2 Hot for the Police & the Fireman

When is Hot Too Hot?

Radiant heat produced by wood and gas fireplaces warms any object in proximity to the heater.

Most of the time this heat is welcomed by us because it feels like the sun and warms us to the bone.  The byproduct of radiant heat is the gradual heating-up of walls and mantels close to the heat source.   In many cases after a fireplace has been on for awhile, nearby surfaces can be too hot to leave your hand on them or “too hot to the touch.”

Fortunately, the fact that they are too hot to touch does not mean that they are unsafe or likely to ignite.  Going back to the sun, have you ever walked on a wood deck drenched in the sun in your bare feet?  It’s like walking on hot coals.  Still, there is no record of a deck spontaneously igniting.

Once a surface gets much beyond our body temperature of 98.6 degrees F, it starts to feel hot to the touch.  Over the years, organizations like the National Fire Protection Association (NFPA),

Underwriters Laboratories (UL) and the National Bureau of Standards (NBS) have established standards for allowable temperatures on a variety of combustible materials.

Research tells us that for wood to ignite it must reach a minimum temperature of 482 degrees F.

There is evidence that if wood (and some other combustible surfaces like sheetrock) are exposed to sustained high temperatures over long periods of time their ignition temperature can be lowered.  It is at this point that codes & standards converge to tell us what is acceptable.

All this research and testing tells us that the critical temperature for potential danger is 187 degree F or approximately 117 degrees F above ambient room temperature.  It is to this temperature that heating appliances are tested.  When a fireplace is installed, the parameters for safe clearances are laid-out in the installation manual by the manufacturer.  The common denominator is that under extreme testing conditions, the nearby surfaces never reach the 187 degree temperature.

While walls and mantels remain safe at these temperatures sometimes the finishes (like paint or sealants) do not hold-up.  They can blister or peel in some cases.  This does not mean there is a safety concern, but it is a good idea to know what the temperature limitations are of the finishes applied to surfaces near fireplaces.

Too hot to the touch is a physical barrier we learn from childhood, but it is not necessarily a legitimate method of gauging the safety of a fireplace installation.

 

March 20, 2017

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