Does a Fireplace Insert Need a Full Chimney Liner?

What the Insert's Own Manual Says

Start with the paperwork, not the chimney. Chapter 10 of the 2023 Florida Building Code, Residential, covers masonry fireplaces, factory-built fireplaces and chimneys, and it says fireplace accessories are installed according to their listing. It has no section that spells out how an insert connects to the flue. That job falls to the insert's listing and its installation manual.

The Regency i2450-2 wood insert is a good example of how plain those manuals are. Its manual says the insert must be installed with a continuous chimney liner of 5-1/2 or 6 inch diameter, extending from the insert to the top of the chimney. For the United States it names the chimney it expects: a UL 103 type HT chimney or a code approved masonry chimney with a flue liner. It also recommends at least 15 feet of chimney above the flue collar.

Other inserts carry their own wording, so the model on your insert's label is the one that counts. When we are asked whether a liner is needed, we read that manual first and answer from it.

Full Liner, Direct Connect and Slammer, Side by Side

Three ways of joining an insert to a masonry chimney turn up when an older installation is opened. Only the first matches the manual quoted above.

• Full liner: a stainless pipe runs from the insert's collar, up through the damper area and the clay flue, to a top plate and cap at the crown.
• Direct connect: a short pipe runs from the collar past the damper and stops a little way into the first flue tile, leaving the rest of the old flue to carry the smoke.
• Slammer: the insert is pushed into the fireplace with no pipe at all, and the smoke has to find its own way through the old damper and smoke chamber.
• Mixed: two appliances sharing one flue, which the insert manual forbids.

Why the Pipe Has to Reach the Top

An insert is a closed firebox with a controlled air supply, so it depends on the chimney's pull to work. The Regency manual warns that weak draft causes back puffing into the room and lets smoke leak out through the appliance and connector joints. A liner sized to the insert's collar gives the smoke one sealed path of the right size, from the firebox to the open air.

A short stub leaves that smoke to slow down in a clay flue and a smoke chamber built for an open fire. Creosote forms where wood smoke cools, and an oversized, cool space above a stub is exactly that kind of place. It is also a space nobody can see into without pulling the insert. For safety reasons, we do not install an insert without the connection its manual asks for.

Insulating the Liner

The homeowner FAQ of a national chimney safety organization explains that liners serving solid fuel appliances need insulation, while liners for gas and oil appliances do not. The DuraLiner instructions follow the same line for wood: insulation is not mandatory from the appliance through the damper area to the first flue tile, it is required above the first flue tile, and the maker recommends insulating the whole flex run.

Those instructions call for at least 1/2 inch of blanket wrapped around the liner and covered with a protective mesh before it goes down the flue. They also say no insulation or other material beyond what they specify may be packed around the liner. Our page on what may go around an insert covers the surround.

What You Can Check From the Ground

You can learn a good deal without a ladder.

If smoke comes into the room when the insert is burning, stop using it and leave the chimney to a crew. Roof work and a smoking insert are both ways people get hurt.

• Read the label on the insert, usually behind the faceplate or on the side, for the make and model, then find that manual's chimney section.
• Look at the top of the chimney from the yard: a round stainless pipe with its own cap usually means a liner; a square clay tile with no pipe means none reaches the top.
• Look around the fireplace face for a small metal tag. The Regency manual tells the installer to fix one there when the fireplace has been altered to take a liner.
• Check whether any other appliance, such as an old furnace, vents into the same chimney.

What Our Crew Checks Before Relining

The liner instructions and the insert manual set our order of work. We measure the flue, the fireplace opening, the damper opening and the total height, then size the liner to what the insert's maker specifies. We inspect and clean the old flue first, because the DuraLiner instructions ask for tar glaze creosote to be removed and for cracked, loose or missing bricks and mortar to be repaired before the liner goes in.

The old damper usually comes out, since the liner instructions call it not needed, while the insert manual says not to remove bricks or mortar from the fireplace. We work inside both. Then the liner is lowered, joined to the insert and fixed to a top plate with its cap, and we check the clearances once more.

Next Step for a Spring Hill Insert

If you are not sure what is above your insert, book a free inspection. Our chimney liner installation crew will tell you what is there, show you the insert manual's requirement and give you a written estimate. We are a local, family-owned company, our experienced crew does the relining itself, and we are Proudly American; we look after inserts across Spring Hill and Hernando County.

If you are planning a new insert, the liner is part of the same job on our fireplace insert installation visit. Ask the Proudly American family crew that will do the work, before anything is bought.