The Benefits of Closed Combustion Wood Burning Fireplaces

slowcombustion

One of the best ways to heat your home is using a closed combustion wood burning fireplace.

  • Provided they are properly installed, closed combustion fireplaces  are very efficient.  A very large proportion of the heat generated is useful.  So they will warm up the room very nicely, even relatively small units.
  • They burn wood, which is a relatively cheap fuel.  But more importantly, wood can be regrown and is thus renewable, unlike other fossil fuels such as coal and gas, which once burned, are gone for ever.  So provided you use wood from responsible sources, these fireplaces are a more ecologically sound way of heating your home as well.
  • Closed combustion fireplaces control the combustion process by limiting the oxygen that reaches the fire.  This makes the fire burn more slowly, and completely.  This slower burning results in more heat being transferred to the room, instead of just rushing away up the chimney, which is one of the ways in which these units improve efficiency.  The slower more complete combustion also results in much lower emissions – virtually no smoke is given off, and very little ash remains afterwards.  The increased combustion efficiency results in far less CO2 being released as well.  The slower combustion also reduces the rate at which you burn fuel, so the costs are reduced.
  • The closed unit is much safer,  because the fire in enclosed inside a steel, or cast iron container, with a special glass door.  With the door closed, no flames or sparks, smoke, or gases, can escape into the room.  Its actually safe to leave the fire unattended.
  • They don’t use electricity.

How Closed Combustion Improves Upon the Traditional Fireplace

The main problem with a traditional open fireplace is that the vast majority of the heat produced just rushes away up the chimney.

Another problem is that you cant really control the combustion process, because the fire is open and lots of air can flow into the fire and away up the chimney.  This results in the fire burning very quickly.   The combustion process tends to be so fast that its not complete, and thus not that hot.  A lot of smoke and soot is produced (smoke and soot are essentially  partially burned fuel particles).  So the traditional fireplace tends to produce a lot more emissions in the form of smoke, soot and left over ash due to the incomplete combustion, with a lot of the energy in the fuel being wasted or unused, while the heat that is produced tends to flow away from the room up the chimney.

A closed combustion fireplace has two main technological advantages:

  1. The fire burns in a closed container.  The amount of air that can enter the container is controllable (usually by means of adjustable holes in the fireplace).  In this way the combustion is slowed right down (which is why these units are sometimes  called “slow combustion” fireplaces), so there is time for the fuel to be burned more completely.  This also results in the fire burning hotter, while using fuel at a much slower rate (one or two logs per hour).
  2. The closed combustion fireplace is normally a stand alone container that can be positioned inside the room, instead of tucked away inside a chimney space.  By burning the fire in a steel or cast iron container inside the room, the heat produced can be more efficiently transferred to the room through convection from air flowing around the fireplace and then circulating throughout the room, instead of the hot air being contained mostly inside the chimney alcove, or even worse, flowing away up the chimney.

 Other Technological Advances Used in Closed Combustion Fireplaces

Most closed combustion fireplaces have a nice glass window in the door which allows you to see the beautiful flames.  In order to stop those flames from smoking up the glass, most units use an “air wash” system in which some of the air flowing into the chamber is directed across the glass so the flames are blown away from the glass.

Another way in which efficiency is improved is “secondary combustion”.  If you look at the flames produced by a closed combustion fire, you will often notice that some of the flames are in the top of the chamber, completely away from any wood.  Those flames are actually burning gas, which is released by the hot wood.  Secondary combustion is enabled by the high temperatures in the chamber and the way air flows around the chamber so that it can reach those hot gases and thus enable combustion of the gas before it flows away up the chimney.  This is achieved in different ways.  Some units have separate air channels that direct the air to different parts of the chamber.  Other units rely on a swirling air flow which can thus reach the different parts of the chamber as required.

Installation

The key installation issue is making sure that the flue “draws” properly.  What this means is that the products of combustion should naturally flow cleanly away up the flue.  If this doesn’t happen correctly, then air will not be drawn into the fire as designed, and thus the fire will not burn properly, resulting in excessive smoking or smouldering.

The exit of the flue should also stand sufficiently proud of the top of the building so that the products of combustion flow cleanly into the atmosphere and do not swirl down around the house.

In order to get a good “draw”, the flue should go straight up as much as possible, avoiding excessive corners or horizontal sections which might cause problems.  One should also realise that the flue can get very hot, so it will need to have suitable insulation applied where it pierces the ceiling etc.

It is also preferable to position the fireplace somewhere where air can circulate around it nicely, to heat up the room efficiently.  For example, it should be positioned with a suitable gap behind the fireplace so air can circulate effectively.  It should obviously not be positioned too close to flammable items such as thin curtains or near any appliances or surfaces that might be damaged by the heat.

Lastly, one should also consider including a suitable place to store the logs close to the fire for convenience, when using the fire.

Suitable Wood

Firewood needs to be dry.  If the wood has any significant moisture content, most of the energy released by combustion will be consumed boiling off the water in the wood – so your fire will burn, but you wont feel much heat.  Plus the combustion will be inefficient with lots of soot and other emissions.

Good firewood has normally been left to dry for a year or so.  Logs that have dried out properly normally split open along their length, which is one sign that they have been properly seasoned.  If the logs are packed in plastic bags, there should be no condensation inside the bags.

Please ensure that you do not burn slow growing indigenous hard wood.  It is very difficult to replace on a sustainable basis.

Buying wood from unknown sellers on the side of the road is probably a bad idea because you don’t know whether it has been properly dried, nor whether it was grown and harvested on a sustainable  basis.

Operation

The video below offers some useful tips on how to operate a wood burning fireplace effectively.

Wood Burning Fireplaces

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