Our Testing Philosophy

The ideal situation for any heater efficiency analysis would be to have a steady-state firing condition and to be able to account for every Btu that entered the picture. This situation does basically apply to oil, gas, kerosene, or LP gas heaters. Unfortunately, the testing of wood burners adds in a number of complicating factors. Wood species, moisture content, piece size, stacking method, total amount of wood, ignition sequence (top to bottom, bottom to top, front to back, etc), effluent products that are difficult to measure repeatably (creosote) are some.

As pieces of wood break and fall or as new fuel ignites, surges and dips in the instantaneous output result. Examine the Calorimeter Room test results of any normal wood burner and you'll see an erratic sawtooth pattern that is effectively unrepeatable.

Some of the inherent characteristics of JUCAs allow testing that avoids many of these sources of error. The units were designed for large, full logs. These large pieces generally only burn between themselves in an environment that encourages constant output. The area burning at any instant only changes very slowly, as does the flame/wood temperature. Where normal woodstoves with brand-type fires have output fluctuations on the scale of tens of seconds, a JUCA with 8" logs has fluctuations on the scale of tens of minutes. Instead of the stove walls constantly increasing and decreasing in temperature (a dynamic situation and hard to accurately quantify) we generally experience a relatively static situation for good fractions of hours at a time.

This situation approaches the ideal condition referred to above. Given these characteristics, we have plenty of time to accurately collect and re-collect all data. Instead of having a situation where the instantaneous efficiency keeps rapidly changing in response to dynamic parameter changes, we experience very repeatable results (given the same fuelwood). Our approach is then to:

  1. Determine the total input Btu available over the course of the fire, based on species, moisture content, and total fuel weight.
  2. Determine appropriate corrections for relative humidity, ambient temperature, baro. pressure, etc.
  3. Relatively continuously monitor the useful heat output, flue temperature, and flue CO2 content.

Since our units operate as warm-air furnaces, generally 90% of the useful output is as easily measurable warm air. The useful radiation component has a higher error figure but since the radiation contribution is so much less than the convection contribution the overall result accuracy is not materially affected by inaccuracies in determining this radiation component. In contrast, a bas-relief on a radiant wood stove could materially affect total output one way or the other.

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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.

The JUCA Home Page is at: juca