# Q&A X degrees plus ambient...what is X?

Posted By QandA, Dec 1, 2007 at 2:17 AM

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### QandA New Member 2. ```NULL ``` Staff Member

Nov 27, 2012
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Question:

Someone told me that a way to check to see if a surface was getting too hot was to take the ambient temperature plus a certain number of degrees. Does anyone know the number of degrees for this equation?

I assume you are talking about combustible materials close to a fireplace or stove; such as a wooden mantle or trim, or a combustible wall. The allowable temperature is 117 degrees F plus ambient or room temperature. Some wood burning standards simply refer to 190 degrees F as the maximum which is roughly the same thing in most cases.

You can use one of those new fancy and inexpensive infrared thermometers to get an idea of the temperature.

Some things to keep in mind....

If it gets anywhere near those temperatures, it should be protected. It is unlikely that any of your normal "testing" fires bring the surface up to it's highest possible temperature...in other words, it is likely to get a LOT hotter at some point then the temp you see.

Dark materials soak up radiant heat much faster.

In the spirit of keeping a great margin of error, many people use the "rule of thumb" which states that you should be able to put your hand on a combustible surface for a couple of seconds without being burnt. Since 120 degrees is about the maximum comfortable temperature for your skin to touch something, this give a decent margin of safety.

Some other comparables:

Water at 130 degrees will burn you. The temperature of radiators and hot water heating pipes are normally a maximum of about 180 degrees.

Although wood ignites at temperatures of over 500 degrees, prolonged exposure to temperatures as low as 250 degrees causes charring of the woods and lowers the ignition temperature.

My figures may not be exact, but they are close. The rule of ambient plus 117 would normally equal about 190 degrees. Again, keep in mind a good margin. Even much lower temps can affect paint and finishes. If it were my house, I'd try to keep things below 130.