Final Report

September 2011

New York State Energy Research and Development Authority

Review of EPA Method 28 

Outdoor Wood Hydronic Heater 

Test Results

No. 11-17
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Review of EPA Method 28 

Outdoor Wood Hydronic Heater Test Results

Final Report

Prepared for the

New York State

Energy Research and

Development Authority

Albany, NY

www.nyserda.org

Nathan Russell

Assistant Project Manager 

Prepared by:

Brookhaven National Laboratory

Dr. Tom Butcher

amd

Nathan Russell

Assistant Project Manager

NYSERDA NYSERDA September 2011

Report 11-17
ii

Notice

This report was prepared by Tom Butcher, Ph.D. in the course of performing work contracted for and sponsored by 

the New York State Energy Research and Development Authority (hereafter “NYSERDA”). Nathan Russell also 

contributed to writing the final report. The opinions expressed in this report do not necessarily reflect those of 

NYSERDA or the State of New York, and reference to any specific product, service, process, or method does not 

constitute an implied or expressed recommendation or endorsement of it. Further, NYSERDA, the State of New 

York, and the contractors make no warranties or representations, expressed or implied, as to the fitness for particular 

purpose or merchantability of any product, apparatus, or service, or the usefulness, completeness, or accuracy of any 

processes, methods, or other information contained, described, disclosed, or referred to in this report. NYSERDA, 

the State of New York, and the contractors make no representation that the use of any product, apparatus, process, 

method, or other information will not infringe privately owned rights and will assume no liability for any loss, 

injury, or damage resulting from, or occurring in connection with, the use of information contained, described, 

disclosed, or referred to in this report.
iii

Preface

EPA began Phase II of the Voluntary Hydronic Heater Program after October 15, 2008. To qualify under this 

“White Tag” program, appliances are tested using EPA Method 28 Outdoor Wood Hydronic Heater (M28 OWHH) 

and must achieve a weighted emissions rate of 0.32 lb/mmBtu (output) less. In late 2010, EPA recognized that the 

efficiency values determined using M28 OWHH did not represent actual efficiencies of qualified White Tag units 

and removed the efficiency values from the website while the issue was reviewed. 

The work described in this report was conducted as a screening-level review of the M28 OWHH test results for the 

23 units that were qualified under Phase II of the program at that time. Personnel from Brookhaven National 

Laboratory, NYSERDA, the Northeast States for Coordinated Air Use Management (NESCAUM), New York State 

Department of Environmental Conservation (NYSDEC), Vermont Department of Environmental Conservation, 

Maine Department of Environmental Protection, and Massachusetts Department of Environmental Protection 

participated in the review of the White Tag qualification data to identify possible sources of error. 

Concurrent to this effort was EPA’s stakeholder process to make improvements to M28 OWHH that included 

representatives from state air agencies, NYSERDA, and BNL as well as manufacturers in the Voluntary Program 

and the commercial test laboratories that conducted the original tests. Many of the recommendations found in this 

report have already been implemented in EPA’s test method revision, Method 28 Wood Hydronic Heater (M28 

WHH).
iv

Acknowledgments

This research was supported by the New York State Energy Research and Development Authority. NYSERDA 

acknowledges the contribution of Louis Fontaine, Maine Department of Environmental Protection; Phil Etter, 

Vermont Department of Environmental Conservation; Marc Cohen, Massachusetts Department of Environmental 

Protection; Randy Orr and John Barnes, NYSDEC; Lisa Rector, NESCAUM, and Chris Brown, BNL. We also 

acknowledge extensive dialogue on these test results coordinated by the EPA to modify the test method. Participants 

included the EPA accredited test labs, manufacturers of OWHHs as well as those listed above. The modified test 

method M28 WHH which resulted from EPA’s stakeholder process can be found on the Burn Wise website.
v

Table of Contents

1. Introduction .............................................................................................................................................................1

2. Overview of Test Method 28 OWHH (M28 OWHH)...........................................................................................2

3. Review of Test Results.............................................................................................................................................4

4. Implementation of Review ......................................................................................................................................8

5. Findings of the Review ..........................................................................................................................................10

5.1. Impacts of Sizing...........................................................................................................................................11

5.2. Effect of Improper Hydronic Heater Sizing on Operational Load, Efficiency and Emissions Rate..............14

6. Conclusions ............................................................................................................................................................18

6.1. Energy Output and Efficiency Measurements...............................................................................................18

6.2. Emissions Rate Measurements......................................................................................................................18

6.3. Impacts of Sizing...........................................................................................................................................19

6.4. Improving the Accuracy of M28 OWHH......................................................................................................19

6.5. Additional Considerations for the NSPS.......................................................................................................20

7. References ..............................................................................................................................................................22
vi

List of Figures

Figure 1. Generic piping arrangement in Test Method 28 OWHH. The appliance (also hydronic 

heater or supply) side is to the left of the heat exchanger while the load side is to the right of 

the heat exchanger..................................................................................................................................................2

Figure 2. Illustration of reported particulate emissions vs. output for selected units. Emissions generally 

increase significantly with decreasing load on the heating system, even exponentially higher below 

20%. Removing the lowest load from the test via substantial revisions to the test method weighting 

factors and load limits will greatly reduce the calculated average emissions rate for the hydronic heater, 

creating a situation in which the test method is even less representative of actual operating conditions. ..............7

Figure 3. PM2.5 emissions using M28 and SR weighting factors. By removing the lowest load, the 

emissions rate using the SR parameters is reduced by an average of approximately 19%...................................12

Figure 4. Efficiency using M28 and SR weighting factors. By removing the lowest load, the efficiency rating 

using the SR parameters is increased by an average 5.6% (absolute). .................................................................13

Figure 5. Bin-hour analysis for a typical built ranch in Syracuse, NY. .......................................................................15

Figure 6. Comparison of emission rates based on weighting factors determined by M28 OWHH and the 

sizing factors with respect to the Syracuse bin hour analysis. As the sizing factor is increased the 

emissions tend to increase, sometimes substantially. The blue columns show that sizing to the design 

day heat load can yield significant reductions in emissions. ................................................................................16
vii

List of Tables

Table 1. Example of a White Tag qualified hydronic heater test data spreadsheet used for this review 

(part 1 of 2).............................................................................................................................................................8

Table 2. Example of a White Tag qualified hydronic heater test data spreadsheet used for this review 

(part 2 of 2).............................................................................................................................................................9

Table 3. Weighting factors for M28 OWHH and a case where substantial revisions (SR) were made .......................11

Table 4. PM2.5 Emissions rate reduction when using SR weighting factors and load limits......................................12

Table 5. Efficiency improvements when using SR weighting factors and load limits ................................................13

Table 6. Expected loads on the hydronic heater based on climate...............................................................................14

Table 7. Weighting factors using Syracuse profile and SR revised categories............................................................15

Table 8. Weighting factors using Syracuse profile and M28 OWHH categories ........................................................16
viii

Glossary

Appliance (Hydronic heater or Supply) side—Everything to the left of the heat exchanger in Figure 1. This is a 

closed loop system. 

Aquastat—A control device for the heater that determines when it turns on and off and determines the temperature 

of the water leaving the heater.

Closed Loop—A system of piping where the water is recirculated. For example on the hydronic heater side, water 

leaves the hydronic heater, passes through the heat exchanger, and returns to the hydronic heater to be 

reheated. 

Condensing operation—Occurs when the heat exchanger or flue surface temperatures are below the flue gas dew 

point causing primarily water vapor to condense on the cooled surfaces. Condensing increases efficiency, 

however, the condensate is corrosive. If it occurs for extended periods of time the hydronic heater will fail 

prematurely due to corrosion of hydronic heater components. Condensing becomes a concern when return 

water temperatures are below 130 F and the degree of condensing increases as return water temperature is 

reduced.

CSA B415—Canadian Standard B415. This standard is analogous to Method 28 OWHH and used to evaluate the 

performance of hydronic heaters.

Design Day Load—This is the highest heating load the building is expected to see and should be used to properly 

specify the hydronic heater size. To be accurately estimated an analysis of the building design is necessary 

to evaluate such characteristics as: square footage, windows, air leakage, insulation, orientation, solar gain, 

climate, and so on. Using only square footage can provide extremely inaccurate results.

Load (Cooling Water) Side—Everything to the right of the heat exchanger in Figure 1. Typically, cooling water is 

sourced from city supply or a well, passes through the heat exchanger, and is drained. It provides a medium 

to transfer energy (heat) from the hydronic heater loop.

M28 OWHH—Method 28 Outdoor Wood Hydronic Heater. The test method for evaluating hydronic heaters.

M28 WHH—Method 28 Wood Hydronic Heater. Revised version of M28 OWHH.

Nominal Output—The maximum hourly output of the hydronic heater.

Open loop—A system of piping where recirculation does not occur. The load side is typically open loop.

Stack Efficiency—Efficiency as determined using the temperature and gas concentrations of the exhaust and the 

chemical composition of the fuel (wood).

Thermal Efficiency—Efficiency as determined using the input/output method described in M28 OWHH

Thermal Storage—Typically a large water-filled tank that serves as a buffer between the building heat load and the 

heater output. It can rapidly absorb the energy from the heater and gradually release it as it is needed 

thereby allowing the heater to operate at higher loads for the majority of the time.

Thermopile—A series of thermocouples used to measure a temperature difference. Accuracy is improved over 

measuring the temperature difference using only two thermocouples..

Two-stage unit—A specific design of hydronic heater that enables the combustion process to be split into two 

stages. Typically there are two combustion chambers for split-wood units. In the first stage, combustion 

occurs under oxygen starved conditions at relatively low temperatures to promote gasification of the 

volatile compounds. In the second stage the gases are combusted under oxygen rich conditions at relatively 

high temperatures to promote complete combustion. Typically an oxygen and/or temperature sensor is used 

to support optimum operation.
ES-1

Executive Summary

Under a Hydronic Heater Voluntary Partnership Program, the U.S. EPA has developed Test Method 28 OWHH for 

Measurement of Particulate Emissions and Heating Efficiency of Outdoor Wood-Fired Hydronic Heating 

Appliances (known as M28 OWHH) for evaluation of the particulate emissions and efficiency of wood-fired 

hydronic heaters. Currently 23 units have been approved under the Phase II emission levels of this program. Meeting 

this Phase II level requires achieving an annual average particulate emissions rate of 0.32 lb/MMBtu heat output or 

less. As part of the M28 OWHH testing, the thermal efficiency of the units is reported. Recently, significant 

concerns were raised due to the reported efficiencies for some of the units being questionably high. The EPA 

suspended qualification of units in the Voluntary Program until the source of the error could be determined and 

remedied.

Figure 1 shows the generic hydronic system arrangement in the M28 OWHH test. The test installation is arranged to 

enable measurement of energy output from the hydronic heater on both the supply (appliance) side of the heat 

exchanger shown or on the load (cooling water) side. Energy input is based on the heating value of the wood and the 

amount consumed based on the change in the mass of the hydronic heater during a test. Efficiency is simply energy 

output divided by energy input. Energy output is based on the supply side water flow rate and temperature change 

across the heat exchanger. Energy output can also be measured based on the load or cooling water side flow rate and 

temperature rise across the heat exchanger and these measurements were included in the test method to provide a 

quality control check on the test results.

Figure ES-1. Generic piping arrangement in Test Method 28 OWHH. The appliance (also hydronic 

heater or supply) side is to the left of the heat exchanger while the load side is to the right of the 

heat exchanger.
ES-2

Manufacturers seeking to be partners in the Voluntary Program are required to submit the qualification test data, via 

independent test labs, to EPA for review and approval. A request was made to EPA for the data from qualified 

White Tag wood hydronic heaters and personnel from the Northeast States for Coordinated Air Use Management 

(NESCAUM), Brookhaven National Lab (BNL), New York State Energy Research and Development Authority 

(NYSERDA), New York State Department of Environmental Conservation (NYSDEC), Vermont Department of 

Environmental Conservation, Maine Bureau of Air Quality, and Massachusetts Department of Environmental 

Protection reviewed the data in order to identify possible sources of error that could explain the high efficiencies.

Many aspects of the data were analyzed including: testing parameters such as water temperatures, flow rates, stack 

temperatures, and wood moisture content; precision requirements related to flow meters and temperature sensors; 

and efficiency and emissions calculations on the hydronic heater side compared to the load side of the heat 

exchanger in the test apparatus. The group did not review aspects of particulate measurements. Efficiency was also 

estimated via a stack loss analysis for comparison with the reported results. In some cases parameters had to be 

estimated for the stack loss analysis as they were missing from the obtained test data. The efficiency based on stack 

loss as calculated in this analysis is reasonably accurate under steady-state conditions i.e. full load (Cat IV) testing 

and overestimates efficiency under cyclic loading such as in categories one through three (Cat I-III). Rather than 

using the efficiency based on stack loss analysis to establish an efficiency rating for a specific test, the purpose of 

this analysis was a high-level check to determine if any reported efficiencies for a test run in M28 OWHH were 

unreasonably high from a thermodynamic perspective. For this evaluation, any reported efficiency by M28 OWHH 

that was greater than two percent less than the combustion efficiency was considered thermodynamically impossible 

and a flag was raised on the analysis of the subject test run.

An underlying issue with M28 OWHH was the use of water temperature and flow rates from the supply side of the 

heat exchanger for determining hydronic heater output energy. In this review it was often found that the supply side 

temperature drop across the external heat exchanger was low and water flow rate was high, particularly when test 

runs conducted at lower firing rates were evaluated. This in turn led to an increase in error of the output calculation.

For example, in one case the temperature drop across the heat exchanger was only one degree while the precision of 

the temperature sensor was half a degree thereby providing an error of 50%. However, on the load side of the heat 

exchanger, it is possible to maintain low flow rates (although high enough with respect to flow meter precision) and 

high temperature differences relative to the precision of the measurement instruments thereby reducing the error 

associated with the efficiency measurement drastically. There was often a significantly large difference between the 

energy output values determined using measurements from the hydronic heater side of the heat exchanger compared 

to the efficiency values determined using measurements from the load-side of the heat exchanger.

The review of M28 OWHH test results was performed at the screening level and the intent was to identify any test 

results where a more in depth analysis may be warranted. The M28 OWHH test results of (23) qualified hydronic 

heater units were evaluated and 21 (90%) were found to have missing or questionable data. The specific test issues 

included missing data, testing outside of the prescribed heat-load categories, and error associated with temperature 

and flow measurements described above. The specific recommendations to reduce uncertainty and improve quality 

control on M28 OWHH from this analysis are as follows:

• Use the water temperature and flow rates from the load side of the heat exchanger for energy output 

measurements. The error associated with the precision of the instruments is drastically improved 

(reduced) due to lower flows and higher temperature differences across the heat exchanger

• Integrate overall efficiency calculations based on the CSA B-415 Stack Loss Method as a quality 

control check on efficiency ratings. An input/output method for annual efficiency based on M28 

OWHH load side measurements should still be the primary method
ES-3

• Revise measurement and calculation methods for wood fuel moisture. Ensure the moisture content of 

the wood is accurately recorded by increasing the number and locations of measurements. EPA is 

considering a rigorous evaluation of the wood moisture and energy measurement method as part of the 

New Source Performance Standard

• Specify the hydronic heater temperature range to avoid possible condensing conditions that become a 

concern at return water temperatures less than 130 °F. Condensing will improve efficiency ratings for 

the test, however, if the hydronic heater is not designed for condensing but operated under such 

conditions, it will fail prematurely due to corrosion within the hydronic heater itself. Consider 

requiring typical operational settings for traditional hydronic heating systems: supply temperature 180 

°F (range 160-200 °F) and return temperatures 160 °F (range 140-170 °F)

• Revise methods for temperature and flow measurements. Use a thermopile for temperature 

measurements to improve accuracy. Place the flow meter on the input side of the load side as the 

temperature of the water entering the heat exchanger should be reasonably constant

• Revise the frequency for data collection from 10 minutes to < 15 seconds for values such as water 

temperatures, flow rates, stack temperature and gas concentration. Temperature changes of >10 

degrees were observed over a ten minute period. During cycling a ten minute period may not 

adequately represent the peaks and valleys of the cycle. A shorter time period will allow a better 

output measurement

• Ensure water density calculations are based on the temperature nearest the flow meter and that the 

water density is based on temperatures over the data collection period (i.e. 15 seconds) as opposed to 

the average for the entire test (i.e. eight hours)

• Require reporting of CO2 data. This will help to characterize combustion performance and provide a 

check on efficiency values

• Require reporting of CO data. CO presents a safety concern and is particularly important for units 

installed indoors that will become more common in the market and are tested under M28 WHH.

Similar to CO2, it also characterizes combustion performance.

In light of the development of the New Source Performance Standard (NSPS) that is currently underway, this report 

contains a section that compares and contrasts the emissions and efficiency values determined using the four M28 

OWHH categories with those that would result from substantial revisions (SR) to weighting factors and load limits.

These methods differ in the load on the hydronic heater for their respective test categories. While M28 OWHH uses 

four categories with the lowest being <15% full load, the SR method uses only three with the lowest being <35% 

full load. Moreover, in M28 OWHH, the lowest load (category I) is weighted the most of all four categories due to 

the amount of time OWHHs spend at idle. Moving to fewer and higher loads in the test method creates a situation in 

which the test method is even less representative of actual operating conditions. Because emissions generally

increase significantly with decreasing load on the heating system, even exponentially higher (Figure 2), removing 

the lowest load from the test will greatly reduce the calculated average emissions rate for the hydronic heater. Using 

data from 20 of the hydronic heater tests, overall efficiency was found to increase by more than 5% and the 

emissions rate decreased by 0.048 lb/MMBtu representing a greater than 19% average reduction in emissions ratings 

when the M28 OWHH test data from categories II, III, and IV were used to determine the efficiency and emissions 

ratings per SR method.
ES-4

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0 20 40 60 80 100 120

PM - lb/MMBtu output

Ouput - % of max

Unit 5

Unit 10

Unit 15

Uniit 17

Unit 20

Unit 21

Unit 22

Unit 23

Figure ES-2. Illustration of reported particulate emissions vs. output for selected units. Emissions 

generally increase significantly with decreasing load on the heating system, even exponentially 

higher below 25%. Removing the lowest load from the test via substantial revisions to the test 

method weighting factors and load limits will greatly reduce the calculated average emissions rate 

for the hydronic heater, creating a situation in which the test method is even less representative 

of actual operating conditions.

The section also investigates the impacts of hydronic heater output with respect to building design heat loads. The 

average output of the White Tag units when using the load side data is 175 kBtu/hr. Assuming representative design 

heat loads for a very cold (North Dakota), cold (New York), and moderate climate (Maryland) are 65, 40, and 20 

kBtu/hr respectively leads to sizing factors ranging from 2.7—8.8 respectively. In the very cold climate this 

represents a maximum hourly heater output of 37% and 11% for a moderate climate. The design conditions typically 

occur for less than two percent of the year and for 90% of the time the heating load on the building is at 70% of the 

design load or less (BNL, 2004). For a very cold climate, the heater output will be at or below 26% for 90% of the 

time, similarly for the moderate climate the output will be at or below 7% for 90% of the time. Based on the trends 

in figure 2, operation at these loads can lead to a substantial increase in emission rates.

Finally, with respect to the NSPS, it is important to recognize that M28 OWHH was designed for a particular woodfired heating technology, the outdoor wood hydronic heater. Two-stage combustion wood-fired heaters, designed 

with two distinct combustion chambers and oxygen and/or temperature sensors, combined with external thermal 

storage provide an opportunity to achieve much higher efficiency and lower average emission rates by avoiding 

cycling at low loads (NYSERDA 2011; 2010; 2010A). These units operate at much higher loads than OWHHs, 
ES-5

typically at greater than 50% load for cord-wood fired hydronic heaters and greater than 30% for wood pellet-fired 

hydronic heaters.

Concurrent with this effort was EPA’s stakeholder process to make negotiated improvements to M28 OWHH that 

included representatives from state air agencies, NESCAUM, NYSERDA, and BNL as well as manufacturers in the 

Voluntary Program and the commercial test laboratories that conducted the original tests. Many but not all of the 

recommendations found in this report have already been implemented in EPA’s test method revision, Method 28 

Wood Hydronic Heater (M28 WHH). Those recommendations that were not included in the M28 WHH revisions or 

those that were beyond the scope of the revisions are recommended for consideration as the NSPS is developed and 

include:

• Carefully consider the impact of sizing as it impacts the weighting factors for each load. As the 

hydronic heater nominal output increases with respect to the design load, the time spent at lower loads 

increases

• Carefully consider test category load limits, particularly with the lowest load limit. Emissions can 

decrease exponentially as loads are increased, which may artificially inflate performance ratings and 

stray farther away from representative operation

• Consider the applicability of the method, particularly the category load limits and associated 

weighting factors, with the next generation technology. Installing two-stage, gasification units with 

thermal storage promotes high output operation to avoid low outputs. The suitability of M-28 OWHH 

for testing 2-stage combustion units was the subject of another NYSERDA report (NYSERDA, 

2010A). M-28 OWHH does not properly address these advanced technologies.
1

1 Introduction

Under a Voluntary Partnership Program, the U.S. EPA has developed Test Method 28 OWHH (M28 OWHH) for 

evaluation of the particulate emissions and efficiency of wood-fired hydronic heaters. Currently 23 units have been 

approved under the Phase II emission levels of this program. Meeting this Phase II level requires achieving an 

annual average particulate emissions rate of 0.32 lb/MMBtu heat output or less (US EPA, 2008). 

Currently, several states are implementing regulatory programs that affect these same units. Some of these states 

established regulations that accept outdoor wood hydronic heaters qualified under the EPA Voluntary Partnership 

Program. A limited number of states have established their own certification programs, which may require 

additional review, and provides them with the flexibility to accept or reject results of prior M 28 OWHH tests under 

their individual state certification process.

The purpose of the study described here was to conduct a review of the available Method 28 OWHH (M28 OWHH) 

test results to evaluate: completeness of the data sets, consistency of the test results, error magnitudes where they 

can be estimated, and areas in which the test procedure can be improved. The analysis done in this review is 

considered to be at the “screening” level. It is intended to identify units where significant problems exist with the 

testing and for which more detailed review may be warranted.
2

2 Overview of Test Method 28 OWHH (M28 OWHH)

Under Method 28 OWHH, wood-fired hydronic heaters are installed in a test lab, on a weigh scale with a closed 

loop / heat exchanger arrangement for imposing a thermal load. At the present time a significant modification to 

M28 OWHH is nearing completion and the review discussed in this report is based on the method that has applied 

prior to these modifications. 

Figure 1 shows the generic hydronic system arrangement in the M28 OWHH test. The test installation is arranged to 

enable measurement of energy output from the hydronic heater on both the supply (appliance) side of the heat 

exchanger shown or on the load (cooling water) side. Energy input is based on the heating value of the wood and the 

amount consumed based on the change in the mass of the hydronic heater during a test. Efficiency is simply energy 

output divided by energy input and output is based on the supply side water flow rate and temperature change across 

the heat exchanger. Energy output can also be measured based on the load or cooling water side flow rate and 

temperature rise across the heat exchanger and these measurements were included in the test method to provide a 

quality control check on the test results. 

As part of the revisions to M28 OWHH, which are nearing completion, a decision has been made to change from 

using the supply side measurements to the load side water temperature and flow rate measurements for the energy 

output calculation. The primary motivation for this is the greater magnitude of the temperature change on the load 

side that provides greater accuracy in the overall calculation. In addition, since the load side is an open flow, it is 

easier to include regular calibrations of the flow measuring device against a weigh scale during a test program.

Figure 1. Generic piping arrangement in Test Method 28 OWHH. The appliance (also hydronic 

heater or supply) side is to the left of the heat exchanger while the load side is to the right of the 

heat exchanger.
3

In M28 OWHH, particulate emissions are measured using a dilution tunnel system as defined in ASTM E2515 

Standard Test Method for Determination of Particulate Matter Emissions Collected in a Dilution Tunnel. The 

emissions rate is calculated by dividing the particulate matter mass by the energy output described above.

Testing is done with the heat output adjusted at four different levels as follows:

Category I: A heat output of 15% or less of Manufacturer’s Rated Heat Output Capacity

Category II: A heat output of 16% to 24% of Manufacturer’s Rated Heat Output Capacity

Category III: A heat output of 25% to 50% of Manufacturer’s Rated Heat Output Capacity

Category IV: Manufacturer’s Rated Heat Output Capacity

Testing is done “hot-to-hot” i.e.—at the start of each of the four test runs the hydronic heater is at operating 

temperature and the test is completed when the added fuel charge is consumed. 

From the test data, annual values for both particulate emissions and efficiency are determined based on a weighted 

average of the results of each of the category tests. The weighting values are:

Category I: 0.437

Category II: 0.238

Category III: 0.275

Category IV: 0.050

The test method is intended to provide a replicable and common measurement method to rank appliance 

performance. It is not intended to necessarily represent the full range of potential variations in actual in-use 

operation of an OWHH. The defined test fuel is red or white oak that has 19 to 25% moisture content on a dry basis. 

Each piece is cut to a 4” X 4” cross section and the length was dependent on the firebox dimensions. Fuel heating 

value may be measured or a default value can be used. 
4

3 Review of Test Results

A review of the reported test data for all 23 units has been performed. The approach taken was to conduct this at a 

screening level to provide a rapid comparison of the factors that may affect the accuracy of the reported results and 

compliance with the M28 OWHH test procedures. For some of the tests, data was available in a spreadsheet file that 

facilitates analysis. In other cases only a printed form of the raw test data was available. 

In the review the focus was placed on the following categories:

• Accuracy of the energy output value calculated based on the supply-side water temperature and flow 

rate measurements

• Comparison of the energy output results determined by supply side and load side water temperature 

and flow rate measurements

• Comparison of reported efficiency with efficiency estimated from stack gas conditions for each 

category test

• Measured moisture content of the wood used in the test runs compared to the range specified by M28 

OWHH

• Actual energy output relative to the specified energy output (i.e. load) in each category

• Recalculation of the particulate emissions rate based on load side output.

A discussion of each of these follows.

1. Accuracy of Energy Output Based on the Supply Side Measurements. In M28 OWHH it is specified that 

the supply side flow meter shall be “A totalizing type water flow meter with a resolution of 0.1 gallon and an 

accuracy of 2% of the volume recorded or a flow meter with an accuracy of ±0.1 gal/min.” In the test reports, 

the supply side water flow rate ranges from 3.4 gpm to 21.7 gpm. Using the ±0.1 gal/min criteria, applied to the 

lowest flow rate, 3.4 gpm, the accuracy of this measurement is 2.9% and this is the highest it could be. For the 

purposes of this review it is simply assumed that the accuracy of the supply side flow measurement is 2%.

Energy output on the supply side is the product of flow rate multiplied by the temperature difference across the heat 

exchanger multiplied by the specific heat. M28 OWHH states that the “temperature difference measurement shall 

have an uncertainty of ±0.50 °F”. The percentage error in the temperature difference measurement then can be taken 

as . Where is simply the temperature difference across the heat exchanger on the supply 

side. This error can become quite significant when the Delta T is small. 

2. Comparison of the Energy Output Results Determined by Supply Side and Load Side Water 

Temperature and Flow Rate Measurements. From the data reported for each unit test, a comparison can be 

made of the output determined from the supply side and load side flows and temperatures. These would not be 

expected to be identical but they should be closer than the range that would be achieved if both measurements 

were at the limits of accuracy prescribed in M28 OWHH. For the load side measurement, the accuracy required 

is specified by “A totalizing type water flow meter with a resolution of 0.1 gallon and an accuracy of 0.5% of 

volume recorded or a flow meter with an accuracy of ±0.01 gal/min”. In the test data, load side cooling water 

flows range from a low of 0.118 gpm to 14.9 gpm. At the lowest flow rate an accuracy of ±0.01 gal/min in a 

flow meter would provide an accuracy of 8.5%. For the purposes of this review it has been assumed that the 
5

accuracy of this flow is 0.5%, based on the requirement for a totalizing flow meter. The thermocouple accuracy 

requirement is the same as for the supply side.

For the purpose of the data review, the actual difference between the supply side and load side energy outputs was 

calculated and compared with the maximum possible difference, assuming that the measurements were made at the 

specified accuracy limits as discussed above. The expression for the maximum possible difference between the two 

output measurements, expressed as a percent, is:

Of the 23 White Tag qualified hydronic heater units for which test results were reviewed, two were eliminated from 

the process due to incomplete data. Of the remaining 21 units, 17 have a difference between the supply side and load 

side outputs that is greater than that determined from this equation, raising concerns about the accuracy of the 

results. 

3. Comparison of Reported Efficiency with Efficiency Estimated from Stack Gas Conditions. For each unit 

and category test, the test reports include a value for the efficiency based on the supply side output results. In 

some cases these results are very high, over 90% and even exceeding 100% in one case, and therefore 

unreasonably high thermodynamically. To achieve efficiency levels this high the flue gas temperature would 

have to be very low and the units would be in a condensing mode. To provide a rough check on the reported 

efficiency levels, efficiency was calculated using a stack loss method. This approach assumes that the efficiency 

of the hydronic heater is 100 percent, minus losses that are associated with hot flue gas vented through the 

chimney. This calculation considers the ultimate analysis of the dry fuel (Carbon, Hydrogen, Oxygen, and 

Nitrogen), the actual as-fired moisture content, the flue gas temperature, and the excess air level as determined 

by the measured flue gas CO2 concentration. This value for efficiency is higher than for the direct input/output 

measurement because it does not include jacket losses and does not consider energy lost with unburned fuel. For 

these reasons, if the efficiency, reported based on the supply side output measurements is higher than 2% less 

than the efficiency determined from the stack loss method in a steady state test, the results can be considered 

thermodynamically impossible. In some of the tests results reviewed, efficiency from the supply side output far 

exceeded the efficiency calculated from the stack loss method. 

The heat losses in the stack were calculated using a program developed at BNL which included the following 

thermal factors:

mean heat capacity of dry flue gas

mean heat capacity of flue gas water vapor

latent heat of flue gas water vapor

For each Category test evaluated, the stack loss efficiency was based on a simple average of the flue gas temperature 

and CO2. While this approach is reasonable for a high load test (i.e. Cat. 4), for other burn categories one would not 

expect to see stack loss calculations compare closely with measured efficiencies due to cycling. Nevertheless, for a 

cyclic test this is expected to bias the calculated stack loss efficiency to the high side and the measured efficiency 
6

should never approach the calculated stack loss efficiency. With the purpose of identifying cases where the reported 

efficiency is higher than could be possible based on stack loss efficiency, this approach is seen as unlikely to 

incorrectly identify a case where the reported efficiency is too high. Based on this approach, the review flagged any 

category test where the efficiency reported was higher than efficiencies calculated with the stack loss method. For 

cases where the efficiency is seen as a key issue, a detailed analysis should be done in which flue gas temperature, 

CO2, and fuel consumption rate during each reported time interval is analyzed to determine the run average stack 

loss efficiency. This was impractical in this case in part because only some of the data was provided in a spreadsheet 

file format with the balance provided in a printed table form. In addition, CO2 was not reported for more than half of 

the units reviewed. In this case a typical excess air level, 30%, was assumed for the analysis. 

4. Actual Moisture Content of the Wood Used. For each test, the reported average moisture content was 

compared to the M28 OWHH allowable range of 19-25% dry basis. With the exception of pellet units, the 

moisture content was always within this range. The method does not specify a moisture requirement for pellets 

and the reported values were near 5%, which is reasonable with respect to what is found in the field.

5. Actual Output Relative to the Specified Output in each Category. For each Category test, the actual energy 

output expressed as a percentage of the maximum measured heat output capacity (full load) was plotted with the 

particulate emission rate determined for that category. The M28 OWHH categories are specified as full load 

(100%) for Category IV; 25-50% for Category III; 16-24% for Category II, and <15% for Category I. The 

particulate emissions rate values are quite sensitive to the actual energy output. Figure 2 illustrates this 

sensitivity for selected units and characterizes the trends observed from the data set. For most units, particulate 

emissions rates increase with decreasing load. In some cases, emissions rates increase exponentially in Category 

I and II loads. This increase is most notable and highest for units that have the highest annual emission 

averages. For most of the units tested, the actual test output was within the category specified in M28 OWHH. 

Where there was an exception, an extrapolation was made using the trends illustrated in Figure 2 for the specific 

unit in order to estimate the emissions rate. 
7

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0 20 40 60 80 100 120

PM - lb/MMBtu output

Ouput - % of max

Unit 5

Unit 10

Unit 15

Uniit 17

Unit 20

Unit 21

Unit 22

Unit 23

Figure 2. Illustration of reported particulate emissions vs. output for selected units. Emissions 

generally increase significantly with decreasing load on the heating system, even exponentially 

higher below 25%. Removing the lowest load from the test via substantial revisions to the test 

method weighting factors and load limits will greatly reduce the calculated average emissions rate 

for the hydronic heater, creating a situation in which the test method is even less representative 

of actual operating conditions.

6. Recalculation of the Particulate Emissions Rate Based on the Load Side Output. For all hydronic heater 

units, the reported particulate emissions rates were recalculated based on the output as measured on the load 

side. As noted above, the load side output is generally considered to be more accurate because of the higher 

temperature change across the heat exchanger. 
8

4. Implementation of Review

The review was implemented by creating a test data spreadsheet for each of the 23 White Tag qualified hydronic 

heaters. Table 1 and 2 show an example of the analysis done for one hydronic heater unit (not identified). In the first 

section, “Inputs”, the relevant data extracted from the test report is summarized. In section 2 the calculations 

associated with the basic review are implemented. Section 3 included color coded “flags” that indicate one or more 

areas of the review show cause for concern. A red flag is a higher level of concern than a yellow flag. No color 

indicates that the evaluated parameter is acceptable. Section 4 (Table 2) shows the comparison of a recalculation of 

average efficiencies (annual and Cat IV) and particulate emissions rate based on the load side energy output with the 

values originally reported and also the stack loss efficiency. Section 5 is the implementation of an energy output 

analysis comparing supply and load side energy outputs determinations for each of the M-28 OWHH categories. The 

average percent of full-load value and efficiency for each category are also reported. Section 6 is simply an 

implementation of supporting calculations on the impacts of a change in unit temperature between the start and end 

of the test. 

Table 1. Example of a White Tag qualified hydronic heater test data spreadsheet used for this 

review (part 1 of 2).

Referring to Table 1, Section 3, row 33: red “flags” are indicated in the cells as a result of high maximum potential 

error on the supply side. The yellow flag represents a condition that is cause for concern but not as severe as a red 

flag. This high potential error is due to a low temperature change across the heat exchanger on the supply side. The 

numbers in these cells indicate the actual temperature change (see rows 25 and 26). 
9

Section 3, row 34 includes red flags showing the difference between supply side and load side outputs are out of 

range. Here the difference between these two outputs is greater than the difference could be if the specified M28 

OWHH accuracy requirements were met (see rows 28 and 29). 

In Section 3, row 35 there is a yellow flag, indicating that the reported thermal efficiency (row 21) is higher than 

expected based on the stack loss efficiency (row 30). The yellow flag indicates the reported efficiency is higher than 

2-percentage points below the stack loss efficiency but not higher than the stack loss efficiency (which would lead to 

a red flag).

In Section 3, row 36 there are no flags, indicating all moisture measurements were within the required range. In row 

37 there is another evaluation criterion and flag set for low return water temperature. The relevant version of M28 

OWHH does not include a requirement for a minimum return water temperature. This evaluation criterion was 

included here because of discussions of including this in the next revision of M28 OWHH. In this case a flag in this 

row would simply be cautionary.

In Table 2, Section 5, there is also a red flag in row 49, which indicates that this specific test was done at a load 

higher than the range specified in M28 OWHH. 

Table 2. Example of a White Tag qualified hydronic heater test data spreadsheet used for this 

review (part 2 of 2).
10

5 Findings of the Review

The review identified significant errors in some of the data, with the most common as follows:

• Incomplete data

• Reported efficiency values were higher than possible based on the stack loss method (one unit 

reported efficiency levels over 100% in several categories)

• High error in the energy output due to a low temperature differential across the heat exchanger on the 

supply side

• Large difference between the energy output values determined on the supply- and load-sides of the 

heat exchanger indicating the tests were not done within the required precision of the method

• Tests performed outside of the load specified for the category

For the 23 White Tag qualified hydronic heater units included in the review, the results can be summarized as 

follows:

• Full summary sheets could only be completed for 20 hydronic heater units. Three units were missing 

significant data and the review could not be completed. Critical parameters such as water flow rate 

data on either the supply or load sides were not included, although they are required to be collected 

under M28 OWHH and reported as part of the Voluntary Program agreement

• For 14 of 23 hydronic heater units analyzed, the reported efficiencies were higher than the efficiency 

determined based on the stack loss method. Of those units, eight reported efficiency higher than the 

efficiency based on analysis of stack losses during the category IV tests, which are essentially full 

load, steady firing rate. It is thermodynamically impossible for a unit to have a reported efficiency 

higher than the stack loss efficiency. At full load operation the reported efficiency can approach the 

stack loss efficiency, however it can never be greater

• For 21 of the 23 hydronic heater units, the accuracy of the energy output measurement based on the 

supply side data was low, caused primarily by a low temperature change across the heat exchanger. In 

one case, to cite an extreme example, the temperature difference was close to 1 °F, leading to a 

nominal error in the output of 49%. The criterion used for a low accuracy in this analysis was a 

temperature difference of 10 °F or lower, leading to an error of approximately 7%

• For 15 of the 23 hydronic heater units, the difference between the supply side energy output and the 

load side energy output measurements exceeded the maximum possible level by more than 1-

percentage point, based on the nominal precision of the method, indicating that the testing may not 

have been done in accordance with the test method. This maximum error calculation, as discussed 

above, assumes that all temperature measurements and flow rate measurements are made within the 

accuracy required by the method. If both supply side and load side measurements were made within 

these accuracy constraints, this maximum error could not be exceeded. A low supply side temperature 

difference here does not necessarily cause this metric to be exceeded and a concern to be raised. A 

small temperature difference on the supply side, which is allowed under the existing M28 OWHH, 

leads to a high maximum possible error level. This metric does not provide information on the specific 

measurements that caused the high supply side/load side difference, but rather indicates that the test 

does not appear to have been done within the accuracy specifications detailed in M28 OWHH
11

• For four of the 23 hydronic heater units, tests were conducted at an output level that is higher than the 

category range specified in M28 OWHH. As discussed above, this can lead to lower emission rate 

values. This conclusion is based on the energy output as determined from the supply-side output 

calculation. If the load-side energy output value is used, an additional three units can be considered 

out of category. The supply-side energy output value has been the nominal required metric for 

determining load and the load-side energy output value has been used only as a quality control check. 

The load side energy output value, however, is considered to be more accurate based on the higher 

temperature difference across the heat exchanger.

• Other issues noted in the evaluation that were allowed under the test method included low return water 

temperature, inconsistent aquastat settings between category tests, data recording intervals that are too 

large to accurately characterize performance, the use of a default heating value for the wood fuel, and 

moisture content determinations that were outside of the range specified by M28 OWHH.

5.1. Impacts of Sizing

The ASTM method uses M28 OWHH and CSA B-415 (Canadian Standard analogous to M28 OWHH) as a starting 

point and is currently undergoing revisions. In the US, there seems to be support among manufacturers to increase 

the category load limits and the weighting factors for higher loads similar to CSA 415. For example, the load limit 

for Cat I is < 30% compared to < 15% for M28 OWHH. Table 3 shows the category load limits and weighting 

factors for M28 OWHH and a case where substantial revisions (SR) are made. The revised weighting factors would 

translate to evaluating the hydronic heating units only at Cat III and Cat IV under the current M28 OWHH test. As 

performance typically improves under higher loads, the overall emissions and efficiency performance ratings would 

increase significantly. In order to illustrate the impacts of load limits and weighting factors, an analysis was 

performed using the data gathered in the M28 OWHH review.

Table 3. Weighting factors for M28 OWHH and a case where substantial revisions (SR) were made.

M 28 OW H H SR

L oad weighting factor L oad weighting factor

Cat I <15% 0.437 Cat I <35% 0.5

Cat II 15-24% 0.238 Cat II 35-53% 0.4

Cat III 25-50% 0.275 Cat III 100% 0.1

Cat IV 100% 0.050

It is necessary to discuss some of the assumptions and details behind the analysis. The data only compared load side 

values and ignored supply data as it is expected results are to be reported based on the load side. It was assumed a 

Cat I SR evaluation would be carried out very near the 35% limit, which falls virtually in the middle of a M28 

OWHH Cat III test. In most cases the M28 OWHH Cat tests were carried out very near the upper limit so a M28 Cat 

II test was conducted near 24% load and a Cat III was conducted near 50% load. The average of these two tests is 

37% and it is assumed performance at 37% is very similar to 35%. With this in mind, the performance of an SR Cat 

I test was considered equal to the average of a M28 OWHH Cat II and Cat III test. The SR Cat II test results were 

considered the same as an M28 OWHH Cat III test as the performance difference between 50 and 53% are expected 

to be minimal. The SR Cat III test load matched up exactly with the M28 OWHH Cat IV test so the results are 

expected to be the same between the two. The data for three hydronic heater units 2, 11, and 18 were incomplete so 

they were not included in the analysis.
12

In Figure 3, the impacts of the SR weighting factors on the emissions ratings are depicted. The average decrease in 

particulate emissions rate was 0.048 lb/MMBtu and from a relative standpoint represents an average reduction of 

almost 20%. In an extreme case the emissions were reduced by 68% when using SR weighting factors or 

approximately from 0.24 lb/MMBtu to 0.07 lb/MMBtu. In four cases emissions increased under SR.

Figure 3. PM2.5 emissions using M28 and SR weighting factors. By removing the lowest load, the 

emissions rate using the SR parameters is reduced by an average of approximately 19%.

• Units 2, 11, 18 were not included in analysis

• Assumes Cat I of SR is the average of Cat II 

and Cat III of current M28

• Negative value indicates SR method 

increased PM

• Absolute reduction defined as M28—SR; 

Relative reduction defined as Absolute/M28

• Revised SR weighting factors will reduce 

emissions by 0.048 lb/MMBtu on average

Table 4. PM2.5 Emissions rate reduction when 

using SR weighting factors and load limits.

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

lb/mmBtu

Unit

PM 2.5 emissions using M28 and substantial revisions to load limit and 

weighting factors

Load Side M28

Load side SR

PM 2.5 R eduction

Absolute 

lb/M M B tu R elative %

average 0.048 19.2

median 0.029 22.5

min -0.021 -15.2

max 0.159 67.6

st dev 0.051 20.8
13

Figure 4 depicts the impacts of the SR weighting factors on efficiency. For 17 of the hydronic heater units the 

efficiency increased using the SR weighting factors. The average absolute improvement was about 5.6% and the 

average relative improvement was about 10%. For clarification of absolute vs. relative an example follows. If a unit 

is rated at 50% efficiency it would be expected to jump to 56% efficiency, but the relative gain would be (56-

50)/50*100 = 12%. An extreme case showed a relative increase in efficiency of 29% and an absolute increase of 

15.1%. In three cases the efficiency decreased: in two cases the absolute decrease was about 2% and in the third it 

was about 4%.

Figure 4. Efficiency using M28 and SR weighting factors. By removing the lowest load, the 

efficiency rating using the SR parameters is increased by an average 5.6% (absolute).

• Units 2, 11, 18 were not included in analysis

• Assumes Cat I of SR is the average of Cat II 

and Cat III of current M28

• Absolute efficiency improvement defined as: 

SR Eff - M28 Eff

• Relative efficiency improvement defined as: 

Absolute efficiency/M28 efficiency

• Negative value indicates SR method reduced 

efficiency- occurred with 3 units: 3, 17, 19

• Revised SR weighting factors will increase 

efficiency by 5.6% for each unit

• Efficiency improvements of >=20% (relative) 

in four units. Improvements of >= 10% 

(relative) for six additional units

Table 5. Efficiency improvements when using

SR weighting factors and load limits.

0

10

20

30

40

50

60

70

80

90

100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Efficiency

Unit

Efficiency using M28 and substantial revision to load limit and 

weighting factors

Load Side M28

Load Side SR

E fficiency I mprovements

Absolute % R elative %

average 5.57 9.71

median 4.78 8.34

min -4.43 -6.10

max 15.1 29.2

st dev 5.65 10.3
14

5.2. Effect of Improper Hydronic Heater Sizing on Operational Load, 

Efficiency and Emissions Rate

There are also concerns about the weighting factors associated with each load when sizing practices are taken into 

account. The design day heating load for an average home in the US is expected to be less than 65,000 Btu/hr. This 

value is difficult to generalize as there are a wide variety of factors that come into play associated with the local 

climate and design of the house. There is no doubt that there are residences with higher design day demands, for 

example, a large, poorly insulated home in North Dakota, but there are also several regions in which homes have 

significantly lower design day demands due to milder climates. In addition, the design day heating load for a 

specific home will be determined not just by climate but also by building design. It is also important to note that 

M28 OWHH is for space heating and does not account for loads such as pool heating and driveway snow melting.

Table 6 below depicts example design day heat loads based on climate. Generally speaking, North Dakota would be 

a very cold climate, New York would be a cold climate, and Maryland would be a moderate climate (Lstiburek, 

2004). For improved performance, the heater’s nominal output should be matched to the design load. As the heater’s 

nominal output increases beyond the design day heat load, the time spent operating at lower loads increases. The 

average output of the White Tag units when using the load side data is 175 kBtu/hr. This leads to significant 

oversizing ranging from 2.7—8.8 times the design load depending on the climate. In the very cold climate this 

represents a maximum hourly heater output of 37% and 11% for a moderate climate. Nevertheless, the design 

conditions typically occur for less than two percent of the year and for 90% of the time the heating load on the 

building is at 70% of the design load or less (BNL, 2004). For a very cold climate, the heater output will be at or 

below 26% for 90% of the time, similarly for the moderate climate the output will be at or below 7% for 90% of the 

time. Based on the trends in figure 2, operation at these loads can lead to a substantial increase in emission rates.

Table 6. Expected loads on the hydronic heater based on climate.

V ery C old 

C limate 

C old 

C limate 

M oderate

C limate 

Design Day (DD) Heat load 65 kBtu/hr 40 kBtu/hr 20 kBtu/hr

Sizing factor 2.7 4.4 8.8

Hydronic heater load based on DD 37% 23% 11%

70% of DD load 46 kBtu/hr 28 kBtu/hr 13 kBtu/hr

Hydronic heater load based on 70% DD 26% 16% 7%

The domestic hot water (DHW) load for a four-member family is roughly 45 kBtu per day (ACEEE, 2011). Similar 

to the design day heat loads above, this is a characteristic value and can vary depending on behavior, number of 

occupants, and piping arrangement among other factors. When averaging throughout the day the DHW load 

becomes about 2 kBtu/hr or just over 1% of the average White Tag unit’s nominal output. Accounting for the 

impacts of the DHW load variation (for example higher demand in the morning and evening, and lower demand 

during the day and night) throughout the day instead of evenly distributing the load is beyond the scope of this 

report, but it is expected to yield minimal if any performance gains considering the DHW load is very small relative 

to the White Tag unit’s nominal output. Furthermore, operation of a hydronic heater for DHW-only production, i.e. 

summertime operation, without adequate thermal storage is expected to yield extremely poor performance due to 

this load/output mismatch.
15

To further illustrate the impacts of hydronic heater sizing to a home, an energy model of a code-built home located 

in Syracuse, NY was executed. Some characteristics of the home include R-13 walls, 2500 square feet, and a design 

day of -17 F corresponding to a design load of 55,000 Btu/hr. A bin-hour analysis (Figure 5) was completed on the 

home which identified the frequency of time spent at each load.

Figure 5. Bin-hour analysis for a typical built ranch in Syracuse, NY.

The loads were broken up into 5% full-load increments and it was assumed that the heat would not be turned on for 

loads less than 5% of the design load or less than 2,750 Btu/hr. The red and yellow bars represent the impact on 

hydronic heater output for sizing factors of two and three respectively. At a sizing factor of two a hydronic heater’s 

maximum hourly output is expected to be about 50% while a sizing factor of three yields 35%. The model shows 

this is expected to occur for about two percent of the time. At 70% design load, the hydronic heater’s hourly output 

is expected to be about 35% and 25% of the hydronic heater’s output for sizing factors of two and three respectively.

With this in mind, the average White Tag unit would be expected to operate at or below 25% output for 90% of the 

time. 

Table 7. Weighting factors using Syracuse profile and SR revised categories

W eighting factor s using Syracuse profile and SR revised categories

Sizing Factor ---> SR 1 2 3 4 5 6 7 8

Cat I <35% 0.5 0.388 0.892 1 1 1 1 1 1

Cat II 35-53% 0.4 0.360 0.108

Cat X 53-95% 0.231

Cat III 95-100% 0.1 0.021
16

Table 8. Weighting factors using Syracuse profile and M28 OWHH categories

W eighting factor s using Syracuse profile and M 28 OW H H categor ies 

Sizing Factor --->

M 28 

OWHH 1 2 3 4 5 6 7 8

cat I <15% 0.437 0.077 0.301 0.482 0.799 0.923 0.971 1.000 1.000

Cat II 15-24% 0.238 0.134 0.370 0.410 0.201 0.077 0.029

Cat III 24-50% 0.275 0.459 0.329 0.108

Cat X 50-95% 0.308

Cat IV 95-100% 0.05 0.021

Table 7 and 8 show the weighting factors of each category load for both the SR and M28 OWHH standard for the 

Syracuse house. A Category X is added to capture the gaps in both methods and to see where the sizing factor of an 

additional weighting factor would fall. The column before the sizing factor of one shows the weighting factors for 

both SR and M28 OWHH in Table 7 and 8 respectively. For the SR weighting factors it can be seen that the average 

White Tag unit, with a sizing factor of three, is fully operating in a Cat I mode.

Figure 6. Comparison of emission rates based on weighting factors determined by M28 OWHH 

and the sizing factors with respect to the Syracuse bin hour analysis. As the sizing factor is 

increased the emissions tend to increase, sometimes substantially. The blue columns show that 

sizing to the design day heat load can yield significant reductions in emissions.

Figure 6 shows a comparison of annual PM2.5 emission rates based on weighting factors from M28 OWHH and 

those determined from the Syracuse bin hour analysis. The emissions for Cat X, at loads of 50-95% were 

approximated by taking the average of the Cat III and IV emission rates as Figure 2 shows a linear change in 
17

emission rates between these two loads for most units. Sizing the hydronic heater to the design day heat load reduces 

the emission rate by about 25% on average. A sizing factor of three yields an emission rate about 7% higher on 

average than predicted by M28 OWHH. Although the weighting factors were determined for Syracuse, it can still be 

expected that reducing the sizing factors will carry similar emission rate reductions regardless of location.

It is recommended that any revisions to the weighting factors and load limits, particularly if it supports performance 

ratings using higher loads, are carefully evaluated by EPA during the NSPS process as the impacts of sizing are 

significant. As the units are increasingly oversized, the amount of time spent in the lower loads increases and 

performance at these loads is typically lowest. As efficiency decreases, PM emissions increase greatly, often in an 

exponential manner below 25% load. The Syracuse model shows that for a reasonably harsh climate, the average 

output of a White Tag unit provides a sizing factor of three, which yields about 90% of the operation at a load of 

25% or less. It is also important to consider that these units are installed nationwide where milder winters such as in 

the South could yield sizing factors much larger than three, hence pushing the hydronic heater into even lower load 

operation.
18

6 Conclusions

The technical review of the test results for the 23 White Tag qualified hydronic heater units presented here found 

more than 90 percent of the existing tests had questionable results for efficiency and/or particulate emissions rates or 

were missing data necessary for their determination. Additionally, of the units reviewed, a significant number were 

not conducted within the precision required under the method and some testing was conducted outside of the 

prescribed heat-load categories. 

6.1. Energy Output and Efficiency Measurements

There are significant concerns about the efficiency measurement method and results of the M28 OWHH tests. For 

many of the units tested, the accuracy of the energy output value derived from water temperature and flow rate 

measurements on the supply side of the heat exchanger is poor, and the reported efficiency levels are considerably 

higher than those based on stack loss measurements. Where this occurred, the efficiency results are either very 

inflated or simply not thermodynamically possible. For example, for one unit in a Category IV test (steady, full load) 

the efficiency based on stack loss measurements is 88.11% and the reported efficiency based on the supply side 

output is 95.2%. For this same test the nominal maximum error on the supply side is only 4.44%. Accounting for the 

error provides an efficiency range with a lower bound of 90.8%, which is still greater than the stack loss efficiency 

and thermodynamically impossible. Given the extensive issues with existing test data and method, output and 

efficiency ratings based on the original M28 OWHH tests as published cannot be considered accurate or valid. 

6.2. Emissions Rate Measurements

While the majority of the issues in the analysis are related to energy output and efficiency measurements, the review 

also found issues with emission rate measurement and reporting. The following items outline general areas of 

concern for the accuracy of the emission rate results in the test method:

• Load side energy output rate vs. supply energy output rate—the energy output rate directly impacts 

reported particulate emissions rates. Energy output rates determined using the supply-side energy 

output rate in M28 OWHH were too high, resulting in erroneously high efficiencies and emissions 

rates that were too low

• Emission factors are strongly affected by the load as a percent of full load with particulates increasing 

strongly as load is decreased in some units. In many cases, while there is a range of loads for each 

category, tests are conducted very near the upper limit of the category and in four cases tests were 

conducted at loads outside those prescribed by the category

• The procedure used to establish full-load rated output directly affects the load range over which the 

Category I, II, and III tests can be run. As discussed above, in some units the particulate emission 

factor is very strongly affected by actual load. For marginal units this factor can easily cause the 

emissions rate of a unit to incorrectly be determined above or below the emissions limit of 0.32 

lb/MMBtu (output)

• Tests not conducted within precision—this also affects the output and test load as a % of max load 

(categorization)

• Missing data—for some of the units currently listed and approved there is simply missing data which 

prevents completion of a proper accuracy analysis
19

• Data anomalies—for some units, questions arose regarding wood fuel moisture content measurements 

and Btu values. EPA is considering a rigorous evaluation of the wood moisture and energy 

measurement method and this should be conducted. 

Determining the emission rates for hydronic heaters requires accurately measuring both PM2.5 as well as energy 

output. The review here supports improving the accuracy of energy output but it did not encompass PM2.5

measurement methods. Under another review, questions arose about the wash, probes, and weights with respect to 

the emissions leading to concerns about the emission factors. With this in mind it may be prudent to conduct a 

similar review to identify areas of improvement for M28 OWHH to ensure accurate measurement of PM2.5 and 

emission rates.

6.3. Impacts of Sizing

The review of the test data highlights the very strong dependency of particulate emissions rates on load. An 

important factor in achieving low emissions is proper matching of the heating system capacity to the actual load of 

the building to be heated. The common heating industry practice is to use Manual J (residential load calculation) of 

the Air Conditioning Contractors of America. The average White Tag unit’s nominal output will provide a sizing 

factor of about three to nine depending on the climate and building design where the units output will be 26-7%, 

respectively, of nominal load for 90% of the time. Heater outputs less than about 25% are of particular concern as 

the trends in figure 2 show that emissions tend to increase exponentially as output decreases. The analysis on the 

Syracuse house showed that emissions would be expected to decrease by about 25% if a unit is properly sized to the 

design load. Although the analysis was done for a model house in Syracuse, NY it is expected that similar 

performance improvements would be expected regardless of location as a bin hour analysis will show a similar 

amount of time spent at each percentage of the building design load. Ultimately it is essential to carefully consider 

sizing practices and climate with respect to category load limits and weighting factors.

6.4. Improving the Accuracy of M28 OWHH

Modifications to the existing test method are recommended to significantly improve the measurement of energy 

output, efficiency, and particulate emissions rate based on the findings in this review of 23 qualified White Tag 

hydronic heaters. They include:

• Use the water temperature and flow rates from the load side of the heat exchanger for energy output 

measurements. The error associated with the precision of the instruments is drastically improved 

(reduced) due to lower flows and higher temperature differences across the heat exchanger

• Integrate overall efficiency calculations based on the CSA B-415 Stack Loss Method as a quality 

control check on efficiency ratings. An input/output method for annual efficiency based on M28 

OWHH load side measurements should still be the primary method

• Revise measurement and calculation methods for wood fuel moisture. Ensure the moisture content of 

the wood is accurately recorded by increasing the number and locations of measurements. EPA is 

considering a rigorous evaluation of the wood moisture and energy measurement method as part of the 

New Source Performance Standard

• Specify the hydronic heater temperature range to avoid possible condensing conditions which become 

a concern at temperatures less than 130 °F. Condensing will improve efficiency ratings for the test, 

however, if the hydronic heater is not designed for condensing but operated under such conditions, it 

will fail prematurely due to corrosion within the hydronic heater itself. Consider requiring typical 

operational settings for traditional hydronic heating systems: supply temperature 180 °F (range 160-

200 °F) and return temperatures 160 °F (range 140-170 °F)
20

• Revise methods for temperature and flow measurements. Use a thermopile for temperature 

measurements to improve accuracy. Place the flow meter on the input side of the load side as the 

temperature of the water entering the heat exchanger should be reasonably constant

• Revise the frequency for data collection from 10 minutes to < 15 seconds for values such as water 

temperatures, flow rates, stack temperature and gas concentration. Temperature changes of >10 

degrees were observed over a ten minute period. During cycling a ten minute period may not 

adequately represent the peaks and valleys of the cycle. A shorter time period will allow a better 

output measurement.

• Ensure water density calculations are based on the temperature nearest the flow meter and that the 

water density is based on temperatures over the data collection period (i.e. 15 seconds) as opposed to 

the average for the entire test (i.e. eight hours)

• Require reporting of CO2 data. This will help to characterize combustion performance and provide a 

check on efficiency values

• Require reporting of CO data. CO presents a safety concern and is particularly important for units 

installed indoors, which will become more common in the market and are tested under M28 WHH.

Similar to CO2, it also characterizes combustion performance.

For the revisions to M28 OWHH currently planned, it is recommended that the energy output calculations based on 

the supply side measurements of water temperature and flow rate be replaced with energy output determined using 

temperature and flow rates measured on the load side of the heat exchanger. With a greater temperature difference, 

the accuracy is expected to be considerably better as demonstrated in this report. In some prior tests, a recirculation 

system on the load side was used, which led to low temperature differences but high flows. Such a system decreases 

the accuracy and should be discouraged. In addition, flue gas CO2 measurements and a stack loss efficiency 

determination, which are not currently required in M28 OWHH should be added. Incorporating these revisions is 

expected to provide greater accuracy and consistency with the certification tests. It is anticipated that the revised test 

method will be posted upon completion of new partnership agreements with EPA.

6.5. Additional Considerations for the NSPS

Looking forward to the NSPS, the applicability of the test method to the next generation technology needs to be 

analyzed as distinct operational differences come with the advancements. Two-stage, gasification wood hydronic 

heaters are common place in Europe and beginning to enter the market in the United States. These units incorporate 

advanced control strategies to ensure high performance. For example, the units often use a temperature sensor and 

oxygen sensor in the stack coupled, via a control system, to a variable speed blower that regulates combustion air in 

order to ensure combustion parameters are continuously optimized. In doing so, these units can achieve efficiencies 

near 85% and emissions on the order of 0.05 lb/MMBtu. These units are capable of maintaining similar performance 

levels across a wide range of outputs, typically as low as 30% and 50% outputs for pellet and splitwood fuels 

respectively. To ensure outputs do not drop below these levels, advanced algorithms and thermal storage are used. 

The advanced algorithms reduce output as the hydronic heater nears its setpoint to prevent sudden and drastic output 

changes that typically carry performance penalties. For example, as the water temperature approaches the setpoint 

the hydronic heater will gradually ramp down its output as opposed to firing at 100% and then entering an idle 

mode. By coupling the system to thermal storage, an artificial load is imposed on the hydronic heater allowing it to 

fire at a high output regardless of the building’s heating load. In a sense, it functions as a thermal battery that can be 

charged and then drawn down as the building requires heat.

The use of the control system, oxygen and temperature sensors, and thermal storage system allow the two-stage 

units to operate under a different profile than their single-stage counterparts as time spent at idle and lower loads is 

drastically reduced. Whereas the single stage units typically operate at lower outputs, with which M28 (O)WHH 

attempts to capture via a cumulative weighting factor of 0.95 for outputs of less than 50%, the two-stage units when 
21

coupled with thermal storage typically operate at higher loads for the vast majority of the time. Therefore, a serious 

disconnect is realized when applying M28 category loads and associated weighting factors to two-stage units 

coupled to thermal storage. A review of the operational profiles of these units should be conducted in order to 

determine appropriate load limits and weighting factors.

Since January of this year EPA has been engaged in discussions with states and industry representatives to revise 

test method M28 OWHH with the goal of increasing the accuracy of efficiency and particulate emissions rates 

measurements. Many of the changes in the new method (M28 WHH) will address the issues identified in this 

review.
22

7 References

ACEEE, 2011. “Water Heating” Accessed, August 2011. http://www.aceee.org/consumer/water-heating

Brookhaven National Lab, 2004. Hydronic Baseboard Thermal Distribution System with Outdoor Reset Control to 

Enable the Use of a Condensing Boiler. Report Number BNL-73314-2004-IR. Prepared by Thomas 

Butcher, Ph.D.

Canadian Test Method Workgroup, 2010. Performance Testing of Solid-Fuel-Burning Heating Appliances. CSA

Standard B415.1-10. Mississauga, Ontario.

Lstiburek, Joseph. Builder’s Guide to Cold Climates. Canada: Building Science Press, 2004.

NYSERDA, 2011. Environmental, Energy, Market, and Health Characterization of Wood-Fired Hydronic Heater 

Technologies. NYSERDA Report, to be determined. Prepared by the U.S. EPA (Brian Gullet Ph.D. et al) 

for NYSERDA: Albany, NY. At the time of writing, this report was going through a final review.

NYSERDA, 2010A. Stage Combustion Biomass Boilers: Linking High-Efficiency Combustion Technology to 

Regulatory Test Methods. NYSERDA Report 10-19. Prepared by Brookhaven National Lab (Thomas 

Butcher Ph.D.) and Richard Gibbs Ph.D. P.E., for NYSERDA: Albany, NY (August).

http://www.nyserda.org/publications/10_19_staged_combustion_biomass_boilers_acc.pdf

NYSERDA, 2010. European Wood-Heating Technology Survey: An Overview of Combustion Principles and the

Energy and Emissions Performance Characteristics of Commercially Available Systems in Austria, 

Germany, Denmark, Norway, and Sweden. NYSERDA Report 10-01. Prepared by Bioenergy 2020+ 

(Birgit Musil-Schlaeffer) for NYSERDA: Albany, NY (April). http://www.nyserda.org/programs/

Research_Development/10-01_european_wood_heating_technology_survey.pdf

U.S. EPA. 2008. EPA Hydronic Heater Program Phase 2 Partnership Agreement. Washington, D.C.

http://www.epa.gov/burnwise/pdfs/owhhphase2agreement.pdf
NYSERDA, a public benefit corporation, offers objective 

information and analysis, innovative programs, technical 

expertise and funding to help New Yorkers increase 

energy efficiency, save money, use renewable energy, 

and reduce their reliance on fossil fuels. NYSERDA 

professionals work to protect our environment and 

create clean-energy jobs. NYSERDA has been 

developing partnerships to advance innovative energy 

solutions in New York since 1975.

To learn more about NYSERDA programs and funding 

opportunities visit www.nyserda.org.

New York State

Energy Research and

Development Authority

17 Columbia Circle

Albany, New York 12203-6399

toll free: 1 (866) NYSERDA

local: (518) 862-1090

fax: (518) 862-1091

info@nyserda.org

www.nyserda.org

NYSERDA, a public benefit corporation, offers objective 

information and analysis, innovative programs, technical 

expertise and funding to help New Yorkers increase 

energy efficiency, save money, use renewable energy, 

and reduce their reliance on fossil fuels. NYSERDA 

professionals work to protect our environment and 

create clean-energy jobs. NYSERDA has been 

developing partnerships to advance innovative energy 

solutions in New York since 1975.

To learn more about NYSERDA programs and funding 

opportunities visit www.nyserda.org.
New York State Energy Research and Development Authority

Vincent A. Delorio, Esq., Chairman | Francis J. Murray, Jr., President and CEO 

State of New York

Andrew M. Cuomo, Governor 

Review of EPA Method 28 

Outdoor Wood Hydronic Heater 

Test Results

Final Report No. 11-17

September 2011