Let’s start with a few definitions.
“Things” are properly called “appliances.” This means fireplaces, woodstoves, furnaces, boilers, pellet stoves or hot water heaters etc. They’re all individual appliances. And even though the question always comes across as “how many on one chimney?” let’s make sure to discuss “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 either; a damper is something that can block the flue.)
The answer to the question is: “It Depends.” The rules are found in various NFPA standards and in the IRC (International Residential Code.) This article is general in nature but for those who want to drill down into the details, most of the information can be found in IRC chapters 10, 13, 18 and 24.
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.
IRC M1801.12 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, period. It goes without saying, I hope, that gas or oil appliances cannot be vented into a flue which also vents a solid fuel appliance. EVERY SOLID FUEL APPLANCE GETS ITS OWN VENT!
How about hooking up a woodstove into an existing masonry fireplace flue? That’s OK as long as:
- The fireplace has been blocked off. Remember, only one appliance per flue!
- 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 factory-built systems. The manufacturer’s listing and instructions will preclude 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:
- The rules apply to listed appliances. While I have never seen an unlisted gas or oil furnace in my life, if you have one, you are referred back to the rules for solid fuel burning appliances- one per flue.
- If venting two or more appliances on the same flue, you have to know the flue can handle it, as determined but 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 your home.
- The connectors for the appliances have to be offset. They can’t 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, don’t mix “natural draft” appliances and “fan assisted” appliances on the same flue. This rule is more complicated than this, but if this is your case, be sure you refer to the manufacturer’s instructions. Call and HVAC company and make them show you to your satisfaction it’s right. Don’t take anyone’s word for it, see it in writing.
The NFPA 54 (Gas) and the NFPA 31 (Oil) show diagrams in great detail, and cover sizing the connectors as well (connectors are the smoke pipes that carry the fumes from the appliance to the chimney flue.)
Read another helpful article by the American Society of Home Inspectors.