An Evaluation of Overall Efficiency for EPA 

Certified Non-catalytic Wood Heaters 

Prepared for the Hearth Patio and Barbecue Association (HPBA) 

By: Robert Ferguson 

Ferguson, Andors & Company 

July 21, 2011 
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Introduction 

In order to provide a meaningful response to EPA’s recent disclosure of their intent to add 

overall efficiency limits (passing grades) to the proposal for the revised New Source 

Performance Standard (NSPS), the Hearth, Patio & Barbecue Association (HPBA) has 

undertaken an efficiency data gathering effort. Although EPA has indicated that they will 

propose that all regulated product categories will include overall efficiency allowables, this 

report focuses on wood heaters. 

Wood heater manufacturers were asked to provide CSA B415.1-2010 overall efficiency data for 

their certified wood heater models, along with corresponding EPA emissions certification values 

for currently produced models that meet Washington State emission limits. In some cases, the 

data were provided by test laboratories after receiving permission from several manufacturers. 

The efficiency results we have received represent the only known overall efficiency database for 

currently certified wood heater models. Every effort was made to insure that a high level of data 

quality was achieved. However, the database is by no means perfect or complete. Many of the 

efficiency values were determined by using pre-existing data in conjunction with the new CSA 

B415.1-2010 efficiency calculation spreadsheet. A few of the results that were received were 

determined to be close surrogates and therefore usable for the intended purpose. For example, 

one company had conducted only three test runs spanning the operating range for each model 

rather than the usual four. The majority of the data were generated by EPA accredited 

laboratories although some of the data came from manufacturers’ R & D labs. In all cases, the 

weighted average overall efficiency was determined using the same burn rate probability 

weighting scheme required in EPA Method 28 when determining the EPA weighted average 

particulate emissions rate. This allows a direct comparison of emissions and overall efficiency 

for the datasets. 

However, it is important to note that the precision of the CSA B415.1-2010 test method, and 

more specifically the stack loss efficiency determination portion of that method, has not been 

determined. And the EPA emission test methods have been previously shown to include a high 

level of variability1

. Therefore, the unknown uncertainty of the efficiency test results presented 

here in conjunction with the emission result variability must be considered when drawing 

conclusions from these data. 

Data 

Table 1 includes data from 68 non-catalytic heater models. At the request of the manufacturers 

as a means of protecting confidential business information (CBI), the manufacturer and model 

identity is masked. 

 

1 Curkeet, R. and Ferguson, R, EPA Wood Heater Test Method Variability Study - Analysis of 

Uncertainty, Repeatability and Reproducibility based on the EPA Accredited Laboratory Proficiency Test

Database, October 6, 2010.
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Table 1 

Overall efficiency results for seven catalytic models were received. Results were in the range of 

72.1% to 82.4%. The overall efficiency values for these models were determined using the 2000 

version of CSA B415.1. It is expected that these values as reported will be within ± 1 or 2% of 

the values using CSA B415.1-2010. 

Data Analysis 

The data for non-catalytic models were evaluated in several ways. First, a frequency distribution 

was created. Figure 1 is a bar chart showing the distribution of overall weighted average 

efficiencies by efficiency range for the 68 non-catalytic heater models. It should be noted that 

ranges shown are equal to or greater than the first number and less than the second. For 

example, the range 65 – 70 is actually 65.0 to 69.9. 

Figure 2 is a plot of weighted average overall efficiency versus weighted average particulate 

emissions for the 68 non-cat models. The purpose of this evaluation was to see if there was any 

relationship between emissions performance and efficiency. A linear regression line was fit to 

the data and a corner score2 analysis conducted. 

 

2 Ref.: Mark’s Standard Handbook for Mechanical Engineers, Eighth Edition, 17-22
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Figure 1 
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Figure 2 

Discussion 

Although it should be noted that some of the models in this database might postdate the 

enhanced EPA certified wood heater database that was submitted to EPA by HPBA in February, 

2010, it is estimated that this database represents more than 60% of the 96 non-catalytic models 

with emissions of 4.5 g/hr or under included in the enhanced EPA certified wood heater 

database. Of the 68 non-catalytic wood heater models in this database, 21 models have weighted 

average overall efficiencies that are below the proposed level of 70%. This represents 31% of 

the heater models in the database 

The average of all 68 models is 71.0% with a standard deviation of ±3.8%. The median 

efficiency is 71.8%. 
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The weighted average emission rates for the 21 models below 70% overall efficiency range from 

1.2 to 4.5 g/hr with an average of 3.6 g/hr and a standard deviation of ±0.8 g/hr. The weighted 

average emission rates for the 47 models with overall efficiency equal to or greater than 70% 

range from 0.7 to 4.5 g/hr with an average of 3.0 g/hr and a standard deviation of 1.0 g/hr. 

The precision of the CSA B415.1-2010 stack loss efficiency test method has not been 

determined. 

The linear regression line has a very poor R2 value indicating no relationship between particulate 

emission performance and overall efficiency. 

The corner score test confirms there is no correlation or association between particulate emission 

performance and overall efficiency. 

Too little data (n = 7) were obtained to allow any meaningful analysis for catalytic wood heaters. 

However, the limited data available suggest less risk for this category of heaters.