Glossary of Terms
The industry terminology we use, to make sure we’re all talking about the same thing.
A masonry fireplace is what most people think of when they picture a fireplace. Constructed on a solid concrete foundation, they are massive towers of brick, stucco or stone usually seen rising up an exterior wall of a house. Some factory-built fireplaces are constructed to look like masonry fireplaces. The key difference is that a masonry fireplace will have solid, laid brick and mortar side walls and back while the factory-built will have cast cement panels lining a sheet metal box.
A factory built fireplace (can be wood or gas) is constructed of metal, and is designed to go into framing, with its own dedicated metal chimney. Sometimes they are called “zero clearance” fireplaces but this is something of a misnomer, as no fireplace is truly zero clearance.
A masonry thimble is a round, terracotta cylinder extending through the chimney wall into the flue. The hole is usually 6 or 8 inches in diameter and is designed to accept stove pipe.
A “Class A” thimble is also a 6 or 8 inch diameter hole in the wall, but projects several inches into the room, and has black metal trim and a smooth metal interior. It usually has a crimped metal flange for fitting the metal stovepipe. A Class A thimble is used in situations where the wall doesn’t have the required 18” of non-combustible material between the thimble and the wood framing of the house. A Class A thimble is insulated like other Class A chimney components, and therefore requires only 2” clearance to combustibles.
A wood or pellet stove is freestanding, burns solid fuel, and can vent out the top or the back. A top-vented stove is generally installed with either its own Class A chimney or into a masonry/Class A thimble, while a rear-vented stove is often installed in front of an existing open fireplace and connected to a metal flue liner.
A wood or pellet insert functions just like a stove, but is designed to slide into your existing fireplace and connect directly to a liner, with no stovepipe required. It usually has some sort of surround faceplate to block off the remaining fireplace opening.
A high efficiency wood-burning fireplace functions exactly like a stove except that it can go into framing. They are clean-burning, meet EPA standards for emissions, and are efficient as well. These fireplaces can be equipped with gravity or forced air kits to distribute heat to other rooms of the house. These fireplaces are nearly as efficient as wood stoves and can potentially be used as primary heat sources. They require Class A chimney that must vent vertically above the roofline. Code requires a minimum of 3′ above the roof line AND 2′ above any point within 10′. In some cases high efficiency fireplaces can be used in conjunction with masonry fireplaces and flues.
A Direct Vent Appliance is a completely sealed propane or natural gas burning heating or decorative appliance, with a glass front. Combustion air comes from the outside of your house, and the flue gases vent to the exterior of your house through a separate pipe. These differ from freestanding Gas Logs, which sit in your open fireplace without a sealed glass front, and get their combustion air from your home, just as a regular wood fire does. Direct Vent appliances can be divided into:
- Zero-Clearance Fireplace
Built into the framing of your wall, gas fireplaces don’t always have to have a chimney running above the roofline, as a wood fireplace requires. Each gas fireplace has its own venting requirements, but many of them can vent either directly to the rear, or up a foot or two and then out the back wall (provided, of course, that the wall is an exterior one).
Most freestanding gas stoves are designed to look like wood stoves, but some are more contemporary in appearance, with glass or rock firebeds instead of ceramic logs. Gas stoves and fireplaces both use the coaxial venting system, which has a smaller pipe inside a larger one to separate combustion and exhaust air (see diagram above).
- Gas Fireplace Insert
Like a wood insert, a gas insert is specifically designed to slide into your existing masonry or factory-built fireplace. Since the venting has to run inside your existing chimney, gas inserts use a colinear venting system, which separates the air streams with two 3” or 4” flexible metal tubes.
Wooden Sun only sells and installs fully-vented gas logs (an operational chimney is required), but many people have freestanding gas logs in what are called “unvented fireboxes”. These “fireplaces” are no more than metal boxes set into the wall, and have no chimney that could be used for an insert or replacement logs. This doesn’t mean that you can’t get a new gas appliance, merely that it will change the scope of work required.
Designed to keep rain, animals, and other debris out of your flue, there are multiple types of caps available for your flue or chimney (see our blog post on maintaining your chimney crown for more information on caps for terra cotta flues and masonry chimneys). If you have a factory-built appliance with its own chimney, there are caps available for those as well. Most manufacturers will list their accepted venting components in their installation manuals. If you already know the brand of chimney you have, please let us know; otherwise, please ask one of our sales or service technicians how to find this information.
Flue sizing, direct connects explained, and other in-depth technical explanations.
A direct connect is when an insert or a woodstove is installed into a masonry fireplace without a full liner. Instead it will have a short length of stainless liner (often an oval or rectangular shape) extending just to the bottom of the flue (the terra cotta tiles).While building code allows for a direct connect, the NFI does not allow them and Wooden Sun does not install them, for the following reasons:
- The appliance will not draft as well because the flue is so much bigger than the flue collar (see the flue size FAQ on our “more information” page).
- The appliance will take a lot longer to start up and begin drafting properly because heating up a larger flue takes significantly longer than heating up a stainless liner wrapped in an insulation blanket (see our post on drafting issues for more information on the physics of your chimney).
- Because of the drafting issues, the appliance will never be as efficient, meaning that it will take more wood to heat the same amount of space.
- These types of installations can be dangerous and are more prone to chimney fires.
- Direct connects produce a lot more creosote because the flue gases cool down so much more, forming glazed creosote (also called third degree creosote) which can only be removed with a rotary sweep. Third degree creosote catches fire relatively easily, and burns very hot, very fast.
- Unless the direct connect is completely removed (a big job requiring moving the appliance and taking apart the direct connect), the area where the direct connect goes into the flue cannot be reached to clean–a lot of creosote forms in these spaces and if it is not swept properly the creosote buildup is significant. This can catch fire and be the beginning of a major structural fire.
- Fully lined chimneys are much safer if a chimney fire were to occur. With terra cotta tiles only, it is possible for the chimney fire to spread through voids in the mortar or cracks in the flue tiles. A full liner is a chimney system within a chimney system–the fire can be contained by the stainless liner and insulation, with the added back up of the terra cotta and masonry chimney.
- In summary, direct connects are the height of “penny wise and pound foolish.” While they are initially a cheaper installation, in just a few years the full liner will have paid for itself in increased efficiency and decreased cleaning costs, not to mention dramatically improved safety.
In houses built prior to the 1950s, masons often did not use terra cotta flue liners. Chimneys that are brick, stone, or concrete block only, without terra cotta flue tiles, are unsafe for use. Gaps form where mortar has fallen out, and these gaps may allow excessive heat to come into contact with combustibles, or noxious flue gases to enter the home. Chimney fires can easily start house fires, and produce carbon monoxide and other deadly gases. Our blog post on HeatShield Flue Repair goes into more detail on this subject.
When a wood stove is hooked into a thimble, (a round opening where a woodstove connects to a masonry chimney, usually made of terra cotta or metal) running directly into a terra cotta lined chimney, sometimes the flue is too big for the appliance to draft properly. NFPA (the National Fire Protection Association) standards require a flue area no bigger than twice the size of the flue collar on the appliance for an exterior chimney (built on an outside wall of the house, surrounded by the outside air on three sides), or three times the size of the flue collar for an interior chimney (built in the middle of the house, surrounded by the house on all four sides). The area of a six inch flue collar (the most common) is just over 28 square inches. The area of an eight inch flue collar is just over 50 square inches. Terra cotta tiles vary a lot in their thickness, so it is a good idea to measure the interior dimensions to get an accurate read. Typically an 8×8 flue tile has an area of 49 square inches, an 8×13 flue tile has a free area of 84 square inches, and a 12×12 flue tile has an area of 121 square inches.