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Fireplaces Are Terribly Inefficient (but Can Be Made Better!)
This article discusses:
how a fireplace works,
how to improve it’s energy efficiency,
what typically goes wrong with fireplaces, and
the routine maintenance you should do for your fireplace.
How A Fireplace Works
Despite their appearance of being all warm and cozy, fireplaces tend to be very inefficient for home heating. In fact, the energy efficiencies of fireplaces typically range from only 10% to a minus 10%. The reason the efficiency can be below zero is that the fire draws in heated air from inside the home and vents it up the chimney. In other words, most traditional fireplaces remove more heat from a home than they actually produce!
The major components of a fireplace are the firebox, the chimney, the fuel supply (wood) and the air needed for proper combustion. To see a diagram showing the components of your fireplace system (for example the flue liner, ash door, damper, smoke shelf, etc.), see the diagram below.
A running fireplace needs constant airflow and fuel. As the fire runs, the hot combustion air goes up the chimney, which rises because the hotter air is less dense than the cooler air outside of your chimney. It’s important to note that all of the air that goes up your chimney has to come from inside your house. For an average fire, this is about 300 cubic feet per minute of air . . . which is the equivalent of three good size bathroom fans all operating at the same time.
So where does all of this air for your running fireplace come from? Well, it comes from openings in your home, such as your bathroom and kitchen fan vents, and all of the small openings around your doors, windows, and around pipes penetrations of your walls. And this means on a cold winter day, when your fireplace is running in one room of your house, that it is pulling in 300 cubic feet per minute of ice cold air from outside your house into the other rooms where you have vents, etc . . . and since these are in areas away from your fireplace, it means that your central heating system has to work harder to heat up all of this in-coming cold air.
Here are some basic operating tips for your fireplace:
Slightly crack open a window that is closest to your fireplace, so that the fireplace can get the air it needs in the room where it will be heated up by the fireplace (rather than bringing cold air into other areas of your house, which then have to be heated by your central heating system). Also, by cracking a window in the room, it will help reduce the chance of smoke coming back into your house during start-up, when the air in your chimney is cold, and the heated air hasn’t started a strong enough draft to carry the smoke up the chimney.
When starting a fire, use small pieces of kindling wood, rather than a lot of newspaper, as paper can create flaming scraps, which can be carried up and onto your roof (see types, costs, and reviews of kindling wood). Place wood criss-crossed above the light kindling, to allow easy air flow.
Build fires in the back of the firebox, to help keep the fire from falling out of the fireplace.
Make sure the damper is open before starting your fire, and then close the damper when the fire is completely out. Leaving the damper open when the fireplace is not in use allows cold air to come down your chimney and into your house.
Never use gasoline, lighter fluid or a butane torch to start a fire.
If your fireplace is equipped with glass or metal doors, make sure they are closed before you leave a fire unattended.
Firewood can attract termites into your home, which can do thousands of dollars of damage, so you will want to store your firewood at least 20 feet away from the side of your house. And to help keep termites and other pests from infesting your wood, your woodpile should be supported at least 5 inches from the ground by non-wooden supports, such as concrete blocks, metal rails, rocks, etc.
Your woodpile should be kept covered at all times to keep rain and snow from dampening the wood (helpful accessory: wood pile covers). But don’t wrap it tightly, or this will keep the wood’s moisture from evaporating and will delay the seasoning (drying) process.
If you store some of your firewood on a storage rack inside of your home, you should carefully inspect wood thoroughly for bugs before you bring it inside (helpful accessory: firewood racks). And then this indoor area needs to be inspected from time to time for any signs of pest infestations. You should always burn only “seasoned” firewood (see below in the the “Routine Maintenance” section of this article.)
Improving the Efficiency of Your Fireplace
Although beautiful to watch, traditional fireplaces are very inefficient. So here are some suggestions for how you can make your fireplace more energy efficient:
Crack open a window nearest your fireplace
As mentioned above, a roaring fire consumes a lot of air from your home which needs to be replaced with air from the outside. So rather than allow the air to come in from rooms far away from your fire (air that needs to get heated by your home heating system), if you slightly open a window as close as possible to your fireplace, this air can be heated by the fireplace itself.
Turn on central or room fans to circulate heat
One way to make better use of the heat from your fireplace is to use room fans or even the fan from your central heating system in “manual” mode to circulate the heat from your fireplace into other rooms of your house.
Use hard wood for fuel
Harder woods (such as oak, hickory, ash, madrone, eucalyptus, walnut and hard maple) are more dense, and therefore they provide more “fuel” for your fire in the same space, versus using softer wood. Soft woods (such as pine or spruce) generally don't burn as well or provide as much heat as harder woods.
Add a damper (if your fireplace does not have one)
The damper is a metal plate in your chimney that regulates airflow through the chimney. Some dampers fit snuggly when they are new, but can warp from the heat of your fireplace within a year or two, producing a loose fit and allowing air to leak past them. If your fireplace does not have a damper, one should be to install one so that you can close off the chimney when the fireplace isn't being used. Non-flammable open and closed signs that hang from the damper handle make it easy to remember if a damper is open or closed.
Use an inflatable flue plug
An inflatable chimney flue plug can stop warm air from going up the chimney when the fireplace is not in use (see types, costs, and reviews of inflatable flue plugs). It expands like a balloon to seal much tighter than your damper, and therefore you have less heat loss when your fireplace is not in use. They can cost about $50-70.
Install a top-sealing chimney damper
Chimney cap dampers close the entire top of the chimney, so they reduce heat loss when the fireplace is not being used, but don’t improve efficiency otherwise (see types, costs, and reviews of chimney cap dampers). They can cost around $400-500.
Bring outside air directly into the firebox
A great way to improve the energy efficiency of your fireplace is to duct outside air directly into the firebox. This is even more effective when combined with the installation of glass doors, so that then the fireplace does not use room air for combustion at all (helpful accessory: fireplace back plates). With direct ducting of outside air, the vents on your glass doors can be sealed completely.
Install a fireplace insert
To make your fireplace truly efficient, you can install an insert approved by the U.S. Environmental Protection Agency (EPA). A fireplace insert is basically a woodstove that fits into a masonry fireplace (see types, costs, and reviews of fireplace inserts). Over the past 15 years or so, fireplace inserts have become even more energy efficient, and can typically achieve 60-70% efficiency. To ensure safety, a fireplace insert needs to also have a stainless steel liner installed in the chimney. A fireplace insert can also include a blower to push the heat further out into the room.
Install a wood stove
Another option is to install a wood stove in front of fireplace (assuming that your hearth size is sufficient) and then running the stovepipe up the existing chimney. Although not as attractive as an open fire, a wood stove at 75-85% efficiency, is significantly more energy efficient than a typical open fireplace (10% to minus 10%)!
Install air circulation ducts around the firebox
If you are building a new fireplace, install ducts around it so that cool room air is drawn in, circulated around the firebox, and ducted back into the room. The ducts are completely self-contained, and the air never mixes with the chimney smoke. If you want to have the warmed air pushed out further into the room, you can install a fan in the duct system.
Install a fireback
A fireback is simply a heavy sheet of metal (traditionally cast iron) behind the fire. In addition to protecting masonry in the back of the fireplace, a fireback reflects heat into the room (instead of all the heat going up the chimney).
Install tube heat exchanger
If you have an existing fireplace and are not able to build a duct system around it, you can install a small-scale duct system in the firebox. This heat exchanger consists of hollow tubes bent into a shape that fits around the fire. Cool air from the floor enters the bottom of each tube, is heated by the fire, and convected out the top of the tube and into the room. Tube heat exchangers can increase fireplace efficiency by about 5-10%, however, they will need to be cleaned of soot from time to time to maintain there efficiency.
Install glass doors
Installing glass doors on your fireplace help reduce the amount of heated room air that will be escaping up the chimney. The fire still draws air through vents below the doors, but only enough to burn the wood. Glass doors improve the overall efficiency of a fireplace and still enable you to see the fire, but they also reduce the direct radiant heat you feel from the fire, especially as they get dirty.
What Can Go Wrong With a Fireplace
There are five major risks from improper operation of your fireplace:
Carbon Monoxide Poisoning
According to the U.S. Consumer Product Safety Commission, fireplaces and chimneys cause more than 25,000 house fires every year, resulting in at least 10 deaths annually.
To reduce the risks of a chimney fire, you should have a professional clean ("sweep") and inspect your chimney each year. And to reduce the risks of room fires, you should use a fire screen in front of your fireplace, and not place any combustible materials anywhere near your fireplace. And to reduce the risk of roof fires, you should install a “spark arrester” on the top of your chimney. You should also install carbon monoxide and smoke detectors in your home, and of course, be sure that they are operating properly (see types, costs, and reviews of smoke and carbon monoxide alarms).
Also, it is important to burn only the appropriate materials in your fireplace. The following are examples of what NOT to burn in your fireplace:
Painted or varnished wood, trim or other wood by-products
Pressure-treated lumber (due to the treatment compounds)
Driftwood (salt from salt water driftwood is corrosive)
Plywood or compressed or composite materials (due to glue compounds used in the layering process)
Household trash (which could produce a variety of toxic emissions)
Styrofoam containers (disposable cups, plates or food packaging)
Glossy or colored papers (magazine pages, product packaging)
Any plastics or wrapping products
Gypsum board (sheetrock, etc.)
And to protect from insect infestation, you should properly store and protect your firewood.
Routine Fireplace Maintenance
Routine maintenance for your fireplace involves tasks for four areas: annual chimney cleaning and inspection; wood maintenance; damper; ash dump (see types, costs, and reviews of fireplace tools).
Annual chimney cleaning and inspection
Each year, you should have your chimney cleaned and inspected by a trained professional. Below are the items that you should be sure they include on their checklist:
The chimney lining should be checked for creosote buildup, cracks and voids. If more than one-fourth inch of creosote has accumulated, the chimney should be cleaned.
The smoke chamber should be smooth and free of cracks, voids and creosote.
The damper should open smoothly and close tightly.
The chimney cap and screen should be inspected to be sure that the cap is in good shape, and the screen still prevents animals from entering the chimney.
The exterior chimney should be checked for cracks and deterioration.
The crown or chase top should be water tight.
The flashing and counter flashing should be examined.
The hearth should be checked for any deterioration.
It's important to ensure the wood you burn in your fireplace is properly seasoned. “Seasoning” is the process where moisture in the wood is given the opportunity to dry out, so that the wood burns more effectively. The moisture content of the wood determines how hot your fire will burn, and also the amount of creosote which accumulates in the chimney. Some woods can have up to 50% of its weight in water when freshly cut. If wood is burned when it is still too "wet," the fire will be smoky and slow-burning, if it burns at all. Water content of seasoned wood should be under 20%. Wood needs at least six months (and many experts suggest at least a year) of drying to reach the target 20% moisture level that is recommended for a good fire.
So how can you tell if your wood is properly seasoned? Seasoned wood has a darker exterior and has more cracks in the end grain. Wood that hasn’t been properly seasoned will have the bark tightly attached and will look lighter on the ends and edges. When you knock two well-seasoned logs together, you will hear a hollow sound, not a dull thud.
Seasoned wood typically costs 30-35% more than green or freshly cut firewood, since the seller must store the wood on their premises for a long period of time before they can sell it.
And as mentioned above, you should check from time to time to be sure that your woodpile is not in contact with the ground, that it is properly covered from rain and snow, and that it is not infested with termites.
After the end of the heating season, you should check your damper to be sure it is tightly closed, and as discussed above, you should consider using an inflatable stopper to tightly seal your chimney.
Many traditional fireplaces will have a small “trap door” at the bottom of their fireboxes, which can be used to push cold ashes into. Below the firebox (typically in the basement) there will be a access door where the dumped ashes can be cleaned out from time to time.
So if you have a fireplace (or are considering buying a house which has a fireplace), hopefully this article has helped you to better understand: how a fireplace works (and why they are so inefficient); some options for improving fireplace energy efficiency; what can typically go wrong with fireplaces; and the routine maintenance you should do for your fireplace.
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