Performance Requirements and Educated Guesses
The design requirements in UL 1482 are not the most important from the point of view of safety; the performance requirements are. The fire tests, for example, specify that during testing the stove must not cause temperatures on exposed walls to rise more than 117° F above ambient temperature. Similarly, "a chimney connector furnished as part of the assembly shall not break, disassemble, or become damaged to the extent that it is unacceptable for further use after being subjected to a longitudinal force of 100 pounds." Unlike design specifications, which often leave the decisionmaking to industry, performance requirements are created entirely by UL's engineers. UL decides which aspects of performance to test and how to go about doing so.
On the first score, UL 1482 is quite comprehensive. It tests several aspects of normal stove operation as well as performance following various mishaps (for example, a heavy impact to the chimney connector). On the other hand, 1482 is far from scientific. According to UL, "sound engineering principles, research, records of tests and field experience … [and] information obtained from manufacturers, users, and
others having special experience" form the basis for its standard. Most test methods in UL 1482 reflect two factors not included in this list: educated guesses and concessions to the practicalities of product testing.
Guesswork abounds in UL's standards, although it is usually disguised by the exactitudes of scientific language. For woodstoves with glass doors, for example, the impact test for glazing calls for a steel sphere of 1.18 pounds mass and 50.8mm diameter to be dropped against the glass from a pendulum arc with a height of 16.25 inches. Research results certainly do not form the basis for this test. The explanation provided by a UL engineer is that a two-inch ball bearing—the steel sphere described more "scientifically" above—swung from around 45 degrees "seemed about right." The idea, according to this engineer, was to simulate an accidental jab against the glass with a fireplace poker. A larger ball would, of course, simulate a more serious jab. But absent any information on real-world experience with jabs and related mishaps, this educated guess lives up mainly to the second half of its name. A representative of a major glass manufacturer takes exception to UL's explanation. "We could never get [UL] to tell us why [the falling ball test] has these specific requirements." This engineer believes that the requirements came from some other UL standards. In fact, rather than simulating a specific hazard for woodstoves, the two-inch ball bearing test would more accurately be described as UL's generic impact test. It appears in similar form in a host of product standards, including the standard for portable video recording systems.
Even the most basic requirement of the three fire tests—that the temperature not rise more than 117° F on exposed surfaces and 90° F on unexposed ones —is largely guesswork. "There is some evidence to support these temperatures," observes a widely respected woodstove expert, but "they are still doubtful. Lots of different temperatures are plausible." A challenge to the rationale for these specific temperature tolerances was raised in the canvass process, but UL was unable to provide a specific response.
The practical necessities attendant to laboratory product testing help explain why guesswork often takes the place of science. In writing performance requirements, UL's engineers are sensitive to the cost of the endeavor. UL will not undertake costly scientific experiments where guesswork seems satisfactory. The engineers also try to keep the cost of the test itself within reasonable limits. For example, in UL 1482 a photoelectric method for measuring wisps of smoke was abandoned in
favor of less accurate visual observation tests because the former was considered too costly. Similarly, since walls and ceilings are made of a variety of materials, the most realistic test method would include separate tests for each material. Striking a balance that favors economy over realism, the tests are done with only one building material.