After the results are in, we look for closure of the numbers. We know how much energy was available over the whole test period. We accurately know the useful output. We know how much sensible and latent was wasted up the flue.
Closure in our tests is generally very good. Commonly the discrepancy is within two or three per cent total. This means that we have accounted for all but that much of the original energy produced in the fire. This discrepancy is caused by a combination of measurement errors and unaccounted for by-products. This last refers primarily to carbon monoxide, ashes and creosote. Since our results show a pattern of negative as well as positive closure, we believe that our accuracy may be better than suggested and that the bulk of the non-closure is due to variation in the wood and other measurement variations. This suggests that the combustion efficiency is probably about 98% or higher, and agrees with the observations of many JUCA owners of remarkably little creosote and ash production and accumulation.
Our test results suggest that JUCAs produce only 7 to 12% as much creosote and pollution as an airtight woodburner would. Since such airtight products have been tested as having combustion efficiencies in the 70% to 80% range (at low firing levels), this does seem to agree. Instead of 20% to 30% wastage, JUCAs seem to only leave one-tenth as much unburnt, or 2 to 3%. Figures for both are better under certain, especially laboratory) conditions.
The design of the JUCAs is based on a non-airtight principle of operation. We planned back in 1973 and 1974 to allow the fire to burn cleanly by allowing excess air supply. Since this would cause less creosote in the smoke produced, we would be able to extract heat from the smoke down below the threshold limit of creosote condensation. Since there was little creosote in the smoke which could condense, we could actually CAUSE it to condense in the JUCA. Very little creosote laden smoke gets to the smokepipe and chimney under normal conditions. Unfortunately, some of our JUCA owners have been so trained to think in terms of airtight operation that they try to operate it as an airtight, even going so far as to gasket the doors. Those users are likely to develop rapid creosote deposits in their smokepipe and chimneys.
The JUCA design is meant to allow very complete combustion (due to free access to draft air) possibly in the 98% range, and at the same time allow very extensive and efficient heat exchange so that the end result is a high net (overall) efficiency. In addition, the use of large logs allows constancy of output without having to suffocate the fire.
The approach we use is not very applicable to competitive airtight woodstoves. Measurement of radiated energy is more difficult than measuring warm air without a complex Calorimeter Room. Large non-closure due to unknown creosote, pollution and carbon monoxide losses, rapidly fluctuating conditions due to small-sectioned fuelwood and other matters complicate the picture for testing those devices. If we had to test such products, we would probably use a constant-firing heat source, such as an adjustable gas log burner. Heat transfer efficiencies would then be easy to measure. Combustion efficiency effects of the thermostatic (suffocation) draft control could be studied to be applied back to wood fuel.