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Direct-Vent Fireplace Anatomy

Direct-vent fireplaces burn natural gas or propane, not wood. But direct-vent models are different from conventional gas-fired fireplaces, which require a through-the-roof Class B flue to carry away hot combustion gases.

Manufactured Fireplace Construction

Direct-vent fireplaces burn efficiently, extracting most of their heat from combustion gases and sealing the combustion off from interior rooms.

A direct-vent model has a glass door that is securely sealed to prevent leaks of combustion products into the room. Air to feed the flame enters from outside, and the relatively cool combustion gases are exhausted through the wall-mounted vent (if you prefer, you can also vent them through a roof).[GARD align=”left”]

Most of these units have passageways that direct room air around the firebox/heat exchanger and then return it to the room. Some come with fans to boost this circulation; with others, fans are an option.

These fireplaces garner efficiency ratings that approach those of gas-fired furnaces—in the 78 percent range. (This means they convert 78 percent of the fuel’s potential heat to usable heat.)

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Masonry Fireplace & Chimney Anatomy

Typical Masonry Fireplace Construction

Classic masonry fireplaces are built in place by masons, using brick, stone, and other masonry materials. Building such a fireplace is expensive—typically costing $5,000 or more—and time consuming.

Real masonry fireplaces require a heavy-duty concrete foundation to bear and distribute the significant weight of the fireplace and chimney. For more about how they are built, see How Fireplaces Work. They’re almost always built where a home is under new construction (a new home or major addition). Traditionally, these fireplaces burn wood but, because of the pollutants they create, many masonry fireplaces are now fitted with gas-burning log sets.[GARD align=”left”]

It’s important to note that many fireplaces that appear to be masonry are not actually made entirely from brick, stone, or other masonry materials. Instead, they have a facade of masonry materials but their interiors and chimney flues are high-efficiency appliances manufactured from steel, as discussed below. Bottom line is that this is just a much easier, more affordable way to build a fireplace that is far more efficient than its masonry counterpart.

Construction of a Masonry Fireplace

The construction of a typical classic fireplace, from top to bottom, is shown here. The chimney cap keeps objects from falling into the chimney and, with a spark arrester, keeps sparks from escaping.

The chimney flue, made from insulated metal or terracotta, safely carries combustion gases up the chimney.

The smoke dome and wind shelf are designed to work together to funnel smoke and gases out the chimney.

The damper, a door made of cast iron or steel, regulates the draft up the chimney.

The firebox contains the fire and sends smoke upward. It also maximizes heat radiation into the room.

The ash pit is a fireproof storage area for ash. It’s accessed through a cleanout, a small metal door outside.

The foundation supports the weight of the fireplace and chimney, distributing it evenly to the ground.

What is a Chimney Cricket?

Where a masonry chimney penetrates the roof, you’ll find a chimney cricket. You may ask what—or perhaps who—is a Chimney Cricket?”[GARD align=”right”]

Although the word “cricket” immediately conjures up visions of a large insect, on a house it refers to a peaked-shaped structure on the roof—a small, false roof behind a projection such as a chimney. The term probably originated with the games of cricket or croquet, which were first played back in the time of Henry VIII. A roof cricket is shaped like the arch—or peaked-shaped wicket—used in both of these games.

Also called a saddle, a cricket is designed to shed water away from the chimney and other roof projections; it’s usually capped with a large piece of sheet-metal flashing. Sometimes a large flashing that collects water where two roof slopes meet is also called a cricket.

 

Masonry Fireplace & Chimney Anatomy

Typical Masonry Fireplace Construction

Classic masonry fireplaces are built in place by masons, using brick, stone, and other masonry materials. Building such a fireplace is expensive—typically costing $5,000 or more—and time consuming.

Real masonry fireplaces require a heavy-duty concrete foundation to bear and distribute the significant weight of the fireplace and chimney. For more about how they are built, see How Fireplaces Work. They’re almost always built where a home is under new construction (a new home or major addition). Traditionally, these fireplaces burn wood but, because of the pollutants they create, many masonry fireplaces are now fitted with gas-burning log sets.[GARD align=”left”]

It’s important to note that many fireplaces that appear to be masonry are not actually made entirely from brick, stone, or other masonry materials. Instead, they have a facade of masonry materials but their interiors and chimney flues are high-efficiency appliances manufactured from steel, as discussed below. Bottom line is that this is just a much easier, more affordable way to build a fireplace that is far more efficient than its masonry counterpart.

Construction of a Masonry Fireplace

The construction of a typical classic fireplace, from top to bottom, is shown here. The chimney cap keeps objects from falling into the chimney and, with a spark arrester, keeps sparks from escaping.

The chimney flue, made from insulated metal or terracotta, safely carries combustion gases up the chimney.

The smoke dome and wind shelf are designed to work together to funnel smoke and gases out the chimney.

The damper, a door made of cast iron or steel, regulates the draft up the chimney.

The firebox contains the fire and sends smoke upward. It also maximizes heat radiation into the room.

The ash pit is a fireproof storage area for ash. It’s accessed through a cleanout, a small metal door outside.

The foundation supports the weight of the fireplace and chimney, distributing it evenly to the ground.

What is a Chimney Cricket?

Where a masonry chimney penetrates the roof, you’ll find a chimney cricket. You may ask what—or perhaps who—is a Chimney Cricket?”[GARD align=”right”]

Although the word “cricket” immediately conjures up visions of a large insect, on a house it refers to a peaked-shaped structure on the roof—a small, false roof behind a projection such as a chimney. The term probably originated with the games of cricket or croquet, which were first played back in the time of Henry VIII. A roof cricket is shaped like the arch—or peaked-shaped wicket—used in both of these games.

Also called a saddle, a cricket is designed to shed water away from the chimney and other roof projections; it’s usually capped with a large piece of sheet-metal flashing. Sometimes a large flashing that collects water where two roof slopes meet is also called a cricket.

 

Types of Direct Vent Fireplaces

Direct-vent gas fireplaces are made in many different designs.

Gas fireplaces come in all shapes and sizes. This three-sided “cyclone” accents the end of a bar. Photo: Heat-N-Glo

Standard one-sided models are the norm, but you’ll also find two-sided, three-sided (peninsula), and four-sided (island) styles, as well as bay-window shapes and corner units. Heat-N-Glo even makes a three-sided Pier Bar unit that can go at the end of a bar.

Sizes vary. You’ll find units from about 30 to 48 inches wide and about 24 to 30 inches high. They are typically quite shallow—from 13 to 18 inches deep.

Nearly all manufacturers make both top- and rear-venting models. Where you need to vent a unit upward because there isn’t a straight shot out a wall, top-venting types work well.

Depending upon the model, vents may run vertically or horizontally for quite a distance—up to 25 feet or more.

Direct-Vent Fireplace Heat Capacity

Gas-fired fireplaces, like other gas appliances, are measured by their “Btu”-per-hour input or output capacity, depending upon the manufacturer. A Btu (British thermal unit) is equal to the amount of heat required to raise one pound of water 1 degree Fahrenheit.[GARD align=”left”]

Input capacity refers to the amount of gas burned without taking into consideration the percentage of heat lost through the flue. Output capacity refers to the total usable heat generated. Most manufacturers use the input capacity figure because it’s always larger than the output.

If you’re concerned about how much heat a fireplace will generate, the output figure is the one that’s important. If you want to know how efficiently it will use its fuel, figure the Steady State Efficiency rating by dividing the output rate by the input rate.

Input ratings vary, and some units have a range, depending upon the flame’s setting. If supplemental heat is important, choose a model with plenty of output. The Mendota DXV fireplace, for example, has a thermostatically controlled, variable two-level burner that can deliver from 25,000 to 40,000 Btus per hour with 75 percent efficiency.

Direct-Vent Fireplace Options

Controls and options for direct-vent fireplaces vary from one manufacturer to the next.

Some models have a standing safety pilot light that burns continuously to light the main burner whenever the gas is turned on.

Others have electric spark ignition to save energy or a piezoelectric ignition, often chosen when there isn’t an electrical hookup at the fireplace. Some of these types use a millivolt generator so the fireplace can operate even if the power goes out.

But if the pilot isn’t lit, the main gas valve won’t open. In fact, Blaze King’s Split Second(tm) Safety System shuts off the gas within one second if the pilot light goes out.[GARD align=”left”]

You can use a wall switch, a thermostat, or a remote control to operate many types of direct-vent systems. Some remote controls have adjustable thermostats, blower-speed controls, and flame-height adjustments.

Other options for direct-vent fireplaces include variable-speed fans to boost circulation, special trim kits, propane gas conversion kits, decorative screens, firebrick-style fireboxes, and more.

Vent-Free Fireplaces

In some parts of the country, codes allow the use of vent-free fireplaces. Vent-free or “no-vent” fireplaces, made by several manufacturers, have an oxygen-depletion sensor to shut off the gas if the oxygen level ever drops below a preset level. In addition, their burners produce only very low levels of carbon monoxide.

Because all their heat is recirculated into the room, these have very high efficiency ratings. On the downside, they have a much smaller and less realistic fire than other fireplaces. Also, product directions usually require you to leave a window slightly open during use.

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Gas Burning Manufactured Fireplaces

Gas fireplace presents a very realistic fire and efficient heating. Photo: Napolean Builders

[/media-credit] Gas fireplace presents a very realistic fire and efficient heating.

Unlike traditional mortar-and-brick masonry fireplaces with foundations and chimneys, direct-vent gas-burning units take advantage of zero-clearance technology.

They are prefabricated from metal and designed to be installed in wood-frame construction without a foundation or—and this is the big difference between these and any other fireplace—a chimney.

They burn so efficiently, they can be vented directly out a wall. This means they’re much easier and less expensive to install than masonry units and you have much greater flexibility in their placement.

Because they are gas-fired, these units address the serious problem of wood-smoke pollution that grew to crisis proportions in some regions about a decade ago. Unlike wood-burners, these gas fireplaces give off virtually no particulates.[GARD align=”right”]

Like all gas-burning fireplaces, they’re much easier to use than wood-burning units because you don’t have to buy, haul, chop, start, and continually feed wood to the fire. And you don’t have to scoop out the ashes or sweep the chimney. With gas, you just turn on the fire with a knob, a switch or even a remote control. Or you can let a thermostat do the job. When in use, they only cost pennies per hour to operate.

Of course, all gas-burning fireplaces have artificial logs, so the big question is: Does the fire look like a wood fire? The only way to set your mind to rest on this issue is to visit a couple of dealers and check out their offerings. You’re likely to discover that, although early gas fireplaces had phony-looking fires, new burners and artificial logs are surprisingly realistic. Many produce a very authentic fire with tall, dancing flames and have logs and embers that glow. And, to heighten the effect, you can even buy incense that imitates the aroma of a wood fire.

fireplace insert for warmth and efficiency

[/media-credit] Gas-fired fireplace insert offers efficiency and ease of use.

When shopping for a new gas fireplace, be sure it is lab-certified by an organization that is compliant with your local codes, such as the American Gas Association (AGA). Also be sure that the fireplace is installed according to the manufacturer’s directions. If you intend to put it in a bedroom, a mobile home, or at a relatively high elevation, be sure it’s approved for that usage.

Prices for direct-vent fireplaces, of course, vary widely, depending upon size, style, finish, and other options. You’ll find they start at about $450—a small price to pay for the value, comfort, and enjoyment a new fireplace will bring your family.

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How Fireplaces Work

Radiant heat from a fuel-burning fireplace warms people an objects with radiant heat.

Here you can see the main parts of a fireplace’s facade; on the following pages, you’ll find the internal workings.

In the case of combustion fireplaces, smoke and combustion gasses are vented away from interior living areas through a flue or chimney, and oxygen-filled combustion air is drawn into the burning chamber. To pull smoke and gasses away by natural convection, a chimney is typically tall—from fireplace to a couple of feet above the roof (though some high-efficiency fireplaces don’t require full-on chimneys for venting).

Unless a fireplace has glass doors and a vent that draws combustion air from outdoors, it can extract more warmth from a home than it delivers. Those same convection currents that carry smoke up the chimney can also pull expensively-heated interior air from the room, sending it out through the chimney.

A fireplace’s hearth and facade may be made of brick, rock, concrete, marble, granite, tile, or other related, non-combustible materials. Codes and common sense restrict how close to the opening combustible materials—such as wood paneling, wood flooring, or wallboard—may be located.[GARD align=”right”]

The rest of the fireplace may be constructed in a variety of ways, depending upon the type. On the following pages,  you can see the typical anatomy of a masonry and zero-clearance manufactured fireplace.

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