Heat Shields for Clearance Reduction
In addition to using proper materials, adequate clearances are one of the most important safety features of any wood stove installation. A properly installed and maintained wood stove can be one of the safest appliances in your home. But cheating on clearances can create a very dangerous situation.
Wood stove clearances cannot be eliminated entirely, but they can be reduced significantly by using a properly constructed heat shield. It's critical that heat shields are designed properly, and that rated clearance reductions for the type of shield are followed.
NFPA-211, the industry standard for wood stove installations, describes three different types of heat shields. (Note: NFPA-302 is the standard for boats, which is a bit different from the rules outlined in NFPA-211 and this article. The rules in NFPA-302 are less detailed than NFPA-211, so we suggest following NFPA-211 standards unless you're installing on a boat.)
Dwarf Stove Clearance Requirements
Distances to Combustibles Without Shielding
- 18” from back of the stove
- 16” from sides of the stove
- 36" from the top of the stove to the ceiling
- 18” from single-wall pipe
- 2" from insulated pipe
Distances to Combustibles With an Air-Cooled Heat Shield on the wall or pipe (2/3 reduction)
- 6” from back of stove
- 5.33” from sides of stove
- 18" from the top of the stove to the ceiling
- 6” from single-wall pipe
- Read about building an air-cooled heat shield
Distances to Combustibles with an Air-Cooled Heat Shield Attached to Stove(1/2 reduction)
- 9” from back of stove
- 8” from sides of stove
- 9” from single-wall pipe
- Read about building an attached heat shield
Distances to Combustibles with an Insulated Heat Shield (1/2 reduction)
- 9” from back of stove
- 8” from sides of stove
- 9” from single-wall pipe
- Read about building an insulated heat shield
Distance to Combustibles with a Masonry Heat Shield (1/3 reduction)
- 12” from back of stove
- 10.66” from sides of stove
- 12” from single-wall pipe
- Read about building a masonry heat shield
How to Calculate Clearances with a Heat Shield
Clearances are calculated from the heat source to the combustible surface. So an 18" clearance reduced to 6" with a shield that stands 1" away from the wall would be at least 5" from the shield to the heat source.
How Big Should Heat Shields Be?
Clearances are calculated in all directions in a straight line. The easiest way to figure out what surfaces a shield needs to cover is to cut a dowel to the rated clearance you're testing.
For example, from the back of the Dwarf stove, the rated clearance is 18" to combustibles. With an 18" dowel, measure all points on the wall that you can touch with one end of the dowel, while the other end is touching the back of the stove. All those points must be covered by the heat shield.
Do the same for the stovepipe and the sides of the stove at their respective clearances.
You'll note a couple of things on this exercise. First, the heat shields will tend to be significantly larger than the profile of the stove or stovepipe. Second, the further away the stove or stovepipe is from the combustible material, the smaller the heat shield needs to be.
Don't Trust Pictures
We get a lot of questions about photos of stoves that appear to have much closer clearances than allowed. "How are they able to do that?"
Sometimes it can be hard to tell how close a stove is to a wall from a photo. Clearances may be OK, but appear in a photo to be closer than they actually are.
People do frequently violate clearances and post the results online. Just because someone did it doesn't mean it's safe.
In some cases, stoves are "staged" in a way that they could not be installed, and then corrected later. For instance, on Tiny House Nation S5E15, they didn't have time to install the flue system before filming the episode, so the set dressers just put the stove where they thought it looked best. Photos on the episode show the stove way too close to the wall, but if you look closely, you'll see that the stove was not hooked up yet.
Pyrolysis - Why Clearances are So Large
Clearances are important because wood stoves get very hot while in operation. Combustible materials that are too close to the stove can heat up past their autoignition temperature, and catch fire. A spark is not required to start a fire, just heat, fuel, and oxygen.
In most cases, clearance violations will not cause a fire immediately. As material is repeatedly heated, it deteriorates on a molecular level and its autoignition temperature begins to drop in a process is called pyrolysis. After months or years of repeated heating, a surface that "hadn't had a problem yet" can spontaneously burst into flames.
To be safe, exposed combustible materials around the stove should never exceed 117 degrees F over ambient temperature, and unexposed areas (under the hearth, for example) should never exceed 90 degrees F over ambient. If you observe potentially combustible materials around a stove discoloring, that can be an indication that pyrolysis is occurring. But you won't always be able to see it.
Note for Residential Spaces
Spaces that are subject to building codes generally have specific requirements for clearances. If a stove is UL listed for residential spaces, you need to follow the manufacturer's instructions in the manual. However, most tiny stoves are not UL listed, so you may need to follow the local building code's guidelines for unlisted stoves, which typically require 36" clearances to combustibles in all directions.
If you are using a heat shield to reduce clearances in a project subject to building codes, the shield cannot reduce clearances from the appliance to the wall less than 12" with an air-cooled shield or less than 18" with an insulated shield unless the shield and stove are specifically tested and listed for that purpose. Connector clearance can be reduced to no less than 6" with an air-cooled shield or 9" with an insulated shield.
What Happens When you Cheat
Clearance violations are an especially dangerous problem because they often don't cause a fire immediately. Sometimes pyrolysis is visible as discoloration or charring on the surface of combustibles, but sometimes the pyrolysis can occur inside a wall. You could use your wood stove with no problems for months or years until one day, your wall catches fire from the inside without warning.
Cheating clearances can also be caused by using improper materials. For instance, if you use pellet pipe instead of proper Class A chimney pipe to penetrate your roof, and you follow the manufacturer's suggested clearance to combustibles, you'll create a dangerous clearance violation. Pellet pipe is rated for much lower temperatures that wood stoves produce, so the clearances are calculated assuming those lower temperatures. Connect pellet pipe to a wood stove, and the manufacturer's rated clearances are no longer adequate. Not to mention pellet pipe is very toxic when heated to wood stove temperatures.
How to Construct Air-Cooled Heat Shields
By far the most effective heat shields are the air-cooled type. These shields are constructed with a sheet of 24 gauge or thicker sheet metal, or 1/2" or thicker cement board, with 1" of air space behind the shield and around the perimeter to allow free airflow. 1" heat shield spacers are available for this purpose.
Try to avoid placing a spacer directly between the center of the stove and the wall, since spacers can conduct heat through the shield to the combustible wall. It's usually best to place the spacers around the perimeter of the shield.
The airflow behind the shield is critical for its success. As the shield is heated, the natural convection of air passing behind the shield will keep the shield and the material behind it cool. Enough of the perimeter of the shield must be left open to allow air to enter the bottom of the shield and exit the top. Acceptable airflow strategies are:
- Leave all four sides of the shield fully open.
- Close out the left and right sides of a shield, but leave the entire top and bottom open to allow air to enter the bottom and exit the top.
- If the shield is installed on a flat wall (i.e. not around a corner), you can close out the bottom of the shield if you leave both sides and the top fully open. Air will enter the bottom at the sides and exit the top and sides. This design can't be used if the shield goes around a corner.
A proper air-cooled heat shield reduces single wall pipe clearances by up to 2/3 (18" becomes 6") when used as a wall protector, or up to 1/2 (36" becomes 18") when used as a ceiling protector.
Air Cooled Shields Attached to the Stove and Stovepipe
The industry standard NFPA-211 guidelines for wall and ceiling shields does not cover attaching shields directly to the stove or pipe, but it is often possible to reduce clearances with attached shields in some situations.
Attaching air-cooled heat shields directly to the body of Dwarf stoves using our factory heat shield kit cuts required clearances in half, so 16" from the sides becomes 8", and 18" from the back becomes 9".
Keep in mind that adding a shield to the stove itself will only reduce clearances for the stove. In most cases, you'll also need to add a heat shield for your stovepipe. Attaching air-cooled heat shields to our single-wall pipe using our stovepipe heat shields, cuts clearances to 6" to combustibles with the heat shield in between instead of the standard 18".
The easiest way to add an air-cooled heat shield to your stove pipe is the Tiny Wood Stove Pipe Heat Shield. They attach to your stove pipe and give a sleek, minimalistic look all while offering a peace of mind for the safety of your setup.
Alternatively, you can make air-cooled heat shields for stovepipe by cutting a piece of single-wall stovepipe lengthwise that is 1" larger than the stove's pipe (e.g., a stove with 4" pipe would use 5" pipe as a shield). Then, screw the shield to the pipe using 1-1/4" heat-proof screws and 1" ceramic or metal heat shield spacers. You can also slip the entire shield pipe around the stovepipe and use screws and spacers to attach it.
Check out Nick's 5er installation video for an example of using shields attached directly to the stove and to the pipe to reduce clearances with minimal visual impact.
If your stove is produced by a different manufacturer, attaching a DIY shield directly to the stove or stovepipe may not be permitted, and factory heat shields may or may not be available. So be sure to check with the manufacturer.
Insulated Heat Shields
Insulated heat shields are rarely used for small space installations because they require additional material, and are less effective. Insulated shields are identical in appearance and construction to air-cooled shields, but instead of 1" of air space behind the shield, you use 1" of fireproof insulation like rockwool or ceramic fiber. A shield of this type can reduce wall clearances by 1/2 or ceiling clearances by 1/3.
Masonry Heat Shields
Masonry shields offer a classic "fireplace" look but are less commonly used for tiny living applications because they are no easier to construct than air-cooled shields, are heavy, and they are less effective. Masonry heat shields are essentially a standard brick wall, which uses 3-1/2" thick masonry blocks with no air space. Shields of this type can reduce wall clearances by up to 1/3, and are not generally used for a ceiling.
Shields That Are Not Shields
Tile attached directly to the wall with no insulation or air space is not a heat shield. Tile will conduct heat directly through to the combustible surface behind it. Unless it's at least 3-1/2" thick, a layer of tile provides no rated clearance reduction.
Sheet metal attached directly to the wall with no air space will also conduct heat straight through it, and provides no rated clearance reduction.
Any shield type other than the three types mentioned in NFPA-211 should generally not be used for clearance reduction unless that configuration has been tested. While shields with an amount of air space or insulation other than 1" or shields with a double-layer of sheet metal would likely provide some benefit, the amount of safe clearance reduction they would provide is unknown.
It should also be noted that no testing has been conducted on how effective using multiple shields is. While combining an air-cooled wall mounted heat shield and stove mounted shield is tempting to reduce clearances further, we strongly recommend not furthering reductions beyond what the wall-mounted shield will allow (2/3 reduction).
Want to run your ideas by an expert? Or check to make sure your plan meets the clearance requirements for a safe wood stove installation? Contact us at email@example.com and we will be happy to offer advice and help you plan.