Fireplaces
By: Amanda Campbell
Updated: August 12, 2010
Nothing adds character and ambiance to a room like a fireplace. Most people think immediately of a wood-burning fireplace, but you do have options that can help you avoid the mess and maintenance of wood-burning fireplaces. We'll point out some basic rules of fireplace design and we'll point out some innovations that will help you create the perfect atmosphere with the least amount of maintenance.
Most people now live in houses with some form of centralized heating, so you don't really have to rely on a fireplace as the sole source of heat in a room. If you do, you're better off with a wood-burning stove - they put out a lot more heat.
Almost everyone these days wants a fireplace for the atmosphere it creates - heat's a secondary consideration. There are lots of factory-built choices available now (as opposed to the old-fashioned type of fireplace built with bricks into the house). Factory-built boxes are designed to fit directly into a "chase" (a hole pre-built in the framing stage).
Wood vs. Gas
Wood FireplaceWood creates the classic, traditional look & feel that everyone's after, but it can be messy & hard to control. Gas fireplaces can closely mimic the look of burning wood, but gas is much more controllable. Heat from a gas fireplace is steady, adjustable & even. If a firebox is designed to only burn gas, never wood, then it's considered a gas appliance.
Airflow
Fireplaces (burning wood or gas) generate hot air, which rises. Unless your fireplace is the only source of heat in a room, the air that rises out of that room from a roaring fireplace is air that you've already paid to heat. On the other hand, lots of mechanical devices in a "tight" house vent air to the outside (fans pumping central heat, dryers, range hoods or downdrafts, bathroom & kitchen vents, etc). These devices always draw air out of the living area. That "negative pressure" can take air that's trying to rise through a flue or chimney and pull it back down into a room, so smoke and hearth odors can enter a room.
Solutions to Airflow Problems
The best way to counter these problems is to separate the airflow feeding and venting the fire from the airflow in the house. Shutting glass doors over a firebox will help, especially at the start & end of fire. That's when the fire's energy is lowest and the pressure of hot air rising is weakest. Now we'll take a look at a few options that go one step further.
Let's start with a comparison of airflow paths in gas appliances:
Direct Vent Boxes burn outside air and vent it back outside.
Vent Free Boxes burn inside air that remains inside after it burns.
Traditional Fireplaces & See-Through Boxes burn inside air and vent it outside.
Direct Vent Boxes
VentThese boxes utilize a sealed combustion chamber with a sealed glass front. All air for combustion comes from outside through a double-walled chimney. The double-walled chimney exists solely to supply air to the combustion chamber. The inner ring vents smoke & heat; the outer ring brings air in for combustion and cools the inner ring. Outside air never flows into the room; it's used for (sealed) combustion only. Indoor air never reaches the combustion chamber to burn. The room heats up primarily because room air circulates around the firebox in a gap behind the burn area and reenters the room. So indoor air is heated because of the combustion of outdoor air, even though those two different streams of air never mix. Additional heat radiates into the room off the glass door.
Vent Free Boxes
These boxes use room air for combustion just like a stove-top burner. This burned air remains in the room. There's absolutely no need to vent it, because the American Hearth vent-free boxes we featured in our segment burn gas cleaner and more efficiently than stove tops. Since there's no venting, there's no need to worry about the direction of airflow. Vent Free Boxes have an ODS (oxygen depletion switch) pilot which automatically shuts off in the event of a problem. For instance, if the oxygen level in a room drops, the ODS shuts off, but this will never happen on these boxes because they always pipe fresh air into the room.
Glass FireplaceSee-Through Fireplaces
These are designed differently than the other gas appliances we've mentioned so far. They're not sealed like the direct vents, but the glass front is fixed so the glass doors do not open. This allows room air in for combustion. A pipe above the see-through fireplace vents the burned air to the outside. Some models of see-through fireplaces have doors that can open. If you have one of those models, keep both doors in the same position - either both opened or both closed - so you don't create a negative draft from one side of the box to the other, or from one room to the other.
Fresh Air Plus System - This system was invented & patented by American Hearth. Both their direct-vent & vent-free boxes feature a six inch duct that brings outside air into the home while a fire is burning. The fresh outdoor air mixes with indoor air in the gap behind the burn area, so both airstreams get heated before they flow into the interior room. This constant introduction of fresh (& heated) air improves indoor air quality. The system also uses the (cooler) outside air to "dilute" the room temperature with a manual damper. By diluting room air with cool outside air, you can lower the temperature of air heated in that gap up to 40%. That means you can let this appliance burn longer, and sit closer to it longer, without heating the room to an uncomfortable level or feeling like your legs are burning up.
Gas Appliance Options
Logsets (made of ceramic or concrete), the color & pattern of gas flames, and the "burning embers" (fiberglass pieces that glow when heated) offer more realism nowadays compared to when gas fireplaces first hit the market.
Consumer Tips for Designing a Traditional Fireplace
When designing & locating a fireplace for a new house, keep in mind a few related design concepts to keep the draft in a chimney headed to the outside. Remember, Cold Air Always Chases Hot Air.
Keep the chimney inside the living area of the house. If a chimney's on an outer wall, the cooler outside air will cool the chimney below room temperature. That will cool the warm room air rising in that chimney, and it could cause that exhaust to spill back into the room.
The top of the chimney should be higher than the highest section of the living area of the house. If hot air inside the house is taller than the cold outside air next to the chimney, then that cold air will want to enter the chimney and chase after the hot air in the living area. That creates a draft in the wrong direction.
Chimneys should penetrate the roof line at the highest point possible. If a chimney is kept warm by being inside the house as much as possible, it will always create a stronger draft in the correct direction than a chimney with a lot of exposure to the cooler outside air.
Taller chimneys produce a stronger draft. Draft is created when cold air chases hot air. A taller chimney holds more hot air. That gives the cold air more hot air to chase. That creates a stronger draft.
Most people now live in houses with some form of centralized heating, so you don't really have to rely on a fireplace as the sole source of heat in a room. If you do, you're better off with a wood-burning stove - they put out a lot more heat.
Almost everyone these days wants a fireplace for the atmosphere it creates - heat's a secondary consideration. There are lots of factory-built choices available now (as opposed to the old-fashioned type of fireplace built with bricks into the house). Factory-built boxes are designed to fit directly into a "chase" (a hole pre-built in the framing stage).
Wood vs. Gas
Wood FireplaceWood creates the classic, traditional look & feel that everyone's after, but it can be messy & hard to control. Gas fireplaces can closely mimic the look of burning wood, but gas is much more controllable. Heat from a gas fireplace is steady, adjustable & even. If a firebox is designed to only burn gas, never wood, then it's considered a gas appliance.
Airflow
Fireplaces (burning wood or gas) generate hot air, which rises. Unless your fireplace is the only source of heat in a room, the air that rises out of that room from a roaring fireplace is air that you've already paid to heat. On the other hand, lots of mechanical devices in a "tight" house vent air to the outside (fans pumping central heat, dryers, range hoods or downdrafts, bathroom & kitchen vents, etc). These devices always draw air out of the living area. That "negative pressure" can take air that's trying to rise through a flue or chimney and pull it back down into a room, so smoke and hearth odors can enter a room.
Solutions to Airflow Problems
The best way to counter these problems is to separate the airflow feeding and venting the fire from the airflow in the house. Shutting glass doors over a firebox will help, especially at the start & end of fire. That's when the fire's energy is lowest and the pressure of hot air rising is weakest. Now we'll take a look at a few options that go one step further.
Let's start with a comparison of airflow paths in gas appliances:
Direct Vent Boxes burn outside air and vent it back outside.
Vent Free Boxes burn inside air that remains inside after it burns.
Traditional Fireplaces & See-Through Boxes burn inside air and vent it outside.
Direct Vent Boxes
VentThese boxes utilize a sealed combustion chamber with a sealed glass front. All air for combustion comes from outside through a double-walled chimney. The double-walled chimney exists solely to supply air to the combustion chamber. The inner ring vents smoke & heat; the outer ring brings air in for combustion and cools the inner ring. Outside air never flows into the room; it's used for (sealed) combustion only. Indoor air never reaches the combustion chamber to burn. The room heats up primarily because room air circulates around the firebox in a gap behind the burn area and reenters the room. So indoor air is heated because of the combustion of outdoor air, even though those two different streams of air never mix. Additional heat radiates into the room off the glass door.
Vent Free Boxes
These boxes use room air for combustion just like a stove-top burner. This burned air remains in the room. There's absolutely no need to vent it, because the American Hearth vent-free boxes we featured in our segment burn gas cleaner and more efficiently than stove tops. Since there's no venting, there's no need to worry about the direction of airflow. Vent Free Boxes have an ODS (oxygen depletion switch) pilot which automatically shuts off in the event of a problem. For instance, if the oxygen level in a room drops, the ODS shuts off, but this will never happen on these boxes because they always pipe fresh air into the room.
Glass FireplaceSee-Through Fireplaces
These are designed differently than the other gas appliances we've mentioned so far. They're not sealed like the direct vents, but the glass front is fixed so the glass doors do not open. This allows room air in for combustion. A pipe above the see-through fireplace vents the burned air to the outside. Some models of see-through fireplaces have doors that can open. If you have one of those models, keep both doors in the same position - either both opened or both closed - so you don't create a negative draft from one side of the box to the other, or from one room to the other.
Fresh Air Plus System - This system was invented & patented by American Hearth. Both their direct-vent & vent-free boxes feature a six inch duct that brings outside air into the home while a fire is burning. The fresh outdoor air mixes with indoor air in the gap behind the burn area, so both airstreams get heated before they flow into the interior room. This constant introduction of fresh (& heated) air improves indoor air quality. The system also uses the (cooler) outside air to "dilute" the room temperature with a manual damper. By diluting room air with cool outside air, you can lower the temperature of air heated in that gap up to 40%. That means you can let this appliance burn longer, and sit closer to it longer, without heating the room to an uncomfortable level or feeling like your legs are burning up.
Gas Appliance Options
Logsets (made of ceramic or concrete), the color & pattern of gas flames, and the "burning embers" (fiberglass pieces that glow when heated) offer more realism nowadays compared to when gas fireplaces first hit the market.
Consumer Tips for Designing a Traditional Fireplace
When designing & locating a fireplace for a new house, keep in mind a few related design concepts to keep the draft in a chimney headed to the outside. Remember, Cold Air Always Chases Hot Air.
Keep the chimney inside the living area of the house. If a chimney's on an outer wall, the cooler outside air will cool the chimney below room temperature. That will cool the warm room air rising in that chimney, and it could cause that exhaust to spill back into the room.
The top of the chimney should be higher than the highest section of the living area of the house. If hot air inside the house is taller than the cold outside air next to the chimney, then that cold air will want to enter the chimney and chase after the hot air in the living area. That creates a draft in the wrong direction.
Chimneys should penetrate the roof line at the highest point possible. If a chimney is kept warm by being inside the house as much as possible, it will always create a stronger draft in the correct direction than a chimney with a lot of exposure to the cooler outside air.
Taller chimneys produce a stronger draft. Draft is created when cold air chases hot air. A taller chimney holds more hot air. That gives the cold air more hot air to chase. That creates a stronger draft.
