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Many people have a love-hate relationship with their fireplace. There are an estimated 35 million fireplaces in the United States, which means nearly 1 billion fireplace fires burn each year. Most of your neighbors have fireplaces. and if your home lacked a fireplace and you installed one, you’d recoup more than 75 percent of your investment at resale.
All of this, despite the fact that your fire place is by far the least efficient piece of equipment in your home; 90 percent of the heat it creates goes—literally—up the chimney. It pulls four to 10 times more air out of a room than what’s required to keep the fire going. Even when the fireplace is not operating, the chimney sucks warm air from your home. Furthermore, the smoke from your fireplace contains methane, carcinogens, carbon monoxide and other toxic gases. When conditions aren’t right, the fireplace backdrafts, sending that sooty smoke into your house, blackening a wall and stinking up the place for days. Plus you have to cut, split, haul and store all that wood.
Still, you wouldn’t give up your fire place for anything. Why this twisted love affair?
___ A: Standard masonry fireplace: Fireplaces
provide “heat highways” for the warm air of your home to escape. When
the fire is burning, cold air drawn in from around windows, doors, vents and outlets is heated and then propelled up and out the chimney. When
not in use, rusted or loose-fitting dampers still allow warm air to escape.
Quick Tip: Close that damper: Keep the flue damper tightly closed when the fireplace isn’t in use. An open damper is an open escape path for warm air—whether it’s generated by your fireplace or furnace.
A brief history of the fireplace
By the 1200s, fireplaces roughly resembling yours were being built. These fireplaces, grossly inefficient, would warm the front sides of those huddled about them—while their back sides and the rest of the room froze.
Despite the fireplace’s crucial role in both cooking and heating, improvements came slowly. In the 1400s, fire- backs—metal sheets that reflected heat back into the room—came into use. In the 1600s, metal doors were added to prevent room air from escaping up the chimney, baffles for circulating heated air into the room were experimented with, and coal came into use. In the early 1700s, someone by the name of Count Rumford designed a fireplace with a sloping back, angled sides and an improved chimney that greatly increased efficiency. Still, in the mid-1700s Ben Franklin bemoaned, In common chimneys the strongest heat from the fire, which is upwards, goes directly up the chimney and is lost—five- sixths of the heat is wasted.”
And truth be told, your 20th-century masonry fire place still burns at about the same efficiency as one built 200 years ago.
Why are fireplaces such losers?
Let’s look at what happens when you fire up your fire place on a winter night that’s a brisk 10 degrees F.
Place crumpled newspaper, kindling and logs on the grate. As you open the damper, heavy, cold outside air pushes its way down the chimney and into the room—creating a reverse draft. You light the newspaper and stand back. One of two things may happen: The fire could create a strong enough updraft and pull smoke up the chimney. But not only is your house built fairly tight, your daughter is cooking in the kitchen with the range hood on, your son just finished a shower and left the bath fan running; and the gas furnace and water heater are sucking up air for combustion. There’s simply not enough air to go around, and the house becomes a vacuum. The fireplace loses this air tug-of-war, and smoke is pulled into the room. You crack a window to let in more air. (If your house was built in the ‘70s or later, building codes require a fresh-air intake for combustion air to the fireplace.)
Eventually a strong draft is created. Your fire begins pulling and exhausting air from the surrounding room at a rate of about 450 cubic feet per minute. The house, still unable to provide enough air, sucks in air from around windows and doors, through dryer vents and electrical outlets and along sill plates. This 10-degree-F outside air mixes with heated room air, is drawn past the fire, and then is exhausted up the chimney.
When the fire is well established, you — if you remember — will close that open window and partially shut the damper to slow the rate of burn. Now is when your fire will burn most efficiently, returning a whopping 20 per cent of the heat it generates back into the room. However, if you place a spark screen in front of the opening, this number will drop to about 12 percent.
As the fire dies down, you turn in for the night. As you sleep, the dying fire generates little heat—but the draft up the chimney creates a virtual freeway for heated house air to escape. The net result? You reached your main objective of spending a comfy hour or two reading in front of the fire. But all in all, the fire drew more heat out of your house than it generated. A working knowledge of why your fireplace performs so poorly offers clues as to how to make it perform better.
Good fire-building habits
You can improve the efficiency of your fireplace by simply improving your habits. You should burn hot, blazing fires because the combustion of wood and gases is more complete and the fire and surrounding bricks radiate more heat more efficiently. You should also get rid of your raised grate and use good old-fashioned andirons. These metal stands allow the burning logs to fall into the hot bed of coals, where they burn more efficiently.
___ B: Glass doors: Doors slow the flow of room
air up the chimney—but also block the fire’s heat from entering the room.
For best results, keep the doors open while the fire is burning hot:
close them in the waning stages of the fire.
Energy-Saving Goof! Caution: Highly foolhardy I came home early one spring afternoon to find it was a bit cool in the house, so I decided to build a fire in the fireplace. I hate to admit it, but sometimes we use a little charcoal lighter fluid to get the fire started. This time, however, the can was empty. I went to the garage to find a substitute and spotted a can of starting fluid that said “highly flammable.” Just what I wanted, or so I thought. I brought it into the house and sprayed some onto the logs. Big mistake. I lit a match, and before I could even get it into the kindling, there was a thunderous explosion. Blue flames shot out of the lower vent, hitting my shins just above my shoes and scorching my socks. Luckily I escaped without injury or any major dam age to my house. I never told anyone, not until now. My advice: Start your kindling only with a little newspaper and a match! |
Dampers and glass fronts
When a fire is burning, adjust your damper to the smallest opening that’s possible without smoke spilling back into the house. and shut it completely when the fireplace isn’t in use. Still, your built-in damper—like most—is notoriously leaky. If the masons who built your fireplace never put a cap over it, years of rain and condensation may have rusted and puffed the damper. and high heat may have slightly distorted it. To fix this, you could install a chimney-top damper—a spring- loaded cap that seals off the top of the chimney to block air leakage when the fireplace isn’t in use. It can be opened to varying degrees when the fireplace is in use.
You could also install tempered glass doors (around $500) with adjustable air intake slots (___ B). They’ll help prevent room air from escaping up the chimney when the fireplace isn’t in use, and slow the mass exodus of room air up the chimney in the waning stages of a fire. But if you leave the doors closed during a fire, they’ll block over half the heat from entering the room.
Wood-burning inserts
You can increase the output of your fireplace up to five-fold with a wood-burning insert (___ C). This metal unit sits inside the fireplace and uses the old chimney as a chase for a new metal liner. Faceplates cover the space between the insert and fireplace opening. The unit’s heat-exchange chamber, which warms and circulates room air, is part of what makes these so efficient. Their other advantage is you can adjust the air intakes to closely control the amount of room air allowed into the fire chamber during combustion and close it off completely when not in use. Some inserts come with glass doors for fire viewing; others have metal doors.
___ C: Wood-burning inserts: These inserts allow
you to control the amount of room air entering the fire chamber. Air
circulated and warmed through the outer heat-exchanger makes efficient
use of the fire’s heat.
If you go this route, you’ll need to go through some serious contortions, snaking the 25-ft., flexible stainless steel liner through the existing chimney and attaching it to the insert. Most damper openings are only about 6 in. wide, so you may need to use several different connectors and elbows.
A less complicated insert you could install is a tubular grate/glass door variety. C-shaped hollow tubes, the ends of each protruding above and below the glass doors, sit in the firebox. Room air enters the bottom of the C, is heated as it flows past the fire, then exits through the top of the C as heated air. Tubular grates alone—not used in conjunction with glass doors or blowers—won’t appreciably increase the efficiency of a fireplace. The warmed air is apparently sucked back into the fireplace and up the chimney.
Gas-fired inserts
If you were building a house today, you’d have the option of installing an enclosed, gas fireplace (___ D). These sealed units are airtight, meaning combustion air is brought in, heat is created, and exhaust gas is exhausted, all within the confines of the firebox. Blowers circulate air up and around the units, expelling warmed air into the room. No room air is used for combustion or escapes up the chimney.
True, you couldn’t burn wood or roast marshmallows in your gas fireplace, but neither would you have to haul in wood, haul out ashes or lose massive amounts of heat. If you installed a direct vent unit—one that vents directly through the wall behind the unit—your gas fireplace could be 75 percent efficient. If you really wanted to go first class, you could operate it by remote control.
___ D: Gas-fired inserts: These inserts use the existing chimney
to draw in fresh air and exhaust spent gases. The combustion chamber
is sealed off from the room, and the outer heat-exchange chamber makes
these units up to 75 percent efficient.
But even with your existing house and masonry fire- p lace, you can make use of this technology. You can slide a top-venting gas fireplace insert into your old fireplace— and still have the brick hearth, mantel and fairly natural- looking flame—and 50 percent efficiency. The fresh-air intake and exhaust pipes would run up through your existing chimney.
You would pay at least $1,500 for the insert and pipes. You’d have to remove the existing damper or permanently clamp it in place for safety. and you’d be wise to hire a licensed plumber to install the gas line to the unit and tie into the existing gas lines—but the rest of the installation you could do yourself.
Making sure
Before you get too excited about any changes, have a professional chimney sweep come out to clean and inspect your fireplace. Some chimneys are so encrusted with creosote deposits, they present a fire danger.
Also, consult your local building inspector. He or she may require the installation of an outside air intake near your wood-burning insert. Or, if you go the gas insert route, you’ll have to abide by the many regulations regarding the distance between the outside vents and windows, the ground and the gas meter.
Next: Understanding ice dams: Escaping heat creates higher energy bills and harmful ice dams