Reducing Clearances to Combustibles
Air-Cooled Radiation Shields Clearances are designed to keep combustible materials around a wood burning appliance below 90°C (190°F), even during a chimney fire. It is possible to build an air-cooled radiation shield, which can reduce clearances because moving air is a very effective cooler.
When radiation from an appliance hits a shield and heats it, the shield in turn heats the air behind it. Since hot air rises, the air moves up and exits through spaces at the top of the shield, and cooler air is drawn in at the bottom of the shield to replace it. The hotter the shield gets, the faster the heated air rises, and the more air passes by the shield, cooling it down.
A shielding system made of small sheet metal panels is less likely to buckle than one made of a single large sheet. The edges of the metal should be hemmed to improve its strength and appearance. This kind of system is inexpensive and easily adapted to many situations.
Wall shields must extend at least 450 mm (18 “) beyond each edge of the appliance and 500 mm (20”) above the appliance. To allow air to flow behind the shield, there must be a space of at least 25 mm (1 in.) at the bottom of the shield and a 21 mm (7/8 in.) space between the shield and the wall. (If the shield must extend up the wall to shield the flue pipe, a 75 mm (3 in.) space must be left at the top so the cooling air can escape).) The shield should be permanently mounted on the wall using screws through non-combustible spacers.
Shield Construction Rules
- Minimum space between shield and combustibles: 21 mm (7/8 in.).
- Minimum clearance along the bottom of shield: 25 mm (1 in.).
- Maximum clearance along the bottom of shield: 75 mm (3 in.).
- Minimum clearance along the top of shield at ceiling: 75 mm (3 in.).
- Shield extension beyond each side of appliance: 450 mm (18 in.).
- Shield extension above appliance: 500 mm (20 in.).
- Edge clearance for ceiling shields: 75 mm (3 in.).
- Adhesives used in shield construction must not ignite or lose adhesive qualities at temperatures likely to be encountered.
- Mounting hardware must allow full vertical ventilation.
- Mounting hardware must not be located closer than 200 mm (8 in.) from the vertical centre line of the appliance.
- Mounting hardware which extends from the shield surface into combustibles may be used only at the lateral extremities of the shield.
Air-cooled shields for ceilings should have a 75 mm (3 in.) clearance from adjacent walls.
Certified Commercial Shields
If you do not want to build an air-cooled radiation shield, several different styles of metal and brick or stone commercial shields are available. Be sure that any commercial shield you buy has been certified and labeled by one of the three recognized testing agencies. The manufacturer’s instructions will specify how to install the shields, and the clearance reductions that can be achieved.
What Can Shields Be Made Of?
- Sheet metal with a minimum thickness of 0.33 mm (0.013 in.). This is only 30 gauge -26 gauge is better. The edges should be hemmed (safety edge).
- A solid brick wall at least 21 mm (7/8 in.) away from the wall and vented at the top and bottom.
- Brick or tile slices mounted on an approved non-combustible board with a 21 mm (7/8 in.) air space behind it.
- A certified commercial shielding system.
Note: If a brick wall is built against a drywall or other form of combustible wall. The measurement for clearance is taken from appliance to combustible wall. You would consider the brick not to be there when determining distance. That is why it is important to know the requirements before constructing heat shields to ensure that they will comply with WETT Inspection.
The first step in reducing clearances is to determine the minimum clearance, either from the appliance label or from the table of clearances for uncertified appliances. Then, calculate the permissible clearance reduction for the type of shield you plan to use from the table on clearance reduction. The channel spacers shown are the most effective type to use because they give good support to the shield and do not transmit heat through the mounting hardware to the combustible wall. Metal wall strapping, available from most building supply stores, is made of light steel channels that work well as shield spacers. Note that the bottom of the channel is notched to allow cool air to enter. The shield must extend 450 mm (18 in.) beyond each edge of the appliance and 500 mm (20 in.) above the top of the appliance.
The floor pad must be made of a durable, noncombustible material, such as sheet metal, grouted ceramic tile, or mortared brick. Floor pads must normally extend not less than 450 mm (18 in.) in front of the loading door and 200 mm (8 in.) beyond the other sides and back. Floor pads must not be installed on carpet unless the pad is structurally supported so that it does not move or distort. Most Fireplace shops now carry prefabricated ULC approved floor pads which can be easily placed in front of inspection.com/tag/fireplace/” title=”View all articles about fireplace here”>fireplace is hearth is not the required size.
Floor Pad Size
The floor pad protects flooring from hot embers that might fall from the appliance during fuel loading or servicing. The pad should extend at least 200 mm (8 in.) beyond the sides and rear and 450 mm (18 in.) in front of the loading door. The floor pad must be a continuous, non-combustible surface. The floor pad must not rest on the carpet unless it is strong enough to resist bending or cracking. The best floor pads are laid on the sub-floor so that their finished level is flush to the floor, so there is no edge to trip on.