The criteria for the phase 2 tests were formulated with the help of OWB manufactures. I have spoken with the EPA and with different EPA wood boiler test labs. The tests may give good results in an OWB but the same test may perform bad in a gasser.
see this link for the actual test criteria.
http://www.epa.gov/burnwise/pdfs/owhhphase2agreement.pdf
It is long but it will shed light on what the test is all about. The phase 2 test is voluntary. If you pass you can get listed on the EPA site
http://epa.gov/burnwise/owhhlist.html after parting with 10-15K per boiler for the test at one of the 4 independent labs approved by EPA in the US. Each boiler size eve nof the same model must be checked which is different that the safety tests UL 391 where the larger size can be checked and the approval can be used in a similar model of smaller size. A state or area may require this "voluntary test" to be able to sell in a particular area. the test and "certification" and it is only good until EPA phases it out like they did for phase 1
quoted from the test document...
Qualification of Phase 2 emission level qualified models will end 5 years after the date of the EPA letter acknowledging qualified status and use of Program and use of Program permanent labels and hangtags on these models must be discontinued for units manufactured after that date. Partner may choose to re-qualify a model per the Program qualifying process, or may seek a waiver (see Partnership Agreement Attachment 1).
Effective Date and Duration of Agreement
This Agreement will become effective when signed by both Partner and EPA, and may be amended by mutual written agreement. Partner and EPA understand that this Agreement is wholly voluntary and may be terminated in writing by either Partner or EPA at any time and for any reason with no penalty. EPA may terminate the Agreement, including the authorization to use qualifying labels and hangtags if Partner fails to act in accordance with any part of this Agreement, including its attachments. Unless amended or terminated sooner, the terms of the Agreement will remain in effect until the termination of Phase 2 of the Program by EPA.
...end quote
It is very expensive to get the test and with EPA able to terminate the program or phase it into new one, requiring a different standard I can see how the list of qualified boilers would remain low.
Most gassifiers do not want dimensioned lumber to be burned because during the course of the burn it can cover the nozzel especially if all the wood is dimentioned as per the phase 2 test. Some manufactures specifically say not to burn dimensioned wood in their manuals. The epa phase 2 test uses dimensioned oak lumber with 3/4 x 1 1/2 spacers between 4x4 lengths and is stacked as per the document.
The test also allows a coal bed to be started before the actual smoke and particles are measured with a pre test burn cycle with 2x2 pieces as recommended see section 12.4.1
Once the coal bed has reached its target value then the test begins after loading the dimensioned lumber.
Take time to read over the actual test and think how many people ever use their OWB like this. A gassifier you can see could fail with this test. If you stack wood like the test says in a typical gasser firebox chamber the fire really will not burn as the manufacturer has designed.
If an appliance is made for split seasoned wood of a certain size the test should be conducted with the wood the manufacturer has designed it to be used for. The lighting phase should also be as recommended by the manufacturer. Otherwise what good is the test.
Think about it, most of the smoke in burning is made at start up. this test allows the boiler to have a pre test burn to make a coal bed and the coals raked out evenly before the actual test is done.
Some of the gasser manufactures are looking at the phase 2 tests but more from the standpoint of the free publicity of being listed as an approved boiler etc. this is based on information coming with discussions I have had with the test labs.
Bottom line, in my opinion a one size fits all test just doesn't give accurate results. when we compare OWB, including the efficient ones, and indoor gassers its kind of like comparing as they say apples and oranges.