Chimney guides

Fitting a Gas Insert? It Needs a Liner

Air district restrictions have pushed a lot of California households from open wood fires to gas inserts, and the liner has been left out of a lot of those installations.

A stainless liner being fed down a masonry flue to a gas insert

Short answer

A gas insert is an appliance with a specified vent diameter, usually matching its flue collar. Venting it into an open masonry flue several times that size means the gases cool and slow, the appliance does not draw as designed, and combustion products condense inside the chimney. A correctly sized stainless liner is part of installing an insert properly, not an upsell — and most manufacturers require it.

Why the open flue does not work

An open fireplace needs a large flue because it is pulling a large volume of room air up the chimney. An insert is a far more contained appliance moving a fraction of that through a small outlet.

Put the insert on the fireplace flue and the same modest amount of heat now has to warm several times as much masonry. The gases cool, slow down, and condense on the flue wall. That damages the masonry and it means the appliance never performs the way the specification says it will.

There is a safety dimension too. An appliance that will not draft reliably is an appliance that can spill combustion products into the room, and with gas that means carbon monoxide.

What the manufacturer says

Read the installation manual rather than taking anyone's word. Most insert manufacturers specify a continuous liner from the appliance to the termination, sized to the collar. Fitting it without voids the warranty as well as creating the performance problem.

Why this is so common in California

Two air districts have changed how people heat, and the chimney work has not kept up.

The Bay Area runs Spare the Air days through winter when wood burning is prohibited. The San Joaquin Valley operates mandatory curtailment with no- burn days. Both publish daily status, and both have pushed households toward gas inserts as the way to keep using a fireplace at all.

An insert fitted quickly, by someone who fits appliances rather than chimneys, frequently goes in without the liner. It works, in the sense that it lights. What it does not do is vent correctly.

How to tell if yours has one

Look up into the fireplace above the insert if you can, or ask for the installation paperwork. A liner terminates at a top plate at the crown, visible from the roof. If nobody was on the roof during the installation, there is no liner.

Doing it properly

A flexible stainless liner sized to the appliance, insulated where the chimney runs cold — exterior stacks and long runs — terminating at a top plate and clamp at the crown.

It is also the moment to deal with the crown and cap, because the access is the same. A liner secured to a crumbling crown is a liner that will need revisiting.

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Frequently asked questions

Can I burn wood on a no-burn day if I have a gas insert?

A gas insert is not affected by wood-burning curtailment — that is largely the point of fitting one. Check your air district's rules for the specific appliance type.

Does a wood insert need a liner too?

Yes, and for the same reasons. Any insert has a specified vent size and venting it into an open flue is the same fault.

What if the chimney bends?

Flexible liner handles a modest offset. A severe one may require opening the chimney at that point, which changes the job — and is a reason for a camera survey before anyone quotes.