Air-Cooled or Insulated: The Two Kinds of Metal Chimney

What the Walls Are For

A metal chimney has to carry hot smoke while keeping its outside cool enough to sit near framing. Makers do that in two ways. Air-cooled pipe uses layers of metal with moving air between them, so outside air flows in and carries heat away. Insulated pipe packs a blanket of insulation between the inner flue and the outer casing.

The Florida Building Code, Residential, ties each kind to a different test. Section R1005.4 says chimneys for factory-built fireplaces comply with UL 127, the same standard the fireplace itself is tested to. Section R1005.3 says chimneys serving solid-fuel appliances, such as wood stoves, comply with UL 103 Type HT. The number of walls is a design choice inside each listing.

The Two Types Side by Side

These are the differences that matter when you are looking at a pipe on your roof or in your attic.

• Listed to: air-cooled fireplace pipe to UL 127 with its own fireplace; insulated all-fuel chimney to UL 103 HT.
• Built as: air-cooled pipe with metal layers and air paths; insulated pipe with a ceramic blanket or similar between the walls.
• Serves: air-cooled pipe only the fireplace model it was tested with; UL 103 HT pipe the stoves, inserts and appliances its own instructions name.
• Parts: each system only with its maker's listed parts, never mixed.
• Air spaces: both keep a stated clearance to combustibles that must stay empty.

An Air-Cooled Fireplace Chimney

The Majestic MPL11 wood fireplace manual describes a chimney section with three parts: the inner stainless flue, an outlet air duct around it, and an outside inlet air duct. That is the triple-wall, air-cooled design many people picture. The same manual says to use only parts listed for that fireplace, to keep a 2 inch air space to combustibles, not to pack the air spaces with insulation, and that the fireplace and chimney must not vent a wood or coal heater or an insert without the maker's written authorization.

The Superior Pro Series wood fireplace uses double-wall snap-lock pipe instead: a galvanized outer pipe, a stainless inner flue and a wire spacer that holds them apart. Two walls, three walls, same idea, and each is part of its own fireplace's listing.

An Insulated Stove Chimney

A wood stove chimney is built for a hotter, longer fire. DuraVent's DuraPlus page describes a triple-wall chimney with two insulating layers, a ceramic blanket plus an air space, listed to UL 103HT and meant to stay cool outside while keeping the draft hot inside. DuraTech, from the same maker, is a complete chimney system listed to UL 103 HT for wood stoves, fireplaces and other appliances that need that listing.

So a triple-wall pipe on a roof may be an air-cooled fireplace chimney or an insulated stove chimney. The label on the pipe and the paperwork, not the number of walls, tell you which.

Mixing Brands and Types

Every manual we opened says the same thing in its own words. DuraTech says to use only its own parts to keep a listed system and not to mix or match with other products. Superior says never to use substitute materials not approved by the maker. Majestic says not to substitute parts. A section from another brand may look like it fits, but its locks, spacing and airflow were never tested with the rest of the chimney.

Swapping types is the bigger mistake. Hooking a wood stove to a prefab fireplace's air-cooled chimney puts a UL 103 HT job on a pipe tested for an open fireplace, and the fireplace manual forbids it. Even an insulated chimney goes on a zero-clearance fireplace only when, as the DuraTech instructions put it, it is approved for use with that appliance.

Checking What You Have

From the attic or inside the chase, a flashlight may show a label on the pipe; the Superior manual even asks installers to wrap a warning label around hidden sections. Note the maker and part number and match them to the fireplace's rating plate. Do not pull insulation away from the pipe, and do not climb onto the roof to look. If the label is missing, the pipe is damaged, or sections from different makers meet, have it checked before the next fire.

Repairs With the Right Parts

Our prefab fireplace repair work replaces damaged chimney sections, caps and spacers with parts listed for your fireplace, and our wider fireplace repair work covers the firebox below. After a free inspection we write up what the pipe is, what it is listed for and what it needs.

Matching parts to a listing takes an experienced crew, and ours comes from a local, family-owned company that is Proudly American. Spring Hill families get a Proudly American crew that reads the label before it picks up a tool.