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Updated Revisions to Method 28

2011 August 11

Revisions to EPA’s Method 28 Outdoor Wood-Fired Hydronic Heaters (OWHH) have recently been finalized.  The Method’s title has dropped “Outdoor” because it now allows for the qualification of indoor hydronic heaters, too.  The new name is EPA Method 28 WHH and the revisions will improve reproducibility and accuracy of results.  The revised method can be found at and start on page 17.  The revisions to EPA’s Hydronic Heater Partnership Agreement at this same address will be completed next month.

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10 Responses leave one →
  1. Gerry McDonald permalink
    August 16, 2011

    How do the most efficient gasification systems with heat storage get certified? These units emit virtually no visible smoke after startup, and run continuously with little to no cycling because all excess heat is stored in an insulated heat storage tank. By design, they do not operate at low heat outputs, only operate for several hours at a time and are emitting nothing between charges since required domestic heat is drawn off the storage system. How do these most efficient units compete if there is no certification test that allows them to become “EPA certified.”


  2. August 24, 2011

    Hi Gerry,

    We’re able to qualify these systems using M28 OWHH as well. If the hydronic heater’s particulate emissions are 0.32 lbs/MM Btu or less, it can qualify for EPA’s voluntary program. More information can be found at


  3. Gerry McDonald permalink
    August 24, 2011

    Dear lherring,

    I read the updates to method 28 and I don’t see where heat storage tanks are addressed. The revisions to method 28 still require testing a gasification unit at less than 25% rated output and the results from the low output categories 1 and 2 testing are weighted as 70% of the overall emissions factor for year round use.

    From what I understand, gasification units are not designed to operate at less that 50% rated output. When operatied according to manufacturer’s instructions, the excess heat, over and above the demand of a household heating load, is diverted to a thermal storage unit (i.e. a several hundred gallon insulated storage tank). A properly configured system is plumbed so that the heat storage tank can accept heat at a rate similar to that generated by the gasification system. A gasification typically burns a full load of wood with little or no cycling.

    Your method doesn’t allow for this and I would like an explanation why the most efficient gasification wood heating units with integral heat storage cannot be certified by EPA with the test methods currently available. If we are really looking to encourage use of the cleanest wood heating units, EPA should provided a method to certify these units as they are intended to be used.

    Are there other state government officials out there with similar concerns?


  4. Gerry McDonald permalink
    August 24, 2011

    I also wanted to include the link to the New York State Department of Health’s website, which provides information on wood gasification heating units and heat storage water tanks.


  5. August 25, 2011

    Hi Gerry,

    I’ve received the following response to your question from our OWHH point person, Amanda Aldridge. Feel free to contact her if this answer doesn’t suffice- or 919-541-5268.

    Method 28 refers to ASTM E2618X1 for units with full thermal storage. This is essentially what it does:

    With full heat storage there is a disconnect between heat production and heat use. i.e. the heat output goes into storage at a high rate but can be supplied to the load at almost any rate. To make the test process correspond to what happens with regular boilers, the process includes a data analysis system to equate the emissions and efficiency results to the 4 output categories used for regular boilers. The physical test requires 3 replicate runs. This establishes the total heat stored and the average total emissions. An additional test is then done to determine that rate of heat loss from the unit itself (stand-by loss). Since we then know the total amount of stored heat and the heat loss per hour, we calculate how long that appliance would produce each of the 4 heat output levels and the efficiency and emissions rates over those time frames. These results are then used in the standard weighted average scheme.

    Here’s an example without the stand-by loss included for simplification.

    Suppose the burning of 1,250,000 Btu’s worth of wood result in storage of 1,000,000 Btu’s (80% Efficient) and produces 40 grams (0.09 lb) of PM.

    The manufactures rated output is 250,000 Btu/hr

    Category IV – output =250,000 Btu/hr, Output time = 4 hours, PM = 0.022 lb/mmBtu Out, = 40/4 g/hr.
    Category III – output = 250,000 x 37.5% = 93,750 Btu/hr, Output time = 1,000,000/93,750 = 10.67 hrs, PM = 0.022 lb/mmBtu, = 40/10.67 = 3.75 g/hr.
    Category II – Output = 250,000 x 20% = 50,000 Btu/hr, Output time = 1,000,000/50,000 = 20 hours, PM = 0.022 lb/mmBtu, = 40/20 = 2 g/hr
    Category I – Output = 250,000 x 15% = 37,500 Btu/hr, Output time – 1,000,000, 37,500 = 26.67 hours, PM = 0.022 lb/mmBtu = 40/26.67 = 1.5 g/hr.

    Adding in the stand-by loss will reduce the time that the appliance can produce a specific output rate as well at lowering the total output. So this will result in some increase in the lb/mmBtu and g/hr emissions rates as the output rate is reduced. In most cases the standby loss rate is quite small and the adjustment is therefore also small. But this process is fair and allows for equal basis for comparison of performance between heat storage units and normal boilers.


  6. Gerry McDonald permalink
    August 26, 2011

    I still have a couple questions,

    First, how does this work for a high efficiency gasification unit coupled to a remote heat storage water tank? Some high efficiency gasification units only contain 50 or 60 gallons of water and are coupled with remote storage tanks that may hold 600 to more than 1000 gallons of water. How would a stand by heat loss calculation be applied in this type situation? To both the boiler and the tank?

    Speaking of stand by loss, I don’t see where stand by loss is calculated for outdoor boilers in Method 28. I would think the application of stand by loss should be reversed. Outdoor boilers lose heat to the outdoors and this heat is wasted, so a constant rate stand by loss calculation would make sense to include in the calculations. However, high efficiency gasification boilers and heat storage water tanks are located indoors, so the heat loss is actually lost to the indoor environment and is realized heat, so stand by heat loss calculation doesn’t seem to make sense.

    Can you help me better understand this and how it relates to my initial concern, how do the most efficient gasification systems with heat storage get certified?


    lherring Reply:

    Hi Gerry,

    I’ve asked Amanda to reply directly to you. You should hear back from her within a few days.

    Take care,


  7. October 6, 2011

    (This response is from Rick Curkeet, PE/Chief Engineer-Building & Hearth Products of Intertek Lab)

    We are now seeing OWHH designs that do very well on emissions throughout their operating range. Emissions rates are as low as 0.2 lb/mmBtu output even at 15% of nominal output. Year round weighted average values under 0.1 lb/mmBtu are coming soon. Some of these units have heat output ranges from 20,000 to 150,000 Btu/hr but produce emissions rates of 5 – 6 grams/hour – on par with and EPA certified wood stove. However, heat storage is also a viable technology which can allow the appliance to operate in its “sweet spot” in terms of clean and efficient combustion much of the time. Almost all appliances burn cleanest and most efficiently at higher output rates.

    The test method we use for large heat storage systems is ASTM E2618 Appendix X1. Essentially the appliance is coupled to the required heat storage and starting from the normal low storage tank temperature (about 130 F) a full fuel load is burned and all the heat is routed to the storage. The storage mass and temperature rise provide the measure of heat output. This process is done 3 times to generate an average value. Then a stand-by loss test is done. Essentially this just requires measuring the storage temperature over time (at least 8 hours) and calculating the energy loss rate. The result is a factor in terms of Btu/hr/deg. F of temperature difference between the storage tank and ambient temperature. The data accumulated in this test process provides the factors needed to essentially pro-rate the emissions measured while the fuel burned to the heat delivered under the four output rate categories specified in M28 WHH. E.g. if the storage hold 1,000,000 btus and we draw heat at 20,000 Btu/hr the system would provide heat for 50 hours. Thus there would be 50 hours of stand-by loss. Factoring this in results in a bit shorter duration. So there’s a bit of math involved, but the process is sound and allows us to rate heat storage emissions and efficiency using the same output rates and weightings that apply to regular OWHH units.

    The stand-by loss procedure is actually in ASTM E2618 which is referenced by M28 WHH for heat storage systems. For both regular and heat storage units, environmental energy losses occur, but to lab ambient temperatures of 70-80 F. With regular OWHH units, a low output test can last for 30 or more hours. During that time heat is lost from the system at a nearly constant rate so the stand-by loss is actually occurring during the test. There would be, as you have noted, higher losses for outdoor units at winter temperatures. However, these units are quite well insulated and the direct losses are typically fairly small. For a storage tank, insulation values are commonly R-10 to 15 and the result is a loss rate that is usually less than about 0.3%/hr. In the lab the average temperature difference between the appliance of storage tank is about 90 to 95 F. At a mean outdoor temperature the difference would be 130 to 135 F. about 40% greater. So the environmental stand-by loss would be about 40% higher as well or about 0.4%/hr. Individual models may be better or worse, but these losses occur in the lab as well, so the relative performance rankings would not be affected.

    We currently only have recognized methods for rating non-heat storage appliances and appliances with sufficient storage connected to accept the output of a full wood load. Adding a smaller amount of heat storage would almost certainly be beneficial, but developing a test method that properly measures the effect has proved to be very difficult. We have a couple of promising ideas, but so far no one has been able to find the resources to actually conduct experimental tests to determine if the proposed methods are workable and valid.


  8. October 6, 2011

    Hi Gerry,

    Thanks for the reminder. It has been posted.


  9. Gerry McDonald permalink
    October 13, 2011

    Leigh, thanks for posting Rick’s thoughts on this. I would ask that you please post my response on the blog also.

    I agree and it totally makes sense that batch load units operate most efficiently in their “sweet spot.” Therefore it makes sense that high efficiency dual stage gasification units that are designed and recommended to be operated in their sweet spot at 100% output with heat going to a remote heat storage tank should not be penalized because they do not operate well at low outputs and therefore cannot pass the Method 28 certification. They just aren’t supposed to operate at low output. Why require they be tested that way?

    I would disagree that the stand by heat loss calculation process is sound. The loss calculated at the highest storage tank temperatures (first 8 hours) is different from what happens in real life and results in an artificially high heat loss calculation over the period the heat is used. As the tank temperature drops over say a 40 hour period from heat usage, the delta T would be smaller later in the time period and the heat loss would be proportionally smaller. Using laboratory temperatures, a storage tank delta T would more likely be 60 F to 100 F. Regardless, calculating loss from an INDOOR storage tank doesn’t make sense. It is “realized” heat. I have never heard of anyone that needed to vent excess heat from a properly insulated thermal storage tank. I would guess that properly insulated tanks have similar R-values as outdoor OWHH units. In addition, calculating the loss from OWHH units at lab temperatures also doesn’t make sense, the losses should be calculated based on OUTDOOR winter temperatures and maximum operating temperatures since OWHH units operate continuously. The delta T based on lab temperature is likely 90 to 95F, but in real use it should be 140 to 200 F. These type heat loss calculations appear to handicap the technology-advanced, most efficient units twice. Based on the differences in how these units are sited and used in the real world, I disagree that with the assumption that performance ranking is not affected by heat loss calculations.

    One additional point, Method 28 requires the moisture content of wood to be 19% to 25% for test pieces used in Method 28. This is tailored for OWHH units where wood is typically stored outside, with lower fuel batch load temperatures and higher moisture content. High efficiency gasification units specifically recommend wood moisture content requirements of 20% or less and my experience is that these units work best when moisture content is closer to 15%. This is readily achievable when wood is stored indoors for a period of time prior to use with the added bonus that the batch load temperature of the wood fuel is 40 to 70 F higher than if the wood is stored outdoors.

    I want to thank you for your discussion on this important matter. I think everyone’s goal, including EPA’s should be to promote the cleanest most efficient technology. Test Method 28 and ASTM E2618 should not steer technology, rather let technology flourish and develop a new test method that puts all units on an even playing field (delivered heating efficiency, average daily and short term real time emission rates). There is a lot at stake here, including states’ ability to meet NAAQS and lack of an adequate test method should not have the result that the cleanest most efficient wood heating units cannot get EPA certification because the test method is only designed to test the more crude technology. I find it unacceptable that EPA does not have the resources to develop a new test that evaluates all boilers on an even basis. The Europeans have developed technology over the past 30 years as a result of environmentally conscious regulations that guided industry to design and manufacture units that are both clean and efficient. The US should be able to do the same or at least not discriminate against the best.

    Gerald McDonald, Chief
    Indoor Health Assessment Section
    Bureau of Toxic Substance Assessment
    New York State Department of Health


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