hydrocarbons are combustible. Wood also contains noncombustible components in the form of water and mineral compounds. The amount of water in the wood affects how well it burns. If your firewood has 20% to 25% water in it, then it should burn just right. A moisture content higher than that results in poor burning. Lower moisture can cause the wood to burn too fast. The minerals in the wood consist of calcium and potassium oxides and carbonates, and trace amounts of other elements. These do not burn and make up most of the ash you shovel out of your wood stove or fireplace.

Different species of trees produce wood with different compositions and, therefore, different burning characteristics. For example, oak contains more heat per pound than pine. Here is a link to a chart listing the characteristics of some common wood varieties.

Less than half of the potential heat available from wood comes from the wood, itself, burning. Most of the heat potential is in combustible gases that the wood gives off while burning. These gases consist mostly of carbon monoxide (CO), Hydrogen, (H2), methane (CH4), and various aromatic hydrocarbons. The combination of these combustible gases is called wood gas or producer gas. Wood gas can be used to run internal combustion engines but that is another story. The problem with inefficient wood burning appliances like open fireplaces and badly designed wood stoves is that the combustible gases go up and out of the chimney before being burned, thereby wasting much of the potential heat available.

The key to burning wood gases before they escape is summarized by the three T’s – time, temperature, and turbulence. The longer you can keep the gases in the firebox the better chance of it burning. Increasing turbulence mixes the air with the combustible gases within the firebox so they can ignite before going up the chimney. These gases need to be exposed to high enough heat in order to ignite. Keeping them inside the firebox longer and burning a hot fire helps.

Some stove manufacturers use a catalytic combustor to lower the burn temperature of the gases. This involves putting a honeycomb-like ceramic element coated with a catalyst just below the flue connection. The gases go through this device and catch fire. I don’t like this method. The catalytic combustor is not something you can make. They are expensive. They wear out. It makes the stove harder to clean. It’s better to just keep the gases in the firebox longer and burn hot fires.

Two ways to keep the gases in the fire box longer are to provide a longer path for the smoke to get to the chimney and to control the flow of air into the firebox. The simplest way to make the smoke path longer is by putting a thick steel shelf between the fire and the flue opening positioned so that the smoke has to go around it to get to the chimney. 

Turbulence is created by providing multiple entry points for air to get into the stove. Primary air is provided via openings low in the firebox, like at the front and back. Secondary air is provided via openings higher in the firebox. The secondary air puts air directly into the upper part of the fire box where the unburned gases have just risen from the burning wood. The air mixes with the gases, creating turbulence that keeps the gases in the firebox a little longer giving them an opportunity to burn before heading up the chimney. A common way to add secondary air to a wood stove is by putting two or three perforated steel tubes across the top of the firebox. The ends of these tubes are connected to air from outside the stove. Air gets sucked into the firebox through these tubes. The video below shows the gases burning just below one of the secondary air tubes in our stove and the great turbulence that is burning the gases.

 

Since burning wood requires oxygen, sufficient air must be supplied to the firebox. This is called “draft.” The simplest way is to create a natural draft with a vertical chimney. You can also create an artificial draft using a powered fan. A principle of physics is that hot air rises. With natural draft the hot flue gases naturally rises up and out. The rising air in the chimney draws fresh air into the firebox via the air inlets.. To get a good draft you should have a minimum of 6 feet of total rise in your chimney but more will be better. Stoves generally draft well when the rise is more like 10 to 12 feet. There is a lot more to efficient chimney design and that will be the subject of a future article.

Combustion air flow can be controlled using “dampers” at the air inlet or in the chimney. The best way is to control the air at the inlet. If you do that, you don’t need to put a damper in the chimney. It’s easier to clean the chimney without one. Using the dampers you can slow the flow of air which will keep the gases in the chimney longer.

It is a good idea to draw the air used for combustion from outside your building envelope. If you don’t provide for an exterior air source for your stove all the air that feeds the fire has to come from inside your house which will suck cold air from every possible leak in your building envelope – under doors, around windows, through recessed lighting cans in the ceiling, and even through electrical outlets. Open fireplaces with large chimney diameters can draw so much air from your house that the cold air replacing it can make your house colder even when the fire is going.

Wood stoves heat your home by radiant heat transfer. That is to say that the fire makes the stove hot and that heat radiates into the house. You get more even heat with increased mass. The fire heats the mass of the stove and the mass radiates the heat into your house. With sufficient mass, you can still be getting heat for hours after the fire has gone out.

Modern wood stoves are made out of either cast iron or 3/16” to ¼” thick steel plate lined with fire brick. The thick steel provides mass and is a good conductor of heat. The firebrick is a dense, fired ceramic that also holds heat. Firebrick is the standard way of doing it but there are other possibilities. Masonry, stone, rammed earth, cob, etc. can all be used as mass to absorb and re-radiate heat into your home. The important thing is that the material is dense and non-combustible.

our pile of split walnut drying in the sun

Wood is a dirty fuel, as compared to propane or natural gas. The non-combustible portion of the wood (minerals, dirt, etc.), as well as unburned hydrocarbons, produces ash that will fill your firebox and coat the inside of your chimney over time. It should be easy to clean every part of the stove and chimney. This is very important. Complex chimney paths, obstructions, or tight bends within the stove will make it very difficult to keep clean. A dirty stove will provide less heat and, once the systems starts to clog, will get worse quickly.

Burning wood for heat has been done for millennia. A lot has been learned about efficiency since those first campfires.  Now that you know the basics you can get the most heat out of the wood you burn. To learn more about burning wood the EPA has a great PDF online with more details than I have presented here.

 The next article about wood burning will cover how to install a wood stove and chimney safely.

Seven Reasons Why I Don’t Like Forced-air Furnaces

Most houses in the U.S. have forced-air furnaces. The only advantage to a furnace is it can deliver even heat to every room in your house. They have way more disadvantages than advantages. Electric furnaces are even worse than gas ones because of the cost of using electricity to produce heat.  If you can build or adapt a house to use single-point interior heat sources like free-standing wood or gas heaters you will be much better off. Here is why I don’t like forced air furnaces:

Furnace in unheated basement with uninsulated ducts

1.       Inefficient. Although gas furnaces have a high efficiency rating, as high as 90%, this refers to the efficiency of combustion plus the efficiency of the heat exchangers inside the unit. The delivered efficiency, which is the amount of heat delivered into your house through the duct system is much lower. How much lower depends on how well the duct system is constructed. The heat is delivered to the rooms via ducts in the attic, crawlspaces and inside walls. These ducts leak heat, air, and are usually outside the insulated areas of your house. If you already have a house with a forced air furnace, check the ducts. Where are they? Are all the joints completely sealed with tape? Are the ducts wrapped with insulation? If not, you are losing a lot of heat. 

      Another source of inefficiency is how a furnace cycles on and off. Before the blower turns on, the gas burner runs until the heat exchangers get hot. Once the blower is on, the ducts need to get heated up before maximum heat is delivered to your rooms. Then the thermostat turns off, the whole system cools down and the cycle starts all over again. This results in wasted heat as the system heats itself up over and over again.

dirty and clean air duct

2.       Unhealthy air. Old duct systems, particularly leaky ones, eventually get dirty. Mold grows in them, dust accumulates, rodents and insects get in your ducts through larger leaks and poop and pee, make nests, and die and rot in there. Whenever your furnace fan is on, all that junk gets blown into your house. Yuck! Some chronic allergies and health issues are causes solely by having dirty ducts in your home. If you have an older forced air system you should have your air ducts cleaned. There are services that do this. Furnaces have filters that help to mitigate this problem but you have to remember to change them often and check that they are installed correctly. A dirty filter can seriously block the air flow causing even more of the hot air to leak out.

3.       No power, no heat. The furnace delivers heat to your house via an electric blower, which needs power to work. If your power is out, not only will your furnace not light (electric ignition circuits and relays turn your furnace on and off), it can’t deliver any heat to your house. None at all.

4.       Blowing air is less comfortable. When air blows against a moist surface like your skin, it cools your skin. Even the warm air from your furnace will do this. You feel warmth but this is in spite of the heat it delivers. You will feel much warmer if the same amount if heat is delivered via direct radiation (like from a wood stove) or radiant floor heating rather than from blowing hot air.

5.       Furnaces are noisy. Your thermostat turns your furnace on and off many times during cold days and nights. Every time it turns on, the fan motor and the flow of air makes noise. This cycling on and off is not only noisy, it is intermittent noise which is harder to get used to.

6.       Heating unused rooms. Unless you close the heat registers in rooms you don’t spend much time in, you will be heating areas of your house that don’t need to be heated much. It is a waste of energy. Since the registers are either up by the ceiling or on the floor, it is not easy or convenient to close and open them all the time so people mostly don't do it.

7.       Expensive. Buying and installing a furnace with all the ducting and air vents costs a lot of money. Installing a gas stove or wood stove costs a lot less. Unless you put a stove in every room of your house, the furnace will probably be more expensive.

Instead of buying a furnace, design your house with a fairly open floor plan and get one or more single point interior heating appliances like free-standing gas stoves and/or wood stoves. The gas stoves are only slightly less efficient than a furnace in the combustion and heat exchanger departments but all that heat gets into your house because the whole thing is inside your house. No heat loss or dirty air blowing through ducts or leaks because – no ducts!

They can run without external electricity. They have thermostats that run on the tiny voltage created by a thermopile which uses the heat of a pilot light to generate the electricity. You can get them with fans to circulate air, but they will run just fine without a fan. If you want to circulate air, get a ceiling fan or two. On low, a ceiling fan uses very little power. It isn’t necessary, though, and you will still have heat if there is a power outage.

Gas stoves are a lot quieter than furnaces. The only sound you hear is the flame burning. The radiant heat coming from a gas stove is more comfortable and you can always go stand right next to the stove to get toasty warm.

You will need either natural gas or propane to run a gas heater and there are costs for that. There is also the possibility that the gas grid goes down or propane becomes unavailable for a time due to a disaster. If you are worried about that, get a wood stove, at least as a backup heater.

Wood stoves have many of the advantages of a gas stove and none of the problems that furnaces have. They are more work to use, do not have thermostats, and need to be attended and cleaned regularly but you can still have heat in some part of your house if you are unfortunate enough to lose both electricity and gas supplies.

If you already have a house with a forced air furnace get a wood or gas stove as a backup heater in case of a power outage. You may find you like the backup heater so much that you will turn off your furnace for good.