• Active since 1995, Hearth.com is THE place on the internet for free information and advice about wood stoves, pellet stoves and other energy saving equipment.

    We strive to provide opinions, articles, discussions and history related to Hearth Products and in a more general sense, energy issues.

    We promote the EFFICIENT, RESPONSIBLE, CLEAN and SAFE use of all fuels, whether renewable or fossil.
Not open for further replies.


Hearth.com LLC
Staff member
Oct 18, 2013
Installing a Wood Burning Fireplace Insert - Part #1

Please note: This article is intended as an overview of the installation - it is very important to obtain and use your owners manual for specific details which may apply to your fireplace insert model.

Open fireplaces generally produce very little heat - and also pull air which you already paid to heat up the flue. A wood burning fireplace insert can solve the problem of low efficiency, as well as provide longer burn times and greater safety…..BUT it must be planned and installed correctly.

As a first step, please become familiar with the definitions of a fireplace or hearth stove. A good article for the basics is located here.

Next, determine the type of fireplace which exists in your home. There are these basic types:

Masonry based fireplace - This includes the following sub-groups:

1. 100% masonry site-built fireplaces - these are usually constructed by a mason on-site using a combination of concrete block, brick, firebrick and clay flue tiles. Your masonry fireplace is probably lined with clay flue tile - an orange, red or yellow terra-cotta material. You can see these tiles, which are usually square or rectangular, from the top and/or bottom of the fireplace.

2. Heatform - or Metal fireplace with masonry chimney:


A Heatform is a metal firebox which is built into a full masonry chimney. These were very popular in the period from 1960-1980 as it eliminated the need for a trained mason to build sloping firebox walls.

3. 100% prefabricated masonry - Some MASONRY fireplaces are now built at a factory and hauled out to the job site to be set on a concrete foundation. Many of these also have special masonry chimneys made of pumice (volcanic rock). Brand names include Isokern and Firerock as well as many smaller local companies.

Metal Fireplace - also called Pre-fabs, Zero Clearance, etc.

Don’t let the appearance of the fireplace front fool you - in some cases the only way to identify a prefab metal fireplace is to look up the flue from below. Fireplaces with round metal flues - usually from 8” to 11” ID - are prefab fireplaces.

Installing an Insert into a Prefab Fireplace

If you have determined that your fireplace is prefab, your options are quite limited. Many prefab fireplaces are very small and it is difficult to find a unit which will fit….or is approved and UL listed for such an application. You are left with the following choices:

A. Most economical - buy a fireplace insert that BOTH fits and is approved for use in pre-fab fireplace line chimney to the top with new stainless liner and connect to stove. Note that in very rare cases, some newer prefab fireplaces may have an HT (high temperature, UL 103) chimney and not require a full liner. However, it is the authors recommendation for all existing prefab chimneys to be fully lined.

B. Rip out ENTIRE framing assembly, facing, firebox and chimney and replace with new metal chimney and stove or built-in high efficiency fireplace.

In most cases, you cannot take just any old stove and place it in front of or in such a fireplace and pipe it up. It must be an insert approved for such use. This is because either the chimney in many prefabs is not tested with anything other than the open fireplace and is not as heavy duty. Or, there is wood in the walls a few inches from your fireplace and a stove in front with its connecting pipe will not meet the required clearances.

As a practical matter, installing most inserts into a prefab fireplace will require a unit with a very small firebox.

Installing a new insert into a Masonry Fireplaces - any style (see #1 to #3 above)

Choosing a Model

Before choosing a model, take stock of your existing fireplace. You’ll want to measure a number of dimensions as well as take note of the age, surrounding materials, hearth and other factors which may enter into your purchasing decision. Some of these are as follows:

* The height, width and depth of the opening and at the back of the fireplace

* The width of the damper opening, also known as the throat (with damper open)

* The distance that the hearth (in front) comes out from the front wall of the fireplace

* The distance to the adjacent wood trim and the mantle. Pay attention to the stove manuals recommended clearances

* The age and condition of the entire fireplace and chimney

A professional inspection by a chimney sweep or other fireplace pro is often the best way of answering these questions in a complete manner. If you are uncertain about the fit of the insert or freestanding stove, make a cardboard mockup to assure a good fit.

Once you have dimensions to work with, you can go about the process of choosing an insert - big or small? All the way back in the fireplace, or out on the hearth? Some of these questions relate to style and others to performance. Here are some links to help you make those various decisions:

https://www.hearth.com/talk/calculators/burn-time - Wood stove output and burn time calculator

choosing_a_wood_stove - General advice on choosing a stove and sizing

Whichever insert you choose, be sure to keep in mind that you have to have room to work with the smoke outlet. This means if you are buying a freestanding stove to sit in front of the fireplace, the rear flue outlet must be lower than the fireplace opening. If your unit is top vented and will slip back into the fireplace, it is always nice to have a couple inches of room between the insert or freestanding stove and the fireplace opening. Some inserts can be installed by setting the pipe into position, sliding the unit back in, and then pulling the pipe into the flue collar from inside the stove.


Sizing up the Chimney

Rockford Chimney Supply - We are the best source for Chimney liners and Stainless steel chimney liner kits to install your insert or stove. Everything we sell ships FREE, same day to all lower 48 US States! Buy online and save. We guarantee customer support that will not end until you are satisfied with your installation. Visit our complete site today.

*Note: The chimney must be thoroughly cleaned before installing the liner and insert. Arrange to have this done prior to installation. *

Many modern fireplace inserts and stoves require a FULL relining of the existing fireplace chimney using stainless steel piping. This pipe is available in both flexible and rigid lengths.

Here is how you determine whether your chimney needs to be relined:

Current NFPA codes require that the area (in square inches) of the fireplace flue cannot be more than 3x the area of the appliance (stove) flue of the interior chimney and 2x the area of the exterior chimney. Let’s look at the most common fireplace flue sizes:

12x12 - this flue size is about 11 x 11 inches inside, or a total of 121 square inches

8x12 - this flue size is about 7 x 11 inches inside or a total of 77 square inches

Common insert/stove pipe sizes are 6” and 8” round or 28 sq. inches (6) and 50 sq. inches (8).

A sample calculation would go like this - exterior 12x12 chimney/insert with 6” flue 28/121 - the chimney is well over 2x the sq inches of the flue, so this would require lining to the top with stainless steel.

Example #2 - interior 8x12 chimney/insert with 6” flue 28/77 - the flue less than 1/3 the size of the flue outlet, so NFPA would not require a full lining. Note that it is possible that BOTH owners manuals, local officials and fireplace installers will allow some wiggle room on these numbers. However, there are many other advantages to a full relining of the chimney, so prospective buyers would be wise to consider doing the job even if not 100% required.

Deteriorating chimneys

If the chimney is not lined with clay tile or in poor condition with lots of cracks and other problems, you may have to go further than a simple lining. In these cases, the liner must be insulated with a special wrap. The insulation holds the heat into the liner and prevents it from soaking through the masonry and overheating nearby wood framing members. Another method for repair is a poured-in liner.

There are also products and treatments which can be used to restore and waterproof the exterior masonry and crown (cement top) of your chimney structure if it needs repair.

What liner? Oval or Round - Rigid or Flex?

If you have decided to do a full chimney relining, you must decide exactly what type of stainless materials you will use to do so. If the insert uses a 6” flue, then chances are that you can work with round piping and if necessary, ovalize it slightly to get it though the damper frame. However, if your insert/stove uses 8” pipe, you will have to do some additional planning. 8” must be heavily ovalized in order to fit through the standard 5” wide damper throat.

There are also many choices for liners - various thicknesses and types of flex liner are available, as well as rigid. All things being equal, rigid makes for a better installation because it is thicker and has a smooth inner wall (better smoke flow - less creosote). However, it is difficult to use 100% rigid in most fireplace installations - some installers use rigid for the straight shot, and then switch to flex for the bottom 8-10 feet when they have to go though the smoke chamber and damper.

Planning the Plate

A block off plate which sits in the upper area of the fireplace is suggested for the most efficient insert installation. This provides a positive seal so that room air does not get sucked up the chimney along with the stove exhaust - and also serves to help block cold downdrafts when the stove is not in use. Before installing your unit, you’ll want to plan the block off plate - instructions for the fabrication are here.

Confirming the Hearth or Hearth Extension

Most fireplace inserts need 18” of non-combustible material on the floor (hearth) in front of the door opening. In some cases, this hearth (and the existing hearth) must also meet certain R or K insulation values. These values are specified in the owners manual and are required to avoid overheating of the combustible materials underneath the hearth. Note that many stove manuals will show 16” as the hearth extension (in front), while the newest NFPA codes require 18”. The manuals have not yet caught up to the code - so use the 18” whenever possible. Read some of our other articles on hearth clearances and construction for tips on building or buying an extension.

Installation Day

All of your planning and materials come together for the big day when you or your professional installer actually line the chimney and place the insert. As with many projects, the more planning and measuring you have done beforehand the less likely you are to run into unanticipated problems. Please see Part #2 (coming soon) of this series for some pics and descriptions of the actual fireplace insert installation.
Last edited:
  • Like
Reactions: Mel W
Not open for further replies.