by Norbert Senf
ASTM E-1602-94 "Standard Guide for Construction of Solid Fuel Burning Masonry Heaters" has recently come due for a mandatory 5 year renewal. The ASTM subcomittee has agreed to resubmit the existing standard guide as is, with minor editorial changes.
The subcommittee also agreed, for future versions, to expand the clearance requirements to include a wider range of masonry heaters. Since the document does not yet specify firebox wall thicknesses, a masonry fireplace "code wall", ie. 2.5" nominal firebrick lining and 8" overall nominal wall thickness, is currently assumed.
In order to have the necessary information to specify clearances to combustibles for a wider range of heater wall constructions, a fueling protocol for a standard safety test is needed. It was agreed that a new ASTM standard test method should be proposed for this. Although there is an existing UL 127 protocol for factory built fireplaces, it is not clear that this is an appropriate method for masonry appliances. UL 127 specifies a fixed burn rate, achieved by adding a standard "brand" at 7 minute intervals. This continues until temperature stabilization, ie., no further rise in the exterior temperature surface of the masonry, is observed.
The thermal mass of masonry adds a heat storage component that a metal factory fireplace does not have. As a result, temperature stabilization, as required by UL, can take a long time. It is unclear whether this represents real world conditions or not. It was agreed that further testing is required to answer this question. Three sets of tests have taken place recently - one set on masonry fireplaces, and 2 sets on masonry heaters.
OMNI Environmental Services (Beaverton, OR) conducted thermal testing on a standard masonry fireplace last March, using the UL protocol. The purpose of this test was to study combustibles in contact with fireplace sidewalls, something not allowed by codes, but often seen in practice.
With an 8" firebox sidewall, combustibles in contact with the exterior reached the UL limit 10 hours into the test. Fueling was stopped at 12 hours, at which point the UL limit was exceeded by 11F. The highest temperature recorded was 27F above the limit, 1 hour 50 minutes after the end of the test.
With a 12" sidewall, the highest temperature was 25F above the UL limit, 2 hours 40 minutes after the test was completed.
It is not clear how applicable these results are to masonry heaters. Masonry heaters are fired with a large charge of fuel, rather than receiving regular limited reloadings as fireplaces do. From a safety perspective, operating masonry heaters with non-design firing conditions is a concern. Of most concern would be a heater that is regularly reloaded with a full fuel load. Many of the senior members of MHA have personal experience with heaters that have been fired under these conditions. For example, I recently examined an over-fired heater. There was clear evidence of damaged refractories. The owner stated that she had been absent from her house for an extended period, and that the heater had been operated by tenants who lacked operating knowledge. Since the masonry heater was the main heat source for the house, they simply kept reloading it until the house was warm, not realizing that there is a time lag in the heat output.
There have been suggestions to adopt existing European protocols for safety testing. It should be realized, however, that European heaters are typically smaller room heaters, and that whole-house heaters are a recent North American innovation. Also, there is a long standing masonry heating tradition in Europe, and the heater builder or manufacturer is not expected to assume liability for abusive operation, as he may be required to in the United States.
To obtain preliminary data, we conducted a test last January on an unfaced contraflow heater core at Lopez Labs. The heater was fired with 2 back to back loads of 50 lbs each. The firebox wall thickness, minus the facing, was 4.5". A plywood panel was set with a 4" airspace from the heater, as per ASTM E-1602. The surface temperature on the plywood reached the UL limit when the heater surface temperature was 350F, after 4 hours.
Another set of tests was done at the annual Masonry Heater Association (MHA) meeting last April. A test heater was constructed with 3 different firebox wall sections. The rear wall was 4.5" firebrick, identical to the Lopez Labs test, above. The left sidewall was 8" soapstone, and the right sidewall was a 8" (nominal) brick "code wall", including a 4.5" firebrick liner. The fuel used was hardwood cordwood with 14% moisture content, and 4 loads of 50lbs each were burned back to back. The masonry surface temperature peaks were as follows:
8" brick (code) 210F
8" soapstone 410F
With this fueling protocol, ie., 400% overfiring, one can reasonably conclude that the code firebox wall is safe at the ASTM E-1602 clearance of 4" to combustibles.
Yet to be discussed by the ASTM subcommittee is the appropriate overfiring percentage that is required to adequately insure public safety.
Further information on the above testing is available on the MHA website at