FAQClearances

What are clearances?

When you install any kind of fuel-burning appliance you are going to be faced with understanding what is meant by the word “clearance” and adhering to the rules, or “codes”, that concern your particular type of installation. When discussing hearth appliances (stoves, fireplaces, chimney, etc.) we talk of many different clearances and in almost every case the definition is the same. For our purpose of safely installing hearth appliances and accessories we define “clearances” as follows:

The distance between the surface that will get hot and the surface of any combustible material.

Technically, any distance between two items can be called clearance. But we are mostly interested in the application of clearances to maintain a safe surface temperature for the combustible materials. So, we usually use the term “clearance” to refer to the space between a hot item and a combustible item. We may also occasionally use the term when referring to other distances, such as the distance to the edge of a hearth or the distance above a roof, but usually “clearances” are referring to the distance to a hot surface of a stove or pipe, or maybe a masonry chimney or fireplace.

What are the most common clearances that must be maintained when installing a hearth appliance?

Since most of the time we are concerned with the distance to combustible surfaces from a hot surface, the most common clearances are closest to the burner and exhaust system. These include:

Stove clearance (from all surfaces of the stove)

Connector pipe clearance (from the surface of any exposed pipe)

Chimney pipe clearance (from the surface of a metal factory-built chimney system)

Masonry Chimney clearance (from the surface of masonry constructed chimneys)

Associated with each of these common clearances are more detailed clearances that may, or may not, come up in any particular installation. Some of these are as follows:

Stove clearances:

Floor clearance (hearth protection), wall clearance (wall protection), ceiling clearance, mantel clearance, furniture clearance, alcove clearance.

Connector clearance:

Wall clearance (wall protection), ceiling clearance, mantel clearance, alcove clearance, building structure clearance.

Chimney pipe clearance:

Building structure clearance, adjacent building clearance, roof clearance, ceiling clearance, wall clearance.

Masonry Chimney clearance:

Building structure clearance, adjacent building clearance, roof clearance, ceiling clearance, wall clearance.

How are safe clearances determined for each category of hearth product and how are these clearances enforced?

Safe clearances are determined by testing in a laboratory and the application of accepted standards for consumer products. In the case of hearth products most standards are administered through either the UL or ANSI standards organizations. Testing is carried out by certified laboratories to determine that each product meets the accepted standards. The manufacturers of these products publish the tested clearances and certify that their products are “listed” by the standards authority. The building codes adopt these standards and safe clearances are enforced through the building code authorities.

The “standards” for clearances are created by scientific application of known principles of thermal conductivity and thermal resistance. The hot surface is brought near the combustible surface and measurements are taken under varying conditions. These measurements help determine how the hot surfaces of an appliance or chimney will effect the combustible material. A clearance appropriate to maintaining a safe temperature on the combustible surface is adopted and then “listed” by the standards organization. All manufacturers must meet the same standards, so, if they don't get the clearances they wanted the first time the product is tested, they must work to make changes to the product so that the desired clearances can be achieved.

The building codes adopt the manufacturers instructions, as backed up by laboratory testing and listing, as the “code” for the installation of that particular hearth product. It is up to the installer and the building code officials to follow the manufacturer's instruction for installation so that the product can be considered within the safety standards. The standards organization requires the manufacturer to publish an instruction manual for installation to help ensure that the installation will be according to the safety standard. All appropriate clearances must be outlined in the instruction manual.

What about fireplaces and chimneys that are built on-site and are not subject to laboratory tests and manufacturer's instructions? How are clearances determined for these types of products?

In the case of site-built masonry fireplaces and chimneys there may not be any “manufacturer's instructions” to enforce. (There are such instructions for most pre-fab masonry structures or “engineered site-built products” that are cast and assembled on site but designed and tested elsewhere.) In this case it is up to the constructor and the building code official to follow the clearances as outlined in the general building code. The approach to setting this building code standard is similar to that taken for manufactured and listed products. Certain known scientific principles are used to establish rules to prevent overheating of nearby combustible materials.

In cases where unlisted (not tested) appliances or products are to be installed the building codes allow for the application of clearances to “unlisted” products. Most unlisted products are similar to some tested product (such as a metal stove surface or a kiln, etc.) and the NFPA has determined safe clearances for these types of on-site products, again using known scientific principles about thermal transfer and resistance. Where a product does not compare favorably to some other known material it will be up to the constructor to prove to the building code officials that the design and installation of the product can be enforced under the rules for “unlisted” products.

What is a typical clearance to the stove surface?

Actually, there is no “typical” clearance for UL Listed hearth products. The clearance required from any particular stove may be significantly different from other stove models even within the same category. However, we can generally say that there a three types of stoves and they have a general association with a certain range of clearances when compared with the other types. These three general types of stoves are wood stoves (cord wood or coal), pellet stoves (wood pellets, bio-mass), and gas stoves (natural gas or propane. (There are also oil stoves, or electric stoves, etc. but these types of stoves are less common and not “typical”).

In general, wood and coal stoves have a higher clearance than either pellet or gas stoves. This is because most wood stoves are designed to heat the entire mass of the appliance and radiate more heat in all directions. Pellet stoves are usually quite cool in the rear but very hot near the top and front, where the heat exchanger is located. Gas stoves also are quite cool in the rear and bottom, and usually do not require any wall or floor protection, but they can be very hot on the front near the glass. A wood stove may need to be between 12” and 18” away (sometimes more) from a combustible rear wall but a gas or pellet stove may be able to be placed as close as 3” or 4”. A wood stove may also need a certain ceiling height above it or a special thermal protection underneath it. Most pellet and gas stoves do not require these thermal protections.

Can I reduce clearances safely by adding non-combustible materials to the wall or ceiling?

Yes, this can be an acceptable way to reduce the clearances needed between either a stove or a connector pipe and a combustible surface. NFPA 211 outlines various ways to protect combustible surfaces and reduce clearances. However, these clearance reductions are based on standard tests against unlisted products. Bear in mind that these clearance reductions have limits and under no circumstances can you use the NFPA 211 techniques to over-ride the manufacturer's instructions.

In general, if you use the proper reduction techniques you can reduce clearances by between 33 and 66 percent, within the limit established in the NFPA 211 code. NFPA 211 limits clearance reductions to 6” for connector pipe and 12” for a stove. You may place a stove or connector closer than these minimums only if allowed by the manufacturer of the stove. Some stoves have been tested to under 6” to an unprotected wall so in that case the NFPA 211 gives you no advantage.

The manufacturer may give you clearances to both unprotected and protected surfaces. If the owner's manual specifies clearances to protected surface you cannot use NFPA 211 techniques to reduce the clearances further. NFPA 211 can be used only to reduce clearances tested to unprotected surfaces (by adding the properly constructed protection).

What clearances are required for chimneys?

There are many different kinds of chimney and they all have their own unique requirements depending on what type of appliance they are venting. In the hearth industry we usually deal with residential type chimneys and these types of chimneys come in about five or six general types. The most common types of chimney we see in residential use are site-built masonry chimneys and factory-built metal chimneys.

Site-built masonry chimneys can be of two basic types, either designed on-site using commonly available masonry materials (such as brick, stone, mortar) or engineered and designed off-site and partially assembled and shipped to the site for assembly. The clearances for each type of fireplace will be determined either by the standard building code or the designer/engineer of the masonry structure and outlined in an installation document. Most masonry structures, when built according to the accepted standards for construction of masonry materials, require either a one or two inch clearance to all combustible materials of the building structure. Note that some off-site engineered masonry products may require either more or less clearance, depending on the design and the materials used.

The majority of masonry chimneys are built on-site and are not engineered or design tested in a laboratory. For these chimneys, the clearance for safety is directly affected by the methods of construction. For this reason standards for proper construction techniques have been added to the building codes and are enforced by the local code authority. In general, a certain thickness of masonry materials is required between the masonry surface and the combustible surface. When the minimum amount of masonry is used there must also be additional air space clearance (2” for interior chimneys and 1” for exterior). In some cases thermal insulation materials can also be used to protect combustible materials.

Factory-built chimney systems come in about four varieties for residential hearth appliances. These types are generally referred to as “Class A” (UL103 or Type HT), Type PV (pellet vent), Type B (B-vent), or Direct Vent (for gas or pellet appliances). We usually do not refer to the vent types (PV, B-vent, or Direct Vent) as “chimneys” in the traditional sense. Of these types only the “Class A”, is classified as “chimneys” in the same way as the masonry chimneys mentioned previously. However, for the purposes of this discussion on clearances we will treat them all as metal factory-built chimneys. (These vents may also be classified as “Connectors”and are discussed in that chapter in NFPA 211).

Unlike site-built masonry chimneys, factory-built metal chimney systems are tested and listed to be used with specific types of appliances. There are several standards that are tested for depending on the intended use of the chimney. For instance, a “Class A” chimney system may be tested to the UL103 standard for use with solid fuel appliances but may not be tested to the more strict UL103HT standard. Only chimneys tested and listed as UL103HT should be used for modern wood stoves. But some wood fireplaces can use chimneys tested only to the UL103 standard.

In general, the clearance required to a “Class A” chimney is two inches to any combustible. The chimney manufacturer may also require the use of specially designed shields or flashing in addition to the 2” air clearance. While most chimneys are approved for this 2” clearance, in all cases the manufacturer determines the clearances required and no “general” clearance should be accepted as safe. The manufacturer's instructions must be followed precisely.

One often overlooked clearance for a chimney serving a wood or coal stove is the clearance from the end of the chimney to the ceiling or an adjacent wall. The chimney manufacturer may not specifically address these clearances, or they may assume an “unlisted” clearance. The chimney itself may be as close as 2” in most cases but the clearance to the connector pipe may be significantly higher depending on the stove being installed. NFPA 211 states that a ceiling clearance for a connector pipe cannot be less than 9” and a wall clearance cannot be less than 6”. It stands to reason then that the end of a chimney cannot be placed closer than these minimums for the connector. Usually, the placement should take into consideration the stove being installed and the listed clearances for that stove and connector.

Clearances for most pellet and gas vent systems is one inch to combustible walls and sometimes three inches to ceilings or enclosures. The same rule applies with these types of vents – the manufacturer's instructions must be followed. The NFPA 211 can be consulted when the manufacturer's manual does not specify a particular clearance.

Most factory-built chimney systems can be used with any model of stove or fireplace that is listed to be used with that particular type of chimney. However, many fireplace systems are tested and listed to be used only with the chimney systems manufactured by the same manufacturer as the fireplace. These systems are usually tested together as a unit. All the clearances for the chimney and the fireplace are tested and listed as a system and must be followed. All listing will be voided on these types of appliances if non-matching chimney systems are used. If a manufacturer allows the use of any chimney system it will be stated in the manual and then you can use the clearances as listed by the chimney manufacturer for the chimney portion of the installation, when it does not conflict with the instruction manual for the fireplace.

What can be done if I have an existing chimney that is not code-compliant with respect to the required clearances but is otherwise structurally sound? Can I still safely install a stove on this chimney?

Maybe. It will depend on two things. Either the clearance defect needs to be corrected or the type of stove being installed needs to be of the type that does not require a thermally insulated chimney. Most masonry chimneys are not easy to access and correcting the clearance defects is difficult and costly. Some metal factory-built chimneys can easily be re-installed or the clearance defects can otherwise be easily repaired. If adding air space, or thermal insulation materials, to the exterior of the chimney structure is not practical, or desired, there may be a solution using UL listed re-lining materials.

Due to the large number of chimneys that exhibit defects in code-compliance, either through lack of clearance or poor construction techniques, the chimney sweep and repair industry developed re-lining materials that can be used to upgrade a defective chimney and ensure that it can be safely used for it's original purpose. Most of the time the chimney is structurally sound. Only the codes that concern thermal transfer to combustible materials have been broken. The solution in these cases is a thermally insulated liner system that allows the chimney to meet the thermal properties as originally intended by the building codes. The clearances in the code are intended to prevent too much thermal transfer to nearby combustible framing members. A properly insulated liner system can accomplish this and there are many UL tested systems now available and in everyday use.

If you are installing a wood stove on this existing chimney with clearance defects then it is usually essential that you install a UL listed liner system. The codes for clearance were developed with knowledge of what happens when a chimney fire takes place. Chimney fires are the most common cause for extreme thermal transfer and can lead to a house fire. It is essential that this thermal transfer be controlled to prevent catastrophic house fires.

In some cases the installation of an insulated liner is extremely difficult or, due to the size constraints of the existing masonry flue, nearly impossible. Each chimney will have to be assessed and a determination made about the practicality of a re-line with an insulated liner system. Sometimes an uninsulated liner system can be used safely to vent a wood stove. Only a competent chimney professional can determine if the chimney structure can be used without insulation. In general, an uninsulated liner can only be used if there is enough masonry mass to absorb the potential heat and mute the thermal transfer. Some chimneys that lack the required air clearance may provide sufficient masonry mass between the liner and flue so that the heat from a chimney fire can be dissipated through the masonry mass itself. This can be difficult to determine and the best solution to clearance violations is a UL listed insulated liner system.

If a re-line for a wood stove will not fit, or becomes too costly, you may decide to install a stove that burns either pellets or gas. As a general rule, pellet and gas stoves do not require the same thermal insulation for their chimneys as a wood or coal stove requires. In most cases you can simply re-line the existing masonry chimney with the appropriate type and size of uninsulated liner pipe and then hookup the new pellet and gas stove to your chimney. An insulated liner is usually not required. Sometimes it may be desirable to insulate the liner to help the draft performance of the chimney but the insulation is not usually required to correct clearance violations.

Can I reline a “Class A” metal factory-built chimney? Will such a re-line correct any clearance defects?

Yes, you can re-line a metal factory-built chimney. This is usually allowed and can improve the safety and performance of the chimney system. There are situations that require special parts and materials in order to preserve the function of the existing chimney system. Special adapters and UL tests have been developed to allow relining these metal chimneys but each chimney type is unique and should be inspected by a qualified chimney professional prior to re-lining.

Most “fireplace” chimneys are not approved for use with a modern wood stove. Also, there is no testing done with hooking up a free-standing stove to a factory-built fireplace and chimney system (“zero-clearance” wood fireplaces), as there is for similar installations with masonry chimneys. Such installations may be considered safe with a properly installed liner system but there is no specific code. You will need the approval of the local building code official and dose of common sense. Inserts, on the other hand, can be installed to zero-clearance fireplace systems with the proper approval from the stove manufacturer. The zero-clearance fireplace must also allow the use of inserts.

A distinction should made between stand-alone metal “Class A” chimneys and chimneys used with fireplace systems, which may also be considered “Class A” due to their being tested to the UL103 standard. As mentioned, fireplace chimneys are usually not approved for use with wood stoves but they may be relined if approved by the local building code inspector. Stand-alone chimneys are usually approved for use with a wood stove but the chimney may not have been tested to the “HT” standard which is required for all modern wood stoves. A re-line with a UL Listed Type HT liner system will usually allow such a chimney to be safely used with a new wood stove.

All the same clearance rules will apply as with masonry fireplaces and chimneys. Any clearance defects may prevent a safe installation. However, unlike with masonry chimneys there is no UL listing that allows an insulated liner to correct a clearance defect in a factory-built chimney. All clearances must be established for the existing chimney in accordance with the instructions of the chimney manufacturer.