Clearance to Combustibles Testing
MHA Annual Meeting, Wildacres, NC
May 7-13, 2001

Introduction: (Return to top of page)

Efforts are under way to update the clearances to combustibles requirements for the
ASTM Guide to Solid Fueled Masonry Heaters.

In order to obtain baseline data to serve as a basis for discussion, a test heater was built
meeting existing fireplace building code wall thickness provisions (8" at the lower and
upper firebox, firebrick lined,  6" at the downdraft channels).

The left half of the heater (viewed from the rear)  used a mineral wool expansion joint, and the right half
used a cardboard expansion joint. Construction photos of heater

Description of Test (Return to top of page)

The heater was a small contraflow heater with a 36" x 24" overall footprint, and a
14" wide firebox. A total of 142.5 lb of dry red oak (11% moisture) was burned as follows:

Surface temperature readings were taken at 13 points on the heater at 30 minute intervals,
using an infrared thermometer.

The data locations were as follows:

Results: (Return to top of page)

Discussion: (Return to top of page)

The right half of the heater ran hotter, indicating better heat transfer through the cardboard..
The results were somewhat skewed by a slight wind from the right side, resulting
in more wind cooling on the right.

UL-127 Temperature limits on combustible surfaces are:

90F over ambient room temperature for unexposed (ie., in contact and covered) surfaces
and 117F over ambient room temperatures for surface exposed to ambient air.

Testing done at Lopez in January 2000, indicates that in order to obtain 117F above
ambient with a 4" ventilated air space, a heater surface temperature of approximately
350F is required.

Conclusions: (Return to top of page)

The heater suface did not exceed 350 degrees Fahrenheit with 500% overfiring.

However, it is likely that surfaces temperatures would have been higher on an indoor test
with no wind cooling. On the other hand, the wood was very dry at 11%, representing
somewhat more than 500% overfiring with standard 20% wood.