Connecting a Small Wood Stove to a Larger Chimney

Adapting a Tiny Wood Stove to a Larger Flue Size

Does your structure already have an existing solid fuel rated flue system in good shape?  Is your current stove too large for your space, or just needs to be upgraded?  Attaching a new small stove to your existing flue can be an attractive option.  Reusing existing pipe significantly decreases the difficulty of your project, avoids cutting any new holes in your structure, and can cut the overall project cost in half or more.

Are you installing a small stove in a structure subject to building codes?  Most building codes require using UL Listed Class A pipe, and the smallest available pipe meeting that requirement is 5 inch (see our 5" Ventis parts).  If you're using a 3" or 4" stove, you'll need to make sure your stove can handle adapting to the larger size.

Are you looking to save money on your project by using locally available chimney pipe in a larger size?

Can you safely use a larger chimney with your small stove?

Interior after tiny wood stove installation.

Chimney Size and Draft

The size of a wood stove chimney is critical to its performance.  The flue is the "engine" of a natural draft wood stove.  The rising column of hot flue gas actively pulls combustion products from the stove, and pulls fresh air in through the intakes.  This phenomenon is commonly referred to as "draft," and without it, a wood stove will not function.

Decreasing the flue size smaller than the stove's collar (or flue flange) is almost never permitted.  Doing so would restrict the flue system to less than required for proper draft, causing smoke to spill out of the air intakes.  Per NFPA-211 (the industry standard for residential style wood stoves):

12.4.4 (1) The cross-­sectional area of the flue shall not be less than the cross­-sectional area of the appliance flue collar, unless specified by the appliance manufacturer.

However, increasing the flue to a size larger than the stove's collar (or flue flange) is sometimes permitted.  A larger chimney does have a negative effect on draft, since it allows the flue gases to slow and cool before they escape the top of the chimney.  But as long as the chimney isn't too large, the stove will still draft just fine.

How large is too large?

Acceptable Flue Size Increases for Roof Exits

Roof exits are superior to wall exits for creating draft, so a larger chimney can be used for a roof exit than would be possible with a wall exit.
  • Roof exits provide flue gases with a straighter vertical route.  A roof exit with no elbows, or at most two 45 degree elbows, creates very little resistance to the rising column of hot flue gases.
  • Roof exits keep the majority of the stovepipe in the heated envelope of the structure, so the flue gases stay hot on their way out.
NFPA-211 specifies which chimney size increase are acceptable for a roof exit:

12.4.4 (2)  The cross-­sectional area of the flue of a chimney with no walls exposed to the outside below the roof line shall not be more than three times the cross­-sectional area of the appliance flue collar.

So, according to industry standard, the following size increases are acceptable:

Acceptable Flue Size Increases for Wall Exits

Wall exits don't produce as strong of a draft as roof exits, so your options for increasing flue size in a wall exit are limited.

  • A wall exit adds two sharp 90 degree turns and a horizontal section that resists the vertical movement of hot flue gases
  • Because most of the pipe in a wall exit is outdoors, the hot flue gases have to move through a large quantity of cold pipe, which can cause them to cool and slow down.
NFPA-211 specifies which chimney size increases are acceptable for a wall exit:

12.4.4 (3)  The cross­-sectional area of the flue of a chimney with one or more walls exposed to the outside below the roof line shall not be more than two times the cross-­sectional area of the appliance flue collar.

So, according to industry standard, the following size increases are acceptable:

  • A 4" stove can use a 5" wall exit.  We carry a 4x5" adapter.
  • A 5" stove can use a 6" wall exit.  We carry a 5x6" adapter.

Exceptions to the Rules

The NFPA guidelines are based on an otherwise well-designed flue system, and a normal household stove.  There are a few considerations that may make a size increase not function well for a small stove, even if it is theoretically acceptable.

  • When stoves become very small, heat output is limited, and the ratio of pipe surface area to volume increases.  The household rules of thumb start to break down at 3".  In our experience, most 3" stoves will not function reliably well with a wall exit, even if the pipe size matches the stove.  The smallest stove we've seen that drafts reliably well with a wall exit is our Dwarf 3kW Standard, which uses a 4" flue system.
  • Adding "heat robbing" appliances to the flue can negatively impact a stove's draft, and cause an otherwise sound flue system to fail.  Flue water jackets and "heat reclaimer" systems cool and slow flue gases, and can interfere with proper draft.
  • The oven on the Dwarf Cookstove Combo also robs significant heat from the Dwarf 5kW stove, so 6" wall exits are not recommended for this model.

How to Avoid Replacing the Entire Flue System

If you have an existing flue system that is too large for your stove, but you don't want to replace the whole flue, adding a liner inside your stovepipe might be an option.

If you run single-wall stovepipe inside of your existing chimney from the flue flange all the way to the chimney cap, you can effectively reduce the size of your existing stovepipe to match the stove.

Make sure you're using solid fuel rated stovepipe or a solid fuel rated flexible chimney liner inside your existing flue system. For best results, the space between the liner and the existing stovepipe should be filled with ceramic stovepipe insulation.

Using a Small Stove Inside a Masonry Fireplace

A small wood stove can often be used as an insert in a masonry fireplace.  However, the existing chimney is usually too large for the stove to function well.
When adding a small stove to a masonry fireplace, it's best to add a solid fuel rated chimney liner system from the stove's flue flange all the way to the chimney cap.  DIY kits are available that include flexible pipe, insulation, a cap and flashing for the chimney, and a tee to connect to the stove's rear exit.

Need help?

Drop us a line at support@tinywoodstove.com for personal help with your small stove installation.

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