Fireplaces, Chimneys, and Appliance Venting

Fireplaces, woodstoves, furnaces, boilers, pellet stoves or hot water heaters… They’re all individual appliances. And even though the question always comes across as “how many can you have on one chimney?”, let’s make sure to talk about “how many on one flue?”  A chimney is a structure that has one or more flues in it. A flue is simply the chimney passageway that vents the fumes from whatever is attached to it. (A flue is not the same as a damper; a damper is something that can block the flue.)

The answer to this question is: “It Depends.” The rules are found in various NFPA standards and in the IRC (International Residential Code.)

Let’s start with solid fuel burning appliances:

Solid fuel includes coal or corn or cherry pits, but for most of us that means cord wood or pellets. The rule here is easy and clear. Multiple solid fuel prohibited. A solid-fuel-burning appliance or fireplace shall not connect to a chimney passageway venting another appliance. In other words, only one appliance per flue. Also this means, gas or oil appliances cannot be vented into a flue which also vents a solid fuel appliance.


How about hooking up a woodstove into an existing masonry (brick) fireplace flue? That should be OK as long as:

  • The fireplace has been blocked off.
  • The liner for the woodstove has to be properly sized, which generally means the same size as the collar-size coming from the appliance.

Make sure the chimney is clear of combustible materials before inserting the smaller liner.

Gas and Oil Appliance Venting:

Gas fireplaces are usually factory-built systems. The manufacturer’s listing and instructions should mention attaching any other appliances to it.

Multiple gas or oil furnaces or boilers, as well as hot water heaters, can be vented into one flue. There are a few rules to mention with this:

The rules apply to listed appliances as well…

If venting two or more appliances on the same flue, you have to know the flue can handle it, as determined by the BTU input and other factors.

Both or all appliances have to be on the same floor. So, no furnaces in the basement or room heaters on the second level of a dwelling.

The connectors for the appliances have to be offset. They cannot come into the flue at the same height, and especially never directly across from each other.

The smaller of the two connectors go into the flue above the larger one (usually meaning the hot water heater).
As a general rule, do not mix “natural draft” appliances and “fan assisted” appliances on the same flue. The rule may be more complicated than this, but if this is your case, be sure you refer to the manufacturer’s instructions.

We realize a lot of this is technical “jargon”, and that maybe you don’t want to try and figure it all out yourself. So, we always recommend getting a certified professional out to your home BEFORE you add or change anything with your fireplace, chimney or venting issues.