Conclusions and comments
Overall, all tested bell heaters with all modifications have shown very high efficiency numbers, where heat transfer efficiency value of 84.9% (AL-01 & WV-01) was the highest, and 76.8% (DS-01) was the lowest. Both low oxygen levels (the lowest average 8.71%) and low stack temperatures (the lowest average 212F) contributed to high efficiency numbers.
It is a remarkable increase in comparison with efficiency results discovered in OMNI 5-heaters test in 1992. Five heaters tested by OMNI were Biofire, Contraflow KIT, Tulikivi, Swedish Royal Crown, and Grundofen. The net efficiency numbers were as follows: Biofire - 54%; Contraflow KIT-54%; Tulikivi-59%; Grundofen - 60%; Royal Crown-65%. Average net efficiency was 58.4%.
In comparison, net efficiency of tested bell heaters: DS-01: 73%, NH-01: 74%; AU-01: 78%; WV: 80% and AL-01: 82%. Average: 78.6% that is 20.2% higher then heaters in OMNI testing.
The tested bell heaters had different shape and size. All the heaters had firebox of approximately the same size and volume. Test results have proven my idea that larger heaters would have higher efficiency than smaller heaters with the same firebox. It also proves, that in the most common shape of heaters (36x48" rectangular) 18x18" firebox is too large to achieve highest possible efficiency in the available volume (DS-01, heater in such shape and size, had the lowest efficiency number of all tested). Both heaters with extra heat exchange volume (heated benches), AL-01 and WV, had the highest efficiency numbers.
These results allow me to conclude that bell heaters do offer higher efficiency despite of their size, shape and functions.
CO emissions results
All tested heaters, despite of modifications to air supply system, had average CO levels in the range between 0.13% and 0.28% that is within the 0.30% limit set up in the EU draft regulations document (we don't have any guidance for CO levels in North America.).
Heaters featuring under fire air supply (grate) and no preheated secondary air supply thorough the sides of the firebox generally shown higher CO emissions. Heaters with 3 rows of air outlets for the secondary air supply in the firebox walls were burning cleaner even with full under fire grate. Version WV-02 with over fire air supply and secondary air supply (no grate) was the cleanest, featuring average CO levels of 0.13%. However, the cycle was much longer due to the fact that combustion was very slow at the coals stage. Half grate version, on my opinion, gave the best results.
My conclusion is that a small under fire grate and secondary air supply system with wider slots and 3 rows of air outlets are beneficial to facilitate clean combustion. Presence of a small under fire grate doesn't seem to have a great negative influence on emissions. The grate could also be partially covered with one of the logs while loading to create an "automatic grate damper".
In regards to the under fire versus over fire question, it is interesting to note that none of all heaters, kahelofens, cookers etc I have seen at the Austrian Kahelofen Fair this January had under fire air supply. All had over fire air supply at the door with secondary air supply through the walls. The only company that had grates in their heaters, Finnish "Nunnaunni", had a special grate that allowed to open and close air supply thorough the grate. This seems logical considering that Austria and Germany have the toughest CO emissions level limit of 0.17%.
Summary of the test results:
* Temperatures for this heater were measured one extra time 15hrs after the test fire: Surface 155F; Bake Oven 240F; Firebox 220F.
Description of heater ID:
WV-01 Whole Village Heater Grate, Advanced Secondary Air Supply
WV-02 Whole Village Heater, NO Grate, Advanced Secondary Air Supply
WV-03 Whole Village Heater, 1/2 Grate, Advanced Secondary Air Supply
WV-04 Whole Village Heater, Grate, NO DRY JOINT, Adv. Secondary Air Supply
AL-01 Alex's Lower Heater, Grate, No secondary Air Supply
AU-01 Alex's Upper Heater, NO Grate. No Secondary Air Supply
NH-01 New Hamburg Heater, Grate, Advanced Secondary Air Supply
DS-01 Damascus Heater, Grate, Simplified Secondary Air Supply